skin disease

▪ pathology

Introduction

 any of the diseases or disorders that affect the human skin. They have a wide range of causes.

General features

      Although most diseases affecting the skin originate in the layers of the skin, such abnormalities are also important factors in the diagnosis of a variety of internal diseases. There is some truth in the belief that the skin mirrors a person's internal health. Often, the visibility and accessibility of skin make it the first organ of the body to show detectable signs of underlying disease. Abnormalities of the skin frequently suggest metabolic, malignant, and glandular diseases.

      Like other tissues, skin is afflicted by all types of pathological changes, including hereditary, inflammatory, benign and malignant (neoplastic), endocrine, hormonal, traumatic, and degenerative processes. Emotions affect the health of the skin as well. The reaction of the skin to these diseases and disorders differs from that of other tissues in many ways. For example, extensive inflammation of the skin may affect metabolism (metabolic disease) within other organs and systems of the body, causing anemia, circulatory collapse, disorders of body temperature, and disturbance of water and electrolyte balance in the blood. The skin has such vigorous healing properties, however, that widespread injury, as in thermal burns, may be followed by a marked degree of regrowth of the injured or diseased areas, with a disproportionally small degree of scarring.

Diagnosis

      The skin has an inherent region-specific anatomical diversity that may profoundly modify the appearance of a rash. This is apparent when skin transplanted from one area of the body to another (other than a symmetrically opposite area) retains the morphological characteristics of the donor area. Thus the morphology of eczema or lichen planus on the palms and soles may bear little or no resemblance to the same disease in the same individual on the face or scalp. In these instances a biopsy shows the abnormalities of the cells of the skin and the pattern and distribution of any invading cells. The ability to identify immunoreactants (immunoglobulins, or antibodies, that react with specific invading agents, or antigens) in skin biopsies has greatly increased the accuracy of the diagnosis of inflammatory disorders and has clarified their immunologic basis, especially in the blistering disorders.

      The classification of hereditary skin disorders generally has been based upon gross morphological, histological, and electron microscopic findings; however, because a skin disease may not always have a characteristic presentation, the specific diagnosis sometimes has been in doubt. Better understanding of the biochemical defects underlying hereditary skin disorders now allows these conditions to be diagnosed with more precision. One subset of the ichthyoses (ichthyosis), a group of sometimes disabling genetic skin disorders, may thus be delineated from other members of the group, based on biochemical detection of a specific enzyme defect (reduced steroid sulfatase enzyme).

Distribution

      The distribution of a rash depends on factors both intrinsic and extrinsic to the body. Mechanical factors (such as trauma, environmental agents, fungal or viral infections, and drugs) are among the most common extrinsic determinants of distribution. Environmental influences, such as sunburn and light-sensitive, drug-induced reactions, may also play a major role. Psoriasis and the rare hereditary blistering disorders collectively called epidermolysis bullosa owe their distributions to local trauma; lesions that show a predilection for the elbows, knees, and lower back are common in psoriasis, and those found in the hands, feet, knees, and mouth of children are indicative of epidermolysis bullosa. A lesion of an eruption that subsequently develops where a mechanical or other physical trauma was applied is termed an isomorphic reaction. Skin diseases in which isomorphic reactions are characteristic include lupus erythematosus (sunlight), psoriasis, lichen planus, and viral warts (wart) (mechanical trauma).

      Intrinsic rather than extrinsic influences explain the characteristic systematic pattern of a number of rashes. Some rashes with a symmetrical and segmental distribution may owe their pattern to the segmental arrangement of the embryo. In other rashes, it is the distribution of the cutaneous nerve supply; for example, the pattern of the rash of herpes zoster (shingles) is determined by the cutaneous distribution of the infected sensory nerve dorsal root ganglion. The blood supply to the skin has a characteristic anatomical distribution that influences the pattern of certain skin eruptions in which cutaneous vascular narrowing or blood stasis is a major feature.

Appearance

      Skin diseases in which there is an overproduction of epidermal cells or a disorganization of their differentiation often show scaling. Simple benign hyperplasia (overgrowth) of the epidermis such as is commonly seen in infantile eczema often appears as lichenification, a term used to describe a thickening of the epidermis in which the normal surface markings of the skin are greatly exaggerated. Chronic benign or malignant proliferative dermatoses involving the epidermis often have a rough warty surface caused by overproduction by the epidermal cells of keratin.

      There are relatively few skin diseases in which inflammation, including responses to physical injury (such as sunburn), allergy, and infections (such as boils and cold sores), does not play a part. Even cancerous lesions of the skin frequently show some degree of inflammatory response. Inflammation is usually the result of the release of chemical mediators in the skin. These mediators include histamine, which is stored in a preformed state in the skin, and peptides (kinins) and fatty acids (prostaglandins and leukotrienes), which are formed enzymatically in response to injury. In some skin diseases inflammation is the major factor in the morphological appearance of the rash (for example, acute allergic (allergen) contact dermatitis).

