Chapter 73A
Epidemiologic Aspects of Age-Related Cataract
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Cataract is the most common cause of blindness in the world.1 An estimated 17 million persons are blind from cataract worldwide, making it the leading cause of visual loss.1 In countries such as the United States and Great Britain, cataract is still a common cause of visual loss, especially among African Americans and older adults.2,3 As the proportion of persons age 60 and older in the world's population increases, a shift in the burden of eye diseases to age-related causes will occur, resulting in cataract accounting for an even greater proportion of visual loss. By the year 2020, the projected numbers of persons with blinding cataract will exceed 40 million worldwide.4 Unless a preventive approach can be found that will effectively protect against the onset or delay the progression of cataract, it will continue as a leading cause of blindness well into the future.
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Cataract is the most common cause of visual loss in all populations, and no gender, racial, or ethnic group is immune from age-related opacification of the lens. Surgery to restore vision lost to cataract can be done effectively and efficiently with excellent visual outcomes. However, access to surgery and use of services still present problems.

In developing countries, the economic impact of vision loss from cataract is huge, including loss of jobs and increase in custodial care.5 It is estimated that 1500% of the cost of cataract surgery could be generated in 1 year through increased economic productivity.6 In fact, cataract surgery is ranked as one of the most cost-effective public health interventions for the developing world. Worldwide, the evidence for superior visual outcome and high patient satisfaction, coupled with the decline in cost of intraocular lenses (IOLs), has increased the demand for IOL surgery and acceptance of pseudophakia as the standard of care.

Quality cataract surgery has been shown to enhance visual function and quality of life. The National Study of Cataract Outcomes reported that severe complications of surgery were rare in the United States, and achievement of 20/40 or better vision in the operated eye occurred in greater than 90% of patients.7 In other countries, however, some evaluations have suggested that desirable outcomes were not as large as expected because of surgical complications, concurrent diseases, loss of aphakic spectacles, and development of posterior capsular opacification.8–14

Clearly, lens opacification and associated visual loss influences the quality of life for patients. Research has documented the improvement in performance of tasks involving vision and improvement in psychologic well being following cataract surgery.15–18 Some data indicate that the improvement in depressive symptoms following cataract surgery may be a major pathway for the improvement in quality of life in other domains.19

Use of cataract surgery is highly variable, even in developed countries where, presumably, high-quality surgery is accessible and affordable. In the United States in 1992 more than 1.2 million cataract surgeries were performed, at a cost of more than $3.4 billion.20 Nevertheless, there is evidence of underutilization of surgical services in the United States by minority populations, and barriers to access need to be addressed.21,22 Cataract patients deserve high quality, affordable, and accessible surgery. This goal is the centerpiece for the Vision 2020 initiative of the World Health Organization Prevention of Blindness Program and the prevention of blindness community.23 Ultimately, however, the prevention of cataract, or delay in the progression of cataract, will offer the most sustainable strategy for control of blinding cataract.

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The three main types of age-related cataract that have been the focus of epidemiologic investigations are nuclear, cortical, and posterior subcapsular (PSC) cataract. They are distinct anatomically and etiologically. Several systems are in research use for grading the presence and severity of the different types of opacities. The primary systems are the Wilmer Grading system, Wisconsin Grading system, Lens Opacity Case-Control Study (LOCS) series, and the Oxford grading system; each has been used for studies of cataract in numerous countries.24–28 These detailed grading schemes are meant to be used with photographs and trained graders and can be the basis for objective analyses of digital images. The World Health Organization has developed a simple assessment system for nuclear, cortical, and PSC cataract, which incorporates several features of the other grading systems. It is meant to be used clinically in field studies, particularly in less developed countries.29 These grading schemes have been instrumental in epidemiologic studies of environmental and genetic risk factors for cataract.


