nervous system disease

nervous system disease

Introduction

      any of the diseases or disorders that affect the functioning of the human nervous system (nervous system, human). Everything that humans sense, consider, and effect and all the unlearned reflexes of the body depend on the functioning of the nervous system. The skeleton and muscles support and transport the body, and the digestive system, heart, and lungs provide nutrients; but the nervous system contains the epitome of the human—the mind—and commands all perception, thought, and action. Disturbance or malfunction of the functions of the nervous system causes changes felt throughout the body. Although many brain diseases cause disorders of thought or mood, this article discusses only diseases of the nervous system that have organic causes. For a discussion of psychological disorders, see mental disorders (mental disorder).

      The first part of this article describes the neurological examination—the medical history, the physical examination of the patient, and the diagnostic tests and procedures that can be employed to provide a physician with information about a possible neurological disorder. Next the principles used in localizing a disease within the nervous system are explained. The third part of this article provides an overview of pathological processes. Finally, an account is presented of the diseases of the nervous system, using a general classification based upon the primary or major site of the disease.

The neurological examination

Medical history
      An old saying in medicine, “Listen to the patient; he is telling you the diagnosis,” is especially true in neurology. A patient's description of symptoms is a valuable tool that allows the physician to learn about the nature and location of a possible neurological disease. While taking a patient's medical history, the neurologist notes the patient's level of awareness, memory loss, posture and gait, demeanour and expression, speech, and, to some extent, personality. The neurologist also notes symptoms such as pain, headache, loss of sensation, weakness, incoordination, wasting of certain muscle groups, and abnormal movements.

      The ability to notice and react to one's environment is not an on-off phenomenon but a continuum. From full alertness a person can descend through drowsiness to stupor, a condition in which awareness is greatly reduced and the best motor response to stimulation is a groan or other vocal (but not verbal) reaction. Deeper levels of unconsciousness pass through light coma, in which strong stimulation produces only a clumsy motor response, to deep coma, in which there is only a reflex movement or no response at all. Such depression of consciousness occurs when there is impairment of the functions of the brainstem or of the cerebral cortex. Brainstem disorders can cause coma if the brainstem is compressed by other parts of the brain swollen because of disease or if it is afflicted by local disease such as encephalitis, stroke, or concussion. Diseases of the cerebral cortex causing coma include poisoning by sedative drugs, lack of glucose or oxygen in the blood, brain hemorrhage, and certain rare infiltrative disorders in which descent through the levels of consciousness occurs over weeks or months. Brief periods of unconsciousness, of which the patient may not be aware, occur in many forms of epilepsy, narcolepsy, repeated attacks of low blood sugar, and reduction in the blood supply to the brain—particularly the brainstem.

      When pressure inside the cranium is increased, pain-sensitive structures in and around the brain are distorted and cause pain in an ill-localized area but often identifiably in the front or back of the head called a traction headache. Traction headaches (headache) may be caused by brain swelling, infection, bleeding, tumour, stress, or obstructed flow of cerebrospinal fluid. Also, pain may be felt in the head region although the disorder causing the pain is situated elsewhere; an example is the facial pain sometimes felt with lack of blood to the heart. Local disease of such cranial structures as the jaw joints, the paranasal sinuses and teeth, the middle ear, and the skull bones themselves may also generate pain.

      Tension headaches are caused by prolonged excessive contraction of the muscles that run front-to-back over the skull; these headaches are often caused by stress. A persistent pressing or pulling pain, often with a throbbing component, is usually described. Migraine headaches may occur simultaneously with tension headaches and are characterized by throbbing pain with scalp tenderness, nausea, vomiting, and sensitivity to noise and light.

Cognitive changes
      Poor concentration resulting from preoccupation, fatigue, or depression is the most common cause of memory loss, but widespread brain disease, vitamin deficiency, epilepsy, and dementia (loss of intellectual power) are also causes. When the period of memory loss is well defined, prominent causes are trauma to the head, seizures, poisoning (e.g., alcohol), and brief episodes of inadequate blood supply to the brain. Impaired comprehension, reasoning, logical thinking, and ability to plan ahead may also be symptoms of a neurological disorder.

Language and speech (speech disorder) deficits
      Patients with aphasia may know exactly what they want to say, but they are unable to express their thoughts in spoken (and often written) words. They may also be unable to comprehend the meaning of spoken or written language, so that normal speech sounds like a foreign tongue. Strokes (stroke) are the most common cause of aphasia, but any focal brain disease may be responsible.

      Similar problems of language and speech comprehension are apraxia and agnosia. Apraxia is the inability to perform useful or skilled actions; apraxic patients may be able to name an object such as a comb or key, but they may not know how to use it. Agnosia is the failure to comprehend the significance of a nonlanguage stimulus; an agnosic patient may be unable to recognize the origin of a sound from a musical instrument.

      Dysarthria, or difficulty in articulation, usually is caused by an abnormality in the nerves and muscles in and around the mouth or in their connections. Problems in the production of speech sounds, called dysphonia, often indicate a problem affecting the larynx or the nerves and muscles of that structure. Since the cranial nerves supplying these areas originate in the brainstem, neurological disease of this region may also be a cause.

Emotional disturbances
      Alteration in mood is a common sign of neurological disease, as a result of either the pathological process itself or of the patient's awareness of the disease. Although depression is most common, euphoria or mood swings may occur with disease of the frontal lobes of the brain.

Motor abnormalities
      Symptoms expressed in the limbs and trunk that may indicate neurological disease include weakness and wasting of muscles, clumsiness, and unwanted movements. Weakness or clumsiness may be caused by diseases of the cerebellum, basal ganglia, or peripheral nerves. Involuntary and repetitive movements of the body may be due to a number of disorders including Huntington disease and cerebral palsy.

      Although the central nervous system is not supplied with sensory fibres that report sensations of pain, severe pain can result from thalamic strokes. Neuralgia, or attacks of pain along the sensory nerves, are caused by irritation of some of the cranial nerves or spinal roots. Also, many disorders affecting the peripheral nerves and muscles are painful.

Change in sensation
      Alterations of sensation may be positive or negative; the former include tingling, burning, itching, and pain, while the latter consist of diminution or loss of some or all sensations. Sensations carried by large, heavily myelinated fibres, such as position, discriminative light touch, and vibration, tend to be affected together, as do those carried by the smaller, thinly myelinated or nonmyelinated fibres, including pain, crude light touch, and temperature.

Physical examination
      The physical examination begins with an examination of higher cerebral functions and proceeds methodically through an evaluation of the cranial nerves, motor systems, and sensory systems.

Higher cerebral functions
      Tests of concentration, of recent and remote memory, of language, and of ability to follow simple spoken or written commands are usually given in order to evaluate the patient's orientation in place and time. Comprehension, reasoning, and planning can be tested, for example, by asking about the similarities and differences between two objects (e.g., child and dwarf, wall and fence) or by asking the patient to explain the message or moral contained in a proverb (e.g., “People in glass houses should not throw stones”).

      If cortical disease is suspected, tests for apraxias, aphasias, and agnosias; tests measuring the ability to draw and copy simple figures; and writing tests are performed. Standardized tests for examining these functions apply quantitative measurements to any deficit, which can be analyzed further by more sophisticated psychological testing.

      The physician examines the head and neck, looking especially for local tenderness or deformity and for evidence in children of enlargement of the head, which may suggest hydrocephalus. The physician may also use a stethoscope to listen to the major blood vessels in the neck or the skull for sounds indicating unusual communication between arteries and veins. Irritation of the meninges, which may occur with intracranial infection or bleeding, leads to unusual stiffness on passive attempts to flex the neck, as well as to an inability to straighten the bent knee when the hip is flexed (the Kernig sign).

Cranial nerves (cranial nerve)
      The physician tests the olfactory nerve by placing items with specific, mild odours (smell), such as coffee, tar, or lemon, under the nose of the patient. The patient should be able to perceive, though not necessarily identify, the odour if the olfactory nerve is functioning correctly.

      An ophthalmoscope is used to test the optic nerve and to see the optic disk, the retinas (retina), and the small arteries and veins that lie upon them. Visual (vision) acuity is tested with a standard eye chart, and the visual field is examined by asking the patient to signal when he sees an object brought in toward the centre of vision from the periphery. An instrument called a perimeter may be used to determine the central and peripheral visual fields.

      The physician tests the three oculomotor nerves (oculomotor, trochlear, and abducens) together by asking the patient to gaze in different directions on command and to follow a moving object with the eyes only. The shape, size, and reactivity of the pupils—both to light and to close objects—are also tested. The upper eyelid is also examined for signs of drooping.

      The physician examines motor functions of the trigeminal nerve by asking the patient to clench the teeth and by tapping the patient's chin to test the jaw reflex. This is one of the few physiological reflexes that is normally not detected. The sensory functions of the nerve are examined by stimulating the face gently with the finger or cotton for light touch, cold steel for temperature, and a pin for scratch or pain sensation. This procedure is done for the three anatomical divisions of the nerve on each side of the face. Finally, cotton is touched to the cornea to assess the corneal reflex. Normally, an abrupt blink is produced.

      The functions of the facial nerve are examined by the patient's ability to close the eyes tightly, to bare the teeth, and to attempt to whistle. The facial nerve also carries fibres subserving the function of taste on the front of the tongue, so weak solutions of sugar, salt, lemon, or vinegar can be used to test its function. (Flavour—as opposed to the tastes of sweetness, saltiness, bitterness, and sourness—is largely mediated by the olfactory nerve.)

      To examine the cochlear nerve, hearing tests are used to determine the patient's overall acuity to the whispered voice. The Rinne test differentiates the patient's ability to hear the hum of a tuning fork held both beside the ear and on the mastoid bone of the skull behind the ear. If the sound is louder at the latter site, impairment of the conduction of vibrations through the three small bones in the middle ear is likely, while if the former sound is louder, any deafness is likely due to disease of the inner ear or of the cochlear nerve. The Weber test consists of placing the tuning fork on the forehead; the sound is better perceived either in the ear without nerve deafness or, paradoxically, in the ear affected by mild middle-ear deafness. Further testing may be performed in an audiometry laboratory.

      Tests of the vestibular nerve are not routinely performed. The usual screening procedure is to tip the patient's head back and down 45 degrees and await the appearance of vertigo or nystagmus. Further testing may be performed in a laboratory and includes the irrigation of the external ear canals with warm or cool air or water, rotation of the patient, and instruction of the patient to gaze in various directions to assess nystagmus.

      To examine glossopharyngeal and vagus nerve function, the physician tests for the presence of touch sensation on the soft palate and the back of the throat (the latter usually eliciting a gagging reflex), the elevation of the palate on phonation (which should be symmetrical but rises to the stronger side in the presence of weakness on one side), the quality and loudness of the voice, and the normal slowing of the heartbeat when one carotid artery is compressed in the neck.

      The sternocleidomastoid and trapezius (trapezius muscle) muscles, supplied by the accessory nerve, are tested by the patient pushing his head forward and shrugging his shoulders upward against the physician's resistance.

      Atrophy and weakness of the tongue muscles supplied by the hypoglossal nerve can be assessed by measuring the deviation of the tongue when extended toward the weak side.

Motor systems
      The presence of gait and postural disturbances, of abnormal movements, and of atrophy may be noticed when the physician is taking the medical history. Physical examination of the motor systems of the patient may confirm initial suspicion of these abnormalities.

      Inspection of the body may show patterns of muscle atrophy. Depending on the pattern of atrophy, lesions may be present in the nerve roots, in more peripheral locations of the nerves, or in the muscles. Symmetrical atrophy is more likely to indicate primary muscle disease, while unilateral atrophy (i.e., affecting only those muscles receiving their motor supply from a single nerve) usually suggests a lesion of the supplying nerve.

      If accompanied by atrophy and weakness, brief, irregular, involuntary twitches of muscles that do not lead to the movement of a joint but are visible and can be felt by the patient may be a symptom of serious underlying motor neuron disease (see below Diseases and disorders: The peripheral system (nervous system disease)). Other abnormal movements, such as chorea and dystonia, and changes in the skin and joints that may be caused by nerve or muscle disease are also noted.

Muscle tone
      When the physician flexes or extends the joints in a normal, relaxed limb, a certain resistance, known as tone, is detected. This resistance decreases whenever the reflex arc is damaged (usually at the level of the peripheral motor or sensory nerve), but it may also decrease with primary muscle or spinal cord disease. An increase in resistance occurs with the presence of a lesion of the upper motor neurons—that is, anywhere along the pathway from the motor cortex to the ventral horn neurons in the spinal cord—by which the muscles in question are supplied. This hypertonia may increase against resistance and then suddenly decrease (“clasp-knife spasticity”), or it may be constant throughout the range of movement (“plastic” or “lead-pipe” rigidity). In the presence of tremor this latter form of hypertonia produces ratcheted, jerking movements, thus the name cogwheel rigidity. Rigidity may suggest a lesion of the basal ganglia, but spasticity implies disease of the direct corticospinal tracts.

      Power is tested either by examining single muscles if a local or lower motor lesion is suspected or by assessing the strength of several muscles by flexing them or extending joints. The latter method is used if an upper motor lesion is suspected.

Reflex activity
      Three main types of reflex activity are tested: an increase in the speed and strength of the reflex response, a decrease in response, and the presence of abnormal reflexes. Using a reflex hammer, the physician taps a tendon while the patient is relaxed and observes the response—usually a single brief, brisk contraction of the appropriate muscle. Response is normally increased if muscles contract elsewhere in the body. When an upper motor neuron lesion is present, the response is excessive and the muscle may contract repeatedly.

      Tendon reflexes are diminished or absent in the presence of a lesion of the lower motor neuron, of the muscle itself, or of the afferent (sensory) side of the reflex arc. Superficial reflexes that cause the underlying muscles to contract should be elicited by stroking the wall of the abdomen with a thin stick. Unlike tendon reflexes, superficial reflexes disappear in the presence of a corticospinal tract lesion.

      The plantar, or Babinski, response is the only abnormal reflex that is routinely detected. Normally, the big toe curls downward when the examiner draws a stick up the sole of the foot. In the presence of a corticospinal tract lesion, it curls upward instead, and the other toes may fan out.

