The global diabetes epidemic 2: eye disease
In the second part of our series on the global burden of diabetes, Joshua Schulman-Marcus discusses diabetic eye disease
Over the next two decades, the number of adults living with Type 2 diabetes in developing countries is projected to increase by 31%, with the largest increase occurring in working aged adults (1). Many of these people with newly diagnosed diabetes will accordingly have complications and ensuing disability, which has already been anecdotally illustrated in the World Health Organization’s report on chronic diseases in developing countries (2).
One of the most clinically important complications of diabetes is eye disease, is that people with diabetes are twenty-five times more likely to become blind than people who do not have diabetes. (3) The most common form of diabetic eye disease is diabetic retinopathy (DR), which is divided into three categories, all of which may coexist: nonproliferative disease, proliferative disease, and macular edema. While all three manifestations can imperil sight, proliferative disease and macular edema are the most threatening (3). Additionally, people with diabetes are at a greatly increased risk of cataracts (4).
The prevalence and severity of diabetic retinopathy are strongly correlated with duration of diabetes (3). Other major risk factors are poor glycaemic control and hypertension. The UK Prospective Diabetes Study demonstrated a 25% reduction in DR for every 10% reduction in glycosylated hemoglobin A1c (5) and an additional 13% reduction for a 10 mm reduction in systolic blood pressure (6). Therefore, with adequate medical management, DR should be a preventable cause of visual impairment (7).
There have been few comprehensive or longitudinal studies measuring the prevalence of diabetic retinopathy in developing countries. Most available data are derived from single-center or cross-sectional studies, and thus demonstrate considerable variability. In sub-Saharan Africa overall, 21-25% of people with Type 2 diabetes have DR at diagnosis (8), while one study in West Africa found DR in 18% of people with both types of diabetes (9). Overall prevalence in a rural Indian screening camp was 17% (10), in Thai hospitals 31% (11), and at a Beijing referral clinic 27% (12). A substantial proportion of people with diabetes in all studies had moderate or severe DR with visual loss, which was tightly correlated with duration of diabetes and poor glycaemic control. The correlation between income and DR prevalence has been less well-studied in these countries. One urban Indian study found l people with diabetes on low-income were less likely to have DR (in contrast to all other diabetic complications) (13), while Mexicans with DR progress at twice the rate of wealthier Mexican Americans in the United States (14).
Current guidelines emphasise screening and risk factor reduction. The International Diabetes Federation guidelines state that at a minimum all people with diabetes should have annual direct fundoscopy and an examination of visual acuity by a trained provider, with more frequent screening for those with existing visual disease or pregnancy (15). Specialist and general training is important, as the sensitivity of a primary provider with a standard ophthalmoscope is only 50% (16). If DR progresses despite risk factor reduction or becomes significantly impairing, the proven therapy of choice is laser photocoagulation (17). The Disease Control Priorities Project has found annual screening and indicated laser treatment to be relatively cost-effective in all developing countries, costing at most US$700 per quality-adjusted life year (1).
Shortcomings and Implications
Despite the cost-effectiveness and clinical importance of recognising and treating DR, all studies from developing countries report significant under-diagnosis and barriers to care. For example, at a Tanzanian clinic only 29% of people referred with diabetes had a visual inspection in the previous year, and 40% had never had their eyes inspected (18). At an Indian screening camp, half of those with referable DR had never had a fundal examination (10). The major barriers include limited awareness of the problem among people with diabetes and a lack of eye specialists, diagnostic technology, laser treatments, and resources for self-monitoring (9,10,18,19). Rural areas are more likely to be resource-deprived (10,18,19) and have higher rates of severe DR than urban areas (10,19). High cost may also dissuade poor people with diabetes from receiving treatment or attending referral appointments (18,19); a strategy of providing screening camp attendees with free food and transport was shown to be successful in increasing referral (10).
The economic implications and cultural attitudes surrounding visual impairment are substantial in developing countries, and they may influence one’s willingness to seek care when the disease is mild. As DR has not been specifically studied in this regard, assumptions must be drawn based on studies about vision loss in general. The disability of visual impairment frequently results in decreased work opportunities and lost earning potential, which is especially devastating to poor households (1,3,7). Culturally, blindness is one of the most feared disabilities. A major survey in India among sighted people found most respondents feared blindness more than loss of a limb or cancer, and large majorities believed that losing one’s sight is “losing one’s sense of self” (20). The authors note that many beggars are blind or have disfigured eyes, and thus blindness is associated with poverty and stigma. Consequently, fears of the socio-economic implications of being diagnosed as visually impaired or blind may dissuade people with diabetes from early follow-up.
In 2002, diabetic retinopathy was responsible for 4.8% of visual impairment worldwide (21). In most developing countries, it pales in comparison to more common diseases such as cataract, glaucoma, trachoma, and onchocerciasis. However, its prevalence is quickly increasing, having ascended from the 17th leading cause of blindness to the sixth in India over the past twenty years (19). This trend is likely to occur in other developing countries owing to increased prevalence of diabetes and longer life expectancy after diagnosis. Therefore, even though currently many people with diabetes in these countries die too prematurely (mostly from heart disease) to suffer severe sight loss, this is a diabetic complication worthy of increased attention.
Diabetic eye disease is traditionally associated with wealthier countries, and thus has largely been overlooked in developing countries. Yet evidence indicates that the latter will increasingly suffer from this disease over the next two decades. Thankfully, it is a largely preventable cause of vision loss that can be controlled through cost-effective interventions. Success in reducing this growing threat to sight will be contingent on better research, attention to cultural implications, increased access to health resources, and improved health systems.
Joshua Schulman-Marcus
4th year medical student
Mount Sinai School of Medicine
New York
USA
and
2007-08 Sarnoff Cardiovascular Foundation Fellow
Brigham and Women’s Hospital/Harvard School of Public Health
Boston
USA
jschumar@gmail.com
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