In the United States, the primary onset of visually disabling cataract is in the older ages (60 and above), and by age 80, approximately 23% or more of the Caucasian population has had bilateral cataract surgery (compared with 11% of the African American population).21 Research on the prevalence of the different cataract types in various countries has been hampered by the absence of a uniform grading system for lens opacities and their visually disabling endpoint, cataract. Several population-based studies have reported the prevalence rate of the three types of lens opacities in different populations and some distinctions do emerge. The highest rates of PSC opacities appear to be in persons of Chinese origin.30,31 Among those age 45 and older, 9% to 15% of populations in Taiwan and China had PSC compared with 6% in a Caucasian population of similar ages.32 Some of this difference may be due to higher rates of cataract surgery for PSC among Caucasians in the United States compared with populations in China or Taiwan. The prevalence of cortical opacity is higher in African Americans (and African Caribbeans) compared with Caucasians.21,33 In one study among 65 to 84 year olds, the rate of cortical opacities in African Americans was 55%, compared with 24% in Caucasians.21 Except among African Americans, the rates of nuclear opacities are higher than the rates for the other two types.21,32


In the last several years, the understanding of risk factors for the different types of cataract has increased substantially.34–36 Personal factors, such as increasing age and female gender, are well-accepted risk factors for cataracts. There appear to be marked ethnic differences in the rates of the different cataract types that do not appear to be explained by known risk factors. It is likely that exploration into candidate genes and gene-environment interactions will yield the most fruitful research into preventive strategies for cataract.35

Environmental factors have been extensively studied for their role in cataractogenesis. Those factors that are relatively common in the population, such as smoking, exposure to sunlight, or diabetes, are of greatest public health interest because preventive strategies could be targeted to large, relevant groups. Cigarette smoking has now been linked to an increased risk of nuclear cataract in numerous studies, with a dose-response relationship and evidence that current smoking is related to incident opacity and drives progression.37–41 Cessation of smoking appears to decrease the risk, although the risk does not return to the level of nonsmokers for at least 10 years.37,42 PSC may also be linked to smoking, although the data are less well established. In 1995, it was estimated that 25% of the U.S. population were smokers,43 suggesting that as much as 20% of the cataract cases were attributable to smoking. Smoking rates are even higher in other countries, such as China, where 60% of men are smokers and every year there are 3 million new smokers. The projected increase in all smoking-related diseases, including cataract, is high in these countries.

Ultraviolet B (a component of sunlight with wavelengths 295 to 320 nm) is well absorbed by the lens and capable of causing epithelial cell and lens protein damage. Chronic, ocular exposure to sunlight has been solidly linked to cortical cataract in a number of studies.44 The most likely position of the cortical opacity, inferior nasal, is compatible with sunlight as the cataractogenic agent.45 Even with the low levels of exposure typically encountered in the general population, a modest increased risk of cortical opacity can be demonstrated.46

Persons with diabetes are at increased risk for cataract, particularly cortical and probably PSC cataract. Cataract appears to be related to the duration of diabetes and level of glycemic control.47,48 Incidence and progression of cortical opacities is greater in those with diabetes, and poorer level of control may further increase the risk.49 The projected rise in the prevalence of diabetes worldwide, and especially in developing countries, will likely fuel an increase in prevalence of cataract in the future.50

A particularly active area of research has been the relationships to cataract of height, weight, nutrition, and supplement use. The role of nutritional status in cataractogenesis remains unclear, however. Some investigators have found the risk of nuclear opacities was greater in persons with low body mass index51–53 (weight for height squared) and with taller stature.51 An interesting study carried out in a birth cohort of 68- to 78-year-old persons found that nuclear opacities were related to low weight at 1 year of age, adjusted for several confounders.54 Some others have found that higher body mass index, or central obesity, is associated with cortical opacities.51,55,56

In theory, antioxidant defense of lens proteins and membranes from oxidative stress should protect against cataractogenesis. However, studies on the role of diet, supplement use, or serum levels of various antioxidants have provided conflicting data and do not strongly support any one micronutrient, supplement, or food source as particularly anticataractogenic.34 Two clinical trials on the protective effect of supplements containing vitamin E or a cocktail of antioxidants have reported no evidence that the incidence or progression of cataract is lower in the group taking supplements.57,58 At present, the promotion of any vitamin or mineral supplement to retard the onset or progression of cataract is unjustified.