      Tests employed to assess cerebellar function in the limbs include asking the subject to touch, successively, the physician's finger held before him and his own nose, to run one heel down the opposite shin, or to perform piano-playing movements with the fingers. The patient may also be asked to outstretch his arms to see if they properly return to a resting position.

Sensory systems
      Sensory testing helps determine which, if any, modes of sensation (sensory reception, human) are impaired and the degree and area of impairment. Results of careful sensory testing may allow the physician to localize the site of a lesion in the nervous system with great accuracy. To achieve this localization, it may be necessary to compare sensation in areas of the body innervated by different spinal segments or in areas supplied by different parts of the brain or spinal cord.

      The physician's finger, a cotton applicator, or a paintbrush may be used for testing sensations of light touch; steel or glass tubes filled with warm and cold water for temperature; a pin for superficial pain and scratch sensation; a tuning fork for vibration; and gentle movement of a finger up or down outside the patient's range of vision for the sense of passive joint movement. Discriminative touch can be tested by determining whether the patient can identify an object placed in his hand, whether he is being touched by one or two ends of a compass, or where he was touched or what number or letter was drawn on his skin by the physician.

Diagnostic tests and procedures
      Further useful data to arrive at a diagnosis may be acquired from the results of the tests and procedures described below.

       electroencephalography (EEG) is a routine procedure, used mainly to localize the origin of epileptic seizures (epilepsy) but also to localize and, occasionally, indicate the nature of brain diseases. EEG may also be utilized to indicate the degree of brain disease in such metabolic disorders as liver failure and some viral illnesses.

      An electroencephalogram is produced by placing electrodes on or in the scalp and then recording the changes in electrical potential that occur while the subject is at rest or stimulated by flickering light, weak electric shock, medication, or sound. These changes, recorded as waveforms on the electroencephalogram, are very similar in all humans. Their absence, delay, or distortion indicates disease in the central conducting pathways of the nervous system, thus allowing further localization of disease (but not indicating the nature of the responsible cause). Computerization allows the generation of “maps” of electrical activity and the more precise localization of abnormal electrical discharges.

       electromyography (EMG) is the examination of muscular electrical activity by means of fine needle electrodes inserted into the muscle. Muscular contraction produces electrical activity, which increases as the contraction grows stronger. The waveforms recorded with primary disease of muscles differ somewhat from those that occur when the muscles are deprived of motor innervation. Single-fibre EMG (SFEMG) is a technique in which even fewer muscle fibres are examined.

      The speed of conduction of impulses along sensory and motor fibres can be measured with nerve conduction studies (NCS). The muscle is stimulated with a small electrical charge, which generates an impulse. The impulse moves along the nerve fibre and eventually reaches a muscle, which contracts. NCS can localize the site or sites of peripheral nerve disease and may even indicate the nature of the disorder affecting them.

      Examination of the pressure and the composition of cerebrospinal fluid can aid in the diagnosis of central nervous system infections, some tumours (tumour), and multiple sclerosis. In a lumbar puncture, also known as a spinal tap, cerebrospinal fluid is obtained by inserting a needle through the skin in the small of the back (below the termination of the spinal cord) so that it passes between the vertebrae into the fluid sac surrounding the spinal cord and nerve roots. High protein levels in the fluid are often a sign of neurological disease.

      A diagnosis may be made by biopsy, the direct examination of surgically removed nerve, muscle, or brain tissue. Special stains are often used to increase diagnostic accuracy. A number of disorders affecting the central and peripheral nervous systems can be differentiated only by their appearance under the microscope.

X ray (X-ray)
      Diseases affecting the skull (malformations, increased intracranial pressure, some metabolic diseases, tumours, and trauma) and spinal cord disorders can be diagnosed with conventional X rays, but X rays employing the injection of iodine-containing contrast media or air, often under a general anesthetic, into an artery, vein, the spinal cord, or, during a surgical procedure, the ventricles of the brain, may provide more valuable information. Such studies allow better visualization of the spine (myelography), of the ventricles (ventriculography), and of the arteries or veins within the cranium and neck (angiography and venography). In most cases, however, even contrast X rays give only a silhouette of the lesion or its blood supply.

Computed tomography (computerized axial tomography)
      Computed tomography (CT), developed in the 1970s by William Oldendorf and Godfrey Hounsfield, is an X-ray technique that allows for the visualization of 3- to 10-mm (approximately 0.12- to 0.4-inch) sections of the brain, skull, and spinal column (as well as other parts of the body) in two dimensions. A person must lie still during the procedure, but it is painless. Contrast medium is occasionally utilized. The clear distinction between black, gray, and white areas of the image allow pathological diagnosis in many cases.

      Magnetic resonance imaging (MRI) is performed by placing the patient within a magnetic coil and applying radio waves to the part of the body being examined. These harmless waves excite protons that form the nuclei of hydrogen atoms in the brain. The protons then give off measurable electrical energy, which, with the aid of a computer, can be used to construct a map of the tissue. Since MRI poorly visualizes bone, excellent images of the intracranial and intraspinal contents are produced.

      Positron emission tomography (PET) employs inhaled or injected radioisotopes and computer techniques to map the metabolic activity of the brain. PET is of particular value in the diagnosis of certain degenerative and metabolic disorders.

      The blood-brain barrier keeps large molecules from passing into the brain or spinal cord from the blood. When this barrier is destroyed around tumours, blood clots, infarcts, or infections, fluid and dissolved substances can pass into the brain. Some radioisotopes injected into the bloodstream also can cross into brain tissue. Measured by outside detectors, the radioactivity of the isotopes can produce a map of areas where the barrier between the brain and the bloodstream has been destroyed by disease. This technique can detect intracranial pathologies, although the CT scan is more accurate. Isotopes are also used to visualize cerebral blood flow in patients with cerebrovascular disease, as well as the flow patterns of cerebrospinal fluid in patients with dementia or a skull fracture.

Localization of neurological disease
      The nature and pattern of the symptoms and physical signs of neurological disease allow inferences to be drawn about the sites of the lesions causing them.

Lower-level sites
      One symptom indicating muscular disease is weakness, usually symmetrical (that is, affecting both sides of the body) and mainly affecting the proximal or girdle muscles. This type of weakness may be noticed when climbing stairs, arising from a deep chair, brushing the hair, or lifting an object. Facial weakness results in drooling and in difficulty in whistling. Weak masticatory muscles tire easily, so that food is chewed with difficulty, while bulbar muscle involvement leads to problems with phonation, articulation, and swallowing. Diseased muscles may also swell and be tender to the touch, or they may cramp. In the condition known as myotonia they continue to contract even when the individual tries to relax the muscles.

Motor end plate
      Where fatigue and weakness are the symptoms, the underlying cause of disease may be a failure of motor nerve impulses to cross to the muscle end plate at the neuromuscular junction.

Peripheral nerves
      Diffuse disease affecting the peripheral nerves may have a greater impact on either motor or sensory fibres, or it may affect both to an equal degree. Commonly, nerves are affected according to their length, the longest ones “dying back” from the periphery, being least able to sustain vital metabolic processes. In such cases of generalized neuropathy, the signs tend to be symmetrical and most obvious in the extremities. In other cases, individual nerves are affected as a result of compression or vascular disease.

      Symptoms of motor nerve damage include weakness and muscle atrophy. Sensory nerve damage may cause numbness, paresthesia (tingling), shooting or burning pains, and hyperesthesia (painful sensitivity to stimuli). In both motor and sensory neuropathies, reflex activity is reduced or absent.

Spinal nerve roots
      The symptoms and signs of damage to the spinal roots are the same as for peripheral-nerve damage except that the area of involvement is restricted to the area supplied by the spinal roots rather than the nerves. Also, generalized symmetrical sensory loss is not seen in spinal root damage.

      Damage to the spinal cord often results in a combination of the signs of root lesions (often including pain) at the site of the lesion with signs of damage to tracts below that level. For example, injury to the cord at mid-thoracic levels spares the arms, which are innervated by fibres originating from higher segments, but it causes characteristic signs (abnormal posture, spastic tone, weakness, increased deep reflexes, and abnormal plantar reflexes) of damage to motor neurons originating below that level—as well as the loss of bladder and bowel control.

      Loss of function in ascending sensory pathways results in the loss of superficial pain, temperature, crude light touch, and scratch sensations if the spinothalamic tract is damaged, but it will cause loss of joint position, vibration, and discriminative light-touch sensations if the dorsal columns are the site of injury. Because the fibres cross shortly after they enter the cord, spinothalamic-tract lesions on the left side of the spinal cord lead to loss of sensations on the right side of the body below the lesion. This is not true of lesions of the dorsal columns, which carry fibres originating from the same side of the body and cross in the brainstem.

      Damage to sympathetic autonomic fibres that run in the cervical portions of the spinal cord may lead to drooping of the eyelid (ptosis) and a smaller pupil on the same side as the injury (Horner syndrome).

Higher-level sites
      Damage to the brainstem threatens life, since so many of the control centres for many functions, including consciousness, respiration, and blood pressure, are situated there. As with lesions of the spinal cord, localization of the level of the lesion is determined by noting which of the cranial nerve functions are affected.

      A midline lesion of the medulla oblongata is likely to involve the pyramidal tracts (the descending motor pathway) and the medial lemnisci (the ascending tracts relaying sensory impulses from the dorsal columns of the spinal cord). This lesion may produce signs of an upper motor-neuron lesion and dorsal column-type sensory loss at all levels below the medulla. Such signs could theoretically be produced by a lesion located between the midmedulla and the third cervical segment of the spinal cord, but the additional finding of a hypoglossal nerve palsy with atrophy of the tongue accurately localizes the lesion in the midline of the medulla, where the nucleus of this cranial nerve is located.

      A lesion of one side of the medulla spares the pyramidal tracts and medial lemnisci, but it involves the sympathetic pathways, the fibres entering the cerebellum in its inferior peduncle, some of the 8th, 9th, and 10th cranial nerve nuclei, the descending nucleus and tract of the 5th cranial nerve on the side of the injury, and the ascending fibres of the spinothalamic tract from the opposite side of the body. Signs of a lesion, therefore, include: on the side of the lesion, incoordination, drooping eyelids and small pupils, and loss of light-touch and pinprick-pain sensation of the face; vertigo and vomiting; and loss of spinothalamic function (light-touch and pinprick pain again) on the opposite side of the body.

      Damage to the oldest part of the cerebellum, which lies deep in the midline, results in difficulty in maintaining an upright posture. Nystagmus (jerky movements of the eyes at rest) is also likely. The vermis and anterior lobes of the cerebellum developed later in evolution; lesions of these structures particularly affect gait. The lateral lobes are the most recent parts of the cerebellum to develop; if they are damaged, ataxia (incoordination) of the limbs may occur so that arm and leg movements are awkward and impaired by a possibly severe tremor.

Basal ganglia and thalamus
      Thalamic (thalamus) lesions lead to loss of all sensation on the opposite side of the body, sometimes accompanied by extreme pain. Since tumours and strokes affecting this region are also likely to damage fibres in the adjacent internal capsule, signs of damage to upper motor neurons may also be present at all lower levels, thus affecting the cranial nerves as well as the spinal segments. Disorders of eye movement and speech sometimes result from thalamic lesions.

      Lesions of the hypothalamus affect regulation of metabolism, including water and solute control, sexual activity, and appetite for food.

      Basal-ganglion diseases lead to loss of control over movement, resulting in involuntary movements or reduced spontaneity or speed of voluntary movement.

Cerebral hemispheres
      The frontal lobe, which lies rostral to the central sulcus, is involved with many of the components of intelligence (foresight, planning, and comprehension), with mood, with motor activity on the opposite side of the body, and (in the case of the dominant hemisphere) with speech production. Swelling of the underside of the frontal lobe may compress the first cranial nerve and result in the loss of smell. Irritation of the frontal cortex may also cause either generalized or focal motor epileptic seizures, the latter involving the opposite side of the body.

      Damage to the dominant temporal lobe, located inferior to the lateral sulcus, results in difficulty with comprehension of spoken speech. The right temporal lobe (usually nondominant for speech) has a special role in the appreciation of nonlanguage sounds such as music. Irritation of a temporal lobe may lead to auditory or olfactory hallucinations. Memory functions are duplicated in the two temporal lobes; if one lobe is damaged, there may be little effect, but bilateral damage leads to a permanent inability to learn new data.

      In most people the left parietal lobe shares control of the comprehension of spoken and written language and of arithmetic, interprets the difference between right and left, identifies body parts, and determines how to perform meaningful motor actions. Damage to this lobe, located posterior to the central sulcus, leads to forms of apraxia, the inability to perform purposeful actions. The right parietal lobe is concerned with visuospatial orientation, and damage typically leads to deficits such as dressing apraxia (inability to put on clothes), constructional apraxia (difficulty in creating or copying two- or three-dimensional forms), and sensory competition, or sensory extinction, which is an inability to recognize two stimuli when both are presented together on opposite sides of the body—most easily demonstrated in the sensations of touch and vision. Each parietal lobe is also involved with so-called cortical sensation or discriminative touch, the analysis and interpretation of touch sensations originating on the other side of the body. Damage to the parietal lobe can cause a form of agnosia in which sensation is present but interpretation or comprehension is lacking. Irritation of the parietal lobe also leads to tactile hallucinations, the false perception of touch sensations on the other side of the body.

      The occipital lobes, which lie below and behind the parieto-occipital sulcus, are almost exclusively involved with the reception of visual impulses. Damage to one side results in homonymous hemianopia, the loss of all sight in the field of vision on the opposite side. Compression of the optic chiasm, usually by a tumour of the pituitary fossa, may result in the “blinkers” effect. At the optic chiasm the optic nerve fibres from the nasal halves of the right and left retinas cross to the opposite side. Since the nasal retinas “see” the temporal fields (the right nasal retina receiving impulses from objects to the right, the left from objects to the left), a patient with a lesion of the optic chiasm is able to see straight ahead but not to either side. This is called bitemporal hemianopia.

      Irritation of the occipital lobe causes the subject to see hallucinations (hallucination). If the lesion is far back in the lobe, the hallucinations may be of unformed lights, colours, or shapes. However, they also may be vivid and sharply defined pictures, as though a videotape of previous visual experiences were being replayed, if the lesion is farther forward and in the area where the parietal, temporal, and occipital lobes adjoin. This area of the cortex appears to be involved with the analysis and storage of complex perceptions.