      The most important features of inflammation include redness, caused by dilation of blood vessels (blood vessel); heat, caused by increased blood flow; swelling, caused by leakage of fluid from the small blood vessels; and pain or itching. In addition, the affected skin may feel indurated (hardened) because of the deposition of the coagulation protein fibrin and the infiltration by inflammatory blood cells (lymphocytes, histiocytes, and polymorphonuclear leukocytes). Medications designed to counteract inflammation in the skin may antagonize the effects of mediators (e.g., antihistamines). Nonsteroidal anti-inflammatory drugs of the type effective in arthritis, including aspirin and indomethacin, have proved to be of little or no value in the treatment of inflammatory skin diseases.

      Whealing reactions (hives, nettle rash, urticaria) constitute a special form of inflammation in which vascular changes predominate, with little or no inflammatory cellular reaction. Localized fluid accumulation (edema) causes the development of a short-lived wheal associated with intense itching. Its appearance frequently indicates an underlying allergic cause. Intense inflammation may occasionally lead to the formation of blisters, caused by the effects of enzymes released from inflammatory cells, from the resident cells of the skin, or from blood plasma components. These enzymes cause the breakdown of proteins responsible for the structural integrity of the skin. In some blistering (bullous) skin diseases (such as pemphigus), the development of large blisters is the predominant morphological feature.

      Inflammatory diseases of the dermis often evoke a secondary proliferative and scaly response in the epidermis. Dermal inflammatory disorders may be acute, as in hemolytic (blood-cell destroying) streptococcal infections leading to cellulitis (diffuse inflammation of the connective tissues), or chronic and granulomatous, as in chronic cutaneous tuberculosis (lupus vulgaris). In the first instance the changes of acute inflammation discussed above are present but there is normally no epidermal change. In the second, the skin is hardened and thickened, and it has a brownish appearance under the microscope.

      The intrinsic colour of normal skin is pale yellow, but this colour is modified by pigments—both by melanin and by hemoglobin. The intensity of the brown hue conferred by melanin depends on the distribution of the melanin granules within the cells. All races have the same number of melanocytes, although melanin is produced and distributed through the epidermis more efficiently in blacks. Increased melanin pigmentation is a common reaction to prolonged inflammation of the skin. The intensity and shade of pink depend on the state of the cutaneous circulation. When blood vessels are dilated with increased blood flow, as in an inflammatory disorder, the skin is bright red. The vasodilation associated with stagnant blood flow, as in excessive cooling of the skin, gives the skin a purplish hue.

      The predominant symptom of skin disease is itching. Although readily distinguishable from pain, itching appears to be transmitted along the same sensory neural pathways. There are no specialized sensory receptors for itching in the skin; both itching and pain are transduced by free nerve endings located at the dermo-epidermal junction. Although an itching sensation may result in healthy skin from a direct conversion of very light pressure (light stroking of the skin of the lips is a good example), in most skin diseases itching is caused by the release of pharmacological mediators, such as histamine. When itching occurs in normal-appearing skin, as is the case in liver or kidney disease, the itching is probably due to an abnormal accumulation of metabolic products in the skin.

Hereditary skin diseases

      The formation of almost all components of the skin (for example, hair texture and colour and skin pigmentation and thickness) is under genetic control. A large number of common skin diseases also are directly or indirectly determined by a person's genotype (genetic constitution), but their expression may require an external influence or an altered hormonal milieu. The hereditary diseases psoriasis and atopic eczema are examples of skin disorders in which sunlight (as an extrinsic factor) or stress (as an intrinsic factor) activate the condition. Even when heredity has an immediate determining role, other factors may influence the expression of disease. The hereditary blood vessel diseases of the skin, for example, many of which are under direct genetic control, sometimes do not become evident until the hormonal changes of puberty create conditions optimal for disease expression.

      A common genetic abnormality is the nevus, often called a mole or birthmark. Nevi are due to primary abnormalities in the structure or number of skin cells. A local increase in the concentration of melanocytes is termed a melanocytic or pigmented nevus; an area of increased capillary concentration, a capillary nevus. In nevus anemicus, an area of skin is pale because of reduced blood flow, even though there is no evidence of a structural disorder. Although most nevi are benign, malignant transformation may cause melanocytic nevi.

      If an abnormality is inherited as an autosomal dominant trait and one parent is affected, then, from the laws of Mendelian inheritance, each child has a 50 percent risk for the disease. Amniocentesis, often performed between the 12th and 20th weeks of pregnancy, entails little risk to the mother or fetus and makes it possible to diagnose prenatally severe disorders such as epidermolysis bullosa, a blistering and scarring disorder; albinism, or a generalized lack of pigment from the skin; and xeroderma pigmentosum, a precancerous condition in which sunlight-induced skin damage cannot be enzymatically corrected by the affected skin cells.

Generalized skin diseases

Eczema and dermatitis

      The terms eczema and dermatitis are often used interchangeably to denote an inflammatory process in the skin that involves the upper dermis and epidermis. The epidermis exhibits swelling of the keratinocytes and accumulation of fluid between them (spongiosis). In the severe form of spongiosis, blisters form within the epidermis. Childhood eczema in black children frequently is seen as a follicular eruption of pinhead-sized papules. In chronic forms of eczema or dermatitis the prominent changes are thickening of the epidermis and marked hyperkeratosis (keratosis) (thickening of the outer horny layer of the epidermis). These changes lead to lichenification (see above), roughening and scaling of the skin surface, and itching. The function of the horny layer as an impervious barrier may be seriously impaired, with two important consequences: loss of water from the skin leads to desiccation of the horny layer, which in turn leads to cracking, increased scaling, and soreness; and loss of the barrier function causes increased absorption of medications applied to the surface of the skin. For example, the enhanced absorption of topically applied corticosteroids may cause toxic changes in the skin and in distant organs and tissues.