The search for protective agents against the onset or progression of cataract has been disappointing. Initial hopes that aspirin could provide a protective effect were not confirmed in longitudinal studies and clinical trials.34 Interesting cross-sectional data suggested a role for hormone replacement therapy as protective for nuclear cataract in women,59,60 and estrogen receptor alpha has been demonstrated in lens epithelium. However, prospective studies have failed to confirm the association.61 At present, there is no pharmacologic agent that is known to be safe and effective against the onset of age-related cataract.


Nuclear cataract, particularly nuclear cataract mixed with other types, appears to be an independent predictor of early mortality.62–64 The association is present in Caucasians, African Americans, and African Caribbeans. Other predictors of mortality, such as frailty, smoking, diabetes, and other chronic conditions, do not explain this association. Thus the lens may provide an interesting window into the aging process, and there is considerable research interest in finding a common pathway between lens opacification and premortal events at the cellular or subcellular level.

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1. World Health Organization: Programme for the Prevention of Blindness and Deafness. Global Initiative for the Elimination of Avoidable Blindness. WHO/PBL/97.61 1998:1–2

2. Muñoz B, West SK, Rubin GS et al: Causes of blindness and visual impairment in a population of older Americans: The SEE Study. Arch Ophthalmol 118:819, 2000

3. Minassian DC, Reidy A, Desai P et al: The deficit in cataract surgery in England and Wales and the escalating problem of visual impairment: Epidemiological modeling of the population dynamics of cataract. Br J Ophthalmol 84:4, 2000

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8. Murthy GV, Gupta S, Ellwein LB et al: A population-based eye survey of older adults in a rural district of Rajasthan: I. Central vision impairment, blindness, and cataract surgery. Ophthalmology 108:697, 2001

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15. Mangione CM, Phillips RS, Lawrence MG et al: Improved visual function and attenuation of declines in health-related quality of life after cataract extraction. Arch Ophthalmol 112:1419, 1994

16. Steinberg EP, Tielsch JM, Schein OD et al: The VF-14: An index of functional impairment in patients with cataract. Arch Ophthalmol 112:630, 1994

17. Bernth-Pelerson P: Visual functioning in cataract patients: Methods of measuring and results. Acta Ophthalmol 59:198, 1981

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21. West SK, Muñoz B, Schein OD et al: Racial differences in lens opacities. The Salisbury Eye Evaluation (SEE) project.Am J Epidemiol 148:1033, 1998

22. Broman A, Muñoz B, Rodriguez J et al: Factors associated with accessing cataract surgery in a population-based study of Hispanics: Proyecto VER. Invest Ophthalmol Vis Sci 42:2869, 2001

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24. West SK, Rosenthal F, Newland HS et al: Use of photographic techniques to grade nuclear cataracts. Invest Ophthalmol Vis Sci 29:73, 1988

25. West SK, Muñoz B, Wang F et al: Measuring progression of lens opacities for longitudinal studies. Curr Eye Res 12:123, 1993

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35. Congdon NG: Prevention strategies for age related cataract: Present limitations and future possibilities. Br J Ophthalmol 85:516, 2001

36. Taylor, HR: Epidemiology of age-related cataract. Eye 13:445, 1999

37. West SK, Muñoz B, Emmett EA et al: Cigarette smoking and risk of nuclear cataracts. Arch Ophthalmol 107:1166, 1989

38. West SK, Muñoz B, Schein OD et al: Cigarette smoking and risk of progression of nuclear opacities. Arch Ophthalmol 113:1377, 1995

39. Christen WG, Manson JE, Seddon JM et al: A prospective study of cigarette smoking and risk of cataract in men. JAMA 268:989, 1992

40. Klein BE, Klein RE, Lee KE: Incident cataract after five-year interval and lifestyle factors: The Beaver Dam Eye Study. Ophthalmic Epidemiol 4:247, 1999

41. AREDS Report No. 5: Risk factors associated with age-related nuclear and cortical cataract: A case-control study in the Age-Related Eye Disease Study. Ophthalmology 108:1400, 2001

