Diabetes Mellitus: Review of the Common, Rare, and Uncommon (Conceptual and Metaphorical) Types

Diabetes mellitus (DM) comprises a complex heterogeneous group of chronic metabolic disorders that affect various organ systems, resulting in a number of complications, including kidney disease, cardiovascular disease, immune dysfunction, retinopathy, neuropathy, and cancer. Uncontrolled DM may also lead to a coma and confusion, and if untreated, death due to ketoacidosis or nonketotic hyperosmolar syndrome. In 2019, DM was directly responsible for 1.5 million deaths, with an additional 460,000 due to kidney disease, as well as about 20% of cardiovascular-associated deaths. In addition to the well-known types—viz. type 1 diabetes mellitus (T1DM), type 2 diabetes mellitus (T2DM), and gestational diabetes mellitus (GDM)—several other rare conceptual/metaphoric forms have been described, although they are not regarded as formal diagnoses. The aim of this review is to present the various types of DM and summarize their characteristics, clinical features, diagnosis, pathophysiology, risk factors, management, and therapy.