A study shows that middle-aged and elderly people suffering from diabetes and hypertension may be at risk for yet another malady: primary glaucoma.

The disease, also called open-angle or chronic glaucoma, is the most common form of glaucoma in the United States. It's caused by slow clogging of the eye's drainage canals, causing increased pressure. Since it's painless, the disease often doesn't produce symptoms until its later stages. About 3 million people have the disease, including one in 10 people over the age of 80, according to the Glaucoma Research Foundation.

Researchers at the University of Michigan Kellogg Eye Center looked at records of more than 2 million people older than age 40 to see how various components of metabolic syndrome affected glaucoma. Metabolic syndrome affects about 20% of the population and includes diabetes, hypertension or high blood pressure, and hyperlipidemia, characterized by high cholesterol and high triglyceride levels.

They discovered that those with diabetes had a 35% greater risk for developing glaucoma than those without the disease; hypertension upped the risk by 17%. Those with both diabetes and hypertension had a 48% increased risk.

However, hyperlipidemia reduced the risk by 5%, possibly because the medications used to treat this disease may also reduce glaucoma risk. Further testing is underway to find out why this result occurred.

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The study was published in the journal Ophthalmology.

The researchers recommended that those who have diabetes and hypertension have regular eye exams.