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43. Smith SS, Fiore MC: The epidemiology of tobacco use, dependence, and cessation in the United States. Prim Care 26:433, 1999

44. West SK: Ocular ultraviolet B exposure and lens opacities: A review. J Epidemiol 9:S97, 1999

45. Schein OD, West SK, Muñoz B et al: Cortical lenticular opacification: Distribution and location in a longitudinal study. Invest Ophthalmol Vis Sci 35:363, 1994

46. West SK, Duncan DD, Muñoz B et al: Sunlight exposure and risk of lens opacities in a population-based study: The Salisbury Eye Evaluation Project. JAMA 280:714, 1998

47. Klein BE, Klein R, Moss SE: Prevalence of cataracts in a population-based study of persons with diabetes mellitus. Ophthalmology 92:1191, 1985

48. Leske MC, Wu SY, Hennis A et al: Diabetes, hypertension, and central obesity as cataract risk factors in a black population: The Barbados Eye Study. Ophthalmology 106:35, 1999

49. Klein BE, Klein R, Lee KE: Diabetes, cardiovascular disease, selected cardiovascular disease risk factors, and the 5-year incidence of age-related cataract and progression of lens opacities: The Beaver Dam Eye Study. Am J Ophthalmol 126:782, 1998

50. King H, Aubert RE, Herman WH: Global burden of diabetes, 1995-2025: Prevalence, numerical estimates and projections. Diabetes Care 21:1414, 1998

51. Caulfield L, West SK, Barron TV et al: Anthropometric status and cataract: The Salisbury Eye Evaluation project. Am J Clin Nutr 69:237, 1999

52. Mohan M, Sperduto RD, Angra SK et al and the India US Case Control Study Group: India US Case Control Study of Age Related Cataracts. Arch Ophthalmol 107:670, 1989

53. Leske MC, Chylack LT, Wu SY, The Lens Opacities Case Control Study Group: The Lens Opacities Case-Control Study: Risk factors for cataract. Arch Ophthalmol 109:244, 1991

54. Evans JR, Rauf A, Aihie Sayer A et al: Age-related nuclear lens opacities are associated with reduced growth before 1 year of age. Invest Ophthalmol Vis Sci 39:1740, 1998

55. Leske MC, Wu SY, Hennis A et al: Diabetes, hypertension, and central obesity as cataract risk factors in a black population. The Barbados Eye Study. Ophthalmology 106:35, 1999

56. Hiller R, Podgor MJ, Sperduto RD et al: A longitudinal study of body mass index and lens opacities. The Framingham Study. Ophthalmology 105:1244, 1998

57. AREDS Report No. 9: A randomized, placebo-controlled, clinical trial of high-dose supplementation with vitamins C and E and beta carotene for age related cataract and vision loss. Arch Ophthalmol 119:1439, 2001

58. Robman L, McCarty C, Tikellis G et al: VECAT study: The effect of vitamin E on the progression of lens opacities (preliminary results). Invest Ophthalmol Vis Sci 42:2742, S508, 2001

59. Klein BE, Klein R, Ritter LL: Is there evidence of an estrogen effect on age-related lens opacities? The Beaver Dam Eye Study. Arch Ophthalmol 112:85, 1994

60. Freeman E, Muñoz B, Schein OD et al: Hormone replacement therapy and lens opacities: The Salisbury Eye Evaluation Project. Arch Ophthalmol 1119:1687, 2001

61. Klein BE, Klein R, Lee KE: Reproductive exposures, incident age-related cataracts, and age-related maculopathy in women: The Beaver Dam Eye Study. Am J Ophthalmol 130:322, 2000

62. West SK, Muñoz B, Istre J et al: Mixed lens opacities and subsequent mortality. Arch Ophthalmol 118:393, 2000

63. Hennis A, Wu SY, Li X et al and Barbados Eye Study Group: Lens opacities and mortality: The Barbados eye studies. Ophthalmology 108:498, 2001

64. Wang JJ, Mitchell P, Simpson JM et al: Visual impairment, age-related cataract, and mortality. Arch Ophthalmol 119:1186, 2001

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