| Chapter 73A Epidemiologic Aspects of Age-Related Cataract SHEILA K. WEST Table Of Contents |
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MAGNITUDE OF THE BURDEN OF CATARACT SOCIAL AND ECONOMIC IMPACT OF CATARACT AND CATARACT SURGERY EPIDEMIOLOGY OF CATARACT REFERENCES |
| MAGNITUDE OF THE BURDEN OF CATARACT |
| 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. |
| SOCIAL AND ECONOMIC IMPACT OF CATARACT AND CATARACT SURGERY |
| 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. |
| EPIDEMIOLOGY OF CATARACT |
CATARACT CLASSIFICATION SCHEMES 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. PREVALENCE 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 RISK FACTORS FOR CATARACT 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. CATARACT AND MORTALITY 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. |