The World Health Organization (WHO) estimated that in 1990, 38 million people were blind (less than 10/200 in better eye after correction) and a further 110 million people had low vision (less than 20/60).¹ Sixty percent of these people were in India (8.9 million), China (6.7 million), and Africa (7.1 million), with a further 30% being in other developing countries. These figures would be expected to double by the year 2020.

The commonest cause of blindness, according to WHO in 1995,² was unoperated cataract (Fig. 1), somewhere between 40% and 80% in most studies. The global estimates for 1998, taking into account population increases and aging and 1995 service levels, are in excess of 22 million people who are bilaterally blind from cataracts.¹ WHO’s definition of blindness (less than 10/200) is based on a social definition of blindness such that an individual cannot move around independently. Other organizations use less than 20/200 as a definition for blindness, a term also known as economic blindness. If the latter definition is used, then estimates of cataract blindness would more than double.

An example of the magnitude of the problem was presented by the World Bank in 1993. Thirteen million people in India were bilaterally blind from cataract, and 10 million were unilaterally blind.² Forty-five percent of cataract patients were younger than 60 years of age. The World Bank estimates that the number of people older than 65 years of age will increase from 37 million in 1990 to 102 million in 2025.

The incidence (number of new cases) of cataract blindness is not accurately known. A WHO consultation suggests a working approximation of one new case of cataract blindness per thousand population per year,³ but this probably is a conservative estimate, and the possible incidence may be as high as 2 per 1000 per year.⁴

In India, the age-adjusted prevalence of cataract is three times that of the United States.⁵ Lepkowski and associates report a significant proportion (55%) of Indians in the 50- to 59-year age group had some form of cataract in either eye.⁶ The World Bank reports that in three states surveyed in India, 24% of cataract-blind persons were in the 50- to 60-year cohort, and 16% were in the 30- to 50-year group. India had at that time an average life expectancy of 60 years, and 40% of those receiving surgery were in their productive years. The average loss of income reported was between 35% and 60% of earning potential. In addition, totally blind people were thought to experience two to five additional injuries in their life compared with a sighted person, requiring an average of 3 days in hospital. Brilliant and colleagues⁷ report that in some areas of Nepal, the 5-year survival rate for blind people was 50% less than nonblind people. In India, Minassian and coworkers⁸ report a mortality rate double that of the normal population.

Disability Adjusted Life Years (DALY) is a measure of health status and is used by health economists to compare the value of different health interventions. It is an indicator of the time lived with a disability together with the time lost as a result of premature mortality. The World Bank in 1993 estimated that between $15 and $32 per cataract removal could save one DALY, making it one of the most cost-effective surgical interventions. A detailed study in the Lumbini zone in south central Nepal over the period 1991 to 1993 concludes that the cost per DALY was much lower at $5.06. Sensitivity analysis for a worst case scenario increased the cost per DALY to $20.53.⁹ A 10% decrease in mortality rate produces a 10% reduction in the cost per DALY, suggesting that as countries improve their life expectancy, the cost effectiveness for cataract surgery programs improves.

WHO estimates that globally in 1995, over 7 million cataract surgeries were performed with a rate of 1100 cataract operations per million people per year. In Europe, the rate of cataract operations in the established market economies was 3000 to 4400; India, 2000; the former socialist economies of Europe, 1000 to 1500; Latin America and the Caribbean, 500 to 1500; the remainder of Asia and the middle eastern crescent, 600 to 1000; sub-Saharan Africa, fewer than 500; and China, about 200 operations per million per year.

To eliminate the backlog of cataracts by the year 2020, over 12 million cataracts need be removed by the year 2000, 20 million per year by 2010, and 32 million per year by 2020. These latter estimates are for treating people who are bilaterally blind from cataract with vision less than 10/200. Ellwein and Kupfer¹⁰ note that up to one third of cataract operations may be for unilateral cataracts. If this is true, global estimates of people relieved from blindness from cataracts may be grossly overestimated. In developed countries and probably in economically richer areas of the developing countries, many cataracts will be removed at an earlier stage instead of when vision is less than 10/200.

Many factors have been implicated in the development of cataract, but the their role is not always clearly identified.¹¹ These can be conveniently summarized into six groups: demographics, daylight, diet, diseases, drugs, and unknown. Demographics include age, which is the biggest risk factor for cataract. In general, women have an increased risk of developing cataract. One study suggests that hormone replacement therapy may reduce the risk of postmenopausal women developing cataract,¹² although other studies have not confirmed this finding.

A few studies suggest that age-related cataract may be hereditary. The Beaver Dam Study in the United States found that about half of cortical cataract could be explained by the presence of a dominant gene and about one third of nuclear cataract by a recessive gene.^13,14 A study in Australia found an increased risk of both nuclear and cortical cataracts in those with a positive family history.¹⁵ Racial factors also may play a role in people living in the same area. Cortical cataract was four times more common in African Americans than in Caucasians, but nuclear and posterior subcapsular cataracts were only half as common.¹⁶

The association between ocular exposure to UVB radiation (daylight) and cortical cataract seems to be well established and has been confirmed by several epidemiologic studies and by experimental data.^15,16,17 The effect of diet and vitamins is still unclear. On theoretical grounds, antioxidant vitamins could protect the lens from oxidation and reduce the risk of cataract,^{18,19,20,21,22} such as β-carotinoids (vitamin A), ascorbic acid (vitamin C), and α-tocopherol (vitamin E). Although some studies have found a protective effect for one vitamin or another, other studies contradict these findings. It is difficult to assess a lifetime intake of dietary or supplemental vitamins.

Diseases such as diarrhea leading to severe dehydration is, in some studies,^23,24,25 associated with an increase in cataract formation. Diabetes mellitus is consistently associated with an increased risk of cortical and posterior subcapsular cataract.^26,27 Therapeutic drugs associated with cataract include steroids; whether taken orally, topically, or by inhalation, steroids are associated with posterior subcapsular cataracts.^28,29,30 Recreational smoking and high alcohol intake also are implicated in causing cataract.^31,32,33

Other odd risk factors have been suggested by some studies, such as less education, lower socioeconomic status, or rural residence. It seems unlikely that these specific factors themselves would directly cause cataract. It seems more likely that they indicate exposure to, or protection from, some linked factor that the particular study had not identified or measured.

Most of the world’s population lives in rural areas in developing countries, which are countries classified by the World Bank as having low- to middle-income economies measured by gross national product per capita. The economies are divided (1995 figures) into low income ($765 per capita, lower middle income ($766 to $3035), upper middle income ($3036 to $9385), and high income (above $9386). Low and middle income groups are referred to as developing economies.

Many people in developing nations lead a subsistence, agrarian life, growing food for themselves and for their extended families and selling the surplus for cash. Characteristics of developing nations are high birth and infant mortality rates and high rates of curable and preventable blindness. Health services are concentrated in the larger cities, and people living in rural areas often live beyond the logistical services of health care delivery teams. Ophthalmologists working in most developing nations are largely concentrated in urban areas.