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Although diabetes has long been proposed as a risk factor for glaucoma, epidemiologic studies have yielded inconsistent results, making the association controversial. Over the past decade, there has been no systematic review of the literature on the correlation between diabetes and glaucoma or between glaucoma risk and other metabolic abnormalities.
This gap has recently been filled by Myung Hun Kim, MD, at Saevit Eye Hospital in Goyang, Korea, and his colleagues, who conducted an updated systematic review and metaanalysis. They found that diabetes, duration of diabetes, and fasting glucose levels were associated with a significantly higher risk of primary open-angle glaucoma and that diabetes and fasting glucose levels were associated with slightly increased levels of intraocular pressure (IOP).1
Proposed mechanisms. The mechanisms relating diabetes to glaucoma are not clear. Various studies have suggested that diabetes causes microvascular damage and vascular dysregulation of the retina and the optic disc, increasing the susceptibility of the optic nerve head to glaucomatous damage. Diabetes also may disrupt the trabecular meshwork function, thereby elevating IOP.2
Most robust association. Longer duration of diabetes was consistently associated with higher risk of glaucoma across cross-sectional, case-control, and longitudinal studies and was independent of age, race, gender, and other confounders controlled in the original studies. Patients with longer duration of diabetes particularly need to be aware of the importance of glaucoma screening and management, according to the authors.
Authors’ perspective. “We were not surprised to find an association between diabetes and the risk of glaucoma,” said Dr. Kim, “but we were somewhat surprised by the degree of heterogeneity of the associations and the variability between studies. We were reassured that even longitudinal studies, which are less prone to bias than cross-sectional or case-control studies, found positive associations.
“However, since patients with diabetes are more likely to be in contact with eye care providers, diagnostic bias is still an issue that will require additional research. That said, the associations seen in our study were from population-based prevalence studies, which should remove this bias.”
Strengths and weaknesses. A notable strength of this meta-analysis is the large sample size: 47 studies that included 2,981,342 individuals from 16 countries. Other strengths include the evaluation of multiple diabetes-related exposures and glaucoma-related outcomes, as well as the inclusion of prospective studies. Dr. Kim noted that prospective studies are important in providing estimates of incidence and temporal trends and helping to establish the temporal sequence required for causal inference.
The main weakness of the meta-analysis was the substantial variation in the methods and quality of the original studies. “This likely contributed to the high degree of heterogeneity in the results,” Dr. Kim said.
Clinical implications. The results support the recommendation that patients with diabetes be referred to ophthalmologists to check for glaucoma. “In addition to diabetic retinopathy, patients with diabetes should also receive education about glaucoma,” said Dr. Kim. He added that the coexistence of diabetic retinopathy and glaucoma has an important clinical impact.
Areas for future research. Dr. Kim said that additional longitudinal studies are needed. In particular, the authors noted a scarcity of studies on the relationship between glaucoma and glucose biomarkers, prediabetes, and metabolic syndrome and its components. Given the high prevalence of these metabolic abnormalities, the authors recommended that future research evaluate the association between altered glucose metabolism and glaucoma risk.1
“Once the association is clearly understood, specific guidelines for glaucoma screening and management in patients with diabetes can be developed,” said Dr. Kim.
—Gabrielle Weiner
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1 Zhao D et al. Ophthalmology. 2015;122(1):72-78.
2 Nakamura M et al. Ophthalmologica. 2005;219(1):1-10.
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Dr. Kim reports no relevant financial interests.
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