      Dermatitis is classified into several different types, including contact dermatitis, atopic dermatitis, and seborrheic dermatitis. Contact dermatitis may be further classified as allergic or nonallergic. Nonallergic contact dermatitis occurs in response to skin irritants, is common in industrial settings (occupational dermatitis), and usually affects the hands. Acids, alkalis, dirty oils, detergents, solvents, and even repeated exposure to water are among the numerous causes. Genetic predisposition undoubtedly plays a part, however, which explains why some workers, but not others, suffer from occupational dermatitis, despite equal exposure.

      Allergic contact dermatitis is a less common cause of occupational skin disease but is frequently found among the general population. Like primary irritant contact dermatitis, it can be produced and studied under controlled conditions, and therefore more is known about the underlying pathogenic mechanisms. Allergic contact dermatitis occurs only in persons who have, after previous exposure, become sensitized to the offending antigen. Common antigens include nickel, chromium, rubber, and paraphenyline diamine. The dendritic Langerhans cells of the epidermis are responsible for recognizing the antigen, which is usually bound to a homologous protein or polypeptide. This information, conveyed to the local lymph glands via the lymphatic vessels, leads to the formation of specifically sensitized lymphocytes. On subsequent exposure to the offending antigen, these and other secondarily recruited cells release chemical mediators (lymphokines) that evoke dermatitis in the exposed skin. Sensitization usually persists, although it may decline with old age.

      Allergic contact dermatitis is usually suspected from the distribution of the rash, which corresponds to the areas of skin exposed to the antigen. For example, red-tipped match contact allergic dermatitis is caused by phosphorous sesquisulfide released as a vapour following the striking of a match. In pipe and cigarette smokers who use this type of match, the area affected is the side of the face, explained by the smoker's habit of cupping the match and matchbox in the hands during the process of striking. The diagnosis of allergic contact dermatitis is confirmed by a patch test, in which minute concentrations of suspect antigens are placed on areas of the skin. An accurate diagnosis of allergic contact dermatitis is important because the condition can be cured by avoiding contact with the offending substance.

      Atopic dermatitis, which affects a small number of infants, is one of several genetically linked disorders that include asthma, hay fever, and urticaria. This group of atopic diseases is characterized by sensitization of the skin to a wide range of common antigens. In addition to eczematous changes, persons with atopic dermatitis may also often have diminished, absent, or paradoxical cutaneous vascular reactions to vasodilating and vasoconstricting drugs; impaired immunity to fungal and viral infections; and cataracts in the lenses of the eyes. Although there is some controversy over the causes of atopic dermatitis, the role of genetics is undisputed. About 70 percent of patients have a family history of eczema, asthma, or hay fever. Most patients begin to suffer symptoms during the first six months of life, and there is evidence that ingestion of dairy products during this period may precipitate development of atopic dermatitis in a predisposed infant. There is no real evidence that food plays a significant role in the development of atopic dermatitis in older children and adults. The disease usually improves during the second decade of life, although it sometimes persists in adults and may even appear then for the first time.

      Seborrheic dermatitis is a less common form of chronic dermatitis that characteristically affects the scalp and other hairy areas, the face, and flexural areas (groin, armpits, skin behind the ears, the cleft between the buttocks). It is frequently associated with infection of the skin by yeasts (yeast) and bacteria, but a causal relationship with these organisms has not been established. There is no evidence of increased greasiness of the skin (seborrhea). dandruff is a mild form of seborrheic dermatitis that affects most people at some time during their lives. The scaliness of the scalp associated with dandruff is caused by the intermittent shedding of dead stratum corneum cells, which in health are shed continuously.

Psoriasis

      Although the mechanisms of inheritance are not clear, psoriasis, like atopic dermatitis, has been thought to be inherited as an autosomal dominant trait that pursues a chronically remitting and relapsing course. Psoriasis is less common than atopic dermatitis, affecting about 2 percent of the population, and is both a proliferative and an inflammatory disorder. The most important feature of psoriasis is an accelerated proliferation of the keratinocytes, which results in the formation of raised scaly plaques in areas of injury, notably the knees, elbows, buttocks, and knuckles. Streptococcal tonsillitis frequently precedes relapses, especially in children, and the mechanism in these cases is probably immunologic. The isomorphic reaction (see above) is a feature of psoriasis. Persons with the disease may also have a characteristic form of arthritis that affects joints in the fingers and spine. Whether the increased rate of keratinocyte proliferation is due to increased activity of a growth-promoting factor or to lack of a growth inhibitor is unknown.

      As an inflammatory disorder, psoriasis is characterized by nests of neutrophil leukocytes in the epidermis called microabscesses. Possible mediators that attract this cell type are leukotrienes and fragments derived from activated blood peptide components called complement. The skin blood vessels are also abnormal in psoriasis, with increased twisting. The relationship of this dermal change to the epidermal abnormalities is not clear. Although psoriasis is neither infectious nor contagious it is disfiguring.

