by A. Paul Chous, MA, OD
EDITOR’S NOTE: Dr. A. Paul Chous, who himself is diabetic, explains treatment of diabetic retinopathy and stresses the importance of early intervention. When treatment fails, remember that the Diabetes Action Network and the National Federation of the Blind are here to help.
Diabetic retinopathy (DR) is the leading cause of new blindness for Americans between the ages of 20 and 74. It is estimated that an American with diabetes loses his or her vision every 22 minutes. Though clinical research clearly demonstrates that tight control of blood glucose and blood pressure (good metabolic control) substantially reduces the risk of blindness due to DR, many patients develop this eye disease and are at risk for serious vision loss, sometimes even despite outstanding metabolic control. Fortunately, the advent of retinal laser therapy (laser photocoagulation) has greatly reduced the number of patients who lose vision to DR.
Laser photocoagulation refers to the precise and concentrated application of
high energy light, typically of a single wavelength (called monochromatic light).
This light energy (photo-) is absorbed by body tissue and generates
heat which, in turn, causes clotting (-coagulation) of blood and localized
destruction of that tissue. When applied to the retina, the light-sensitive
membrane lining the inside back wall of the eye, laser energy creates focal
“burns” that have two principal effects: (1) they destroy small
areas of the retina and (2) they seal off damaged, leaking blood vessels that
Diabetic retinopathy occurs when high blood glucose damages the smallest blood vessels supplying the retina, leading to leakage of small amounts of blood, serum, blood fats and blood proteins. If significant leakage occurs within the macula, (the most sensitive part of the retina), vision may be affected (diabetic macular edema or DME]—otherwise patients rarely have symptoms at this stage. As DR progresses, new but abnormal blood vessels may form and proliferate on the surface of the retina and optic nerve (proliferative diabetic retinopathy or PDR). These new vessels break easily and bleed profusely (vitreous hemorrhage), leading to vision loss and formation of scar tissue that may tug on the macula or detach the retina from the back wall of the eye, leading to blindness.
When is Photocoagulation Beneficial?
First, retinal laser therapy is beneficial for minimizing fluid leakage from leaky blood vessels within the macula, essentially by cauterizing those leaky vessels. The Early Treatment of Diabetic Retinopathy Study (ETDRS) showed that retinal laser therapy applied to the macula reduces the risk of substantial worsening of vision from significant diabetic macular edema (DME) by 50 percent, if it is performed in a timely fashion, even in patients who have excellent vision when diagnosed. Second, laser therapy causes regression (shrinkage) of the abnormal blood vessels found in proliferative diabetic retinopathy (PDR). The ETDRS showed that one to several thousand laser burns applied outside the macula reduces the risk of severe vision loss from PDR by 50-75 percent, if it is performed in a timely fashion, even in patients with 20/20 vision at the time of their diagnosis. It is thought that sacrificing some of the peripheral retina, by destroying it with laser burns, turns off the chemical messengers causing abnormal blood vessel proliferation, thereby protecting the macula and vision from more severe injury due to PDR.
It is important to understand that retinal laser therapy does not prevent DR, nor is it used to treat mild or moderate DR (for these less severe stages, improving blood sugar and blood pressure control are the mainstays of effective treatment). Rather, it is used to treat only serious, vision threatening DR: significant diabetic macular edema (DME); proliferative diabetic retinopathy (PDR); and sometimes, severe non-proliferative retinopathy (severe NPDR) in people with type 2 diabetes. It is also critical to know that retinal laser therapy for DR works best only when it is performed prior to losing vision, ideally, before the patient has any symptoms whatsoever.
Side-Effects of Photocoagulation
Laser photocoagulation does have side effects. It can reduce a patient’s night vision or his/her ability to function well when going from a lighted environment to a darkened one (e.g., a movie theater). It can cause a loss of peripheral vision or blind spots in a person’s central vision, depending upon where the laser burns are placed. The laser burns themselves can (rarely) become a site for the development of abnormal blood vessels beneath the retina, leading to vision loss (this is the same process occurring when a patient loses vision to “wet” macular degeneration). Finally, and most importantly, sometimes laser therapy does not work, and patients lose vision despite receiving timely treatment.
Future Treatment Options
For all these reasons, a lot of research and effort has gone into developing new medications for treating serious DR before it causes irreparable damage to the retina. While these drugs hold great promise for treating and preventing blindness due to diabetic retinopathy, laser photocoagulation remains the “gold standard.” Despite its shortcomings and limitations, retinal laser therapy has unquestionably saved the vision of tens of thousands of people with diabetes, including the author of this article. Unfortunately, it will not restore vision that has been lost. For that, we must look to the future of molecular and tissue engineering, to the spirit of making the irreparable reparable, and to a host of novel ideas that will, hopefully, become the passion of our best and brightest biomedical scientists.