Pathologies
      Many types of disease affect the nervous system. A short description of these types and an overview of representative disorders follows.

Genetic disease (genetic disease, human)
      Inherited neurological diseases are relatively common and may affect any part of the nervous system. Examples of genetic diseases are: Duchenne and other muscular dystrophies; hereditary motor, sensory, or mixed neuropathies; spinocerebellar degenerations; disorders of the fetal neural tube; certain metabolic disorders; and maldevelopment or premature degeneration of parts of the nervous system, seen in Huntington disease and Alzheimer disease. A genetic component also determines a tendency to epileptic seizures.

Infections
      Although the blood-brain barrier protects the nervous system from microorganisms, it may be damaged by bacteria, viruses, fungi, and other organisms. If damage occurs, resistance to infection of the nervous system is decreased. The major classes of inflammatory disease are meningitis and encephalitis (infections of the meninges, or covering membranes, of the brain and of the brain itself, respectively) and abscesses, which may be situated outside or just below the meninges or in the brain itself. Some viruses lie almost dormant within neural tissue for years before causing destruction. The measles virus may do this, causing a chronic progressive encephalitis that is usually fatal.

      Direct infections of muscles and of peripheral nerves are uncommon except in leprosy. Both the central and peripheral nervous systems may be severely damaged by an immune system reaction after relatively trivial infections elsewhere in the body. Finally, in some infections such as diphtheria and botulism, a toxin liberated by the infectious organism attacks the nervous system.

Trauma
      Traumatic and mechanical injuries can affect all levels of the nervous system. Serious head injuries can lead to compression, laceration, or bruising of the brain inside the cranium. Compression is commonly due to a blood clot inside the brain or formed outside or beneath the dura mater (extra- or subdural hematoma), but inherent swelling of the brain tissue following trauma is also possible as fluid leaks from small blood vessel spaces into the extravascular space (brain edema). Later consequences, such as seizures and alterations in mental processes and personality, indicate damage done to the brain by major or repeated minor head injuries. Less serious head injuries may cause a concussion, a temporary loss of awareness.

      Once destroyed, neurons (neuron) are not replaced. Severed axons sprout growths from the cut end, but they do not form effective connections in the brain or spinal cord. Therefore, destructive injury of, for example, the spinal cord is followed by permanent loss of voluntary movement and of sensation below the level of injury.

      In the peripheral nervous system, axons that have been severed or have degenerated because of disease can grow again. If the nerve remains in continuity, sprouting axons may eventually reestablish effective connections with muscle fibres or sensory organs, but reinnervation is often abnormal because of division and misdirection of growing axons.

Neoplasms
      Both benign and primary and secondary malignant tumours (tumour) can attack the meninges, the brain, and the spinal cord, its roots, and the peripheral nerves. In the latter sites the tumours are usually benign. The brain and spinal cord may also be compressed by tumours arising in adjacent areas, such as the pituitary gland, the spinal column, or the nasopharynx. Tumours destroy the tissue in which they develop, by compressing neighbouring tissue and by increasing intracranial pressure. Those arising outside the nervous system may lead to disease of the brain, spinal cord, nerve, end-plate, or muscle without direct infiltration of these tissues.

Metabolic, toxic, and endocrine disorders
      Numerous complications of disease of other organs affect the nervous system. The largest category includes metabolic disorders, many of which have a genetic basis. The most common neural complications are those that result from toxic agents such as bacterial toxins, drugs, and other chemicals. Neural disorders also can result from mineral and electrolyte dysfunction, from abnormal fluid, or from diseases of the endocrine glands (diabetes, thyroid disease), the kidneys, the liver, the lungs (respiratory failure), and the blood (anemia, leukemia).

Vascular diseases
      The central nervous system has a high metabolic rate and depends upon a liberal blood supply for nutrition and for clearance of metabolic waste-products. Vascular conditions, including interruption of circulation, disrupt nervous function; if circulation is not restored speedily, irreversible damage to neurons results. (The area of dead nervous tissue is known as an infarct.) The peripheral nervous system is much more tolerant of blood supply loss, called ischemia. Another vascular cause of brain damage is hemorrhage due to rupture of an artery.

Demyelinating and degenerative disease
      Demyelinating disease is characteristically scattered throughout the brain and spinal cord, with small foci where the myelin is destroyed and replaced with glial scar tissue. Nerve fibres running through such areas may remain undamaged, but the lack of insulation normally provided by the myelin leads to a blockage of conduction of electrical impulses. In certain very rare diseases, known as leukodystrophies, large areas of demyelination occur in the white matter of the brain as a result of inherited metabolic disorders. Demyelination is also the result of an immune attack upon the central nervous system, which may follow viral infections such as measles, or of immune deficiency states such as pernicious anemia. Schwann cells, which form the myelin surrounding the axons of peripheral nerves, may suffer immune or toxic attack, as in Guillain-Barré syndrome and diphtheria. This also leads to a blockage of electrical conduction. When an injury is primarily to axons, the Schwann cells are also damaged, producing “secondary demyelination.”

      Degenerative diseases are characterized by progressive destruction of neurons without evident cause. Such diseases include Alzheimer disease, affecting primarily the cerebral cortex; Huntington and Parkinson diseases, affecting the basal ganglia and cortex; motor neuron disease, affecting the corticospinal tracts and ventral-horn cells; spinocerebellar degenerations, involving long tracts in the spinal cord, the cerebellum and its connections, and sometimes peripheral nerves; and the hereditary neuropathies with no known metabolic basis, in which motor, sensory, or both classes of peripheral nerves degenerate and lose their function. Most such diseases develop after a variable period during which that part of the nervous system subsequently affected apparently functions normally. As research progresses, the metabolic (and often genetic) bases of these diseases are being successively revealed.

Electrical disorders
      Electrical disorders present a unique problem of classification. Because the nervous system is a biologic electrochemical complex, it is possible to regard every aspect of its function as an electrical manifestation, but it is primarily the epilepsies (epilepsy) that are regarded as expressions of disordered electrical activity. Seizures represent occasional, sudden, rapid, local, and excessive electrical discharges of the gray matter of the brain. There are two main classes, those with a focal origin anywhere within the gray matter and those in which there is no abnormal pathology, but an abnormally low threshold for electrical activation is present. Although certain abnormal movements occasionally have their origin in spinal cord disease, epilepsy is uniquely a brain disease (see below Diseases and disorders: The cerebrum (nervous system disease)). Narcolepsy, a disorder of sleep mechanisms, is not a variant of epilepsy.

Autoimmune disorders
      It is uncertain whether any diseases of the nervous system can properly be regarded as representing an allergic cause in the strict sense of the term, but many conditions characterized by an inflammation of the blood vessels, such as the auto-allergic collagen-vascular diseases, do have such a basis. These, through reduction of blood supply to parts of the nervous system, may induce neurological symptoms.

Diseases and disorders

The skull and spine
Neural tube defects
      The neural tube, the embryonic structure that develops into the central nervous system, normally closes by the end of the third week of fetal growth; severe deficits result if it fails to close. Examples of neural tube defects (neural tube defect) include the absence of brain (anencephaly) and a cyst replacing the cerebellum. The spinal canal or cord may also fail to close up. spina bifida is a neural tube defect that varies in severity. In spina bifida occulta there is only X-ray evidence of damage to the spinal cord. The meningocele form of the disorder is characterized by a meningeal pouch that visibly projects through the skin. Spina bifida meningomyelocele is diagnosed when such a pouch contains elements of the spinal cord or nerve roots. Function of the legs and bladder and bowel control is often severely impaired in individuals with spina bifida. Infants with the defect commonly have hydrocephalus as well.

Cephalic disorders
       hydrocephalus, the accumulation of cerebrospinal fluid in the ventricles, or cavities, of the brain, causes progressive enlargement of the head. The condition usually results from a congenital malformation that blocks normal drainage of the fluid. A tube called a shunt is required to drain cerebrospinal fluid from the brain and prevent further expansion of the skull.

      Macroencephaly is a congenital malformation in which expansion of the brain usually results from a variety of disorders, including hydrocephalus. In Aarskog syndrome the eyes are set unusually far apart, and in craniostenosis the sutures of the skull do not develop, so that the skull grows abnormally. In hemiatrophy half of the skull and face may develop abnormally, in which case the brain also may be unusually small.

      Platybasia, an abnormal shallowness of the base of the skull, is a malformation that may be associated with projection of the vertebral column upward. This condition may also occur in association with bone diseases such as osteomalacia and Paget disease of bone in adulthood. In the Arnold-Chiari malformation, cerebellar or medullary tissue projects downward into the upper cervical spinal canal, causing cerebellar dysfunction, hydrocephalus, or widening of the central canal of the spinal cord with damage to surrounding fibre tracts. Fusion of the upper cervical vertebrae occurs in Klippel-Feil syndrome.

Fractures
      Fractures (fracture) of the skull are common, and no treatment is required if they are linear and not depressed (and thus not liable to irritate the underlying brain). Fractures crossing the middle ear and the nasal sinuses may provide a portal for the entry of microorganisms into the cranial cavity, and those at the base of the skull may damage cranial nerves (cranial nerve).

      Fractures and dislocations of the spine occur most commonly in the neck, with consequent risk of spinal cord damage; at lower levels the thoracic rib cage makes spinal cord compression less likely. Fractures of the odontoid, the bony peg that forms a joint between the upper two vertebrae of the neck, may compress and severely damage the spinal cord; the odontoid process may also separate with the same consequences in rheumatoid arthritis. Compression fractures at lumbar levels may damage the cauda equina, the tail of nerve roots below the level of the spinal cord, but because of the strength of the ligaments, complications are less common.

Tumours
      Tumours (tumour) of the skull base may compress the lowest cranial nerves or the medulla oblongata and upper cervical spinal cord, with the consequences described above in the reference to the Arnold-Chiari malformation.

Vertebral disorders
      The most common disorders affecting the spine (vertebral column) are degenerative, most often following trauma such as hard labour or whiplash. In spondylosis, bony spurs called osteophytes project from vertebrae and become denser, and vertebral disks degenerate and protrude. More commonly protrusion of a vertebral disk causes the spinal canal to narrow, distorts the local ligaments, and compresses an emerging nerve root, resulting in pain, weakness, and numbness in the area. Lumbar spinal stenosis is a condition characterized by the narrowing of a few segments of the spinal canal when the vertebrae, disks, and ligaments protrude, compressing the nerve roots of the cauda equina. Pain also occurs when misalignment of vertebrae causes stretching of the joint capsules at sites where the vertebrae are contiguous. Spondylolisthesis is a disorder in which one vertebra slips forward onto another; it may occur as a congenital deformity or result from trauma.

      Infections, tumours, and bone diseases are also responsible for vertebral disorders, causing pain and damage to the spinal roots and perhaps to the spinal cord as well. Although direct infections of the spinal cord are rare, pyogenic or tuberculous epidural spinal abscesses are treatable diseases. Tumours of the spinal cord are usually secondary to such malignancies as lymphomas or carcinomas of the breast, prostate, or kidney. Benign tumours of the spinal cord may also occur. Paget disease, osteomalacia, and osteoporosis may cause softening of the bone, which then can compress the cord or roots.

The meninges and cerebrospinal fluid
Raised or decreased intracranial pressure
      The circulation of cerebrospinal fluid may be obstructed so that it accumulates in the skull. This condition, called hydrocephalus, may result from congenital stenosis, or narrowing, of the aqueduct of Sylvius, tumours (tumour), meningitis, or blood accumulating within the ventricles. Accumulation of cerebrospinal fluid causes massive enlargement of the skull, degeneration of the brain, and increased intracranial pressure. Symptoms of hydrocephalus may include motor disturbances, cranial-nerve palsies, visible swelling of the head, headache, dementia, and vomiting. Surgical insertion of a tube called a shunt is necessary to drain cerebrospinal fluid from the skull.

      An increase in intracranial pressure can result from any mass lesion in the head (a blood clot, tumour, or abscess, for example) or external compression of the cerebrospinal fluid pathway (such as the Arnold-Chiari malformation). The brain cannot shrink, and compensation is possible only through a reduction in the amount of fluid present in the ventricles or of blood in the vessels. Benign intracranial hypertension is a condition in which there is increased intracranial pressure caused by various metabolic and endocrine diseases; lesions and fluid obstruction are absent.

      Cerebral edema is the presence of excess fluid within either the cells or the extracellular tissues of the brain. This disorder also causes brain swelling and a rise in intracranial pressure. Head injuries, encephalitis, abscesses, lack of oxygen, tumours, strokes, and toxic agents are the most common causes of cerebral edema.

      Cerebrospinal fluid may leak through skull fractures or after a lumbar puncture (spinal tap). As a result of reduced fluid pressure, the brain pulls upon the meninges, causing what is called postlumbar puncture headache.

Blood clots (thrombosis)
      Blood clots lying outside or just below the dura mater (called extradural or subdural hematomas) are other complications of trauma. Extradural hematomas may be complications of fractures of the temporal bone that rupture the middle meningeal artery. Arterial blood, shed under pressure, separates the dura from the underside of the skull bone, forming a rapidly expanding mass that raises intracranial pressure, compresses the brain, and may be fatal unless evacuated surgically. Chronic subdural hematomas expand very slowly and may only be discovered because of seizures, dementia, or other neurological signs.

       meningitis is an inflammation of the meningeal (meninges) coverings of the nervous system, with possible involvement of the brain. Viruses such as mumps, Coxsackie, and ECHO viruses, tuberculosis, fungi, spirochetes, bacteria, protozoa, and some chemical agents may cause the disease. Organisms most often reach the meninges via the blood, but direct spread may occur with skull fractures, middle-ear or nasal-sinus infections, or congenital defects of the meninges. Symptoms depend upon the infectious organism and the resistance and age of the patient, but they usually include lethargy and drowsiness, fever, headache, stiffness of the neck, vomiting, and (in smaller children) seizures. Patients with nonbacterial, or aseptic, meningitis also have fever, headache, and other meningeal signs, but they are not so obviously ill. Residual consequences of meningitis include cranial nerve palsies (especially loss of hearing), hydrocephalus, and brain damage.

      Since treatment with the correct antibiotic is essential to treat meningitis, the most important diagnostic aid is lumbar puncture, the examination of the cerebrospinal fluid. The amount of protein, the number and type of cells, and the glucose level of the fluid confirm the type of meningitis.