Skin cancer

      Skin cancer is one of the most common forms of cancer in humans and, although visible and therefore recognizable at an early stage, it results in significant mortality. The incidence and prevalence of skin cancer can be greatly reduced by simple preventive measures, such as avoidance of exposure to the Sun and to excessive ionizing radiation (radiation injury).

      Primary skin cancers can be divided into two types: epidermal cancers, which originate in keratinocytes, melanocytes, or skin appendages (e.g., sweat glands, the pilosebaceous apparatus); and dermal cancers, which originate in neural, vascular, mesenchymal, or lymphoreticular tissues. Malignant tumours (tumour) arising from keratinocytes or melanocytes are the most frequent skin cancers.

      Basal cell carcinoma, rare in Negroes and Asians, is the most common malignant skin tumour in Caucasians. It arises from the undifferentiated basal keratinocytes of the epidermis. Although multiple basal cell carcinomas may develop early in life as an inherited trait (nevoid basal cell carcinoma syndrome, or as a complication of xeroderma pigmentosum), most arise in middle age and later. These cancers rarely metastasize but may be highly invasive locally; they are then known as rodent ulcers. The lesions occur in fair-skinned persons and on areas of skin that receive the greatest exposure to sunlight. Treatment with inorganic arsenical drugs and exposure to ionizing radiation (X rays, radium) may also contribute to some cases. Avoidance of unnecessary sunlight and careful control of ionizing radiation significantly lowers the incidence of basal cell carcinoma. Although metastases are rare, the cancer may spread locally and invade surrounding tissues. When this occurs, treatment may be difficult and lengthy.

      Squamous cell carcinoma is less common than basal cell carcinoma but has a higher rate of metastasis. It is common in children with xeroderma pigmentosum, who are unable to repair DNA damage caused by ultraviolet irradiation. In most persons this inability is due to the deficiency of an endonuclease enzyme. Incomplete repair of damaged DNA causes mutations that appear as basal or squamous cell carcinomas, malignant melanomas, and keratoacanthomas.

      In adults, squamous cell carcinoma rarely occurs in the absence of an external cause. Protracted exposure to sunlight is the usual cause, but chronic scarring from burns, as well as reactions to vaccinations, radiation dermatitis, and chronic ulceration, may contribute to some cases. Squamous cell carcinoma is also an occupational hazard, as was noted at the end of the 19th century in regard to chimney sweeps who contracted cancer after exposure to tars. Tar-induced squamous cell carcinoma occurs today in workers who distill tar vapour in the manufacture of coal gas and in machinery operators whose clothes and skin become soaked in mineral oil.

      Because of the high rate of metastasis of squamous cell carcinomas, early diagnosis is important, especially in a middle-aged or elderly person with a skin ulcer that fails to heal. Skin lesions that precede squamous cell carcinoma include white patches in the mucous membranes of the mouth, genitalia, or anus; warty lesions called keratoses (which are especially common after chronic exposure to the Sun, when they are called solar keratoses); and the lesions of Bowen's disease—persistent red scaly plaques that on microscopic examination are found to contain grossly abnormal keratinocytes. When squamous cell carcinoma follows Bowen's disease, there is often a history of treatment with an inorganic arsenical drug.

      In Western countries, the mortality from the skin cancer malignant melanoma is increasing by about 4 percent per year. This type of skin cancer arises from the melanocytes of the skin, and the tumour is therefore often, but not invariably, pigmented with melanin. In adults malignant melanoma arises as a new lesion or as a change in a benign pigmented mole. Malignant melanoma metastasizes frequently, and excision of the tumour together with a collar of surrounding healthy skin is curative if done early.

      Primary cancers arising in the dermis are much less common than epidermal malignant tumours. Mycosis fungoides is a malignant tumour of the T lymphocytes of the dermis. Despite the name, fungal infection does not cause the cancer. Mycosis fungoides (which is rare in children) is ultimately fatal, but the tumour grows slowly enough that many patients die of unrelated causes.

      Extrinsic causes of dermal cancers are rare. An exception is the malignant cutaneous vascular tumour called Kaposi's sarcoma. Although there are many varieties of Kaposi's sarcoma, the development of one particular form is a common terminal event in patients with acquired immune deficiency syndrome, and the tumour may therefore result from infection by a retrovirus known as the human immunodeficiency virus (HIV).

Skin infections and infestations

      Healthy skin harbours a resident, harmless bacterial population that includes Micrococcaceae, Propionibacteriaceae, and aerobic diphtheroids. In addition, pathogenic strains of staphylococci may inhabit the skin of healthy persons, who then act as carriers. These strains are often resistant to antibiotics (antibiotic), so that carriers pose a serious health hazard to those with diminished resistance to infection, such as newborn infants and elderly surgical patients. Most infections by resistant organisms occur nosocomially, or in hospitals. Although local measures, such as cleansing of the involved skin, are often effective for minor skin infections, serious infections require antibiotics.

