Glaucoma incidence in an unselected cohort of diabetic patients: is diabetes mellitus a risk factor for glaucoma?

Aims-To evaluate whether diabetes mellitus is a risk factor for the develop ment of primary open angle glaucoma or ocular hypertension (OHT). Methods-A historical cohort study of an unselected population comprising al l residents of the Tayside region of Scotland was performed using record li nkage techniques followed by case note review. Ascertainment of prevalent d iabetes was achieved using the Diabetes Audit and Research in Tayside Study (DARTS) validated regional diabetes register. Glaucoma and treated OHT wer e defined by encashment of community prescriptions and the statutory surgic al procedure coding database. Results-The study population comprised 6631 diabetic subjects and 166 144 n on-diabetic subjects aged >40 years without glaucoma or OHT at study entry. 65 patients with diabetes and 958 without diabetes were identified as new cases of glaucoma or treated OHT during the 24 month study period, yielding a standardised morbidity ratio of 127 (95% CI, 96-158). Case note review d emonstrated non-differential misclassification of prevalent glaucoma and OH T as incident disease (diabetic cohort 20%, non-diabetic cohort 24%; p=0.56 ) primarily as a result of non-compliance in medically treated disease. Rem oving misclassified cases and adjusting for age yielded an incidence of pri mary open angle glaucoma in diabetes of 1.1/1000 patient years (95% CI, 0.8 9-1.31) compared to 0.7/1000 patient years (95% CI, 0.54-0.86) in the non-d iabetic cohort; RR 1.57 (95% CI, 0.99-2.48). Conclusions-This study failed to confirm an association between diabetes me llitus and primary open angle glaucoma and ocular hypertension. A non-signi ficant increase in diagnosed and treated disease in the diabetic population was observed, but evidence was also found that detection bias contributes to this association.