Year : 1998 | Volume
: 46 | Issue : 4 | Page : 221--227
Risk factors for cataract: A case control study
Suresh N Ughade, Sanjay P Zodpey, Vandana A Khanolkar
Department of Preventive and Social Medicine, Government Medical College, Nagpur, India
Suresh N Ughade
C-7 Hemraj, 122 Shivajinagar, Nagpur - 440 010
The present study was designed as a hospital-based, group-matched, case-control investigation into the risk factors associated with age-related cataract in central India. The study included 262 cases of age-related cataract and an equal number of controls. A total of 21 risk factors were evaluated: namely, low socioeconomic status (SES), illiteracy, marital status, history of diarrhoea, history of diabetes, glaucoma, use of cholinesterase inhibitors, steroids, spironolactone, nifedipine, analgesics, myopia early in life, renal failure, heavy smoking, heavy alcohol consumption, hypertension, low body mass index (BMI), use of cheaper cooking fuel, working in direct sunlight, family history of cataract, and occupational exposure. In univariate analysis, except marital status, low BMI, renal failure, use of steroids, spironolactone, analgesics, and occupational exposure, all 14 other risk factors were found significantly associated with age-related cataract. Unconditional multiple logistic regression analysis confirmed the significance of low SES, illiteracy, history of diarrhoea, diabetes, glaucoma, myopia, smoking, hypertension and cheap cooking fuel. The etiological role of these risk factors in the outcome of cataract is confirmed by the estimates of attributable risk proportion. The estimates of population attributable risk proportion for these factors highlight the impact of elimination of these risk factors on the reduction of cataract in this population.
|How to cite this article:|
Ughade SN, Zodpey SP, Khanolkar VA. Risk factors for cataract: A case control study.Indian J Ophthalmol 1998;46:221-227
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Ughade SN, Zodpey SP, Khanolkar VA. Risk factors for cataract: A case control study. Indian J Ophthalmol [serial online] 1998 [cited 2020 Jul 7 ];46:221-227
Available from: http://www.ijo.in/text.asp?1998/46/4/221/24169
Cataract is a major cause of blindness worldwide, particularly so in India. Cataract constitutes 55% of total blindness in this country. It has been estimated that if the development of cataract could be delayed by 10 years, the number of cataract operations could decrease by 45%, considerably reducing the expenditure and burden on the National Blindness Control Programme.
It is generally acknowledged that age-related (senile) cataract is a multifactorial disease. However, owing to inadequacies in epidemiologic understanding of this disease many risk factors have been hypothesised.,, A number of risk factors have been shown to be associated with cataract. These include socioeconomic status (SES), illiteracy, marital status, history of diarrhoea, history of
diabetes, glaucoma, use of cholinesterase inhibitors, steroids, spironolactone, nifedipine, analgesics, myopia early in life, renal failure, heavy smoking, heavy alcohol consumption, hypertension, low body mass index (BMI), use of cheaper cooking fuel, working in direct sunlight, family history of cataract, occupational exposure, and several biochemical variables.,,
However, their relative contribution to the outcome of cataract varies among studies and across different populations. Moreover, the combined effect of these risk factors can help in better prediction of cataract as compared to their individual effects.
The purpose of this case-control study was to estimate the extent of risk associated with the aforementioned factors and to assess their relative contribution in the ultimate outcome of cataract.
Materials and Methods
This study was carried out at the Government Medical College Hospital, Nagpur. The study was designed as a group-matched case-control study. We used two known confounding factors, that is, age and sex, as matching variables.
Sample size was calculated based on the findings of a pilot study of 50 cases and an equal number of controls. With a relative risk of cataract in exposed (low SES) group = 1.62, proportion of exposure in control population = 0.51, desired level of significance = 0.05 and power of the study = 0.90, the sample size was estimated to be 262 cases, with an equal number of controls.
Cases of age-related (senile) cataract were selected from patients attending the Ophthalmology Clinic at Government Medical College Hospital, Nagpur. The criterion for diagnosis of a case was "sufficiently advanced lens opacity that impaired vision".