Tumours (tumour)
      Tumours (tumour) affecting the meninges are usually malignant and commonly spread from cerebral tumours such as medulloblastoma, from distant tumours such as carcinoma of the lung (lung cancer) or breast (breast cancer), and from lymphomas (lymphoma) and leukemia.

      Benign tumours arising from the meninges are called meningiomas. These tumours occur over the convexity of the brain and on the floor of the cranium, where they compress and damage the brain or cranial nerves and may cause seizures. Meningiomas may be removed successfully.

The peripheral system
Neuropathies

Neuronal neuropathies
      Neuronal neuropathies affect the axon or cell body of ventral-horn neurons or dorsal-root ganglion neurons. Damage to the ventral-horn neurons causes reduced muscle tone and power and reduction or loss of reflexes with no change in sensation. Damage to the dorsal-root ganglion neurons also causes reduced reflexes, in this case because the afferent, or sensory, limb of the reflex arc is interrupted. Loss of joint-position sense, discriminative light touch, vibration, pain, temperature, and scratch sensations may be lost or impaired. Pure sensory neuronal neuropathies may be hereditary. Other causes include toxic drugs or other agents, diabetes (diabetes mellitus), herpes zoster, vitamin deficiency, and cancer. Mixed (sensorimotor) neuropathies have similar causes; in most acquired conditions they represent a late stage of a sensory disease.

      Poliomyelitis (polio) is an acute viral infection. Initially it may cause only a brief, febrile illness, but groups of ventral-horn cells of the spinal cord may be destroyed. This may later cause severe pain with further fever, occasional delirium and meningism (due to accompanying encephalitis), and a rapid onset of muscle atrophy, fasciculations, and weakness that may be localized or diffuse, mild or profound, and that may be fatal if the respiratory or bulbar muscles are involved. Only supportive treatment is available for poliomyelitis, but some recovery occurs in the majority of patients who survive the acute stage of the illness. Since the advent of immunization programs in the 1950s, this disease has been rare in the Western world.

Hereditary motor neuropathies
      Hereditary motor neuropathies (also known as spinal muscular atrophies and as Werdnig-Hoffman or Kugelberg-Welander diseases) are a diverse group of genetic disorders in which signs of ventral-horn disease occur in babies or young people. The usual symptoms of muscle atrophy and weakness progress more slowly if the disease begins at a later age (5 to 15 years); at later ages the disease may also pass, leaving only chronic mild weakness and secondary skeletal deformities such as scoliosis (rotation of the spine). Less commonly, muscle weakness occurs in specific patterns—for example, involving only the facial, shoulder, or calf muscles—and the progress of the disease is much slower. Babies with these disorders may exhibit respiratory insufficiency, poor ability to suck, and severe limpness and weakness of all muscles except those of the face and eyes; the muscles of the shoulder and pelvic girdles are primarily affected. Motor development is delayed or absent. Diagnosis of hereditary motor neuropathy is confirmed by electromyography and muscle biopsy. Enzyme deficiencies are the cause of some cases of hereditary motor neuropathy, but in most cases the etiological basis of the disease is unknown.

Motor neuron disease
       amyotrophic lateral sclerosis (ALS) and lateral sclerosis are both motor neuron diseases, progressive disorders of older people that affect neurons of the ventral horns, of the medullary motor nuclei, and of the corticospinal tracts. ALS, or Lou Gehrig disease, is characterized by muscle wasting due to loss of the ventral-horn cells (the lower motor neurons). Lateral sclerosis is the loss of axons in the lateral columns of the spinal cord (the upper motor neurons of the corticospinal tracts). A combination of upper and lower motor neuron signs is associated with these diseases, but muscle weakness and atrophy of two or more limbs is the primary feature. The brain, eyes, and sensory system are unaffected.

Nerve injuries
      Nerve injuries function as neuronal neuropathies affecting the axon far from the cell body. Injuries are of three main grades of severity. In neurapraxia there is temporary blockage of impulse conduction, although the axons remain intact. More severe stretch or incision damage interrupts some axons and is called axonotmesis. Injury that actually severs the nerve is called neurotmesis; surgical reattachment of the severed nerve ends is necessary. Neurosurgery does not guarantee a rapid recovery, since new nerve sprouts grow down the nerve framework at the rate of 1 to 2 mm (0.04 to 0.08 inch) per day at most.

Demyelinating neuropathies
      Demyelinating neuropathies are those in which the Schwann cells, which form myelin (the white, insulating sheath on the axon of many nerve fibres), are primarily affected and migrate away from the nerve. This process causes the insulating myelin of axon segments to be lost, and conduction of nerve impulses down the axon is blocked.

      Acquired demyelinating neuropathies may arise as complications of diphtheria and diabetes (diabetes mellitus), which, partly because of damage to the smallest blood vessels supplying the nerves, are sometimes accompanied by a variety of motor, sensory, autonomic, or mixed neuropathies. Some of these are extremely painful. If sensation is impaired, minor injuries can lead to severely deformed, but painless, “Charcot” joints. leprosy (probably the most common cause of neuropathy in the world), metabolic diseases, cancer, and myeloma or other dysproteinemias also cause demyelinating neuropathies.

       Charcot-Marie-Tooth disease (also known as peroneal muscular atrophy because of the special involvement of shin muscles) is a genetically acquired demyelinating neuropathy. High foot arches, distal motor weakness and atrophy, and reduced reflexes are the main symptoms; sometimes the nerves are greatly thickened. The condition first appears in childhood, though patients have a normal life span.

      Guillain-Barré syndrome is an acute inflammatory neuropathy. In this disease an autoimmune attack upon the myelin sheath of the motor nerves leads to progressive weakness and reflex loss with only slight sensory changes. Weakness rarely may become so severe that the patient needs mechanical help in breathing, but if further complications do not occur, the disease will remit within a few weeks. In severe cases, blood transfusion may speed recovery.

       carpal tunnel syndrome is a common ischemic neuropathy in which the median nerve is compressed at the wrist. Ischemic neuropathies are those disorders in which nerve compression leads to decreased blood supply and subsequent damage to the Schwann cells. The nerve narrows at the site of pressure, although the axon remains intact. Carpal tunnel syndrome causes pain, numbness, tingling, and weakness of the fingers and thumb, especially at night and in the morning. Cubital tunnel syndrome is a similar problem affecting the ulnar nerve at the elbow. Surgical intervention may be necessary to release the entrapped nerve.

      Neuropathies of the autonomic nerves may be hereditary, as in Riley-Day syndrome, or acquired, as in complications of partial nerve injuries, diabetes mellitus, tabes dorsalis, Guillain-Barré syndrome, and other toxic or metabolic disorders (among which alcoholism and certain drug therapies are the most common). Damage to the sympathetic or parasympathetic pathways in the hypothalamus or brainstem may produce similar symptoms—for example, faintness due to disordered regulation of blood pressure and heart rate, disturbances of bladder and bowel control, impotence, and impaired visual accommodation. Some relief may be obtained from medications that replace a deficient neurotransmitter, increase the blood volume, or compress the limbs so that blood no longer pools in the veins.

Disease of the neuromuscular junction
       myasthenia gravis is the most common disease of the neuromuscular junction. At this site the motor nerve impulse normally triggers the release of the neurotransmitter acetylcholine, which diffuses across the synaptic gap between the terminal of the nerve and the specialized end-plate region of the muscle-fibre membrane. In myasthenia gravis, receptors in the end-plate region are partially coated with an antibody, so that the acetylcholine molecules are blocked, depolarization of the muscle fibre cannot occur, and the muscle cannot contract. The amount of acetylcholine released from the nerve terminal is also reduced. As a result, muscle contraction is possible after a period of rest, but sustained contractions quickly weaken. This fatigability is especially present in the eye muscles, causing drooping of the lids on looking upward and to diplopia (double vision). The muscles of the throat, limbs, and respiration may also be involved.

      Myasthenia gravis is diagnosed by electrical studies of neuromuscular transmission and by single-fibre electromyography (see above Neurological examination: Diagnostic tests and procedures (nervous system disease)). Treatment involves the removal of the thymus gland (thymus) (which may produce the antibody) and medications that augment the effect of acetylcholine and suppress the immune system.

Diseases of muscle

Genetic dystrophies (muscular dystrophy)
      Duchenne muscular dystrophy (DMD) is one of the most common genetic dystrophies. DMD is an X-linked disorder that ordinarily affects only males. By the age of three the individual experiences difficulty in walking; progressive failure to run, jump, and climb occurs later, leading eventually to the inability to walk. Because of the infiltration of degenerating muscles with fat, little atrophy may be noticed until late in the course of the disease. Diagnosis is confirmed by testing the blood levels of creatine kinase, an enzyme released from degenerating muscle, and also by electromyography and muscle biopsy. The cause is unknown, and no specific treatment is available, but genetic testing is used to determine whether a case represents a new mutation or has been genetically transmitted by a carrier mother to her son.

      Facioscapulohumeral dystrophy causes weakness and wasting of predominantly the face, shoulder girdle, and arms in teenagers. Other limb-girdle dystrophies also show slower progression and may not declare themselves until adult life. Oculopharyngeal dystrophies first strike the eye muscles, causing drooping of the eyelids and weakness or paralysis of the muscles moving the eyes. Later involvement of the face, bulbar muscles, limbs, and trunk is common.

      Myotonic dystrophy is characterized by weakness and wasting of the face and trunk muscles. In addition, muscles fail to relax after a strong contraction, so that, for example, the patient cannot easily let go after shaking hands. Involvement of other body systems is common. The same failure of relaxation occurs in myotonia congenita but without the wasting features of myotonic dystrophy. Relaxation can be obtained with medications such as diphenylhydantoin and quinine.

Other inherited muscle diseases
      Congenital myopathies cause weakness and poor muscle development in the early years of life, but generally they are not progressive. Diagnosis is determined by muscle biopsy. Lipid storage (lipid storage disease) myopathies are associated with disorders of the metabolism of carnitine, a substance that muscle cells use to convert fatty acids into energy. In these conditions severe muscle weakness progresses slowly. A muscle biopsy shows accumulation of fat in the fibres. In the glycogen storage diseases (glycogen storage disease) glycogen accumulates in muscle fibre, because of a deficiency of an enzyme that helps degrade glycogen into lactic acid for the production of energy. Beginning in childhood, fatigue, pain, and occasional severe muscle cramps during exercise are common. Diagnosis is determined by demonstrating that the exercising muscles do not produce lactic acid as they should.

      Myoglobinuria is a condition in which myoglobin, a substance that stores oxygen within the muscles, spills into the blood and urine. Myoglobin may accumulate in the tubules of the kidney and cause renal failure. This condition, which primarily occurs as a result of muscle damage, can also occur as an inherited metabolic defect or may follow heavy exercise, injury, or toxic damage from drugs or chemicals.

      Malignant hyperthermia is a metabolic muscle disease characterized by high fever and extreme rigidity of muscles, usually caused by certain anesthetics or muscle-relaxant medications given during surgery. Rapid cooling of the patient, correction of the accumulation of lactic acid in the blood (the result of intense muscle contraction), and administration of dantrolene sodium to relax the muscles is necessary for treatment.

      In familial periodic paralyses (periodic paralysis) episodes of weakness occur in association with abnormally high or low blood levels of potassium. Some attacks are caused by a period of rest following heavy exercise, others are caused by carbohydrate or alcohol consumption. Depending on the type of paralysis, treatment includes the administration of potassium, glucose, and diuretics.

Acquired diseases of muscle
       myositis, an inflammatory muscle disease, is associated with some viral infections, causing swelling, pain, and weakness, and with trichinosis and other tapeworm infestations, in which allergic skin rashes commonly accompany the same symptoms. dermatomyositis is an autoimmune disease characterized by swelling, weakness, and tenderness of the proximal, facial, neck, and bulbar muscles in both children and adults. A skin rash is also present, mainly around the eyes but also on the face and limbs. Diagnosis is determined by electromyography, blood enzyme levels, and sometimes muscle biopsy. Treatment includes steroid and immunosuppressant medications. Closely related conditions without the rash are associated with collagen-vascular diseases, such as scleroderma and polyarteritis, or with cancer.

      Polymyalgia rheumatica, another autoimmune disease, mainly affects women over the age of 55. Symptoms include severe muscle stiffness (especially after sleep), malaise, weight loss, muscle tenderness, anemia, and fever. Inflammation of arteries, particularly of the branches of the carotids, may also occur. Blindness may follow if the ophthalmic arteries are involved. Treatment with steroid medications produces relief.

Spinocerebellar degenerations
      Spinocerebellar degenerations are genetically determined conditions characterized by dysfunction of the dorsal columns or of the corticospinal and spinocerebellar tracts of the spinal cord. These conditions usually appear in the first 20 years of life and cause position sensation, gait, limb power, balance, and coordination disturbances. (For further discussion, see below The cerebellum: Genetic diseases (nervous system disease).)

      Myelitis, inflammation of the spinal cord, may be caused by viral or bacterial infections such as mononucleosis, mumps, measles, chicken pox (chickenpox), tuberculosis, and herpes zoster. Symptoms result from the degeneration of the dorsal roots and include a painful girdlelike sensation around the trunk, a loss of motor, sensory, and bladder functions below the level of the inflammation, meningism, and fever.

      Damage to the spinal cord may result from spinal fractures or dislocations. The severity of damage varies with the severity of the injury. Transient weakness and hyperactive reflexes may occur because of damage to the corticospinal tracts, or paraplegia may occur because of damage to the motor and sensory fibres. Spinal cord injuries at high cervical levels may cause paralysis of the diaphragm, resulting in ventilatory failure.

Tumours (tumour)
      Tumours within the spinal cord are called intrinsic, those outside the cord but compressing it are called extrinsic. Intrinsic tumours include ependymomas and astrocytomas, which are malignant, and angiomas and cysts, which are benign. Cancer of the breast, lung, kidney, or prostate gland can cause extrinsic tumours of the spinal cord. Other tumours arising outside the cord include lymphomas, lipomas, neurofibromas, meningiomas, and chordomas. All of these can cause spinal nerve irritation, with pain and tingling, at the level of the tumour. Diagnosis of a tumour is confirmed by myelography, usually with computed tomography (CT) scanning or magnetic resonance imaging (MRI). Raised protein levels or malignant cells in the cerebrospinal fluid may also confirm a tumour. Surgery and radiation therapy are the principal treatments.

