diabetes mellitus

diabetes mellitus [mə līt′əs]

n.

〚ModL, lit., honey diabetes < L mellitus, of honey, honeyed < mel, honey: see MILDEW〛

a chronic form of diabetes involving an insulin deficiency and characterized by an excess of sugar in the blood and urine, and by hunger, thirst, and gradual loss of weight

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Disorder of insufficient production of or reduced sensitivity to insulin.

Insulin, synthesized in the islets of Langerhans (see Langerhans, islets of), is necessary to metabolize glucose. In diabetes, blood sugar levels increase (hyperglycemia). Excess sugar is excreted in the urine (glycosuria). Symptoms include increased urine output, thirst, weight loss, and weakness. Type 1, or insulin-dependent diabetes mellitus (IDDM), an autoimmune disease in which no insulin is produced, must be treated by insulin injections. Type 2, or non-insulin-dependent diabetes mellitus (NIDDM), in which tissues do not respond to insulin, is linked to heredity and obesity and may be controlled by diet; it accounts for 90% of all cases, many of which go undiagnosed for years. Untreated diabetes leads to accumulation of ketones in the blood, followed by acidosis (high blood acid content) with nausea and vomiting and then coma. Careful attention to content and timing of meals, with periodic checking of blood sugar, may manage diabetes. If not, injected or oral insulin is necessary. Complications, including heart disease, diabetic retinopathy (a leading cause of blindness), kidney disease, and nerve disorders, especially in the legs and feet, account for most deaths. Degree of blood-sugar control does not always correlate with progression of complications. Gestational diabetes may occur as a complication of pregnancy.

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▪ medical disorder

Introduction

      disorder of carbohydrate metabolism characterized by impaired ability of the body to produce or respond to insulin and thereby maintain proper levels of sugar ( glucose) in the blood.

Causes and types

       insulin is a hormone secreted by beta cells, which are located within clusters of cells in the pancreas called the islets of Langerhans (Langerhans, islets of). Insulin's role in the body is to trigger cells to take up glucose so that the cells can use this energy-yielding sugar. Patients with diabetes may have dysfunctional beta cells, resulting in decreased insulin secretion, or their muscle and adipose cells may be resistant to the effects of insulin, resulting in a decreased ability of these cells to take up and metabolize glucose. In both cases, the levels of glucose in the blood increase, causing hyperglycemia (high blood sugar). As glucose accumulates in the blood, excess levels of this sugar are excreted in the urine. Because of greater amounts of glucose in the urine, more water is excreted with it, causing an increase in urinary volume and frequency of urination as well as thirst. (The name diabetes mellitus refers to these symptoms: diabetes, from the Greek diabainein, meaning “to pass through,” describes the copious urination, and mellitus, from the Latin meaning “sweetened with honey,” refers to sugar in the urine.) Other symptoms of diabetes include itching, hunger, weight loss, and weakness.

      There are two major forms of the disease. Type I diabetes, formerly referred to as insulin-dependent diabetes mellitus (IDDM) and juvenile-onset diabetes, usually arises in childhood. It is an autoimmune (autoimmunity) disorder in which the diabetic person's immune system produces antibodies (antibody) that destroy the insulin-producing beta cells. Because the body is no longer able to produce insulin, daily injections of the hormone are required. Type II diabetes, formerly called non-insulin-dependent diabetes mellitus (NIDDM) and adult-onset diabetes, usually occurs after 40 years of age and becomes more common with increasing age. It arises from either sluggish pancreatic secretion of insulin or reduced responsiveness in target cells of the body to secreted insulin. It is linked to genetics and obesity, notably upper-body obesity. People with type II diabetes can control blood glucose levels through diet and exercise and, if necessary, by taking insulin injections or oral medications. Despite their former classifications as juvenile or adult, either type of diabetes can occur at any age. Type II diabetes is by far the most common type of diabetes, accounting for about 90 percent of all cases.

      Diabetes mellitus also may develop as a secondary condition linked to another disease, such as pancreatic disease; a genetic syndrome, such as myotonic dystrophy; or drugs, such as glucocorticoids. Gestational diabetes is a temporary condition associated with pregnancy. In this situation, blood glucose levels increase during pregnancy but usually return to normal after delivery. However, gestational diabetes is recognized as a risk for type II diabetes later in life.

