Diabetes mellitus is a metabolic disorder caused by defects in insulin secretion (type 1), insulin action (type 2), or both. It is characterized by chronic hyperglycemia which ultimately may result in dysfunction and damage to various organ systems, including the brain, kidneys, eyes, and peripheral nerves. Diabetic retinopathy may be broadly classified in terms of the presence or absence of retinal neovascularization. The term nonproliferative diabetic retinopathy (NPDR) is used to describe intraretinal microvascular changes that occur in the early stages of diabetic retinopathy. The etiology and pathogenesis of NPDR are discussed in  Chapters 65 and  67. Proliferative diabetic retinopathy (PDR) is used to indicate the presence of newly formed vessels, fibrosis, or both, arising from the retina or optic disc and extending along the inner retinal surface and/or into the vitreous …