Diabetes mellitus (DM) is a group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both. In 2012, DM was present in 12.3% of persons over the age of 20 years in the United States and in 49% of those over the age of 65 years. A substantial percentage of affected individuals have not been diagnosed. Type 2 DM represents 90%–95% of all cases of DM, with type 1 DM and other causes representing the remaining 10%.
Persons with DM are at risk for microvascular complications, including retinopathy, nephropathy, and neuropathy, and are at increased risk for macrovascular disease. Among adult patients, type 2 DM is accompanied by hypertension (in approximately 75%) and hyperlipidemia (in more than 50%). It is considered a cardiac risk equivalent because of the high excess risk it poses for macrovascular disease, cardiovascular disease events, and mortality.
Classification of Diabetes Mellitus
DM is classified into 4 clinical types.
Type 1 DM (<10% of all cases) results from a cell-mediated autoimmune destruction of β cells in the pancreas. It can present at any age; and, because of its variable clinical phenotypes, the diagnosis can be challenging in adults. The rate of destruction of β cells is rapid in infants and children and slower in adults. Therefore, ketoacidosis as an initial presentation is more common in young patients.
Latent autoimmune diabetes in adults (LADA), a subtype of type 1 DM, is characterized by mild to moderate hyperglycemia and, initially, often does not require insulin therapy. Adults with LADA have one or more β-cell–specific autoantibodies and will tend to require insulin therapy sooner than patients with classic type 2 DM. Type 1 DM should be suspected when there is a positive family history, thyroid disease, or other autoimmune disease.
Type 2 DM (>90% of cases) is characterized by insulin resistance followed by defective insulin secretion and loss of β-cell mass. The reason for this β-cell loss is unknown, but programmed cell death in response to genetic and environmental factors has been demonstrated in animal models. Type 2 disease is usually diagnosed in adults, with both incidence and prevalence increasing with age. However, it is becoming more common in children and now accounts for up to one-third of new cases of diabetes mellitus diagnosed in patients between the ages of 5 and 15 years.
This type of DM is associated with obesity, a positive family history, history of gestational diabetes or prediabetes, physical inactivity, and race/ethnicity. African American, Hispanic, and American Indian individuals have a greater risk of developing type 2 DM than white individuals. Type 2 DM may be asymptomatic and remain undiagnosed for months to years.
Gestational DM is glucose intolerance that has its onset or diagnosis during pregnancy (occurs in 5%–20% of pregnancies).
Other types of DM include those caused by genetic defects in insulin secretion or action, pancreatic surgery, disease of the exocrine pancreas (eg, cystic fibrosis), endocrinopathies (eg, Cushing syndrome), or drugs (eg, glucocorticoids, thiazide-type diuretics, and atypical antipsychotic medications).
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.