By A. Paul Chous, MA, OD | July 18, 2006 12:00 am
When it comes to diabetes-related eye complications, the good news is that most cases of severe vision loss due to diabetes are preventable. The bad news is that tens of thousands of people still lose vision to diabetes each year, despite all that is known about prevention and treatment.
Diabetes is the leading cause of new blindness for American adults between the ages of 20 and 74. The American Diabetes Association (ADA) estimates that 12,000–24,000 people in the United States lose their vision to diabetic retinopathy each year, and studies have shown that people with diabetes have a fivefold to twenty-fivefold increased risk of blindness in their lifetimes.
However, these numbers very likely underestimate the true incidence of vision loss and blindness, because diabetes can cause a variety of eye diseases other than retinopathy, several of which can cause severe visual impairment (see “Eye Diseases Associated With Diabetes” ). Moreover, these statistics ignore vision loss less severe than “legal blindness” (defined as vision on the eye chart worse than 20/200 with the use of prescription lenses or severe loss of peripheral vision to within 20 degrees of central vision). But even less severe vision loss can and does substantially affect quality of life for thousands of people with diabetes.
Why do so many people continue to lose their sight to diabetes when so much is known about preventing and treating these complications? This question has dogged eye doctors, diabetes specialists, and public health experts for years, and the consensus answer boils down to four critical and interrelated elements: the epidemic of new diabetes cases, lack of patient education, lack of patient motivation and support, and lack of access to excellent diabetes care.
I do not pretend that these issues are easily solved, but I believe the tools are at hand to greatly reduce all the dreaded complications of diabetes — including blindness — as well as the human suffering they entail. I believe that a collaborative effort among people who have diabetes, health-care providers, and policy makers is the surest and fastest route to that end.
With that goal in mind, here is some of what I have learned in my 36 years of living with diabetes and my 15 years as a doctor of optometry specializing in diabetes care and education. Most of these strategies, in addition to minimizing the risk of eye complications, will go a long way toward preventing all diabetes complications. This is not all that surprising since eye problems caused by diabetes often go hand in hand with nerve, kidney, and cardiovascular disease.
Keep your blood glucose levels as close to the normal range as possible. High blood glucose levels are directly or indirectly responsible for all forms of diabetic eye disease. The landmark Diabetes Complications and Control Trial, published in 1993, showed that in people with Type 1 diabetes, each 10% reduction in average blood glucose levels, as reflected by a person’s glycosylated hemoglobin (HbA1c) level, lowers the risk of developing diabetic retinopathy by roughly 60% and lowers the risk of preexisting diabetic retinopathy getting worse by 43%.
This means, for example, that if your HbA1c level is typically 7.0% (equivalent to an average blood glucose level of 172 mg/dl) and you bring your HbA1c down to 6.3% (equivalent to an average blood glucose level of 147 mg/dl), you have dramatically reduced your chances of developing retinopathy. If you already have some degree of retinopathy and you lower your HbA1c level by this amount, you have substantially lowered the chances of your retinopathy getting worse. These risk-reduction statistics hold true until HbA1c levels are below 5.0% (which is equivalent to an average blood glucose level of 101 mg/dl), at which point the risks of retinopathy development and progression are minimal.
The landmark United Kingdom Prospective Diabetes Study showed similar results for people with Type 2 diabetes. If you can keep your HbA1c results around or under 6.0%, you will greatly lower the odds of having serious eye complications from diabetes.
Why and how does high blood glucose damage the eyes? The “stock” answer given by medical professionals is that high blood glucose levels damage the smallest, most fragile blood vessels throughout the body, and the eyes have lots of small blood vessels. While this explanation is not incorrect, it is somewhat simplistic. The fuller answer is quite a bit more complex. Essentially, high blood glucose has two harmful effects in the eyes:
Of course, the clear benefits of “tight” blood glucose control have to be balanced against the risks of severe hypoglycemia (low blood glucose). Tight control is best achieved with close consultation with members of your diabetes care team, careful attention to diet (especially portion control), regular exercise, medicine if necessary, and blood glucose monitoring on a regular basis.
Living with diabetes is admittedly often like walking a tightrope between high and low blood glucose levels. There is great variability in each person’s response to specific foods, activities, and treatments. The point is that the better you can learn to recognize and react to your unique blood glucose patterns, the closer to normal you can keep your blood glucose levels, and the healthier your eyes will be.
Keep your blood pressure well controlled through diet, regular physical activity, and medicine, if needed. The United Kingdom Prospective Diabetes Study showed that for Type 2 diabetes, controlling high blood pressure lowers the risk of retinopathy and worsening of existing retinopathy even more than tight blood glucose control. A 10/5 mm Hg reduction in blood pressure (for example, from 150/90 mm Hg to 140/85 mm Hg) reduced the risk of severe vision loss by nearly 50%. (In addition, this same blood pressure reduction lowered stroke risk by 44% and death by 32%!)
Elevated blood pressure increases blood flow into the eye, accelerating diabetic retinopathy. It also increases the turbulence of blood flow to the retina and optic nerve, which increases the risk of abnormal blood clotting (as happens in ischemic optic neuropathy and retinal vascular occlusion).
