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The potential association between diabetes and primary open angle glaucoma (POAG) has been studied by many groups, with most studies supporting a weak association between the two diseases. The present population based study by Ellis et al, published in this issue of theBJO (p 1218), has determined anincidence for newly diagnosed glaucoma and ocular hypertension among diabetics and non-diabetics. The authors make the distinction that previous studies assessed prevalence rather than incidence when determining an association. However, with diseases that have a low incidence, such as POAG, determination of prevalence may be preferable, in order to achieve an adequate number of affected patients and the necessary power to come to a more definitive conclusion. This is especially the case with POAG and diabetes, in which the association seems tenuous.
In their study, Ellis et al relied on the detection of new glaucoma prescriptions and glaucoma surgical procedures among their large study population of 6631 diabetics and 166 144 non-diabetics. Case notes of these patients were then reviewed for further categorisation (POAG, ocular hypertension, misclassification). This reliance on detection within the established medical community has the same disadvantage as many earlier studies—namely, the potential for detection bias since diabetics are more likely to receive eye examinations and be screened for glaucoma. However, in the Blue Mountains Eye Study1 the entire study population underwent a detailed eye examination including automated perimetry, stereo optic disc photographs, and applanation tonometry to establish a diagnosis of glaucoma or ocular hypertension. (The need for very large numbers of patients in incidence rate studies would have precluded such detailed screening examinations.) The age-sex adjusted odds ratio (OR) for glaucoma in diabetics compared with those without diabetes was 2.12 (95% confidence intervals (CI) 1.18–3.79), and the authors concluded that there was a real association between glaucoma and diabetes. On the other hand, the Baltimore Eye Study,2which was conducted similarly to the Blue Mountains Eye Study, found an age-race adjusted odds ratio of 1.03 (95% CI 0.85–1.25). In the present study, the relative risk for POAG diagnosis was 1.57 (95% CI 0.99–2.48). The authors acknowledge that detection bias probably contributed to the observed increased risk among diabetics and conclude that an association is not supported.
Unfortunately the question of whether or not the diabetic condition is a significant risk factor for glaucoma development remains open and controversial. The authors make an earnest attempt to address this issue by determining the incidence of newly diagnosed glaucoma but arrive at a relative risk that is somewhat increased but not statistically significant. Most population based studies that have explored the possibility of an association have found similar results and harbour the same concern about detection bias as in this study.
Thus, conclusions that have been drawn are usually tentative and conflicting. In the final analysis, it is clear that the association between diabetes and POAG, if it exists, is not a strong one.
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