1.1Introduction

Diabetic retinopathy (DR) affects 4.2 million Americans over the age of 40 years, 655,000 of whom have sight-threatening retinopathy [1, 2]. Diabetic retinopathy is a microvascular complication of diabetes and is the leading cause of new cases of legal blindness in the United States [2]. Worldwide, in 2010 it was estimated that DR affected 93 million persons, and 28 million were affected by vision-threatening diabetic retinopathy (VTDR). Diabetic retinopathy may become the leading cause of visual impairment globally [3]. It is a progressive disease associated with a decline in best-corrected visual acuity. The Wisconsin Epidemiologic Study of Diabetic Retinopathy (WESDR) noted that 3.6 % of type 1 diabetes mellitus (DM) and 1.6 % of type 2 DM patients were legally blind [4].

1.2Epidemiology

Diabetes is becoming an increasingly important public health problem. In 2010, approximately 285 million persons, 6.4 % of world population, had DM [5]. The International Diabetes Federation has predicted an increase in number of individuals with diabetes to 552 million in 2030, a prevalence of 7.7 % [6]. Previously, DM was considered a problem of affluent nations, but with an increase in urbanization, sedentary lifestyles, population aging, and obesity throughout the world, DM is becoming an important public health issue in developing countries [5, 6]. In the near future, 80 % of the world’s diabetic population will be from low-income and middleincome countries with 60 % from Asia [7]. Most of the diabetic patients in the west are elderly, but diabetic patients in Asia are usually young to middle-aged adults. With the increasing trend of childhood obesity, many young individuals are at a risk of DR.

Meta-analysis for Eye Disease (META-EYE) study [3] collated data for 22,986 subjects from 35 population-based studies from the United States, Australia, Europe, and Asia. The mean age was 58.1 years with median diabetes duration of 7.9 years and median HbA1c 8.0 % (range 6.7–9.9 %). Fifty-two percent were female. The ethnicity was as follows: 44.4 % Caucasian, 30.9 % Asian, 13.9 % Hispanic, and 8.9 % African American. The prevalence of any DR and VTDR in this meta-analysis study was 34.6 and 10.2 %, respectively [3]. Vision-threatening diabetic retinopathy was defined as the presence of proliferative DR (PDR) or diabetic macular edema (DME). There was no difference in the prevalence of DR with respect to gender. The American National Health and Nutrition Examination Survey (NHNES, 2005–2008) reported DR in 28.5 % of diabetic patients and VTDR in 4.4 % [1]. The prevalence of DR is reported to be much higher in developing countries. In rural China, for example, 43 % of diabetic patients have DR, and 6.3 % have VTDR [8].

The prevalence of DR is higher in type I diabetes (a form of DM that results from autoimmune destruction of the insulin-producing beta cells of the pancreas) compared to type II (a metabolic disorder characterized by high blood glucose, insulin