The controls were selected from among patients attending the hospital for conditions other than cataract. The same diagnostic criteria were used to exclude the possibility of cataract in controls.
The study included 21 hypothesised risk factors for cataract namely low SES, illiteracy, marital status, low BMI, history of diabetes, history of diarrhoea, smoking, alcohol consumption, glaucoma, myopia, family history of cataract, hypertension, renal failure, use of drugs like aspirin, nifedipine, steroids, cholinesterase inhibitors, spironolactone, occupational exposure, working in direct sunlight, and use of cheaper cooking fuel [Table:1].
Assessment of marital status, glaucoma, use of cholinesterase inhibitors, myopia early in life (up to age < 21 years), renal failure, use of steroids, spironolactone and nifedipine, use of cheaper cooking fuel, family history of cataract, and occupational exposure (on the military base, the possibility of exposure to microwave (radar) and equipment used for transmission of signals in Air Force station causing microwave exposure) was carried out through interviews and clinical examination.
Determination of low SES was based on the lower and upper-lower categories of modified Kuppuswamy's classification for SES. The subjects who could not read and/or write were categorised as illiterates. Subjects who had dehydrational crisis from severe diarrhoea, so as to require being bedridden for at least 3 days were classified as having a history of diarrhoea. However, subjects were considered exposed only when the episode of diarrhoea preceded the cataract-induced visual disability by ≥3 months. Diabetes was diagnosed on the basis of clinical history. The subject was labelled as a heavy smoker if he smoked ≥10 cigarettes/bidis daily for ≥2 years. Heavy alcohol consumption included high alcohol intake, that is ≥75 gms per day for ≥2 years. Hypertension was defined as a systolic pressure of ≥160 mmHg and/or a diastolic pressure (phase V) of ≥95 mmHg, or history of current consumption of antihypertensive drugs. Subjects with <18.5 BMI were included in exposed category. A crude criterion of sunlight exposure classification has been used in this study, that is ≥6 hours average daily exposure to sunlight for a minimum period of 10 years.
Pearson's chi-square and odds ratios were calculated in univariate analysis, to estimate the risk associated with these factors. 95% confidence intervals were built around these estimates. All the risk factors which showed a p value <0.1 on univariate analysis were subjected to unconditional multiple logistic regression analysis. The risk factors identified to be significant with p <0.05 were included in the final model of logistic regression analysis. Attributable risk proportion (ARP) (defined as the proportion of total disease risk in exposed persons which may be attributed to their exposure) and its 95% confidence limits were estimated for the risk factors included in the final model. Population attributable risk proportion (PARP) (defined as the proportion of disease risk in all persons which may be attributed to the exposure under investigation) and 95% confidence intervals were also estimated for the preventable risk factors.
[Table:1] shows the distribution of study subjects by matching factors. Majority of the subjects were in the 51-70 years age group. The study included 52% males and 48% females.
[Table:2] shows the results of univariate analysis according to risk factors for cataract. The study identified significant association of cataract and 14 risk factors: low SES, illiteracy, history of diarrhoea, history of diabetes, glaucoma, use of cholinesterase inhibitors, myopia early in life, history of heavy smoking, history of heavy alcohol consumption, hypertension, use of nifedipine, use of cheap cooking fuel, working in direct sunlight, and family history of cataract.
Results of the unconditional multiple logistic regression analysis (full model and final model) are presented in [Table:3]. Of the 14 significant risk factors on univariate analysis included in the full model, 9 risk factors were identified as significant (p<0.05). Use of cholinesterase inhibitors, nifedipine, history of alcohol consumption, working in direct sunlight, and family history of cataract were not significant in the full model. In the final model of multiple logistic regression, the remaining 9 risk factors confirmed their significant association with cataract (p<0.05).
ARP and PARP estimates for the significant risk factors are shown in [Table:4]. The etiological role of all the 9 significant risk factors is confirmed from the estimates of ARP. High ARP values showed that much of the disease risk in exposed persons may result from that exposure. The estimates of PARP highlighted the impact of elimination of these risk factors on the reduction of cataract in this population.
