Book Review

The Epidemiology of Eye Disease

N Engl J Med 1998; 339:1482-1483November 12, 1998

Article

The Epidemiology of Eye Disease
Edited by Gordon J. Johnson, Darwin C. Minassian, and Robert Weale. 436 pp. New York, Chapman & Hall, 1998. $99.95. ISBN: 0-412-64310-3

Approximately 40 million people in the world are blind and another 100 million have substantial visual impairment. The principal causes of blindness and visual disability are quite distinct in developed as compared with developing countries, and where they overlap in name (e.g., cataract and glaucoma), the prevailing epidemiology is very different. In underdeveloped settings, cataract, glaucoma, trachoma, onchocerciasis, vitamin A deficiency, and other related sources of childhood blindness predominate as the causes of blindness. Other important causes include leprosy, corneal scarring from infection, and trauma. In the most affected countries of Africa and Asia, the prevalence of blindness may be as high as 1.5 percent, with an additional 5 percent of the population visually disabled. In regions where trachoma and onchocerciasis are hyperendemic, the rates may be higher still.

Blindness in underdeveloped regions is in many ways a reflection of socioeconomic status, since all the chief causes are treatable or preventable. In virtually every survey of blindness performed in these regions, cataract is the principal cause of blindness. In fact, cataract, although highly treatable with surgery, accounts for almost half the world's cases of blindness. In developing countries, blinding cataract is not only very prevalent but is also more severe at an earlier age; unfortunately, the resources available for a curative operation are insufficient. In some regions of sub-Saharan Africa, there is only one surgically trained ophthalmologist for every 1 million people. In India, despite a reasonable supply of ophthalmologists, the number of people who become blind from cataract is increasing annually. Both the backlog and the new cases of blinding cataract far outstrip the capacities of the medical system. Inefficiency and conflicting incentives in India contribute to the performance of relatively few cataract surgeries per ophthalmologist per year, even though cataract may account for as many as 80 percent of cases of blindness. Fortunately, the tide is slowly turning with the emerging recognition that cataract surgery, and other approaches to the prevention of blindness, are not only compassionate and good medicine but also make terrific economic sense. Blindness has repeatedly been shown to be associated with loss of economic productivity, and cataract surgery has been directly linked to the restoration of economic output.

In the 19th century and earlier, trachoma was pandemic in Europe and the New World. It vanished from these regions with improved sanitation and industrialization, without the benefit of specific advances in therapy. Today, approximately 6 million people are blind from trachoma and many more are at risk from current infection. Although curative treatment exists, particularly with the recent availability of azithromycin, the underlying epidemiologic basis of blinding trachoma is unchanged — namely, the chronic reinfection in remote regions where the condition is endemic. Progress has been made over the past decade in controlling onchocerciasis, or river blindness, through a combination of vector control and mass treatment with ivermectin. Although it is still premature to anticipate global eradication, the disease has been effectively controlled with this two-pronged approach in large regions where it was previously endemic. Globally, glaucoma accounts for approximately 5 million cases of blindness. Open-angle glaucoma is the predominant form of the disease among populations of African and European descent, and angle-closure glaucoma predominates among those of Asian origin. The prevention of blindness due to glaucoma in developing regions appears today to be an almost insurmountable task, because the disease is silent until quite advanced, medical treatments are unavailable or too expensive, and access to effective surgery is almost nonexistent in the areas most affected.

In developed countries, particularly Europe and the United States, age-related macular degeneration and diabetic retinopathy predominate as causes of blindness. The prominence of both conditions reflects, in a sense, the flip side of modernization. The prevalence of blindness due to macular degeneration is directly related to the age distribution of the population, and as the age distribution of our population becomes more top-heavy, the absolute rates of blindness from macular degeneration will increase. Similarly, the strongest predictor of diabetic retinopathy is the duration of diabetes, and as longevity increases, so will the number at risk for blindness from diabetic retinopathy, unless more effective prevention or treatment becomes available. In developed countries, cataract accounts for only 2 to 5 percent of blindness, despite the high prevalence of cataract related to older age. In these settings, health services research related to cataract surgery has taken on increasing importance because of the increasing rates of surgery and the associated costs. In the United States, the age-adjusted rate of cataract surgery has quadrupled over the past 15 years. In 1995, cataract surgery, the most common surgical procedure in elderly persons, was performed approximately 1.3 million times in persons receiving Medicare benefits. The technological advances that have occurred in cataract surgery and intraocular-lens design over this period have predictably lowered both patients' and surgeons' thresholds for performing this surgery.

Even with so much eye disease, associated blindness, and economic and human costs, there has until now been no easily accessible source of information on ophthalmic epidemiology. The Epidemiology of Eye Disease fills this void. This book synthesizes the findings of several decades of epidemiologic research into a single, well-organized reference source. The first section of the book addresses principles of epidemiologic methods as applied to ocular disease. As intended by the authors, the content is pitched toward ophthalmologists and other health professionals with an interest but no significant background in epidemiologic methods. The second and chief section of the book contains chapters on the epidemiology of important ocular conditions in both developed and developing countries. This section will be an excellent resource for the entire community of researchers and practitioners interested in the prevention of blindness. Perhaps the only notable omission in this section is a chapter on refractive error, particularly myopia. Uncorrected refractive error is a leading cause of visual disability worldwide. The epidemiology of myopia is particularly fascinating and mysterious in the parts of Asia that have undergone significant modernization over the past one or two generations. For example, the prevalence of myopia among adolescents in Singapore two generations ago was approximately 25 percent, similar to that in the United States. Currently, however, the prevalence of myopia among high school graduates in Singapore is over 90 percent. One can only speculate on the forces underlying this remarkable generational phenomenon. There is hope that epidemiologic methods will provide some answers in the coming years. The third and final section of the book addresses issues relevant to the programmatic prevention of blindness. This section will be especially valuable to those interested in population approaches to the control and prevention of eye disease.

As the population ages in both developed and developing regions of the world, the burden of eye disease will increase. An epidemiologic, population-based approach to meeting the certain challenge of this burden will be critical.

Oliver D. Schein, M.D., M.P.H.
Johns Hopkins University, Baltimore, MD 21287-9019