Subacute combined degeneration
      Subacute combined degeneration, which results from a vitamin B12 deficiency, causes demyelination of the corticospinal and the dorsal columns. Much of the damage is to the large dorsal-root ganglion neurons; the peripheral nerve fibres also demyelinate, so that peripheral neuropathy also occurs. Symptoms include tingling, numbness, weakness, mental disturbances, and vision changes. Treatment with vitamin B12 may lead to improvement.

Vascular disease
      Diseases of the blood vessels (blood vessel) are uncommon in the spinal cord. When blockages of vessels do occur, causes include severe atherothrombotic disease of the aorta, aortic or thoracic surgery, bleeding from an arteriovenous malformation, or arterial compression by a tumour or prolapsed disk.

      Damage to the anterior three-quarters of the cord is common because this area is supplied by only the ventral spinal artery. The dorsal columns and the dorsal-root entry zone usually escape damage because they are supplied by three or more dorsal spinal arteries.

      Radiation therapy for certain cancers may cause a slow but progressive deterioration of the roots and tracts at the irradiated level of the cord. Myelopathy often leads to spastic weakness of the legs and to less obvious sensory changes.

       syringomyelia is frequently associated with obstructions of the roof foramina, or openings, of the fourth ventricle of the brain. Increased pressure of the cerebrospinal fluid enlarges the canal and ultimately causes a cavity, or syrinx, to branch out from the spinal canal and enlarge over years. As a result, sensory fibres, traveling from the dorsal-root entry zone to the opposite spinothalamic tract and to ventral-horn neurons at the same level, are interrupted. In addition, compression of the corticospinal tracts occurs. Interruption of the dorsal-root–ventral-horn connection, which mediates reflex activity, results in loss of stretch reflexes and atrophy of some muscles innervated at that level. Interruption of the spinothalamic fibres, which transmit sensory impulses from both sides of the body to the thalamus, causes a loss of light touch, temperature, and pain sensations, as well as signs of an upper motor neuron lesion below the level of the cavity. Some perception, such as position sense, is retained. This is an example of dissociated sensory loss, which is also found in some brainstem lesions, transection of half of the spinal cord, and certain peripheral neuropathies.

 Diagnosis of syringomyelia is determined by X ray, by CT scanning combined with myelography, or by MRI. Surgical drainage of the cavity, to prevent pressure from distending it further, may result in improvement.

Demyelinating disease
      Demyelinating diseases frequently affect the spinal cord, particularly the corticospinal tracts and dorsal columns. The only features of Devic disease, a variant of multiple sclerosis, are a band of spinal cord inflammation and demyelination and optic nerve involvement.

      Brief, involuntary jerks of the trunk and of the limbs may occur in spinal myoclonus. Many diffuse, metabolic, or local structural causes of myoclonus are possible, and the disease commonly originates in the brainstem or in the cerebral hemispheres.

Cranial nerves (cranial nerve)
      Damage to the olfactory nerve can occur from a head injury, local nasal disease, or pressure from a tumour and may result in reduced sensitivity to smell or a complete loss (anosmia) on the side supplied by the nerve. Damage to the nerve may also result in a loss of flavour perception. Hallucinations of smell may occur in brain disorders such as epilepsy, with the presence of a tumour, or in depressive illnesses.

 Disorders of the optic nerve or of the pathways traveling to the occipital lobe cause visual loss in the affected eye. In the early stages of disease, when the optic process is irritating the nerve rather than decreasing its conducting ability, phenomena such as streaks of light may be seen. When the optic pathways are affected within the brain, the precise location of the disease can be determined by testing the pattern of visual loss affecting both eyes. Pain in the eyes is sometimes due to neuritis of the optic nerves, but it is usually caused by ocular disease. Optic neuritis causes a total or partial loss of vision. It is a condition that is occasionally inherited, but it may also occur because of infection, drug or chemical toxicity, ischemia, or demyelinating disease. Compression of the nerve by a tumour or aneurysm may eventually cause demyelination, which results in optic atrophy. Papilledema is a condition characterized by a swelling of the nerve head with fluid as a result of raised intracranial pressure.

Oculomotor, trochlear, and abducens nerves
      Compression of the oculomotor, trochlear, or abducens nerves may be caused by lesions, diabetes, vascular disease, head injury, infection, or neuropathy. In the brainstem, multiple sclerosis, stroke, Wernicke disease (see below Brainstem (nervous system disease)), and tumours are possible causes of compression. Double vision is the primary symptom; if the oculomotor nerve is affected, the pupil may be enlarged as well. In Horner syndrome, interruption of the long sympathetic fibres passing from the brainstem to the pupil causes drooping of the eyelid and a small pupil.

      Argyll Robertson pupils, small and irregular pupils that do not react to light but constrict on accommodation to close vision, may be associated with syphilis or other brainstem diseases. Adie pupils constrict very slowly in light and dilate slowly when the light is removed.

      In progressive external ophthalmoplegia, a disorder of the central nervous system mechanisms controlling gaze, the eyes may fail to move in one or another direction. Other neurological problems, such as parkinsonism, dementia, or neuropathy, may be associated with this condition. Local lesions of the brainstem may also cause paralysis of eye movement, as may severe myasthenia and myopathies affecting the eye muscles.

      The jerky eye movements of nystagmus usually signify brainstem, vestibular, or cerebellar disease, but they may also be complications of very poor eyesight or may occur as a congenital defect.

      When any of the three oculomotor nerves are affected, the axes of the eyes are not able to remain parallel, so light falls on different parts of the two retinas and double vision results.

Trigeminal nerve
      Numbness of the face is commonly due to compression of the trigeminal nerve caused by a tumour in the cranial cavity or nasopharynx or by a brainstem disorder. Trigeminal neuralgia, also called tic douloureux, is an intense, repetitive, pain felt in the lower half of one side of the face. It occurs primarily in people over 55 years of age. Symptoms may be relieved by medications such as carbamazepine, diphenylhydantoin, or baclofen or by surgical removal of a loop of normal artery where it impinges upon the nerve at its exit from the brainstem.

      The facial nerve is damaged most commonly by swelling within the facial canal in the temporal bone that results from viral infection. This causes Bell palsy, an abrupt weakness of all the facial muscles on one side of the face that is often accompanied by pain around the ear, unusual loudness of sounds on the same side, and loss of taste on the front of the tongue.

      The herpes simplex virus, lesions of the brainstem and of the angle between the cerebellum and pons, middle-ear infections, skull fractures, diseases affecting the parotid gland, and Guillain-Barré syndrome all may cause facial palsy.

      In hemifacial spasm repetitive twitching of one side of the face occurs. Irritation of the facial nerve as it leaves the brainstem appears to be the cause, and in many cases relief is obtained through surgical decompression.

      When both divisions of the vestibulocochlear nerve are affected by disease, symptoms may include ringing in the ear ( tinnitus), a sensation of spinning ( vertigo), and other symptoms such as deafness. Deafness, if not caused by middle-ear disease, suggests damage to the cochlear portion of the nerve. Compression of the nerve at the cerebellopontine angle by a tumour, an aneurysm, a meningioma, certain systemic diseases, drug toxicity, small strokes (stroke), or Ménière disease may cause hearing loss. In Ménière disease an accumulation of fluid in the inner ear produces increasing deafness, tinnitus, and vertigo.

      Benign postural vertigo is characterized by brief severe attacks of vertigo induced by movement, especially turning in bed. The condition is less persistent than vestibular neuronitis, in which severe vertigo persists for days, probably as a result of viral infection of the inner ear or vestibular nerve.

Bulbar nerves
      (In this context, the term bulbar refers to the medulla oblongata, which looks like a swelling, or bulb, at the top of the spinal cord.) Damage to the 9th through 12th cranial nerves, the bulbar nerves, causes impairment of swallowing and speech and weakness of the neck muscles. These paralyses often lead to choking and asphyxia due to inhalation of saliva and food and inability to swallow or clear the airway of secretions that may be aspirated into the lungs. A nasal tone to the voice and weakness of coughing are early signs of damage to these nerves. Causes of bulbar palsy include motor neuropathies, such as diphtheria, poliomyelitis (polio), and botulism, motor neuron diseases, myasthenia gravis, certain muscle diseases, and compression of the nerves by tumours.

      Brainstem lesions produce a number of syndromes depending on their location and cause. In Moebius syndrome, the abducens and facial nerves, which originate in the brainstem, do not develop. encephalitis may affect the brainstem only, with consequent damage to cranial nerves, to cerebellar connections, to the long ascending and descending tracts in the pons and medulla oblongata, and to the reticular activating system and short tracts within the brainstem that control the functions of consciousness and coordinate eye movements. Strokes (stroke), malignant tumours such as gliomas, and multiple sclerosis also may affect this region. In syringobulbia a cavity forms within the brainstem in association with syringomyelia (see above The spinal cord (nervous system disease)) that causes cranial nerve palsies and both cerebellar and long-tract symptoms.

      Wernicke disease is caused by a thiamine (vitamin B1) deficiency and occurs most often in alcoholics; symptoms include ocular palsies, nystagmus, memory disturbance, and peripheral neuropathy. Eye symptoms may be corrected by the administration of thiamine.

      Correction of severe sodium depletion may cause edema, or accumulation of fluid, in the central nervous system. Since many motor nerve fibres cross over and intertwine in the pons, the resulting swelling may lead to their compression and dysfunction, as well as to demyelination, a condition known as central pontine myelinolysis. Unconsciousness and spastic paralysis of the limbs are the primary symptoms.

Disorders of sleep
      The raphe nuclei of the pons and the locus ceruleus, which mediate sleep, are situated in the brainstem. Sleep consists of two phases: rapid eye movement (REM) sleep and non-REM, or slow-wave, sleep. During non-REM sleep an individual progresses from drowsiness through deeper and deeper levels of relaxation, with decreasing ability to be aroused; progressively slower waveforms appear on an electroencephalogram (EEG) during this phase. Periods of REM sleep, during which dreaming occurs, punctuate slow-wave sleep. Paradoxically, although the individual appears deeply asleep, fast activity occurs on an EEG during this phase, and numerous brief, small-amplitude movements are made by the eyes and the muscles.

       narcolepsy is a genetic disorder in which, with little warning, irresistible sleepiness overcomes a person during the day. One form includes vivid hallucinations on awaking or falling asleep, temporary but profound sleep paralysis on awakening that does not affect breathing, and sudden, brief loss of muscle power in the limbs and trunk during emotional moments such as laughter (cataplexy). Increased sleepiness, or hypersomnia, may be the cause of drowsiness or narcolepsy. Narcolepsy may be described as an intrusion of REM sleep into the waking hours. Treatment of the disorder with stimulants and tricyclic medications is often effective.

      Brain death is synonymous with brainstem death, since the control centres for essential functions such as consciousness, respiration, and blood pressure are located within the brainstem. In many countries strict criteria for diagnosis of brain death have been established by common consent among medical, religious, ethical, and legal experts. Signs of brain death include the presence of deep coma with an established cause, the absence of any brainstem functions such as spontaneous respiration, pupillary reactions, eye movements, and gag and cough reflexes. Electroencephalography (EEG) may be a useful confirmatory test. When brainstem death is confirmed, the heart usually stops beating within a day or two, even when other vital functions are artificially maintained.

Genetic diseases
      Spinocerebellar degenerations are a group of inherited disorders characterized by atrophy of the central nervous system and of peripheral nerves. They commonly affect the cerebellum and its connections, as well as the nuclei in the medulla known as the olives, the centres for control of eye movements, the optic nerves, the dorsal columns of the spinal cord, and the corticospinal tracts. Onset is usually in the first two decades of life. Some patients show only minimal signs, although most conditions are slowly progressive. Friedreich ataxia, the prototypical spinocerebellar degeneration disorder, affects the optic nerves, dorsal columns, corticospinal tracts, and cerebellum. Peripheral neuropathy, skeletal deformities, high-arched feet, and rotation of the spine (scoliosis) also may occur.

Inflammatory diseases
      Inflammatory diseases affecting the cerebellum comprise abscesses, which usually complicate chronic infections of the middle ear through the spread of infection along the veins draining back into the posterior fossa, cerebellitis, associated with some viral infections in children, and tuberculomas, inflammatory masses that act as tumours, raising intracranial pressure and causing cerebellar dysfunction.

      Cerebellar hemorrhage may occur with high blood pressure, causing sudden headache, neck stiffness, and cerebellar signs, often with evidence of compression of the brainstem on the side of the bleeding. Prompt surgical evacuation of the blood clot is necessary, and the availability of computed tomography (CT) scanning aids in diagnosis. Infarcts of the cerebellar arteries may also affect part of the medulla oblongata or pons.

Demyelinating disease
      Demyelination frequently affects the cerebellum and its connections. The primary signs of cerebellar disease are nystagmus, ataxia, and scanning speech. (See Unlocalized or multifocal disorders: Demyelinating diseases (nervous system disease).)

Metabolic diseases (metabolic disease)
      Chronic alcoholism, toxicity of diphenylhydantoin (an antiepileptic medication), thiamine and nicotinic acid deficiencies, and hypothyroidism may all cause cerebellar dysfunction. In all of these disorders, typical signs of truncal and limb incoordination, or ataxia, are detectable; treatment usually reverses the deficits. Numerous inherited metabolic defects of metal, lipid, or amino acid metabolism (see below Unlocalized or multifocal disorders (nervous system disease)) and some serum protein abnormalities also cause signs of cerebellar disease.

Tumours (tumour)
      Benign tumours, usually schwannomas on the vestibulocochlear nerve, may compress the cerebellum and lead to dysfunction on one side, but malignant astrocytomas and metastases from cancers are more common. In children, medulloblastomas are fast-growing malignant tumours that destroy the central part of the cerebellum and cause severe gait ataxia. Astrocytomas grow much more slowly and can often be completely removed.