The present case-control study of risk factors for age-related cataract include 21 important risk factors. Although the univariate analysis demonstrated significant association between 14 risk factors and cataract, the multivariate analysis identified 9 risk factors in the final model. This study, which included a large number of risk factors, is one of the largest case-referent studies carried out in central India. An attempt was made to include as many risk factors as possible to study their independent and joint contribution in the ultimate outcome of cataract. The interactions among significant risk factors on univariate analysis were studied by unconditional multiple logistic regression in the full and final model. This is the advantage gained by this study over many earlier studies carried out in this country. Because cataract is a multifactorial condition, interactions among the risk factors and their joint contribution to the outcome of cataract will be helpful in understanding the complex epidemiology and etiology of cataract.
The present study identified 9 important risk factors for age-related cataract in this population. It is not surprising that low SES was significantly associated with senile cataract in this study. The prevalence of the majority of risk factors included in this study was higher in the low SES group, thereby indirectly identifying its significant association with cataract. Although literacy is one of the important components of socioeconomic classification, it has been included separately in this study. Illiteracy was also found to be significantly associated with cataract. Socioeconomic variables related to educational achievement are likely explanation of this finding. As with many diseases, low SES (whether measured by income, education, or occupation) is a risk factor for cataract formation and probably for all subtypes of cataract. Earlier studies also reported its significant association with cataract.
In tropical countries, severe diarrhoea has been proposed as a major cause to account for the excessive prevalence of cataract. The role of diarrhoea in cataract formation has been confirmed by case-control studies conducted in central and eastern India., In these studies severe dehydration episodes were found to be significantly associated with cataracts in a dose-dependent fasion. The large relative risk of 3.68 estimated in this study also confirmed its role in the outcome of cataract. Diarrhoea being a more common and severe disease in this population it is likely to play a significant role in the etiology of cataract. However, other authors have failed to support this association. A case-control study in rural India showed no association nor did an observational study in Bangladesh. The India-US case-control study used confinement to bed for one day as the definition for diarrhoea and also found no association.
Diabetes has been identified as a significant predictor of cataract. The present study also identified its strong association with cataract. However, the support for diabetes as a risk factor for cataract formation is less clear. In the Italian-American cataract study no association between a past history of diabetes and cataract formation was found. In the lens opacity case-control study, a modest association for all cataract types except nuclear sclerosis was demonstrated, and in the India-US case-control study diabetes was one of the listed exclusion criteria. The modest associations found in the case-control studies can also be explained by a selection bias common to all diabetic studies in ophthalmology. Given this bias, the contradiction of large case-control studies with significant power, and the modest association between diabetes and cataract in many of the studies when an association exists, the time-honoured belief that diabetes is a risk factor for cataract formation, must be considered cautiously.
Glaucoma appeared as a powerful risk factor for cataract. The wide confidence limits [Table:3] can be explained on the basis of low prevalence of glaucoma (1.1%) in the control population. The cholinesterase inhibitors which were widely used to treat glaucoma are known to cause cataract, but lenses of glaucoma patients opacify even without this theory, and it appears that the cataract risk associated with glaucoma is increased by a combination of the disease itself and the treatment applied. However, in this study use of cholinesterase inhibitors was significant in univariate analysis but it was not significant in multivariate analysis.
Myopia early in life was a significant risk factor for cataract. This study investigated myopia early in life as a risk factor for cataract because a myopic shift occurs at an early stage in the development of cataract. Moreover, myopia is expected to play a significant additive role in the ultimate outcome of cataract in combination with other significant risk factors.
With the litany of ills associated with cigarette smoking comes evidence that cataracts should also be added to the list. Two case-control studies have failed to find an association between smoking and cataracts, but other cross-sectional, case-control, and prospective cohort studies have found an association and a dose-response relation as well. In this study also history of smoking was significantly associated with cataract. The role of alcohol in cataract formation is less clear. Three large case-control studies,, did not find an association between alcohol consumption and cataract formation. In this study history of alcohol consumption was significant in univariate analysis but not in multivariate analysis.
