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JAMA
15 October 2003
CLINICIAN'S
CORNER
Important
Causes of Visual Impairment in the World Today
Nathan
G. Congdon, MD, MPH; David S. Friedman, MD, MPH; Thomas
Lietman, MD
JAMA. 2003;290:2057-2060.
INTRODUCTION
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
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
Age-Related
Causes of Blindness
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.
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| Figure.
Important Causes of Visual Impairment and Ocular
Structures They Affect
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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
Infectious
Causes of Blindness
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/�L). 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.
Nutritional
and Metabolic Causes of Blindness
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 retinopathy45-46
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.
Trauma
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
Global
Vision 2020 and the Challenge of World Blindness
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
AUTHOR
INFORMATION
Corresponding Author and Reprints: Nathan G. Congdon, MD, MPH, Wilmer
120, 600 N Wolfe St, Baltimore, MD 21287 (e-mail: [email protected]).
Disclosure: The authors state that they have no financial interest
in any of the treatments discussed in this article.
Author Affiliations: Dana Center for Preventive Ophthalmology, Johns
Hopkins University Schools of Medicine and Public Health, Baltimore, Md (Drs
Congdon and Friedman); and Proctor Foundation, University of California San
Francisco School of Medicine, San Francisco (Dr Lietman).
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