The basal ganglia
      Parkinson disease is a progressive disorder caused by degeneration of the cells of the substantia nigra and locus ceruleus (both pigmented nuclei in the brainstem) and of their connections with the basal ganglia. The basal ganglia are nuclear masses situated above the brainstem that are involved with the initiation and patterning of voluntary movements. This motor control system uses the neurotransmitters dopamine and acetylcholine; in parkinsonism there is a deficiency of dopamine as a result of the degeneration of neurons in the substantia nigra. This leads to inhibition of activity within the basal ganglia. Most cases of parkinsonism occur in late middle age. Slowness of movement, muscle rigidity, stooped and flexed posture, and repetitive tremor of the limbs are the main symptoms of Parkinson disease. Depression and loss of intellectual agility are also common. The cause of Parkinson disease is unknown, although encephalitis in early life, many drugs and toxic chemicals, brain trauma, cerebral anoxia, and other degenerative diseases of the nervous system (e.g., progressive supranuclear palsy) cause similar or identical symptoms.

      The medication levodopa, or L-dopa, a precursor of dopamine that crosses the blood-brain barrier into the brain, where it encourages the synthesis of the deficient neurotransmitter, is the primary treatment. Other treatment approaches include anticholinergic medications, which reduce activity in the basal ganglia; medications such as bromocriptine and pergolide, which augment the responsiveness of the receptor sites where dopamine normally exerts its effect; amantidine, which increases dopamine release; and selective inhibitor medications such as deprenyl, which block the action of the enzyme that breaks down dopamine. If treatment with medication is not successful, surgery may be performed to destroy the area of the brain that produces tremors.

Dystonias (dystonia)
      Dystonias, sustained muscular contractions that produce abnormal postures of the face, head, trunk, and limbs, are caused by disease of the basal ganglia. Other abnormal involuntary movements, such as vocalizations, tremors, and jerks, may also occur. Generalized dystonia (e.g., dystonia musculorum deformans) is an inherited condition in which persistent and often painful twisting and writhing movements of any muscle group occur, usually in children. The intensity of the movements may be so severe that self-care is impossible. Although a number of medications are effective in reducing the movements, surgical destruction of the ventrolateral nucleus of the thalamus may be performed to relieve muscle rigidity in severe cases. Focal dystonias include writer's cramp, orofaciomandibular dystonia, and torticollis.

      Tics (tic) are similar to dystonia, but they consist of brief movements or utterances of unknown cause. In severe cases, sudden, explosive speech or movements occur repeatedly, as in people with Tourette syndrome.

       chorea is characterized by brief and involuntary dancelike movements that are usually seen with disease of the caudate nuclei, which are part of the basal ganglia. Huntington disease is an inherited form of chorea that leads to dementia and early death but is usually asymptomatic until adulthood. Presymptomatic diagnosis can be determined by genetic testing. Early symptoms of the disease include personality changes, such as irritability and depression, and an inclination to alcohol abuse. This is followed by intellectual deterioration with paranoia and brief, jerky movements of the limbs and face or, less commonly, muscle rigidity. Medications such as phenothiazine or butyrophenone may reduce the movements.

      Chorea also may occur in a nondementing inherited form as a complication of rheumatic fever in children (Sydenham chorea), of pregnancy (chorea gravidarum), of a few systemic diseases (e.g., hyperthyroidism, lupus erythematosus), and as a toxic effect of certain drugs (e.g., levodopa). Hemiballismus is a form of chorea caused by a small infarct of the subthalamic nucleus; wild, flinging movements occur on one side of the body.

      Tardive dyskinesia is characterized by choreic movements affecting the face, eyes, tongue, and trunk; the disorder usually appears after prolonged treatment with antipsychotic medications such as the phenothiazines. Treatment with reserpine or amantidine may be successful.

       Wilson disease is an inherited condition characterized by cirrhosis of the liver and degeneration of the lenticular nuclei of the basal ganglia caused by copper deposits in the brain, corneas, and liver. The disorder is the result of an inborn error of metabolism that hinders the liver's ability to excrete copper normally. Wilson disease causes chorea, dystonia, athetosis (continuous writhing and twisting movements of the limbs), and tremors. Brown pigment is deposited around the edges of the corneas, and behavioral and intellectual disturbances are common. Treatment includes administration of d-penicillamine and a low-copper diet.

      Athetosis may also result from strokes, anoxia (particularly at birth), and hemolytic disease of the newborn (in which bilirubin derived from damaged red blood cells is deposited in the basal ganglia, causing the formation of glial scar tissue).

Essential tremor
      Essential tremor is an inherited disorder characterized by movements that are interrupted by a regular oscillation of the limbs, as during writing. The tremor is usually absent at rest and often affects the head and voice. Beta-adrenergic blocking medications and primidone are effective treatments.

Craniocerebral trauma
      The concussive and shearing stresses of head injury may cause concussion, contusion of the brain (most often of the tips of the frontal and temporal lobes, called contrecoup injury), or laceration of the brain tissue. In the last two cases, neurological deficits are detected at the time of injury, and with laceration (as in a depressed fracture of the skull) or bleeding into the brain, posttraumatic epilepsy is possible.

      Extradural hematomas, often from tearing of the middle meningeal artery, may result as a complication of a head injury. Arterial blood, pumped into the space between the dura and the inside of the skull, compresses the brain downward through the tentorium or the foramen magnum. Surgical removal of the clot is necessary. Subdural hematomas usually develop more slowly and may sometimes take weeks to form; they follow the rupture of small veins bridging the gap between the surface of the brain and the meninges. Headache, seizures, intellectual decline, and symptoms similar to those of extradural hematomas may occur. Removal of the clot is the usual treatment.

      Other complications of head injury include cranial nerve palsies, subarachnoid hemorrhage, thrombosis of a carotid artery, focal deficits, and cerebrospinal fluid leakage, which may lead to intracranial infection. Later consequences include dementia, seizures, irritability, fatigue, headaches, insomnia, loss of concentration, poor memory, and loss of energy. Repeated minor head injuries, which may occur in some boxers, may also lead to dementia and to a Parkinson-like syndrome.

      The term cerebral palsy encompasses all of the conditions that damage the brain around or before the time of birth—other than developmental causes. Approximately six cases of cerebral palsy occur in every 1,000 live births in the Western world. Hypoxia and asphyxia during a prolonged and difficult labour are the most common causes, but improvements in obstetrical care have reduced the incidence of the condition. Cerebral palsy is characterized by a delay in motor development, spasticity, weakness of the limbs, athetosis, or ataxia. Sensory, visual, and cognitive defects may be detected later in life. Mental retardation occurs in about half of children with the condition.

      Mental retardation is characterized by incomplete or insufficient general development of mental capacity, causing a delay in the normal development of motor, language, and social skills. Behavioral abnormalities and impaired emotional control are also common. Before birth, chromosomal and developmental diseases, genetic and other metabolic disorders, and intrauterine infections or toxicity are possible causes. In the postnatal period, asphyxia, hemolytic disease, nutritional deficits, infections such as meningitis and encephalitis, trauma, and toxins are the most frequent causes of mental retardation.

       Down syndrome is a common chromosomal disorder that produces mental retardation. More than 1 in every 800 to 1,000 children suffer from this condition. Clinical features of the disorder include short stature, smallness of the head, obliquely slanted eyes, a flattened face, wide hands with a single transverse palmar crease, short digits, and weak muscles. The brain is slightly smaller than normal, and the appearance of neurons is also abnormal. After the age of 30, features of Alzheimer disease are almost always present in individuals with Down syndrome. Prenatal diagnosis of Down syndrome is possible by examination of the amniotic fluid.

      Dementia is a condition characterized by the global impairment of intellectual capacity. Common early symptoms include personality changes, loss of interests, impairment of attention and concentration, difficulty with comprehension, and difficulty in handling abstract concepts. Later, increasing impairment of the capacity to retain new information, social withdrawal, unnecessary repetition of words and actions, and emotional disturbances may be present. As dementia progresses, language ability may begin to deteriorate and the individual may neglect to care for himself appropriately.

      Individuals with dementia are at risk of developing a related but more acute syndrome called confusion or acute brain syndrome. In this disorder, individuals lose their orientation in time, space, and person; they may show altered levels of consciousness, emotional instability, physical excitability, agitation, disorderliness, hallucinations, and delusions. Acute brain syndrome is commonly caused by generalized metabolic diseases and intoxication with sedative drugs (including alcohol).

      The most common cause of dementia is Alzheimer disease. The disease is common in the elderly. The entire cerebral cortex, particularly the frontal lobe, is affected. Alzheimer disease has a slight familial tendency, but the cause of the disease is not known. There is no specific treatment for this condition.

      Multi-infarct dementia results from numerous small strokes which impair brain function so that the individual has global intellectual impairment. Diagnosis is confirmed by demonstration of the infarcted areas by computed tomography (CT) scanning.

      Dementia can also be caused by vitamin deficiency, endocrine disorders, metabolic diseases, brain inflammation, trauma, tumours, degenerative diseases, severe multiple sclerosis, arterial disease, hydrocephalus, Pick disease, and progressive supranuclear palsy.

  epilepsy is a chronic disorder in which excessive signaling of the cerebral neurons causes recurrent seizures. Epilepsy is estimated to affect 40 million to 50 million people worldwide. Seizures are divided into two major categories: generalized seizures (a result of abnormal activity in most or all of the brain) and partial seizures (originating in a specific area of the brain).

Generalized seizures
      The onset of generalized seizures is usually in the early years of life and may be caused by a genetic abnormality. One primary type of generalized seizure is tonic-clonic seizures, in which an individual loses consciousness without warning, falls, and stiffens, because of muscle contractions. Rapid jerking of the entire body follows and ceases in approximately one minute. The person regains consciousness a few minutes later. Immediately afterward the individual is usually confused and sleepy.

      Primary generalized, or absence, epilepsy is characterized by repeated lapses in consciousness. Minor movements such as blinking may occur.

      Myoclonic epilepsies are another form of generalized seizures in which single or repeated brief shocklike contractions of the muscles occur.

      Seizures can also result from sleep deprivation, poisoning, certain metabolic disturbances, and withdrawal from drugs or alcohol.

Partial seizures
      Partial seizures originate from a specific area of the brain. In simple partial seizures, consciousness is retained and abnormal sensations, visual hallucinations, or local muscle contractions occur. In complex partial seizures, caused by lesions within the temporal or frontal lobes, unconsciousness, abnormal sensations, hallucinations, and limb jerking or abnormal posture may occur.

Diagnosis and treatment
      A complete physical, blood tests, and neurological examination may be necessary to identify the cause of epilepsy. Electroencephalography (EEG) may be performed to determine any abnormalities in electrical activity. Magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), or magnetic resonance scanning (MRS) may be used to detect any structural or biochemical abnormalities of the brain.

      Valproate, ethosuximide, clonazepam, carbamazepine, and primidone are used to treat epilepsy. Some of these medications selectively block the electrical discharges that lead to seizure activity, while others stimulate the inhibitory effect of the neurotransmitter gamma-aminobutyric acid (GABA). Surgery may be performed if seizures are uncontrolled by medication.

      There are four major varieties of headache, each with its own type and severity of pain, temporal relationship, site and pattern of radiation, nature of aggravating or relieving factors, and associated symptoms.

Vascular headaches
      Vascular headaches include migraines (migraine) and its variants as well as headaches due to abnormal stretching of the arterial walls in the cranium as a result of vessel-wall disease. Migraine headaches are extremely painful recurring headaches that are sometimes accompanied by nausea and vomiting; most migraine sufferers have a family history of the disorder. The pain is typically severe, throbbing or pounding, felt on one or both sides of the head, and aggravated by activity, noise, or bright lights. Visual phenomena such as zigzag lights may precede the onset of pain. Visual loss, weakness of a limb or of one side of the body, speech disturbances, or confusion may also accompany and outlast the headache. In a variant called cluster headache, severe pain is felt in and around one eye that lasts approximately an hour and frequently wakes the patient in the early morning. Such attacks occur about 10 to 20 times a month, with months of no symptoms until the next cluster begins. In the elderly, brief episodes of brain dysfunction resembling small strokes may occur, apparently caused by arterial spasm during migraines.

      Foods such as cheese, chocolate, alcohol, and those containing nitrite or monosodium glutamate are the most frequent causes of migraines. Other causes include excessive or deficient sleep, stress, oral contraceptives, and menstruation. Medications such as ergotamine, antiemetic agents, and pain-relieving agents or even oxygen may be necessary to treat acute attacks. Preventive medications include tricyclics, adrenergic blocking agents, and lithium.

Tension headaches
      Tension headaches are continuous and generalized pains felt from front to back or all around the head; they are generally less severe than migraines. Although stress is the most common cause, arthritis of the neck may also cause such a headache. Tension headaches are often experienced by migraine sufferers, and a combined form of headache exists. Alleviation of stress may reduce symptoms.

Traction headaches
      Traction headaches are caused by the distortion of intracranial pain-sensitive structures. This type of headache may be caused by intracranial bleeding or tumours, brain infections, obstruction of the flow of cerebrospinal fluid, and low pressure of the fluid due to leakage after lumbar puncture. Such headaches are moderately but increasingly severe, involve the whole head, may be accompanied by a throbbing sensation, are worse with movement and coughing and on awaking, and improve as the day progresses but recur for hours.

Referred headaches
      Pain may also be referred to the head (i.e., felt in the head even though the site of disease is elsewhere) by eye disorders such as glaucoma, infections or tumours of the nasal sinuses, dental infections, and arthritis of the neck.

Infections

       encephalitis, an infection of the brain, may be caused by a number of microorganisms including viruses, bacteria, and fungi. In the Western world, viral encephalitis is the most common type of the disorder; it is typically caused by the herpes simplex virus (herpes simplex). Other causes of viral encephalitis are measles, mumps, polio, rabies, and influenza.

      Typical symptoms of encephalitis include the onset, over hours or days, of headache, fever, stiffness of the neck, drowsiness, and malaise. Computed tomography (CT) scanning of the head, electroencephalography (EEG), and lumbar puncture (spinal tap) may be necessary in order to diagnose the disorder. Herpes simplex encephalitis is treated with medications such as acyclovir or vidarabine.

      Cerebral abscess is caused by bacterial infection that may be carried via the bloodstream in cases of generalized or distant infection or may result from infection following a skull fracture. Infection produces a pus-filled abscess, usually localized close to the site of the original infection. Symptoms resemble those of viral encephalitis. Electroencephalography (EEG) and computed tomography (CT) scan may be performed to localize and determine the source of the infection.

