What You Should Know About Diabetic Macular Edema
Blurred vision in patients with diabetes is most commonly caused by fluctuations in blood sugar, which transiently change the refractive state of the eye. Among the non-transient causes of blurred vision in diabetics, the most common cause is diabetic macular edema. In this condition, fluid leaks out of the blood vessels of the retina into the retinal tissue. The retina has a pumping mechanism to get rid of fluid, but if the leakage increases enough, it overwhelms the pump, and the retina swells. When the retina swells, the vision blurs.
The retina lines the back of the eye and is responsible for turning light into nerve signals, which travel to the brain through the optic nerve. Figure 1 shows these tissues in a schematic form.
Figure 1. Anatomy of an Eye
Figure 2 shows the appearance of a normal retina as seen by the ophthalmologist examining the eye. The fatty components of the leakage fluid form yellow deposits within the retina, as shown in figure 3.
Figure 2. Normal Retina
Figure 3. Diabetic Macular Edema
What Causes Diabetic Macular Edema?
The main causes are the duration of diabetes mellitus and the level of the blood sugar. A conscientious diabetic patient can still develop macular edema if the disease has been present for twenty or more years. A diabetic patient who typically has very high sugars will develop macular edema much earlier. The best way to judge control over time is by checking the hemoglobin A1C, also called the glycosylated hemoglobin. A value of the hemoglobin A1C of seven or less indicates good control and represents a worthy goal.
High blood pressure also contributes to diabetic macular edema. Patients should attempt to maintain the top number of the blood pressure (diastolic) under 140 and the bottom number (systolic) under 90. Kidney failure can make diabetic macular edema worse. Unfortunately, some patients respond to certain medicines with an increase in diabetic macular edema. Rosiglitazone and pioglitazone are occasional suspects in this regard.
How is Diabetic Macular Edema Treated?
Initial efforts are directed toward improving blood glucose control. Attempts are also made to normalize blood pressure if it is high. If kidney function is impaired, attempts are made to improve it. Cessation of smoking is advised, when smoking is a habit.
Once these factors have been addressed, laser surgery may be recommended. There are two types of laser surgery used to treat two different types of diabetic retinopathy. Laser surgery for macular edema is called focal laser. The other type is called panretinal laser, used for proliferative diabetic retinopathy, which occurs when abnormal, weak blood vessels bleed and form scars.
In focal laser surgery, intense light is used to burn small spots near the macula, the central portion of the retina, responsible for fine vision. An anesthetic injection is frequently placed in the lower eyelid to keep the eye from moving during this delicate work and to reduce discomfort. After a treatment, the swelling of the retina goes down slowly, and a recheck is made after three to four months.
In cases where focal laser has been tried and the swelling persists, injections of triamcinolone may be recommended for the eye. Triamcinolone is closely related to cortisone, and seems to make the blood vessels less leaky. The effect of these injections lasts several months, and may need to be repeated if recurrences develop. Side effects of these injections include a predictable acceleration of cataract in all patients, and a rise in eye pressure in approximately 25% of patients. Pressure checks are made one week and one month after the injection to detect these rises. Usually the pressure rises can be treated successfully with drops taken over several months.
To properly diagnose diabetic macular edema, and to assess the effect of treatment, ophthalmologists rely on Optical Coherence Tomography (OCT) to measure retinal thickness. OCT is a type of photograph, not an x-ray, and is painless to have done. Another helpful type of photograph is a fluorescein angiogram, which involves the intravenous injection of a small amount of food-coloring dye. As the dye circulates through the blood vessels of the eye, a Certified Retinal Angiographer takes several pictures of the retina. This study helps the ophthalmologist assess the health of the retinal circulation, which guides his treatment plan.
If after reading this brochure you would like to seek further information on your own, an excellent resource is Pubmed, on the National Library of Medicine website, accessible at www.pubmed.com.