      Signs of bacterial infection appear after the organisms have been introduced into the dermis. Some organisms, such as staphylococci and streptococci, produce a pustular infection ( impetigo, boils) in which the main features are acute inflammation and the accumulation of large numbers of white blood cells as pus. Other organisms, including Mycobacterium tuberculosis and Treponema pallidum (the spirochete that causes syphilis), may cause more chronic inflammatory changes without pus but involving a characteristic arrangement of mononuclear inflammatory cells called histiocytes. The term granulomatous is used to describe this kind of cutaneous reaction to infection. Leprosy is a cutaneous bacterial granulomatous disease in which the causative organism, Mycobacterium leprae, resides in the neural tissue in the skin, producing the characteristic features of nerve thickening and anesthesia. Despite its bacterial origin and popular reputation, leprosy is minimally infectious, and isolation of affected persons is rarely justifiable.

      A number of different viral species are epidermotropic; that is, they invade the epidermis, either from the skin surface through a minor abrasion, as in viral warts, or from the bloodstream (viremia), as in varicella (chickenpox), or through the peripheral sensory nerves, as in herpes zoster (shingles). The human wart virus causes epidermal cellular proliferation and hyperkeratosis. When it invades the genital skin and mucous membranes it produces moist, exuberant, highly vascular warts called condylomata acuminata. These warts are transmitted through sexual contact and may become cancerous. By contrast, common viral warts of the hands, feet, and other non-mucosal surfaces are not sexually transmitted and rarely become cancerous. Viral warts on non-mucosal surfaces occur in almost all children, although most disappear spontaneously with the development of acquired immunity.

       herpes simplex (cold sores, fever blisters) and herpes zoster (shingles) are two epidermal viral infections that produce blisters within the epidermis. The severity of these infections is influenced by the state of the person's immune system; they are more severe in injured persons, in the elderly, and in debilitated patients, especially those with cancer of the lymphoid system. Herpes zoster (shingles) and varicella (chickenpox) are caused by the same virus, and patients with shingles have previously had chickenpox. The virus remains dormant in the sensory nerves and is later reactivated, either spontaneously or following the impairment of immune defense mechanisms. The virus, by translocating along the nerve branches, invades the epidermis; the distribution of the ensuing vesicular rash, which is painful, is confined to the cutaneous distribution of the same sensory nerve.

      Fungal infection is a common cause of chronic skin eruption. Several species of dermatophyte fungi may live on the dead horny layer of the skin. The inflammatory changes they produce are partly due to irritant products of the fungus, which diffuse into the skin, and partly to an immunologic reaction by the host in an attempt to eliminate the fungus. Yeasts, of which Candida albicans is the most common, are also a cause of chronic skin or mucous membrane infection. In patients receiving immunosuppressive drugs for organ transplantation, yeast infections may become systemic, and both yeast and dermatophyte infections may be extensive, persistent, and unresponsive to drug treatment. The availability of systemic antifungal drugs, including griseofulvin and the broad-spectrum imidazole group, has revolutionized the treatment of these disorders, but many patients with both minor and serious fungal infections remain resistant to drug treatment.

      Skin infestations are frequent in persons living or working in overcrowded, unhygienic conditions. Pediculosis (crabs, lice, nits), which affects hairy areas, is diagnosed by identifying the egg capsules (nits) that are cemented to the hair shaft. Lice may also be visible near the base of the hair. Scalp, axillary, or pubic (pubic louse) hair may be affected. Scabies, which is caused by the mite Sarcoptes scabiei, is usually acquired through close personal contact, especially sexual contact. It affects all areas of the body except the soles of the feet, head, and neck (in infants these areas are also involved) and causes an itchy eruption that is largely an allergic reaction to the products of the mites. Although most persons develop an immunologic reaction to scabies, acquired immunity is poor or nonexistent, and recurring attacks are common. The condition is treated with benzyl benzoate, and all persons who have had contact with the infected individual must be treated, whether symptomatic or not, because of the high rate of reinfestation.

Urticaria ( hives)

      The skin blood vessels can also show acute, short-lived reactions. Urticaria features an area of central redness, on which is superimposed an irregular wheal, caused by local edema, and surrounded by a bright pink flare. The wheals are usually multiple and remain visible for 30 to 45 minutes. The reaction is due to release of histamine from cellular stores within the skin and is usually accompanied by itching. Common causes include allergies to shellfish, strawberries, and nuts or to drugs such as penicillin; but physical factors such as cold, exercise, and sunlight may also produce the response. Since histamine is an important causative agent in urticaria, most cases respond to antihistamine treatment.

Localized skin diseases

Pigments

      An increase in melanin pigmentation may be caused by an increased density of melanocytes, by abnormal packaging of melanin, or by increased melanin production. Pigmented birthmarks usually reflect local increases in melanocyte numbers, but in certain rare congenital (congenital disorder) pigmentary disorders, such as von Recklinghausen's neurofibromatosis, there is abnormal packaging of melanin within the melanocytes. Pigment production in the skin is regulated by a pituitary peptide hormone, melanocyte-stimulating hormone, and the increase in melanin pigmentation seen with pituitary tumours may reflect overproduction of this hormone by the pituitary.

      Both suntans and postinflammatory pigmentation result from the overproduction of melanin by melanocytes. In suntans melanin overproduction occurs in response to ultraviolet light and protects the skin against sunburn and the carcinogenic actions of ultraviolet rays. Postinflammatory pigmentation follows a number of inflammatory skin disorders, including chronic dermatitis, acne vulgaris, lichen planus, and drug reactions. It is more common in dark-skinned than in fair-skinned persons. In suntans and postinflammatory pigmentation there is usually no increase in the number of melanocytes.

