Updated July 18, 2006

Avoiding Eye Complications

by A. Paul Chous, M.A., O.D.

Retinal perfusion pressure (RPP) is highly dependent on the average pressure inside the blood vessels (known as the mean arterial pressure, or MAP) and somewhat dependent on the internal eye pressure (intraocular pressure, or IOP). Together, these pressures predict the risk of vision loss from diabetes. Both MAP and RPP can be calculated quite simply by knowing your blood pressure and your internal eye pressure, the latter of which is routinely measured at the eye doctor’s office. The formula is as follows:

RPP = 2/3 x MAP – IOP
where MAP = Mean Arterial Pressure = (systolic blood pressure – diastolic blood pressure) ÷ 3 + diastolic blood pressure.

For example, if your blood pressure averages 150/90 mm Hg and your intraocular pressure is 15 mm Hg, your MAP = (150 – 90) ÷ 3 + 90 = 110 mm Hg, and your RPP = 2/3 x 110 – 15 = 58.3 mm Hg.

If your blood pressure averages 110/80 mm Hg and your intraocular pressure is 15 mm Hg, your MAP = (110 – 80) ÷ 3 + 80 = 90 mm Hg, and your RPP = 2/3 x 90 – 15 = 45 mm Hg.

This may look like a lot of math, but it’s worth doing it if it helps you to gauge your personal risk of developing severe retinopathy that threatens your vision. Considerable research has shown that RPP and MAP strongly predict the risk of developing severe, sight-threatening retinopathy in Type 1 and Type 2 diabetes, respectively.

Specifically, the risk of severe retinopathy increases by fourfold to sixfold (400% to 600% increased risk) when RPP is higher than 50.1 mm Hg (in Type 1 diabetes) and MAP is higher than 97.1 mm Hg (in Type 2 diabetes).

Amazingly, in one large study, RPP and MAP predicted the development of severe diabetic retinopathy as well as or better than HbA1c and duration of diabetes. My advice is to know your blood pressure, know your intraocular pressure, calculate your MAP and RPP, and discuss these numbers and their implications with both your eye doctor and your diabetes physician. Since IOP is almost always above 10 mm Hg (the normal range is between 10 mm Hg and 21 mm Hg), keeping your blood pressure at or below 115/75 mm Hg (a level at which MAP = 88.3 mm Hg and RPP is less than 48.9 mm Hg) will greatly reduce the risk of losing vision to diabetes.

Blood lipid control
Check, know, and improve your blood lipid levels through diet, regular physical activity, and medicine, if necessary. Diabetic retinopathy is often more severe in people with abnormal blood lipids, especially elevated low-density lipoprotein (LDL, or “bad”) cholesterol and triglycerides. The risk of ischemic optic neuropathy, retinal vascular occlusion, and cataract is also higher in these people.

Ideally, LDL cholesterol should be below 100 mg/dl, triglycerides should be below 150 mg/dl, and HDL (“good”) cholesterol should be above 45 mg/dl in men and above 55 mg/dl in women.

There is some evidence that the cholesterol-lowering statin drugs may reduce the risk of diabetic eye disease (and strong evidence that they reduce the occurrence of stroke and heart attack) even in people with normal blood lipid levels. Reduced consumption of saturated fats, trans fats, and hydrogenated fats, and increased consumption of foods high in monounsaturated fats (such as olive oil and avocados), omega-3 fats (such as flaxseed or flaxseed oil, cold-water fish, and fish oil supplements), and dietary fiber appear to improve blood lipid levels. So does regular physical activity, such as walking 30 minutes each day. Increased physical activity can also improve blood glucose control which, in and of itself, improves your blood lipid levels.

Also in this article:
Eye Diseases Associated With Diabetes

Dr. Chous is a doctor of optometry specializing in diabetes education in Tacoma, Washington. He is the author of Diabetic Eye Disease: Lessons From a Diabetic Eye Doctor (Fairwood Press, 2003), which can be ordered through the Web site www.diabeticeyes.com.

More articles on Diabetic Complications

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