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Purpose: To evaluate the prevalence and risk factors of lens opacities in a geographically defined population of subjects with type 2 diabetes mellitus compared with a control population.
Methods: Subjects in the community of Laxå with a diagnosis of type 2 diabetes mellitus (n = 275) and a control group (n = 256) participated in the study. Lens opacities were graded with Lens Opacities Classification System II in all participants. Lens Opacities Classification System score ≥2 was considered as significant lens opacity. Anthropometric and blood chemistry data were collected for all participants in connection with the eye examination. For the diabetic population, yearly updated information on glucose control, blood pressure and body mass index was available through medical records from diabetes diagnosis until the time of the eye examination.
Results: The prevalence of significant cortical, posterior subcapsular and nuclear cataract was 65.5%, 42.5% and 48.0%, respectively, in the type 2 diabetes population in Laxå. In logistic regression analyses, all types of lens opacities were strongly associated with age (p < 0.0001). Cortical lens opacity was also associated with a diagnosis of diabetes (p < 0.0001), posterior subcapsular lens opacity with HbA1c (p < 0.0001) and nuclear lens opacity with female gender and higher heart rate (both p = 0.0004). In the diabetic population, all types of cataract were likewise strongly associated with age (p < 0.0001), posterior subcapsular cataract with HbA1c (p = 0.0032), nuclear cataract with female gender (p = 0.0002) and higher heart rate (p = 0.0008).
Conclusions: Our study shows that cortical cataract is associated with diabetes mellitus, not necessarily defined by glucose control, whereas posterior subcapsular cataract is associated with glucose levels. Nuclear cataract is not associated with diabetes mellitus, but is more frequent in women and is also associated with higher heart rate.
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Age was, as expected, the strongest risk factor for all types of lens opacities both among patients with type 2 diabetes and a gender- and age-matched control group from the same geographical area. Besides age, in multivariate analyses, cortical lens opacity was significantly more common among patients type 2 diabetes. There was also a significant association between posterior subcapsular lens opacity and a higher HbA1c. Having nuclear lens opacity showed, however, no association with a diabetic state or glucose control, but was instead significantly related to female sex and surprisingly to a higher heart rate.
In the diabetic population, age was again the dominant significant determinant for all types of cataract. A significant association could also be seen between posterior subcapsular cataract and HbA1c. Nuclear cataract was significantly associated with female sex and higher heart rate.
Several studies have shown similar results, although the association between diabetes and different types of cataract is complex.
The Framingham study showed an increased prevalence of cataract in patients with diabetes (Klein et al. 1985). In the Wisconsin Epidemiologic Study of Diabetic Retinopathy, the risk of cataract surgery was higher for persons having type 2 diabetes using insulin (Klein et al. 1995a).
In the Beaver Dam Eye study, the incidence and progression of cortical and posterior subcapsular cataract were associated with diabetes. Further, HbA1c was associated with increased risk of nuclear and cortical cataract. In a further analysis in this study, longer duration of diabetes also increased the prevalence of cortical cataracts (Klein et al. 1995b, 1998).
In the cross-sectional Blue Mountains Eye Study, posterior subcapsular cataract, but not nuclear cataract, was significantly related to diabetes (Rowe et al. 2000). In a longitudinal study, cortical cataract was related to impaired fasting glucose (Saxena et al. 2004).
In the Barbados Eye study, the 4-year incidence of lens opacities was evaluated. The factors increasing the risk for cortical opacity were old age, female gender, low socioeconomic status and diabetes mellitus; for posterior subcapsular opacity were age and diabetes; and for nuclear opacity were age, female gender, leaner body mass and diabetes (Leske et al. 2002; Hennis et al. 2004).
The Los Angeles Latino Eye Study reported that all types of lens opacities increased with age and that nuclear opacity was more common in women (Varma & Torres 2004).
In the Reykjavik Eye Study, cortical lens opacity was related to age and outdoor exposure and nuclear lens opacity to age and to smoking. In this study, no association of lens opacities to diabetes was found, but the study included relatively few patients with diabetes (Sasaki et al. 2000; Katoh et al. 2001; Arnarsson et al. 2002).
The Tanjong Pagar Survey found diabetes to be related to cortical cataract, posterior subcapsular cataract, any cataract and cataract surgery (Foster et al. 2003).
In our study, the impact of diabetes on the prevalence of cataracts seems to be less important than in many of the studies cited earlier. Possible explanations of this could be the fact that our diabetic population was under good glucose control and most subjects had an early diagnosis of diabetes by opportunistic case finding. This probably led to less difference between the diabetic group and the control group concerning blood glucose levels. As our diabetic group also incorporated all elderly diabetic persons in the community, it might have augmented the impact of age on the occurrence of cataract and in doing so reducing the effect of other variables.
Although cortical and posterior subcapsular opacities showed similarities regarding the associations we found with a diagnosis of diabetes and metabolic control, there were also differences. For cortical cataract, it was having a diagnosis of diabetes, but not glucose control, that proved to be associated with our outcome variable, and for the diabetic population only, neither HbA1c nor fasting blood glucose measured cross-sectional or longitudinally was statistically significant. A possible interpretation of this finding can be that other conditions, tied to the diabetic state, than glucose control may play a role. With posterior subcapsular cataract, it was the other way round. Especially, not only among the whole study population but also among the diabetic population measures at the cross-sectional survey of glucose control were significantly associated with this kind of cataract. This result can indicate a role for glucose levels below the diabetes state in the development or worsening of posterior subcapsular cataract.
Finally, an unexpected result was seen for nuclear opacities. Others have shown that the associations were also found with age and female sex, but the strong and consistent association with a higher heart rate both among the whole study population and among the diabetic group is, to our knowledge, a new finding. If persistent in future studies, one may consider whether some age-related mechanism, other than chronological age, can be responsible or whether, perhaps, nuclear cataract and its relation to heart rate in adult life can be traced back to imperfect foetal development of certain organs, the so-called Barker hypothesis.
The strengths of our study are that we could examine and compare a complete type 2 diabetic population with a control group matched for age, gender and residency and that we also could use longitudinal data on continuously updated glucose, blood pressure and BMI values in the diabetic group.
On the other hand, our study is relatively small, and this may have prevented us from observing associations that would be significant in larger cohorts. Our findings are restricted to subjects of Caucasian origin.
Whether prevention and improved control of diabetes would reduce the burden of cataract remains to be demonstrated. Our data, together with others, indicate a possibility for such a scenario for cortical and posterior subcapsular cataract, but not for nuclear cataract. Future studies on the pathogenesis and epidemiology of cataracts must distinguish between the three types of lens opacities, as they clearly show different patterns.