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Timely diagnosis, intensive diabetes treatment, and consistent, long-term follow-up evaluations for persons with diabetes are essential for effective care, which can preserve vision and substantially lower the risk of vision loss. B. CLASSIFICATION OF DIABETES MELLITUS The following definitions and categories of diabetes are based on the classifications reported by the American Diabetes Association (ADA).84 1. Type 1 Diabetes Mellitus Type 1 diabetes (formerly called insulin-dependent or juvenile diabetes) occurs when the body’s immune system attacks and destroys insulin-producing beta-cells in the pancreas. It accounts for approximately 6 percent of individuals with diabetes in the United States.85 The primary characteristic of type 1 diabetes is absolute dependence on exogenous insulin to prevent profound hyperglycemia and ketoacidosis. Although more frequently diagnosed in children and young adults, type 1 diabetes can occur at any age. It may be caused by genetic, environmental, or other factors, and currently there is no known way to prevent it. Persons with type 1 diabetes may develop other autoimmune disorders such as Hashimoto’s disease, Graves’ disease, Addison’s disease, myasthenia gravis, and pernicious anemia. 17 aoa.org There are two forms of type 1 diabetes, both of which are characterized by destruction and/or loss of secretory function by insulin producing pancreatic beta-cells. One form is an immune-mediated disease with autoimmune markers such as islet cell antibodies (ICAs), insulin autoantibodies (IAAs), autoantibodies to glutamic acid decarboxylase (GAD65), and autoantibodies to the tyrosine phosphatases IA-2 and IA-2β, and ZnT8. About 85 to 90 percent of individuals with fasting hyperglycemia are positive for one or more of these markers. Strong human leukocyte antigen (HLA) associations also exist. The other form of type 1 diabetes, called idiopathic diabetes, has no known causes. Idiopathic diabetes is strongly inherited, but it lacks autoimmune markers and has no HLA association. An absolute dependence on insulin replacement in affected persons may come and go. Only a minority of persons with diabetes falls into this group, and they are predominantly of African or Asian descent. 2. Type 2 Diabetes Mellitus Type 2 diabetes (formerly termed non-insulin dependent or adult-onset diabetes) occurs when the body does not produce enough insulin (relative insulin deficiency) or cannot use the insulin it makes effectively (insulin resistance). Defects in insulin secretion are primarily related to inflammation, metabolic stress, and genetic factors. In contrast to type 1 diabetes, with this form of the condition, autoimmune destruction of beta-cells does not occur. Type 2 diabetes is the most common form of diabetes worldwide and its prevalence is increasing. The risk of developing type 2 diabetes increases with age, obesity, and lack of physical exercise. It currently accounts for about 91 percent of diabetes cases in the United States.85 This form of diabetes develops more frequently in adults than in children; however, the prevalence of type 2 diabetes in children is increasing, especially in high-risk ethnic groups, such as American Indians, Hispanic Americans, African Americans, Alaska Natives, Asian Americans, Native Hawaiians and other Pacific Islanders. Most of these children are between 10 and 19 years old, have infrequent or mild diabetic ketoacidosis, are obese, and have a strong family history of diabetes.86 Children with metabolic risk factors, such as high body mass index (BMI) and impaired glucose tolerance (IGT), are at an increased risk of developing type 2 diabetes.87 3. Gestational Diabetes Mellitus Gestational diabetes mellitus (GDM) refers to any degree of glucose intolerance with onset or first diagnosis during the second or third trimester of pregnancy that is not clearly overt diabetes prior to gestation. It is now recommended that high-risk women found to have diabetes during their initial prenatal visit in the first trimester receive a diagnosis of overt, not gestational, diabetes.84 GDM is caused by the hormones secreted during pregnancy or by a shortage of insulin.
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