Important Causes of Visual Impairment in the World Today

Visual impairment, which may be defined as blindness (best vision of ≤20/200 in the better eye in the United States and <20/400 by the World Health Organization [WHO] definition) or low vision (<20/40 in the United States and <20/60 according to WHO), is one of the most common disabilities: an estimated 40 million people worldwide were blind nearly a decade ago, the time of the last accurate assessment, and 110 million people had low vision.¹ Among persons older than 40 years in the United States, 937 000 people were blind and 2.4 million people had low vision in 2002. Figures for the developing world, where approximately 90% of world blindness exists, and the developed world are expected to increase significantly during the next decades as the world's population ages.²

Refractive error has been identified in a number of population-based studies as the leading cause of visual impairment in the developed world³ and a leading cause of blindness in the developing world as well.⁴ Although the visual impact of myopia (near-sightedness) and hyperopia (far-sightedness) can generally be neutralized by the use of spectacles, contact lenses, or refractive surgery, refractive error is frequently not addressed in the population at large. An estimated 11.8 million Americans 40 years and older have hyperopia (> + 3 diopters [D]), whereas 30.4 million are myopic (< − 1 D) and 5.3 million are severely myopic (< − 5 D).³ Vision may be lost when refractive error is corrected due to bacterial keratitis associated with contact lens wear⁵ or complications from refractive surgery.⁶ Furthermore, severe myopia may be associated with increased risk for a number of vision-threatening disorders, including retinal detachment,⁷ glaucoma,⁸ and cataract.⁹ An estimated $12.8 billion was spent to correct refractive error in the United States in 1990.¹⁰

Cataract is the leading cause of blindness in the world,¹ with an estimated 17 million persons bilaterally blind due to cataracts (Figure 1). Unlike in the developing world, cataract blindness is usually prevented by surgery in the United States, but cataract remains the leading cause of low vision, affecting approximately 20.5 million Americans older than 40 years.¹¹ One in 20 Americans older than 40 years has undergone cataract surgery,¹¹ and cataract care consumes approximately 60% of the Medicare budget for vision.¹² Factors thought to be associated with increased risk of cataract include cigarette smoking, exposure to UV-B light, high alcohol intake, diabetes, medications (including steroids), ocular surgery, and trauma.¹³ Recent studies¹⁴ have cast doubt on the hypothesis that exogenous antioxidants, such as vitamins A, C, and E, may protect against cataract, at least in well-nourished populations during modest periods (7 years). Practical considerations and attributable risk suggest that smoking cessation is the main viable strategy at present for cataract prevention.¹⁵ No medications or other nonsurgical therapies exist at present to prevent cataract, although it has been suggested that delaying the onset of cataract by 10 years could lead to a 50% reduction in surgery.¹⁶ The number of cataract surgical procedures required worldwide is predicted to increase more than 4-fold from 8 million annually to 35 million,¹⁷ as the population ages and the demand for early surgery increases.

Glaucoma is an irreversible optic neuropathy that causes loss of peripheral and ultimately central vision. An estimated 6.7 million people are blind from glaucoma¹⁸ worldwide, with almost 70 million affected by the disease. An estimated 2.2 million Americans currently have glaucoma,¹⁹ with significant increases expected in the coming decades owing to the aging of the US population. African Americans are 3 to 4 times as likely to have open-angle glaucoma as whites, and even greater racial disparities may exist with regard to blindness from this disease.²⁰ Recent clinical trials indicate that lowering the intraocular pressure can prevent the development of glaucoma in individuals with elevated intraocular pressure²¹ and can decrease the likelihood of progression in those with early disease.²² Although angle-closure glaucoma is rare in the United States, this disease may be as important as open-angle glaucoma as a cause of blindness worldwide because of the high prevalence in the large populations of South and East Asia and the greater risk of vision loss compared with open-angle glaucoma.²³

Age-related macular degeneration (AMD) causes irreversible loss of central vision and is the leading cause of blindness among European-derived individuals older than 65 years.² Age-related macular degeneration is strongly associated with aging, increasing in prevalence from less than 1% for whites in their sixth decade of life to more than 15% in the ninth decade of life.²⁴ Current laser treatments, although effective in slowing progression of the condition, frequently sacrifice central vision²⁵ or often must be performed multiple times. The most important treatable risk factor for AMD is smoking, which leads to a 3-fold increased risk of disease²⁶ and may cause as much as 15% of AMD. Age-related macular degeneration is estimated to affect 1.8 million individuals in the United States, which will increase to nearly 3 million by 2020.²⁴ A recently completed clinical trial demonstrated a 30% reduction in the incidence of AMD among individuals at risk taking high-dose antioxidant vitamins and zinc supplementation.²⁷

Trachoma is endemic in impoverished, dry areas of Africa, the Middle East, Australia, and Southeast Asia. It is by far the leading cause of infectious visual loss, accounting for approximately 5 million cases of bilateral blindness worldwide.¹ Recurrent episodes of chlamydial conjunctivitis lead to a cascade of conjunctival scarring, trichiasis (inturned eyelashes), infectious corneal ulcers, and corneal scarring (Figure 1). Surgery for trichiasis can be successful but may fail over time, and subsequent surgical procedures are often more difficult.²⁸ Vaccines have not been shown effective, and in fact there is some suggestion that scarring proceeds more rapidly in a vaccinated cohort.²⁹ Circumstantial evidence links disease specifically to poor hygiene and inadequate access to clean water. However, specific hygiene or water-supply programs have shown only a modest effect, at least in the short term.³⁰ Mass antibiotic administration, most recently with azithromycin, has proven effective in reducing community infection.³¹ Studies are being conducted in Ethiopia and Tanzania to determine if periodic mass distributions of azithromycin can eliminate infection from communities without causing significant resistance and adverse effects.

