Women with Type 2 Diabetes at Risk for Glaucoma

BOSTON, July 12 -- Women with type 2 diabetes have a nearly twofold increased risk of developing primary open-angle glaucoma, the most common form of the disease, found researchers here.

Among more than 76,000 women enrolled in the Nurses' Health Study, those with type 2 diabetes had a relative risk for glaucoma of 1.82 (95% confidence interval, 1.2-2.7) even after the data were controlled for various glaucoma risk factors, said Louis R. Pasquale, M.D., and colleagues of the Massachusetts Eye and Ear Infirmary, and other Harvard-affiliated institutions.

"While obesity fuels the type 2 diabetes epidemic, it appears that factors unrelated to obesity contribute to the positive association between type 2 diabetes and glaucoma; we were surprised to find this," they reported in the July issue of Ophthalmology.

"Our study had a large enough sample to allow us to focus on type 2 diabetes only and to study its relation to newly diagnosed primary open-angle glaucoma cases," they added. "We were also able to correct for other factors that could contribute to glaucoma. Our work suggests, but in no way proves, that factors other than lifestyle behavior contributing to insulin resistance could lead to elevated intraocular pressure and glaucoma."

Primary open-angle glaucoma is the most common form of the disease, accounting for about 60% to 70% of all glaucoma cases. It's one of the leading causes of blindness, and affects more than two million people in the United States, according to the National Eye Institute.

In several large eye studies, type 2 diabetes has been associated with increased intraocular pressure, the most obvious sign of primary open-angle glaucoma. But although diabetes has been conclusively linked to retinopathy, its alleged association with glaucoma is controversial, Dr. Pasquale and colleagues wrote.

To see whether there is an association between type 2 diabetes and glaucoma, the investigators looked at prospective data from the Nurses' Health Study on 76,318 women who were followed from 1980 to 2000.

The women were 40 or older at baseline, did not have primary open angle glaucoma at study outset, and reported having eye exams during the follow-up period.

Potential confounders were determined based on biennial questionnaires filled out by all Nurses' Health Study participants, and a diagnosis of type 2 diabetes was confirmed on a validated supplemental questionnaire.

During the follow-up, 429 women reported that they had primary open angle glaucoma; these cases confirmed were confirmed by medical chart review.

The investigators found that type 2 diabetes was positively associated with a nearly two-fold risk for primary open angle glaucoma (relative risk 1.82. 95% CI. 1.23-2.70), after controlling for the potential confounders of age, race, hypertension, body mass index, physical activity, alcohol intake, smoking, and family history of glaucoma.

They also found, however, that longer duration of diabetes did not strengthen the association with glaucoma. The relative risk for diabetes of less than five years duration was 2.24 (95% CI, 1.31-3.84) compared with five or more years duration (relative risk 1.54, 95% CI, 0.90-2.62).

"In secondary analyses, to evaluate the potential for detection bias we controlled for additional factors such as the number of eye examinations, but type 2 diabetes remained positively associated with primary open angle glaucoma," the authors wrote.

Although the Nurses' Health Study looked only at women, "there is no reason to think that our results are gender specific, but more study of the relation between type 2 diabetes mellitus and primary open angle glaucoma in men is needed," they added.

The investigators also noted that the cohort consisted primarily of white women, and that the results may not be applicable to other racial groups. Other investigators have shown that the rate of glaucoma is about six times higher among African Americans than among Caucasians.

The reason for the lack of association of glaucoma with longer duration of diabetes is not known. The authors speculated that "one possible explanation is that we may have lacked the power to find a statistically significant positive trend between type 2 diabetes duration and risk of primary open-angle glaucoma. Alternatively, hyperglycemia may have a direct adverse effect on trabecular meshwork function that reverses with normalization of serum blood sugars, ameliorating the risk of primary open-angle glaucoma. Further study is needed to prove which, if any, of these interpretations of our time trend data are supported."