Recent research on the dynamics of blood flow to the eyes (called ocular hemodynamics) has given new insight into the importance of blood pressure control. The results of this work can help eye doctors gauge the risk of severe vision loss from diabetic retinopathy for individual patients. Research has demonstrated (using a technique called laser Doppler velocimetry) that at the onset of retinopathy, the volume of blood flow into the retina increases dramatically. As mentioned earlier, this is believed to be caused by high blood glucose, which impairs the ability of retinal vessels to constrict and precisely regulate blood flow. The pressure generated by this blood flow, known as retinal perfusion pressure (RPP), injures the walls of the smallest blood vessels (making them leaky) and redirects blood away from the smallest vessels (capillaries) into larger vessels. This results in destruction of the capillaries and inadequate blood circulation within retinal tissues (a phenomenon known as capillary nonperfusion). This destruction, in turn, leads to the release of biochemical messengers that promote the growth of new but abnormal blood vessels that bleed profusely and can lead to retinal detachment (as happens in proliferative diabetic retinopathy).
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.
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 40 mg/dl in men and above 50 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.
If you smoke, quit. Smoking constricts blood vessels, thereby raising blood pressure and increasing the risk of diabetic eye disease. Nicotine, the primary addictive ingredient in cigarettes, reduces the ability of red blood cells to carry oxygen, resulting in retinal hypoxia (oxygen starvation) that further damages the eye. Smoking also generates free radicals, chemical compounds that damage cells. This is why smokers are much more likely to develop cataracts at a younger age than nonsmokers and much more likely to develop macular degeneration, the leading cause of vision loss in people over 50 in the Western world. We all know that smoking is bad for health. If you have diabetes, quitting smoking is arguably the single best thing you can do for your eyes (as well as your heart, lungs, kidneys, dental health, etc.).
Make sure you get a dilated eye examination every year by an optometrist or ophthalmologist knowledgeable about and experienced with diabetes and diabetic eye disease. It is estimated that 40% to 50% of people with diabetes do not get annual dilated exams. This is tragic, because most cases of severe vision loss from diabetes are preventable with early diagnosis and timely treatment. For example, laser therapy for severe, vision-threatening cases of diabetic retinopathy has been shown to decrease the risk of severe vision loss by 50% to 75%, and vision loss from glaucoma can be largely prevented with drugs, laser treatment, and/or surgery. Unfortunately, by the time people have symptoms of diabetic eye disease, very often, irreversible eye damage has already occurred, and the prognosis for successful treatment is poorer. This is why it is so critical for people with diabetes to have their eyes examined on a regular basis. Remember that all people with diabetes, whether they have Type 1 or Type 2 diabetes and whether or not they use insulin, are at increased risk for losing vision.
Why is dilation of the eyes so important? Dilating, or enlarging, your pupils allows the eye doctor to see more of the inside of your eye, view it more easily, and view it in stereo (in 3-D), factors that greatly improve the chances of early diagnosis. Remember that good vision (on an eye chart test or in the real world) does not mean there is no diabetic eye disease. Many people with diabetic retinopathy and glaucoma, two potentially blinding eye diseases, have 20/20 or better vision and absolutely no symptoms at the time they are diagnosed. If you have diabetes and wait for symptoms of eye disease, it may very well be too late. However, if you follow the guidelines described here, your annual visits to the eye doctor are more likely to end with the words, “Everything looks normal and healthy. Keep doing what you’re doing and we’ll see you next year!”
Seek out health-care professionals who are knowledgeable, communicate effectively, and serve as your advocates. Knowledge is power, and you want a health-care team with plenty of both. Members of your diabetes team should explain their findings and recommendations in understandable language and should encourage you to ask questions and actively participate in the management of your diabetes. You, the person with diabetes, must also strive to become knowledgeable. This will empower you to make educated health-care decisions, to ask better questions, and to get better care.
Effective communication also means that members of your health-care team should communicate well and often with each other, because diagnoses and treatments given by one specialist often affect the treatments and recommendations offered by others. In short, you want your team members to be on the “same page.” Insist that your eye doctor send timely reports to your other doctors and health advisors, and ask them to send reports to your eye doctor. Because good diabetes care requires frequent visits to members of your health-care team, it is very important that each one encourages you, advocates for you, and works with you and the other team members in your best interest. Put simply, this is what good health-care professionals do, and if you’ve got diabetes, you cannot settle for anything less.
Even as science discovers more about the causes and complications of diabetes, and even as more effective treatments become available, the epidemic of diabetes and its complications continues to grow. Perhaps the most effective way to stop diabetes complications, including blindness, is to prevent diabetes in the first place.
The groundbreaking Diabetes Prevention Program, a study that examined what might prevent diabetes, has shown that simply walking 30 minutes each day, five days per week, reduces the diagnosis of Type 2 diabetes in at-risk people by nearly 60%. But for those of us who have already been diagnosed, the risk of severe eye complications can be reduced by as much as 95% if we can work together to educate, motivate, and support ourselves, our health-care providers, and our policy makers to take every action possible in turning the tide. Hopefully, this article will serve as another springboard toward that end.
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