      Loss of melanin pigmentation may be caused by the absence of melanocytes, as in vitiligo; reduced or absent synthesis of melanin by melanocytes, as in albinism and phenylketonuria; abnormal packaging of melanin in melanosomes, as in the Chediak-Higashi syndrome; or insufficient transfer of melanosomes to keratinocytes, as in hypopigmentation secondary to atopic dermatitis or psoriasis. Vitiligo is a common disorder of pigmentation that results in the widespread appearance of depigmented patches, notably around the mouth, anogenital area, eyelids, elbows, knees, and backs of the hands, due to an absence of melanocytes in these areas. Vitiligo is usually a slowly progressive disorder that is probably the result of abnormal immunologic reactions against constituents of the body's own cells (autoimmunity).

      Certain chemicals found in plants, the psoralens, if taken orally or applied to the skin enhance the ability of ultraviolet light to induce pigmentation. Although rarely successful, treatment with psoralens and ultraviolet radiation has restored pigmentation. Persons with vitiligo should avoid sunlight, which accentuates the contrast between normal and depigmented skin.

Connective tissues

       collagen is the major structural protein of skin and is responsible for its tensile strength, elasticity, and pliability. It is synthesized in the dermis by fibroblasts. Scarring is caused by excess production of collagen during healing. When the collagenous proliferation is extensive, a hypertrophic scar, called a keloid, results.

      There are at least five types of collagen and numerous inherited disorders of collagen synthesis. The best known of the inherited disorders is the Ehlers-Danlos syndrome, which includes at least seven varieties of connective tissue defects. Most persons with Ehlers-Danlos syndrome have increased joint mobility and fragile skin, and they bruise easily. Depending on the nature of the defect in collagen synthesis, patients may also have spinal deformities, ocular defects, and ruptures of major blood vessels.

      The connective tissue of the skin may rupture after stretching, even in healthy persons, forming purple linear discolorations. Because of the defect in the continuity of the dermal connective tissue, these striae are soft to the touch. Initially livid, after several months the striae become ivory white and firm because of the formation of scar tissue. Rapid growth during adolescence may cause striae to form on the thighs, buttocks, lower trunk, or shoulders. During pregnancy striae form on the abdomen as the elastic tissue stretches and weakens. Striae may also follow treatment with corticosteroid drugs.

      The skin connective tissue is affected in a group of inflammatory disorders collectively termed the autoimmune connective tissue diseases (connective tissue disease). These diseases, which include systemic lupus erythematosus, dermatomyositis, polyarteritis nodosa, and systemic sclerosis (scleroderma), are characterized by the involvement of more than one organ or system, the presence in the serum of immunoglobulin autoantibodies that react with human tissue or organ components, and inflammatory damage to blood vessels of the skin and other organs. Systemic sclerosis causes hardening of the skin (scleroderma) due to an increase in the connective tissue of the dermis and marked tethering of the dermal connective tissue to subcutaneous tissues. The process is not confined to the skin, and the cause is not known, although an autoimmune basis is thought to be involved because of the associated immunologic abnormalities. There is evidence that the skin fibroblasts of patients with scleroderma produce more collagen than the fibroblasts of healthy persons. It has also been suggested that the connective tissue changes may be caused by vascular abnormalities. No satisfactory treatment is known, although improvement of the circulation with vasodilating drugs may produce temporary relief.

Blood vessels

      Blood circulation in the skin serves both to supply nutrients and as a means of regulating temperature. The structural and functional organization of the skin's vascular system is complex, and there are major regional variations. Skin blood vessel disorders may arise from an increase in the viscosity of blood contents or from abnormalities of the vessels themselves. Increased viscosity may be due to an overabundance of red blood cells (polycythemia) or an increase in one or more fractions of the protein content of the plasma; hyperglobulinemia is an example of the latter. In functional disorders of the blood vessels, nervous control of the vessels may be disturbed, leading to vascular spasm or dilatation (flushing). In Raynaud's phenomenon, the vascular spasm is severe, affecting the extremities and leading to attacks of cold, white fingers. Milder degrees of spasm, as well as increases in blood viscosity, may cause a purple discoloration of the skin (cyanosis), which may assume a reticulate pattern (livedo reticularis).

      Damage to the vessel walls themselves may perturb skin circulation. Causes include atherosclerosis resulting from age-related degenerative changes, disturbance of lipid (fat) metabolism, and inflammation of the vessel wall. Inflammation is characteristic of the autoimmune connective tissue diseases (see above). When disease of the blood vessels is severe, lack of blood in the affected areas may cause skin ulcers. Chilblains (pernio) occur when the skin is persistently chilled, leading to protracted vessel spasm, cyanosis, coldness, and ultimately ulceration of the affected area.

hair and pilosebaceous follicles

      Hair is of little functional value in humans. Many systemic diseases, however, alter the appearance and growth patterns of hair. In diseases of malnutrition, such as kwashiorkor, the scalp hair becomes reddish brown, sparse, and brittle. Acute protein starvation may lead to the cessation of hair growth and constriction of the hair shafts. Chronic iron (vitamin) deficiency may cause diffuse hair loss (alopecia). Since the continuous hair growth of healthy persons involves the cells of the hair root in exuberant mitotic activity, hair is especially vulnerable to the cytotoxic action of drugs. Acute hair loss is an early side effect of the cytotoxic drugs used in cancer chemotherapy.

