DEAR DOCTOR K:

Could you explain how diabetes affects vision?

DEAR READER:

The high blood sugar levels that occur in people with diabetes can have serious consequences throughout the body, including the eyes. Many of my patients with diabetes are most concerned that diabetes will rob them of the precious gift of sight.

People with diabetes are at greater risk of developing cataracts and glaucoma, and keeping blood sugar under control can reduce your risk of developing them in the first place. But if you develop either of these two conditions, fortunately there are effective treatments. These days, no person with diabetes should go blind from cataracts or glaucoma.

However, a third eye problem -- diabetic retinopathy -- is more likely to cause severe vision loss or blindness. Diabetic retinopathy occurs when abnormal blood sugar levels damage small blood vessels in the retina, the light-sensing area in the back of your eyes. The retina sends visual images to the brain.

Diabetic retinopathy begins when the walls of small blood vessels in the retina weaken. They leak fluid into the surrounding tissue, often leaving protein and fat deposits in the retina. The vessel walls also develop tiny bulges or balloons called "microaneurysms" that leak red blood cells into the retina.

As the condition progresses, the abnormal vessels begin to close, robbing the retina of its blood supply. Nerve fibers in the retina that are necessary for vision begin to die from poor circulation and lack of oxygen. (I've put an illustration of this process on my website, AskDoctorK.com.)

These changes may not alter your vision. But if fluid leaks into the center of the macula -- the part of the retina responsible for sharp, central vision -- your sight will be impaired. Swelling of the macula is called macular edema.

As retinopathy advances, the damaged retina tries to repair itself by sprouting new blood vessels. However, these new vessels are very fragile and don't grow normally; they tend to leak blood and break apart. This can cause a sudden loss of vision.

Treatments can help to prevent vision loss, or slow its progression. But there is no cure for diabetic retinopathy. You've got to catch it early and stop it from getting worse.

You can significantly reduce your risk of eye diseases by keeping your blood sugar at near-normal levels. Controlling your sugar levels also will protect against damage to other parts of your body, including your kidneys, heart and brain.

Regular vision testing is also vital. Get a comprehensive dilated eye exam at least once a year. These exams can detect macular edema and diabetic retinopathy in the earliest stages. Prompt treatment can help prevent severe vision loss and blindness.

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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