      Cortical thrombophlebitis may result from an infection that travels along the course of the intracranial veins, which themselves are infected and thrombose, or clot. This disease can occur in the venous channels of the dura mater, resulting in infarcts of the surrounding brain and damage to contiguous cranial nerves. Cortical thrombophlebitis occurs primarily in children suffering from malnutrition and dehydration, but it also may result from some blood diseases and may occur in pregnancy. The symptoms resemble those of a cerebral abscess, but there is a greater likelihood of seizures.

      Antibiotics are usually administered to treat cerebral abscesses and are chosen according to the infectious organism. Anticoagulants may be used to treat cortical thrombophlebitis. Surgery may be necessary to remove scar tissue from the abscess.

Autoimmune reactions
      The human immunodeficiency virus, or HIV, causes acquired immune deficiency syndrome, or AIDS, an infection that greatly diminishes the cell-mediated immune system. Many viral, bacterial, and fungal infections occur as a result. Neurological complications include encephalitis and dementia, caused by invasion of the brain by HIV.

      Following common viral infections such as measles, during bacterial endocarditis (a bacterial infection of the heart valves), or after vaccination against such diseases as smallpox, an acute neurological illness with meningeal irritation, fever, reduction in consciousness, seizures, and raised intracranial pressure may occur. This condition is known as acute toxic encephalopathy and may be an acute allergic reaction of the immune system to the preceding infection. In Reye syndrome, a variation of acute toxic encephalopathy occurring in children, the same features are present, along with persistent vomiting and acute degeneration of the liver. There is some evidence that aspirin may cause the condition, and use of the salicylate is usually not recommended in children.

      Acute allergic encephalomyelitis is a similar illness that is caused by damage to the myelin sheaths of the central nervous system and occurs in the second week after vaccination or viral infection. This condition causes symptoms of encephalitis and inflammation of the spinal cord or nerve roots. Treatment with corticosteroid medications is sometimes effective.

      Syphilis follows infection with the spirochete Treponema pallidum as a result of sexual contact with an infected individual or infection of the fetus in utero. Primary syphilis produces a chancre, or sore, at the site of infection. Weeks later, secondary syphilis develops and causes a skin rash and signs of meningitis. In the third stage, the spirochete invades the nervous system and may cause tabes dorsalis (see above The spinal cord: Inflammation (nervous system disease)) or chronic syphilitic encephalitis, which causes progressive dementia and signs of focal damage to the brain. Diagnosis of syphilis is confirmed by blood tests and by examination of the cerebrospinal fluid. Treatment with penicillin stops the progress of the disease and may relieve some of the symptoms.

Slow viruses
      The infectious disease kuru was once prevalent in people of the Fore tribe of New Guinea. Transmission of the disease was traced to ritual cannibalism. Symptoms included abnormal involuntary movements, dementia, and disturbance of motor functions. The disease was invariably fatal. A transmissible agent called a prion, a deviant form of a harmless protein normally found in the brain, is now known to cause the disease. This and similar agents are known as slow viruses.

       Creutzfeldt-Jakob disease is a rare cause of rapidly developing dementia in adults. Most cases of Creutzfeldt-Jakob disease are either inherited or sporadic. Rare cases of transmission between humans have resulted only from exposure to the infectious agent during medical procedures.

      Subacute sclerosing panencephalitis is characterized by the slowly increasing loss of mental abilities, brief, shocklike jerking of the body, weakness, and spasticity. The disorder primarily affects children and young people. The measles virus is the responsible agent, probably causing an abnormal response of the immune system.

      Progressive multifocal leukoencephalopathy is another disease of the brain occurring in individuals whose immune system is suppressed by drugs or disease. Progressive loss of myelin occurs in the white matter of the brain, cerebellum, and spinal cord. The responsible agent is a polyoma virus.

Tumours
      The direct effects of brain tumours (tumour) are replacement or compression of tissue, which impairs the function of the compressed part and often causes epileptic seizures. Indirect effects of tumours on the brain include raised intracranial pressure, which is particularly common in the case of tumours arising within or compressing the ventricular system.

Benign tumours
      Benign intracranial tumours do not spread within the brain or metastasize to distant sites. The most common benign brain tumours are neurofibromas (tumours of the myelin-forming Schwann cells), tumours of the skull, and meningiomas. Pituitary adenomas arise within the pituitary fossa. By compressing the underside of the optic chiasm, these tumours cause visual deficits, and they raise the intracranial pressure through compression of the hypothalamus and third ventricle. In addition, many pituitary adenomas secrete hormones, which may stimulate such reactions as abnormal growth and lactation. Surgical removal through the nose or through craniotomy (opening up the cranial cavity), is often required; small tumours of the pituitary may be suppressed by the medication bromocriptine.

      Craniopharyngiomas (craniopharyngioma) arise from a vestigial remnant of tissue in the roof of the mouth and extend upward to cause effects similar to those of pituitary tumours. Since they often contain calcium, craniopharyngiomas may be seen with X rays. Craniotomy is necessary for removal.

      Colloid cysts (cyst) may be found within the ventricles. Although they seldom cause symptoms, on account of their small size, they occasionally may block the flow of cerebrospinal fluid, causing an acute rise in intracranial pressure that results in headache and loss of consciousness.

      Other benign intracranial masses include parasitic cysts, granulomas, tuberculomas, and dermoid cysts. These masses may arise in any part of the brain or beside it, in which case there may also be involvement of the skull.

Malignant tumours
      Malignant tumours of the brain (brain cancer) are more common than benign ones; the most frequent malignant brain tumours are gliomas (glioma), which arise from the neuroglial cells. Astrocytomas may arise anywhere in the brain and destroy the function of the tissues that they invade and replace; they also cause seizures and eventually signs of raised intracranial pressure. Treatment includes surgical removal and radiation therapy.

      Medulloblastomas and ependymomas are other varieties of glioma arising, respectively, in the brainstem and in the walls of the ventricles. The former are described above (see The cerebellum: Tumours (nervous system disease)). Ependymomas also grow in the posterior fossa in young people, but more slowly. These tumours tend to cause an increase in intracranial pressure and signs of brainstem and cerebellar dysfunction. Surgical removal is seldom complete, but radiation therapy may be able to shrink the tumour and slow its growth.

      Oligodendrogliomas may occur anywhere in the cerebrum and are particularly liable to cause epileptic seizures. They often contain calcium, which may allow their detection with X rays. Teratomas and pinealomas are more common in young men; arising from the pineal gland and damaging the hypothalamus and upper brainstem, they disturb the control of eye movement, obstruct the flow of cerebrospinal fluid, and cause cerebellar dysfunction. Their effects may resemble those of a benign or of a malignant tumour. Lymphomas such as Hodgkin disease rarely invade the brain, although they may compress it. Reticulum cell sarcomas (microgliomas) do invade the brain diffusely and may cause headache, seizures, personality changes, and various focal signs.

      A history of headaches, seizures, and focal neurological problems may indicate a brain tumour; diagnosis is confirmed by magnetic resonance imaging (MRI), computed tomography (CT) scanning, or radioisotope scanning. Surgery is usually necessary in order to confirm the nature of the tumour, to relieve increased pressure, and to remove all or part of the mass. This also may make radiation therapy more effective. Swelling surrounding tumours can often be reduced by steroid medications.

      Malignant tumours arising elsewhere in the body may metastasize to the brain; common sites of origin of such secondary tumours are the lung, breast, thyroid, and kidney. These are usually multiple tumours, so surgical removal is usually unsuccessful; however, radiation therapy may shrink them and slow their growth.

Anatomy and collateral supply
      Blood follows two pathways to the brain: the two internal carotid arteries, which divide intracranially into a number of branches (the largest being the anterior and middle cerebral arteries); and the vertebral arteries, which fuse at the base of the brain and then divide again to form the posterior cerebral arteries. There are many communications between vessels inside and outside the head, and, in some places, two arteries supply the same territory. Despite this generous arterial network, blood supply is actually precarious due to the huge requirements of the brain for blood and the fact that the last branches of arteries anastomose, or join together, very little if at all. To help mediate the different rates of cerebral blood flow caused by variations in heartbeat, respiration, blood pressure, and posture, a system of autoregulation exists whereby the cerebral blood vessels vary their size in response to such changes. Yet, in diseased states, blood supply to parts of the brain still often fails. The effects vary from minor, transient slowing of activity, due to slight reduction in local blood flow, to strokes (stroke), which are a severe, permanent loss of function when blood supply is completely lost.

Occlusive strokes
      Occlusive strokes, those in which a blood vessel supplying a part of the brain is blocked, are divided into four groups: (1) Transient ischemic attacks (TIAs) are the mildest occlusive strokes; symptoms last for minutes or hours. TIAs are usually caused by small emboli (embolism), such as fragments composed of blood cells or cholesterol, that are swept into the circulation of the brain from the arteries of the neck or elsewhere. (2) Reversible ischemic neurological deficits may have the same pathological basis, but symptoms last for up to three weeks. (3) Evolving strokes are characterized by a stuttering or progressive deterioration. Blockage of the larger arteries in the brain or neck is usually responsible. (4) Completed strokes cause reduction in blood supply (ischemia) and death of brain tissue as a result of severe starvation of blood ( infarction).

      The most common pathological change causing stroke is atherosclerosis of the large- and medium-sized arteries in the neck and brain, which causes ulcers in the wall of the artery, narrowing of the lumen, or blockage of the vessel. Small-vessel disease is characterized by the thickening and degeneration of the walls of the arterioles and is usually due to high blood pressure. Other vessel-wall diseases include inflammatory changes, as in collagen-vascular diseases or after deep X-ray treatment; spasm, as in severe migraine headaches; splitting of the wall of the artery; and direct injury. Larger emboli causing more than just a TIA may arise in the larger arteries and from the valves or lining membrane of the heart.

      Other causes of stroke are a severe global decrease in cerebral blood flow, as may occur with a severe hemorrhage or heart attack; diversion of blood to other arteries, as a result of a narrowing or blockage of a vertebral artery in the neck; cerebral anoxia, a severe reduction in the amount of oxygen in the blood; and abnormal viscosity of the blood due to excess numbers of cells or to increased protein concentration in the blood.

Hemorrhagic strokes
      Hemorrhagic strokes, in which a blocked vessel bleeds, are usually due to small-vessel disease in individuals with high blood pressure. Such intracerebral hemorrhage occurs most often in the deep white matter of the brain, brainstem, and cerebellum. The incidence of such strokes has been reduced in part by the introduction of more effective treatments for hypertension. Bleeding also occurs when a defect in the wall of an artery, called an aneurysm, ruptures. When this occurs at the sites of branching of the larger arteries inside the head, blood spills into the subarachnoid space, causing subarachnoid hemorrhage. Bleeding may also occur with malformations of arteries and veins, into an infarct, and with blood diseases that impair coagulation.

Clinical syndromes
      The features of TIAs (and of any stroke syndrome) depend on which part of the brain is affected. Transient loss of vision in one eye, as if a curtain were being pulled over it, is one common sign of ischemia of the retina. Similar brief episodes of numbness or weakness of a limb or difficulty in speech suggest an attack in the carotid artery, while a brief reduction in consciousness, vertigo, slurred speech, impaired vision in both eyes, or imbalance may signify ischemia in the vertebrobasilar circulation.

       infarction of the brain occurs most often during sleep; the individual awakes to find that he has lost the function of part of his body. The most common site of stroke is in the middle cerebral artery. The usual symptoms are aphasia and other disorders of higher cerebral functioning, hemiparesis (weakness of the face and arm on the other side of the body), and loss of sensation in the same areas. Blockage of the internal carotid artery in the neck produces similar symptoms. Because there is extensive communication between the anterior cerebral arteries on both sides, the area of the brain supplied by the anterior cerebral artery on the occluded side may escape damage. Anterior cerebral artery infarcts produce weakness and sensory changes on the opposite side of the body, but the leg is more affected than the arm. Posterior cerebral infarcts cause loss of vision in the opposite half of the visual field and sometimes difficulty in the comprehension of speech (called receptive aphasia).

      Small, deep infarcts in the white matter cause small cavities of damaged tissue, called lacunes. Infarcts in this area cause a number of syndromes in which the damage is restricted to, for example, pure weakness or pure sensory dysfunction on one side of the body. Intracerebral hemorrhage most commonly occurs in the putamen and leads to sudden headache, severe hemiparesis, loss of consciousness, and deviation of the eyes to one side. Bleeding into the pons causes paresis of all limbs and unconsciousness. Bleeding into the cerebellum produces typical signs of incoordination with headache and stiffness of the neck. Subarachnoid hemorrhage (SAH) may be caused by rupture of an aneurysm and may cause a sudden headache, a stiff neck, a seizure, or loss of consciousness and, depending on the damage done by the blood pumped out of the artery, brain damage.

Diagnosis and treatment
      The diagnosis of cerebrovascular disease is determined by clinical evaluation, but it has been greatly improved by magnetic resonance imaging (MRI) and computed tomography (CT) scanning, which not only show the site of the stroke but also allow determination of bleeding. Additional tests may include angiography, for localizing an aneurysm or detecting the location and nature of disease in the arteries of the neck, and a battery of tests that determine whether any underlying disease (e.g., diabetes, blood cell or clotting diseases, infections) might have led to the stroke.

      Treatment of a stroke is initially directed toward support of the cardiac and metabolic systems. High blood pressure is controlled, raised intracranial pressure lowered, and, in some cases, anticoagulant medications prescribed. Rehabilitative efforts start as soon as the patient's condition is stable and may include cessation of smoking, diabetes treatment, clipping or isolation of a cerebral aneurysm, and, perhaps, surgical removal of a blood clot or the roughened lining of the major arteries in the neck. Medications such as aspirin, which inhibit the tendency of platelets to cause clotting in the bloodstream, have proved effective in the prevention of stroke and fatal heart attacks after TIAs in men.

Unlocalized or multifocal disorders
      Many disorders of the nervous system cannot be localized to a single site, or they represent distant effects of disease of other parts of the body.