      Acute diffuse hair loss (telogen effluvium) may also occur after severe fevers and childbirth. It appears to be caused by an abrupt reduction in the proportion of actively growing hairs, and after a few months the hair regrows normally without treatment. The hair follicles may become inflamed, forming small pustules, which, in severe cases, result in bald, scarred areas with destroyed follicles.

      Both pattern baldness in men and hirsutism in women are under the control of the steroid sex hormones (testosterone). Male pattern baldness occurs to some extent in almost all men. It may start any time after puberty and is distinguished from other forms of alopecia by the pattern of frontal and temporal hair recession and thinning over the crown. On the affected areas of the scalp a downy hair called vellus replaces the former long, sturdy, pigmented terminal hair. Almost as common is the reverse process, in which hair that was formerly vellus alters to the long pigmented terminal type and becomes more visible. When this occurs in the areas of the female body sensitive to male sex hormones, and especially when it follows the adult male pattern of hair distribution, it is known as hirsutism. Some affected women have endocrinologic abnormalities, including ovarian cystic disease or overactive adrenal glands, that elevate the levels of androgen (male sex hormones). These women may also have menstrual irregularities or be infertile. Hormonal treatment or, in less extensively hirsute women, electrolytic depilation may be helpful.

      Among persons with curly hair, and especially among blacks, cut ends of hair may curve and bury themselves in the skin, causing acute pustular reactions. These ingrowing hairs, or pili incarnati, affect 70 percent of black men who are clean-shaven.

Acne

      The hair follicle from which the hair shaft emerges serves as a duct for conducting sebum, the oily product of the sebaceous gland, to the skin surface. The hair follicle and sebaceous gland together form the pilosebaceous unit. Sebum secretion is under hormonal control and increases substantially at puberty, then continues at an increased rate into advanced adult life. An important disease of the pilosebaceous unit is acne vulgaris, which occurs on the face, shoulders, and upper torso of adolescents. The hallmark of acne vulgaris is the comedo, commonly known as the blackhead. The comedo consists of a mixture of keratinous debris and sebum that lodges in the mouth of the pilosebaceous duct. Affected areas of the skin also exhibit inflamed papules, pustules, and cysts. The inflammation is caused by leakage of sebum and keratin debris outside the distended pilosebaceous duct. The bacillus Propionibacterium acnes, which populates the lesions, may also contribute indirectly to the inflammation by metabolizing the sebum to produce irritant fatty acids.

      Eruptions can be brought on by contact with dirty oils or tars and halogenated hydrocarbons. Chloracne, caused by exposure to chlorine compounds, may develop in workers involved in the manufacture of cables, insecticides, fungicides, and herbicides. Industrial accidents involving the release of tetrachlorodibenzodioxin vapour have caused severe community outbreaks of chloracne. Acne rosacea, an acne of middle age that exhibits nodular swelling of tissues in the face, is a disease that occurs rarely among blacks.

Sweat (perspiration) glands

      Most sweat glands in humans are eccrine (i.e., they secrete outwardly) and are under the control of the autonomic nervous system. Eccrine sweat glands are distributed uniformly over the body and function mainly in controlling temperature, although on the hands and feet they assist grip. Inability to secrete adequate sweat (anhidrosis) is usually a congenital abnormality. hyperhidrosis is the abnormal excessive secretion of sweat, usually on the palms and soles and in the axillae. Emotional stress normally stimulates sweating in these areas, but in hyperhidrosis excessive sweating may follow minor stimuli or no obvious stimulus at all. Hyperhidrosis may be an occupational problem for those who handle books, papers, or metallic objects that can rust.

      Most serious disorders of the eccrine sweat glands result from the retention of sweat. Prickly heat (miliaria) occurs in persons living in a hot, humid environment and is due to sweat retention brought on by the combination of increased sweat production and blockage of the duct from hydration of the surrounding horny layer of skin. The gland continues to secrete, and the pressure may rupture the sweat duct at its junction with the epidermis. In heatstroke, another disorder of the eccrine sweat glands, the glands are exhausted and unable to secrete sweat. Hyperpyrexia, an excessive rise in body temperature, may ensue and is sometimes fatal. The most common disorder of the eccrine glands, however, is excessive sweating of the palms, soles, armpits, and face. Emotional stress normally stimulates sweating in these areas, but emotional hyperhidrosis (increased sweating) occurs when excessive sweating follows minor or no obvious stimuli.

      Apocrine sweat glands, which are normally limited to the armpits, perineum, and genitalia, differ anatomically and physiologically from eccrine sweat glands. In the apocrine glands the secretions are concentrated at the free end of the cell, which is cast off along with the secretory products. Apocrine secretion is oily, opalescent, and odourless when secreted, but acquires an offensive smell when it mingles with the bacteria of the skin. The actions of deodorants and antiperspirants are based mainly on an antibacterial effect, on mechanical blockade of the apocrine gland pores, or both.

