How Does Diabetes Cause Blindness?
Diabetes can cause several different problems in the eye, all of which can lead to blindness.The most common of these problems are:
- Diabetic retinopathy
- Macular edema
- Vitreous hemorrhage
- Neovascular glaucoma
Of these, diabetic retinopathy, macular edema and cataracts were the most common afflictions I saw in my 20 year career in ophthalmology.
Image of a Cataract
Diabetes Can Cause Cataracts
The lens of the eye sits behind the iris and is responsible for 30 percent of the eye's focusing power. It is the part of the eye that becomes a cataract as it ages.
In a person with diabetes, the excess sugar in the lens cannot be transformed into fructose quickly enough and accumulates in the lens fibers creating opacities. This forms the cataract and in diabetics, this process happens much more quickly than in non-diabetics.
Symptoms of cataracts usually start with blurry or cloudy vision and worsen progressively. Progression is slow in most patients; however, in diabetics cataracts can form seemingly overnight.
One of the most common signs of the development of a cataract is glare when driving at night. The lights of cars can make a patient with cataracts unable to drive at night safely.
Cataracts can be removed surgically; however, diabetics have a higher incidence of complications during and after surgery.
The term retinopathy indicates any disease of the retina that does not include inflammation. Diabetic retinopathy, as its name suggests, is the result of damage to the retina from the chronic high levels of glucose in the blood stream caused by the disease.
There are two types of diabetic retinopathy:
- Background diabetic retinopathy, or BDR
- Proliferative diabetic retinopathy or PDR
Image of the Retina of a Diabetic Patient
Background diabetic retinopathy (BDR) is characterized by dot blot hemorrhages, microaneurysms (small vascular buds), and lipoid exudates, or fat cells that leak from blood vessels under the macula.
Proliferative diabetic retinopathy is characterized by neovascularization or the growth of new blood vessels into the retina. These blood vessels grow from the optic disc, or branch off from other blood vessels on the retina and are usually very weak.
Because of this, these new blood vessels may break down and leak blood onto the retina damaging the neuronal layer which is comprised of the rods and cones that “see” images and send them to the occipital lobe to be processed. Unfortunately, any damage to this particular layer is irreversible.
Rods and Cones of the Retina
There aren't usually any symptoms of diabetic retinopathy. However, blurry vision can be associated with it. Treatment typically involves multimodality therapy meaning a combination of different therapies. Typically, laser photocoagulation therapy is the primary modality chosen.
Anti-VEGF medications are usually the second choice. These medications attempt to reverse the neovascularization at the genetic level. Unfortunately, they do require ongoing treatment. Injections are typically given every 3 to 6 months or any variation thereof.
Macular edema is swelling of the macula, the area that provides our central vision. In other words, when we’re reading or doing anything that requires great detail, we are using the macula.
Macular edema is generally the result of background diabetic retinopathy, more specifically the lipoid exudates that are leaked underneath the macula. This leakage causes separation of the neuronal and RPE (retinal pigment epithelium which sharpens images) layers of the retina, essentially disconnecting the retina from the cells that send the messages to the occipital lobe.
Symptoms of macular edema include blurred central vision, problems reading, straight lines being warped or distorted and loss of sharp vision.
Surprisingly, macular edema can be reversed if caught early enough. Treatment is typically multimodality and includes laser photocoagulation therapy and either Kenalog injections or anti-VEGF medications.
Kenalog is a corticosteroid that reduces inflammation and swelling. While it is an effective treatment, it also has side effects of its own. Like all steroids, Kenalog can cause cataracts and increased intraocular pressure (also called glaucoma).
Retinal and Vitreous Hemorrhages
Neovascularization inevitably leads to either retinal or vitreous hemorrhages, or sometimes both. The new blood vessels are so brittle, they can easily burst. The result is a pool of blood that destroys the retinal tissue it contacts.
In many cases, the blood will leak into the vitreous or blood vessels will grow into the vitreous and burst causing a hemorrhage. The vitreous is the gel-like substance that gives the eye its rigidity and shape.
Sometimes the body will reabsorb the blood from a vitreous hemorrhage. In my experience however, 50 percent of vitreous hemorrhage cases need vitrectomy surgery, which involves removing the vitreous with a probe and injecting silicon-based oil in its place to keep the retina from detaching.
Vitreous and retinal hemorrhages typically appear suddenly. The "symptoms" are of the hemorrhages themselves. Most patients complain of black spiderwebs in their vision along with flashes of light.
Large hemorrhages may appear as a large black splotch in the vision; smaller ones may look like black dots and may be mistaken by the patient as floaters.
Treatment for retinal hemorrhages involves laser photocoagulation therapy to stop the bleeding and prevent further hemorrhages as well as anti-VEGF medications for the neovascularization.
Behind the cornea and in front of the iris is a fluid known as aqueous. This fluid is replaced constantly and drains through the trabecular meshwork, which is a system of pores that allows the aqueous to leave the eye preventing the fluid from building up in the eye. Increased eye pressure is known as glaucoma.
Symptoms of neovascular glaucoma include:
- severe pain
- sudden hazy vision
- sudden loss of peripheral vision
- sensation of pressure in the eye
- red eyes
- sensitivity to light
Neovascularization can affect the iris as well as the retina. When blood vessels encroach on the iris, the trabecular meshwork can become clogged or obstructed. This can cause the aqueous to build up in the eye causing the pressure to rise.
The Eye's Drainage Structures
Increased pressure in the eye causes damage to the optic nerve, the part of the eye that sends the visual signals to the occipital lobe of the brain. Damage to the optic nerve leads to loss of peripheral vision and eventually complete blindness.
Treatment of neovascular glaucoma depends on severity. If caught early enough, eye drops can be used to keep the pressure under control. Anti-VEGF drugs can also be beneficial in preventing further neovascularization.
However, in more advanced cases (which usually occurs in diabetics), surgery may be needed. Trabeculectomy is the surgery of choice. The procedure creates a permanent drain, called a bleb, for the aqueous to escape the eye preventing the pressure from building up.
This is only a brief introduction to the damaging effects diabetes has on the eye. The disease can cause a plethora of different problems, and if the patient is not examined on a regular basis, the damage from diabetes can cause permanent blindness and do so very quickly.
Unfortunately, the only prevention is maintaining a constant blood sugar level in the optimal range. It is very important for diabetics to monitor their blood sugar as well as their diet and physical activity. Diabetics should also be closely monitored by their physicians to help prevent complications of the disease.
This article is not meant to replace the advice of your physician. It is meant to be informational only. You should see your ophthalmologist right away if you experience sudden loss of vision or any visual disturbances.
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© 2012 Melissa Flagg COA OSC