Effects of systemic disease

      Disorders of the hypothalamus can result in autonomic dysfunction, sleep disturbance, and diabetes insipidus. Pituitary (pituitary gland) tumours (tumour) may cause acromegaly, which may result in peripheral-nerve compression. Hyperactivity of the thyroid gland may cause tremors, muscle weakness, and disturbances of eye movement, while hypoactivity may result in mental dulling, deafness, muscle stiffness, and median-nerve compression. Muscle weakness is a symptom of the parathyroid gland hyperactivity; hypoactivity may cause seizures. Muscle weakness is also a feature of adrenal gland dysfunction in both Addison (Addison disease) and Cushing diseases (Cushing syndrome); both of these conditions also induce abnormal mental states.

       diabetes mellitus causes many neurological complications, including peripheral neuropathy, autonomic system failure, painful nerve root lesions in the lumbosacral plexus, atherosclerosis, stroke, and damage to the retinas (retina). Hyperinsulinism reduces blood sugar levels, which may cause coma.

organ failure
      When the function of the liver, lung, heart, or kidney fails, the nervous system is affected. With liver failure dementia, confusion, aphasia, dysarthria, tremors of the limbs, and coma may result.

       kidney failure is often associated with high blood pressure, which may lead to acute brain edema (distension with fluid), visual loss, and coma. Chronic kidney failure is likely to produce peripheral neuropathy as well as symptoms similar to those of liver failure but with increased likelihood of epileptic seizures.

      Chronic respiratory or heart failure also can cause confusion and raised intracranial pressure. In addition to these symptoms, respiratory failure may cause tremors.

Blood diseases (blood disease)
       leukemia and polycythemia can affect the nervous system, the former mainly through infiltration of the meninges or brain and the latter by increasing the risk of stroke. anemia due to deficiency of vitamin B12 may cause spinal cord degeneration and stroke.

      Many biochemical disorders can simultaneously affect the nervous system. Most of them are genetically determined, and only a few are treatable.

Protein and amino acid disorders
       phenylketonuria (PKU) is a disorder of the conversion of the amino acid phenylalanine (a building block of protein) into tyrosine. The primary result is a delay in mental and motor development. PKU can be diagnosed within days of birth by a simple urine test and is treated with a diet low in phenylalanine, which may allow intellectual development to proceed. Other aminoacidurias, which cause psychomotor delay and a variety of neurological and musculoskeletal problems, include hyperglycinemia, hyperammonemia, Hartnup (Hartnup disease) and Lowe diseases, maple syrup urine disease (so called because of the distinctive smell of the urine), and homocystinuria (a condition that predisposes one to strokes).

Disorders of fat and fatty acid metabolism
      Lysosomal diseases are caused by the accumulation of substances that are normally metabolized in the cellular structures called lysosomes (lysosome). These disorders often produce symptoms of neurological involvement at birth or in the early years of childhood.

      Many of the conditions mentioned in this and the next section are caused by a genetically determined enzyme failure within cells, which leads to the accumulation of substances that would normally be metabolized. These conditions are called lysosomal disorders after the lysosome, the structure in the cell that contains these substances, and often produce symptoms of neurological involvement at birth or in the early years of childhood. Examples of lysosomal disorders include Tay-Sachs disease, metachromatic leukodystrophy, Krabbe disease, Fabry disease (Fabry's disease), Gaucher disease, Farber disease, and Niemann-Pick disease. Psychomotor retardation, seizures, retinal abnormalities, peripheral neuropathies, and musculoskeletal deformities are prominent features of these diseases. Other lysosomal disorders, called sudanophilic leukodystrophies, are characterized by defective formation of myelin and cerebellar and corticospinal tract dysfunction. Adrenoleukodystrophy occurs only in males and leads to blindness, motor weakness, spasticity, and signs of adrenal gland failure. In Refsum syndrome peripheral neuropathy and retinal and cerebellar disorders accompany a dry, scaly skin rash. The restriction of dietary phytanic acid, a substance found in green vegetables, slows the progress of the disease.

      Inherited deficiencies of blood lipoproteins (lipoprotein) also may result in peripheral neuropathy, retinal degeneration, and other motor and skeletal deformities. In analphalipoproteinemia, the tonsils (tonsil) are swollen with accumulations of cholesterol, while in betalipoproteinemia, excessive fat is present in the stools. Lipofuscinosis is a disorder in which lipids (lipid) are stored in the retinas and nervous system, causing visual impairment, seizures, intellectual decline, and spastic weakness.

Disorders of carbohydrate metabolism (carbohydrate)
       galactosemia develops when infants are unable to metabolize galactose, or milk sugar. Cataracts, weakness of the limbs, and psychomotor delay occur unless substitutes for milk are given. In glycogen-storage diseases glycogen cannot be metabolized to yield lactic acid and energy, so that it accumulates within muscle, liver, and other tissues. In mucopolysaccharidoses and mucolipidoses (both lysosomal disorders), fat and carbohydrates are deposited in the nervous system and in other tissues; severe mental retardation, motor and ocular disorders, and skeletal abnormalities are common features.

Deficiency states
      Vitamin A deficiency primarily affects the retinas and skin, but components of the vitamin B (vitamin B complex) group are essential for normal development and functioning of the nervous system. Wernicke-Korsakoff syndrome (Korsakoff syndrome) (common in alcoholics) results from a thiamine (vitamin B1) deficiency and consists of eye movement disorders, cerebellar incoordination, memory loss, and peripheral neuropathy. If peripheral neuropathy is the only symptom of thiamine deficiency, the disorder is called beriberi. In each case, replacement of the vitamin may lead to substantial improvement. When other vitamins (e.g., niacin, pyridoxine) are deficient, symptoms are similar, and signs of spinal cord disease may occur.

      Subacute necrotizing encephalopathy, also called Leigh disease, is a lethal disorder of infancy marked by psychomotor delay, myoclonic jerks, paralyses of eye movements, and respiratory disorders. The precise biochemical defect is unknown, but thiamine metabolism dysfunction may be involved. Seizures in early childhood are the main feature of pyridoxine (6 (vitamin B6)) dependency, an inherited abnormal metabolic requirement for that vitamin.

       vitamin D deficiency may cause the softening of bone, called rickets, may affect the nervous system, and may cause weakness of the proximal muscles.

Fluid, mineral, and electrolyte disorders
      Abnormal secretion of antidiuretic hormone from the pituitary gland leads to confusion and seizures; it may complicate intracranial disease, especially subarachnoid hemorrhage. High or low levels of electrolytes such as sodium, potassium, calcium, and magnesium, as well as disorders of acid-base metabolism, also have neurological effects such as confusion, seizures, muscle weakness, tetany (spasm and irritability of muscles), and cramps.

       porphyria is a disorder of heme, the pigment in red blood cells. One form, acute intermittent porphyria, causes attacks of acute abdominal or limb pain, weakness due to peripheral neuropathy, epileptic seizures, confusion, and increased pigmentation of the skin with abnormal sensitivity to sunlight. Attacks are sometimes caused by certain drugs, such as barbiturates (barbiturate) and some antibiotics.

      Inherited disorders include Menkes disease, a disorder of copper metabolism that causes seizures and abnormal hair growth; Hallervorden-Spatz disease, a condition characterized by iron deposits in the basal ganglia of the brain that produce rigidity, weakness, and abnormal movements; Lesch-Nyhan syndrome, which results in raised levels of uric acid in the blood, abnormal movements, spastic weakness, mental retardation, and a tendency to self-mutilate; and familial hypoparathyroidism, a disorder in which the basal ganglia calcify, resulting in choreic movements.

Neurocutaneous syndromes
      A number of genetically determined disorders of the ectoderm (the outside layer of cells in the embryo, from which the skin and the nervous system develop) are grouped together as the neurocutaneous syndromes.

       neurofibromatosis (von Recklinghausen disease) results in café-au-lait spots on the skin, formation of fibrous tumours (tumour) on nerves and, in some cases, of brain tumours, rotation of the spine, tumours of endocrine glands such as the adrenal gland, and skeletal deformities. The Elephant Man, a resident of the London Hospital in the 1880s, may have suffered from this condition, his skin thickened by large numbers of nerve and skin tumours. Neurofibromatosis is relatively common, but many patients have only minimal symptoms.

      Ataxia-telangiectasia (Louis-Bar syndrome) results in cerebellar incoordination and choreic movements, overgrowth of blood vessels on the conjunctiva (eye membranes), and disorders of the immune system.

      Von Hippel-Lindau disease results in tumours of blood vessels in the brain, especially in the cerebellum and retina, and in other organs.

      Sturge-Weber syndrome is characterized by a large red (“port-wine”) overgrowth of blood vessels of the skin over the upper face and by a growth of the underlying brain. The latter may cause seizures, spastic weakness, and visual-field deficits.

      Tuberous sclerosis is characterized by epileptic seizures, a facial rash resembling acne, and benign tumours of the lining membrane of the ventricles of the brain and other organs.

Toxic effects of drugs (drug), metals (metal), and poisons (poison)
      Perhaps due to the complexity of the metabolic system and to the rapid metabolic rate of the nervous system, the brain is highly susceptible to damage from chemical substances including products of organ failure and abnormal metabolism and bacterial toxins. Among the externally derived poisons are bacterial toxins, such as those in tetanus, which excite ventral-horn cells of the spinal cord, with resulting muscular rigidity; diphtheria, which produce severe loss of myelin from motor nerves, causing limp weakness of the throat and other muscles; and botulism and tick paralysis, which prevent normal transmission of impulses between nerve and muscle.

      Heavy metals (heavy metal) (e.g., arsenic, lead, thallium, gold, manganese, and mercury), synthetic chemicals (e.g., organophosphates, gasoline, and toluene), alcohols (especially ethyl and methyl alcohol), ionizing radiation, and many drugs can all be toxic to the nervous system. In addition, water overload can cause seizures, and oxygen, under high pressure, can induce depression of brain function.

      Toxic encephalopathy, characterized by reduced consciousness, seizures, and delirium, may occur in poisoning with alcohols, sedatives, and many other chemicals and drugs—the latter usually taken in overdose or for a prolonged period. Seizures, dementia, and drowsiness may also occur in isolation as drug and chemical toxicity, and withdrawal from sedatives (including alcohol) may also be a cause of seizures. Syndromes in which damage to the basal ganglia occurs, including acute dystonias, chorea, and parkinsonism, occur with overdose or prolonged use of, or hypersensitivity to, many drugs and as a result of poisoning by such heavy metals as mercury and manganese. Other agents, such as diphenylhydantoin and ethyl alcohol, selectively damage the cerebellum. Signs of damage to the spinal cord may appear following electric shock, deep X-ray treatments, or exposure to heavy metals and some drugs.

      Meningeal irritation may occur with some drugs given by mouth or by subarachnoid injection. Fungal meningitis may complicate chronic suppression of the immune system. Raised intracranial pressure, due to an increase in the water content of the brain, occurs with cortisone-like drugs, and migraine headaches (headache) are frequently caused by specific foods and drugs.

      Many drugs and other agents are capable of causing damage to cranial nerves or peripheral nerves. Trichlorethylene (a solvent) and stilbamidine (a drug) cause numbness of the face by affecting the trigeminal nerve. The cochlear and vestibular portions of the vestibulocochlear nerve are susceptible to toxicity from aminoglycoside antibiotics.

      The autonomic nerves (autonomic nervous system) may be damaged alone or in conjunction with other nerves. The transmission of impulses at the autonomic ganglia may be blocked by the actions of anticonvulsants, antidepressants, or sedatives. A drop in blood pressure on standing up is the most common symptom of blocked impulses at the autonomic ganglia.

      Toxic symptoms affecting the muscles include cramps, weakness, and atrophy or even acute degeneration. Drugs may also reduce levels of potassium in the blood, resulting in secondary weakness of muscles. Both quinine and certain antibiotics may block impulses at the neuromuscular junction.

Demyelinating diseases
      Demyelinating diseases are characterized by the destruction of myelin surrounding nerve fibres. Although demyelination can occur after various viral infections or vaccinations, multiple sclerosis (MS) is the most common demyelinating disease.

      Multiple sclerosis is a progressive disease that is five times more common in temperate climates than in tropical climates, affecting approximately one million people worldwide. Females are affected more often than males, and most patients develop the first symptoms between 20 and 40 years of age. Pathologically, multiple scattered areas of inflammation, called plaques, are found within the white matter of the central nervous system. The cause of this demyelination is not known, but genetics may be a factor in a person's susceptibility to MS. Also, the influence of an environmental agent is strongly suggested by variability between regions of the same geographic area and by the increased risk of developing MS in people who migrate from an area of low MS prevalence to one of high prevalence before 14 years of age. The infectious agent that causes MS is unknown.

      The onset of MS typically occurs in young adulthood, with alternating intervals of exacerbation and remission of symptoms. Typical symptoms include eye (eye, human) pain and vision disturbances; speech disorders; facial numbness or pain; incoordination of gait and of the limbs; spasticity, weakness, and increased reflexes; frequency and urgency of bladder emptying; and alteration in sensation, especially involving the senses of joint position and touch. In severe cases, intellect and memory may be impaired.

      If physical symptoms of MS cannot be detected, magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), or evoked potential testing (which measures the speed of the brain's response to stimuli) may be useful in diagnosing the disease. Examination of the cerebrospinal fluid may also be suggestive.

      A number of medications, such as corticosteroids and immunosuppressants, are used to alleviate specific symptoms, such as spasticity and cramping of the limbs, bladder symptoms, and facial pain. In about 30 percent of MS cases, the disease progresses without remission; however, most people with MS have a normal life expectancy.

William E.M. Pryse-Phillips

Additional Reading
Sources on the pathology, diagnosis, and treatment of nervous diseases and disorders include Clinical Neurology on CD-ROM (2000); H. Houston Merritt, Merritt's Neurology, 10th ed., ed. by Lewis P. Rowland (2000); Maurice Victor, Allan H. Ropper, and Raymond D. Adams, Adams and Victor's Principles of Neurology, 7th ed. (2001); W. Russell Brain, Brain's Diseases of the Nervous System, 11th ed., ed. by Michael Donaghy (2001); Mayo Clinic Department of Neurology, Mayo Clinic Examinations in Neurology, 7th ed. (1998); M.-Marsel Mesulam (ed.), Principles of Behavioral and Cognitive Neurology, 2nd ed. (2000); Oliver Sacks, The Man Who Mistook His Wife for a Hat and Other Clinical Tales (1970, reissued 1998); and William E.M. Pryse-Phillips and T.J. Murray, Essential Neurology, 4th ed. (1992).William E.M. Pryse-Phillips

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