Our data suggested an association between increased levels of blood pressure and cataract. A previous study that included age and sex adjusted analysis reported a positive association between blood pressure and cataract. However studies that include data on duration of hypertension and whether antihypertensive medications were used are needed to investigate further association between blood pressure levels and cataract.
Exposure to cheap cooking fuel is relatively higher in rural areas and urban slums in this country. It has been reported that cataracts are more common among persons using less-expensive cooking fuels (cowdung, wood, coal), a group likely to have lower SES. Related socioeconomic variables may also explain the association between type of cooking fuel used and cataract. However, there are very few studies which have explored cheap cooking fuel as a risk factor for cataract.
No single predictor of cataract formation has been examined more frequently than sunlight exposure. Several ecologic studies have reported an association between cataract and sunlight or ultra-violet exposure.,,, Despite the impressive consistency of these studies, all have the shortcomings of any ecologic study. These include the "etiologic fallacy": the risk of the population need not be the same as the risk in any sector or the risk of an individual. Confounders and effect modifiers cannot be studied and complicated relations cannot be demonstrated. Moreover, in these ecologic studies, the end point was usually cataract formation as opposed to cataract type and this difference has important clinical and public health implications. This study also attempted to investigate this relationship but could not establish a significant association between working in direct sunlight and cataract. This study used a crude criterion of exposure classification, that is, ≥6 hours average daily exposure to sunlight for a minimum period of 10 years. Earlier studies also found it difficult to quantify an individual's lifetime exposure to sunlight.
Renal failure was reported as a powerful risk factor for cataract in the earlier studies. However, this study could not identify its significance; which partly can be attributed to its low prevalence in the study subjects. Similarly, use of steroids, spironolactone, nifedipine and cholinesterase inhibitors were not found to be associated with cataract. This also can be explained on the basis of their low prevalence in the study population.
A possible protective effect of aspirin for cataracts has been suggested previously,, yet other studies have reported no beneficial effect from aspirin usage. A clinical trial among healthy male physicians reported no apparent reduction in the incidence of cataract in the aspirin-treated group. Population-based observational studies, one among diabetes, found no relationship between aspirin use and cataract. Similarly, the present study also did not identify protective association between aspirin use and cataract. In the light of the contradictory reports till now, further studies may be needed to determine whether aspirin therapy can affect cataract development.
While numerous animal studies have shown that nutritional deficiencies can lead to cataract formation, the role of nutritional status in human cataractogenesis is unclear. An earlier study demonstrated that increased body mass index was associated with a decreased risk of nuclear and mixed types of cataract.This suggests that better nourished persons may have a lower risk of developing cataracts. However, previous studies of relationship between nutritional status and cataract have provided no clearcut patterns. This epidemiologic study also did not identify significant association of BMI and cataract.
Family history of cataract has been found to be significant in most studies in which it was not specifically controlled for,, but the obvious recall bias involved in case-control studies is particularly apparent with this factor. However, family history of cataract and occupational exposure to radiations and microwaves were not identified to be associated with cataract in this study.
The full model of unconditional multiple logistic regression included 14 risk factors to study their interactions and joint contributions in the prediction of cataract. However, the final model identified significance of 9 risk factors. This suggests that 9 risk factors (low SES, illiteracy, history of diarrhoea, history of diabetes, glaucoma, myopia early in life, history of smoking, hypertension, and use of cheap cooking fuel) significantly contributed to cataract in this population after adjusting for the effect of other risk factors.
This study estimated higher population attributable risk proportion for low SES, use of cheap cooking fuel, illiteracy, and history of diabetes. Low SES, use of cheap cooking fuel, and illiteracy are interrelated risk factors and are more prevalent in developing countries including India. Moreover, these are preventable risk factors. Thus, considering the impact on prevention of cataract, a more comprehensive strategy based on these factors and more exploratory studies evaluating their role in cataract outcome in different population groups in this country are needed.
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