Diabetes Mellitus Health Article

DIABETES MELLITUS

The term "diabetes mellitus" represents a group of conditions characterized by abnormally high blood glucose levels (hyperglycemia). In 1997, nearly 16 million people in the United States had diabetes; approximately 10.3 million were diagnosed with the conditions, while an estimated 5.4 million were undiagnosed. Diabetes may be complicated by uncontrolled hyperglycemia, and treated diabetes may be complicated by abnormally low blood glucose levels (hypoglycemia). Maternal diabetes is associated with an increased incidence of major birth defects. Over time, diabetes may cause complications involving the eyes (retinopathy), kidneys (nephropathy), and nerves (neuropathy). Diabetes is also associated with an increased incidence of cardiovascular disease, including stroke, heart attack, and peripheral vascular disease. In the United States today, diabetes is a leading cause of birth defects, blindness, kidney failure, and nontraumatic leg amputations. It is also a major contributor to cardiovascular disease. Diabetes is the seventh leading cause of death in the United States, and medical care for people with diabetes is estimated to cost over $100 billion per year.

When diabetes is associated with marked hyperglycemia, it produces characteristic symptoms and signs; particularly increased thirst (polydipsia), increased urination (polyuria), and unexplained weight loss. At other times, hyperglycemia sufficient to cause changes in the eyes, kidneys, and nerves, and to increase the risk of cardiovascular disease, may be present without clinical symptoms. During this asymptomatic period, an abnormality in glucose metabolism may be demonstrated by measuring fasting venous glucose or venous glucose after an oral glucose challenge.

DIAGNOSIS

When a patient is symptomatic and the plasma glucose is unequivocally elevated, a diagnosis of diabetes presents no difficulty. When a patient is without clinical symptoms, a diagnosis of diabetes is more difficult. According to a 1997 American Diabetes Association (ADA) report, there are three ways to diagnose diabetes (see Table 1). All require measurement of venous plasma glucose, and each must be confirmed on a subsequent day by any one of the three methods. In general, the oral glucose tolerance test is not recommended for routine clinical use and is performed only in patients with elevated but nondiagnostic fasting plasma-glucose levels with a high index of suspicion for diabetes.

CLASSIFICATION

Once a diagnosis of diabetes mellitus is established, it is necessary to differentiate the various forms of the syndrome. Prior to 1979, diabetes was

Table 1

classified on the basis of age at diagnosis as either juvenile-onset diabetes mellitus (JODM) or adult-onset diabetes mellitus (AODM). In the late 1970s and early 1980s, a new classification system recognized two major forms of diabetes: insulin-dependent diabetes mellitus (IDDM or type I diabetes) and non-insulin-dependent diabetes mellitus (NIDDM or type II diabetes). In 1997, the American Diabetes Association recommended modifications to this classification system that eliminated the terms "insulin-dependent diabetes mellitus" and "non-insulin-dependent diabetes mellitus" and their acronyms. The terms "type 1" and "type 2" were retained, with Arabic numerals replacing the Roman numerals. Other specific types of diabetes were also recognized.

Type 1 diabetes is caused by pancreatic beta cell (B-cell) destruction. Immune-mediated type 1 diabetes results from cell-mediated autoimmune destruction of the B-cells of the pancreatic islets. This type of diabetes also has strong genetic or human leukocyte antigen (HLA) associations that can be either predisposing or protective. Another form of type 1 diabetes, termed "idiopathic" type 1 diabetes, is strongly inherited but lacks immunologic evidence for B-cell autoimmunity and is not HLA-associated. Most patients with

Table 2

idiopathic type 1 diabetes are of African or Asian descent.

Type 1 diabetes accounts for approximately 5 percent of diagnosed diabetes in the United States—approximately 500,000 Americans have type 1 diabetes. Type 1 diabetes commonly occurs in childhood and adolescence, but it can occur at any age. Patients with type 1 diabetes are prone to ketoacidosis (decompensated diabetes with hyperglycemia and presence of abnormal acids [ketones] in the blood). Many affected patients have no family history of diabetes. Although most patients with type 1 diabetes are lean when they are diagnosed, the presence of obesity is not incompatible with the diagnosis.

Type 2 diabetes is characterized by both impairment of insulin secretion and defects in insulin action. It is often unclear which abnormality is the primary cause of hyperglycemia. Although patients with this type of diabetes may have insulin levels that appear normal or elevated, insulin levels are always low relative to the elevated plasma glucose levels. Thus, insulin secretion is defective in these patients and insufficient to compensate for the degree of insulin resistance. Although the specific origin of type 2 diabetes is not known, autoimmune destruction of B-cells does not occur. Although type 2 diabetes is associated with a strong genetic predisposition, the genetics of this form of diabetes are complex and not clearly defined.

Type 2 diabetes accounts for approximately 95 percent of diagnosed diabetes in the United States (9.8 million cases), and for the vast majority of the cases of undiagnosed diabetes. The risk of type 2 diabetes increases with age, obesity, and physical inactivity. As such, it is often regarded as a disease associated with a modern Western lifestyle. Type 2 diabetes occurs more frequently in women with prior gestational diabetes and in individuals with hypertension and dyslipidemia. Affected patients often have a family history of diabetes. Type 2 diabetes is more common in African Americans, Hispanic Americans, and Native Americans than in non-Hispanic white Americans. Ketoacidosis seldom occurs spontaneously in type 2 diabetes, but it may arise in association with the stress of another illness. Approximately 70 percent of patients with type 2 diabetes are obese.