Nails (nail)

      The most common causes of diseased nails are chronic paronychia—infection of the soft tissue of the fingers between the lateral edges of the nail plates and the lateral nail folds—fungus infections, trauma, psoriasis, impaired circulation, and dermatitis. Chronic paronychia is an occupational hazard of dishwashers, bar workers, hairdressers, and vegetable preparers. When the cuticle is lost, water, bacteria, and yeasts seep under the nail fold and cause inflammation. The lateral nail folds extrude beads of pus when they are squeezed. The infection is commonly caused by the yeast Candida albicans, and when this organism is identified, infection elsewhere in the body should be sought.

      Like the hair, the nails may be useful indicators of skin diseases and of internal disorders. Psoriasis causes pitting on the surface of the nails and an increase in their growth rate. Lichen planus causes a characteristic scarring of the posterior nail fold, leading to fusion of the posterior nail fold with the nail plate. Club-shaped deformity of the nail plate is associated with chronic chest disease and cyanotic congenital heart disease. A reduction in the rate of nail growth, a yellow discoloration of the nail plate, and a smooth curvature to the nails are all features of the yellow nails syndrome, a rare hereditary condition in which the lymphatic drainage system of the body is defective. In chronic anemia the nail plate may become concave (koilonychia). Transverse depressions of the nail plates called Beau's lines result from temporary interruption of nail growth and are seen in a number of acute illnesses, including coronary thrombosis, pneumonia, and severe injuries. Various forms of interference with the blood supply may affect the nail matrix and produce irregular growth of the nails. Drug treatment may cause shedding of the nails. Photo-onycholysis—destruction of the nails from exposure to sunlight—occurs in patients taking photosensitizing drugs such as certain broad-spectrum antibiotics.

Metabolic disorders

      Metabolic disorders may result in the deposition of excess materials in the dermis and thus cause disease. Uric-acid salts are deposited in gout, calcium salts in calcinosis, lipids in xanthomatosis, an abnormal protein in amyloidosis, mucin in myxedema, and porphyrins (porphyrin) in porphyria. The appearances of the skin are usually sufficiently characteristic to be diagnostic.

      A metabolic disorder that dramatically affects the skin is porphyria, an enzyme defect that causes hemoglobin precursors, or porphyrins, to accumulate in the skin. Porphyrins are abnormally sensitive to long wavelength ultraviolet light. The accumulation of porphyrins results in severe sun sensitivity, with blistering, increased skin fragility, and scarring on the areas exposed to light. The most common form, porphyria cutanea tarda, is usually caused by a combination of a hereditary enzyme defect and chronic alcoholism.

Aging and the skin

      More than one-third of persons over 65 years of age have skin problems. Prevalent in the elderly are such common skin conditions as fungal infections, excessive dryness, various benign tumours, seborrheic dermatitis, seborrheic warts, solar keratoses, and hirsutism. Many age-related skin disorders previously viewed as inevitable accompaniments of advancing age are now known to be remediable.

      Skin may age much faster than the rest of the body because of environmental effects, especially sunlight. A fair-skinned woman who habitually sunbathes, for example, may have a senile skin at age 40, whereas her coeval who spends most of her time indoors may not. The predominant features of the aging skin include skin laxity leading to wrinkles, dryness, itching, increased pigmentation, and telangiectasia (i.e., visible dilation of the skin blood vessels). Moles, large seborrheic warts, and small hemangiomas (de Morgan's spots) are more common among whites, while small black warts (dermatosis papulosa nigra) of the upper chest and face and tiny white spots (idiopathic guttate hypomelanosis) are more common among blacks.

      There is no evidence that the rate of reproduction of epidermal cells slows with age. There are, however, marked degenerative changes in the connective tissue, both elastic and collagenous, that cause wrinkling and loss of elasticity. So-called rejuvenating cosmetic skin preparations are ineffective, although simple emollients may provide temporary suppleness. Avoiding excessive exposure to sunlight and using sunblocks, if adopted early in life, should retard the aging process and reduce the incidence of skin cancers. None of these treatments, however, does more than slow the onset of senescence in the skin, and those who survive into the 10th decade of life have age-related changes that are largely independent of their earlier life-styles.

Malcolm W. Greaves

Additional Reading

Introductory works include Walter F. Lever and Gundula Schaumburg-Lever, Histopathology of the Skin, 6th ed. (1983), a standard textbook of the histology of skin diseases; Thomas B. Fitzpatrick et al. (eds.), Dermatology in General Medicine, 3rd ed., 2 vol. (1987), a textbook with an atlas; Etain Cronin, Contact Dermatitis (1980); Robert M. Adams, Occupational Skin Disease (1983); R. Baran and R.P.R. Dawber (eds.), Diseases of the Nails and Their Management (1984); and Peter D. Samman and David A. Fenton, The Nails in Disease, 4th ed. (1986). Also see George C. Andrews, Andrews' Diseases of the Skin: Clinical Dermatology, 7th ed., edited by Anthony N. Domonkos, Harry L. Arnold, and Richard B. Odom (1982); David R. Bickers, Paul G. Hazen, and William S. Lynch, Clinical Pharmacology of Skin Disease (1984); Leonard C. Harber and David R. Bickers, Photosensitivity Diseases: Principles of Diagnosis and Treatment (1981); and Arthur Rook and R.P.R. Dawber, Diseases of the Hair and Scalp (1982). A readable general overview of personal skin care is William Montagna, Frank Parker, and Antonio Tosti, Skin: Your Owner's Manual (1986).

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