Diagnosis and treatment (therapeutics)

      Many people are unaware that they have diabetes. In the late 20th century, for example, it was estimated that more than 5 million of the 15.7 million American cases were undiagnosed. The disease is usually discovered when there are typical symptoms of increased thirst and urination and a clearly elevated blood sugar level, as defined by a daytime level greater than 200 milligrams per decilitre or a fasting level greater than 140 milligrams per decilitre. Rarely a more detailed oral glucose tolerance test is required for diagnosis. The duration and severity of hyperglycemia can be assessed by measuring levels of advanced glycosylation end products (AGEs). AGEs are formed when hemoglobin molecules in red blood cells undergo glycosylation (binding to glucose), and the bound substances remain together until the red blood cell dies (red blood cells live approximately 120 days). AGEs are believed to inflict the majority of vascular damage that occurs in people with diabetes. A glycosylated hemoglobin called hemoglobin subtype A1c (HbA1c) is particularly useful in monitoring hyperglycemia and the efficacy of diabetes treatments.

      Before the isolation of insulin in the 1920s, most patients died within a short time after onset. Untreated diabetes leads to ketoacidosis, the accumulation of ketones (products of fat breakdown) and acid in the blood. Continued buildup of these products of disordered carbohydrate and fat metabolism result in nausea and vomiting, and eventually the patient goes into a diabetic coma.

      Treatment aimed at controlling diabetes can be highly successful with patient compliance. All patients are put on diets designed to help them reach and maintain normal body weight and to limit their intake of sugars and fats. Frequently they are encouraged to exercise regularly, which enhances the movement of glucose into muscle cells and blunts the rise in blood glucose that follows carbohydrate ingestion. Diabetics who are unable to produce insulin in their bodies receive regular injections of the hormone, often customized according to their individual and variable requirements. Beef or pork insulin, made from the pancreatic extracts of cattle or pigs, can be used to treat humans with diabetes. However, in the United States, beef and pork forms of insulin are no longer manufactured, having been discontinued in favour of human insulin production. Modern human insulin treatments are based on recombinant deoxyribonucleic acid (DNA) technology. Research into other areas of insulin therapy include pancreas transplantation, beta cell transplantation, implantable mechanical insulin infusion systems, and the generation of beta cells from existing exocrine cells in the pancreas.

      There are several classes of oral drugs used to control blood glucose levels, including sulfonylureas, biguanides, and thiazolidinediones. Sulfonylureas, such as glipizide and glimepiride, are considered hypoglycemic agents because they stimulate the release of insulin from beta cells in the pancreas, thus reducing blood glucose levels. The most common side effect associated with sulfonylureas is hypoglycemia (abnormally low blood glucose levels), which occurs most often in elderly patients who have impaired liver or kidney function. Biguanides, of which metformin is the primary member, are considered antihyperglycemic agents because they work by decreasing the production of glucose in the liver and by increasing the action of insulin on muscle and adipose tissues. A potentially fatal side effect of metformin is the accumulation of lactic acid in blood and tissues, often causing vague symptoms such as nausea and weakness. Thiazolidinediones, such as rosiglitazone and pioglitazone, act by reducing insulin resistance of muscle and adipose cells and by increasing glucose transport into these tissues. These agents can cause edema (fluid accumulation in tissues), liver toxicity, and adverse cardiovascular events in certain patients.

      There are several other agents that can be highly effective in the treatment of diabetes. Pramlintide is an injectable synthetic hormone (based on the human hormone amylin) that regulates blood glucose levels by slowing the absorption of food in the stomach and by inhibiting glucagon, which normally stimulates liver glucose production. Exenatide is an injectable antihyperglycemic drug that works similarly to incretins, gastrointestinal hormones, such as gastric inhibitory peptide, that stimulate insulin release from the pancreas. Exenatide has a longer duration of action than incretins produced by the body because it is less susceptible to degradation by an enzyme called dipeptidyl peptidase-4 (DPP-4). A drug called sitagliptin specifically inhibits DPP-4, thereby increasing levels of naturally produced incretins. Side effects associated with these drugs are often mild, although pramlintide can cause profound hypoglycemia in patients with type I diabetes.

      The objective of all forms of treatment of diabetes is to keep the level of blood sugar within normal limits and thus reduce the complications, primarily cardiovascular, that account for most diabetes-related deaths. Other serious complications include a condition known as diabetic retinopathy (retinal changes that can lead to blindness), kidney disease, and frequent infection.

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