River blindness, caused by the filarial nematode Onchocerca volvulus, is found principally in sub-Saharan Africa, with small foci in Central America, South America, and the Middle East. In hyperendemic areas, 50% of adults may be blind from the disease. A 2-pronged approach to elimination, directed at both the black fly vector and the infection itself, has proved to be remarkably successful.³² The mass periodic distribution of the microfilaricidal ivermectin has served as a model for treatment of other infectious diseases. However, the long life cycle of O volvulus and the lack of an effective macrofilaricide may delay complete elimination for years. Interestingly, recent reports³³ suggest that the inflammatory reaction that leads to corneal scarring may not be a response to the Onchocerca itself but to Wolbachia species, symbiotic bacteria that are necessary for Onchocerca to reproduce.

In people with human immunodeficiency virus (HIV), usually it is not the primary infection itself that causes blindness but a secondary infection, most commonly cytomegalovirus (CMV) retinitis.³⁴ Cytomegalovirus was an important cause of blindness in people with HIV in the 1980s in the developed world, but the incidence recently has decreased remarkably with the widespread use of highly active antiretroviral therapy.³⁵ Even with the huge HIV burden in sub-Saharan Africa and Southeast Asia, CMV has not yet been recognized as an important cause of blindness. Mortality from infections such as pneumococcal pneumonia and tuberculosis occurs before individuals are at risk for CMV retinitis (which typically occurs with CD4 counts of <50/µL). As treatment improves for other AIDS-related diseases and life spans are extended, it will be important to monitor the prevalence of CMV cases in the developing world.

The earliest sign of vitamin A deficiency, night blindness, can be followed by conjunctival xerosis, Bitot spots, and eventually irreversible melting of the cornea.³⁶ All of these changes are due to the dependence of rhodopsin and normal ocular epithelial development on vitamin A and its related compounds (Figure 1).³⁷ Unless corneal scarring occurs, xerophthalmia is rapidly reversible with timely beta carotene treatment. Sommer and colleagues³⁸ first reported a significant reduction in childhood mortality with single-dose beta carotene in a randomized trial in Indonesia. These results have since been confirmed in a series of 8 trials showing mortality reductions of 30% to 50%, largely due to decreased mortality from measles and diarrhea.³⁹ More recent trials have demonstrated a 50% reduction in maternal mortality with beta carotene dosing during pregnancy.⁴⁰ The attributable mortality from vitamin A deficiency worldwide has been estimated at 1.0 million to 2.5 million children annually,⁴¹ a figure that is likely to decrease as dosing and supplementation programs are implemented in dozens of countries throughout the world.

Diabetic retinopathy is a leading cause of blindness and visual impairment among adults younger than 40 years in the developed world, and it affects older individuals as well (Figure 1). More than 70% of individuals with type 1 diabetes mellitus (DM) have diabetic retinopathy, and nearly 25% have proliferative disease. Rates are approximately half as high for persons with type 2 DM.⁴² Blindness occurs in approximately 4% of individuals with type 1 DM and 1.6% of those with type 2 DM,⁴² although type 2 DM is a significant cause of blindness because of its greater prevalence. An estimated 4.1 million Americans have diabetic retinopathy. The most important risk factors for developing retinopathy are the duration of diabetes⁴³ and the level of glycemic control.⁴⁴ Clinical trials have documented the benefit of early laser treatment in preventing blindness among individuals with diabetic retinopathy^{45 - 46} and have led to a recommendation for annual screening. Screening for and treating diabetic retinopathy can reduce the likelihood of developing bilateral blindness from proliferative retinopathy to 1% at 5 years and have been documented to be highly cost-effective.⁴⁷ Prevalence of diabetes and associated retinopathy⁴⁸ is growing rapidly in urban Asia, making this a global vision problem.

The annual cost of ocular trauma in the United States is estimated at between $175 million and $200 million for hospital care alone.⁴⁹ Statistics from the National Institute for Occupational Safety on work-related diseases and injuries indicate that the 900 000 work-related ocular injuries reported annually in the United States are second only to dermatologic complaints.⁵⁰ It is estimated that 500 000 blinding ocular injuries occur worldwide each year and that ocular trauma is a leading cause of monocular blindness.⁵¹ The age distribution of ocular trauma is bimodal, with the greatest risk occurring among the young and persons older than 70 years.⁵² Men incur approximately 70% to 85% of ocular trauma.^{53 - 54} Occupational injuries accounted for two thirds of ocular trauma in the United States before modern workplace regulations⁵⁵ and still play a leading role in the developing world,^{56 - 57} although the proportion has been reduced to 15% to 40% as reported in recent US studies.⁵⁸ Sports are a common cause of severe ocular injuries in the developed world⁵⁹ and one uniquely prone to preventive strategies,⁶⁰ particularly when nonpowder firearms such as BB guns and airguns are included.⁶¹ Assault accounts for a steadily growing proportion of ocular trauma, particularly in American urban centers, and often with a particularly poor outcome.⁶²

In the last 2 decades, highly effective treatment or prevention strategies have become available for all of the important causes of visual disability except macular degeneration and trauma. Lack of access to eye care and incomplete knowledge among both primary care physicians and at-risk populations have prevented these strategies from achieving their full potential. With the growing threat from diseases of aging, such as cataract, glaucoma, and macular degeneration, to a rapidly aging world population, it will be necessary to make better use of existing knowledge, as well as find new strategies, to prevent a large increase in the number of blind persons in the world. Much hope for reducing the burden of blindness resides in the WHO-sponsored Global Vision 2020 program, which aims to eliminate two thirds of the world's preventable blindness by the year 2020.⁶³