Diabetic Retinal Disease

Understanding Diabetic Retinal Disease
Diabetes mellitus is a disease in which the body cannot effectively use its main energy source, a simple sugar called glucose. This disorder has traditionally been differentiated into two types, insulin-dependent (Type I) and insulin-independent (Type II). In Type I diabetes mellitus, the body cannot produce enough (or any) insulin, a hormone normally secreted by the pancreas that is critical for permitting glucose to enter the cells of the body where it can metabolized for energy. This disease usually strikes younger individuals, and the classic patient is generally thin. Insulin-independent diabetes usually strikes older, often overweight individuals. These patients actually produce abnormally large amounts of insulin but their cells seem unable to use the insulin effectively. Whereas insulin-dependent individuals must take injections of insulin every day, insulin-independent individuals more often take oral medications. In addition, weight-loss and careful dieting in these patients often can lower or even eliminate the need for medication. In either condition, however, when the glucose in the blood is elevated and not brought under control, biochemical changes occur in the smallest blood vessels of the body, the capillaries, and cause secondary widespread damage to the body. The retina, located at the back wall of the eye and containing the neural tissue involved in transmitting visual information to the brain, is filled with tiny capillaries and is therefore often ravaged by the effects of diabetic capillary disease.

Diabetic Effects on Small Vessels
Although the actual mechanism is unclear, it is known that diabetes makes the tiniest blood vessels in the body, the capillaries, extremely unhealthy. It is believed that elevated levels of blood glucose chemically modify proteins in the cells lining the inner surface of these thin vessels. These cells become unable to function correctly and ultimately die. As this occurs, the blood vessels lose their structural integrity and expand. In addition, they lose the ability to prevent blood from clotting within them and become clogged. Ultimately, this leads to a severe loss of small blood vessels and slow starvation and oxygen-deprivation for the portions of the body they normally would supply. In the retina, this leads to sickness in the cells involved in transmitting light images to the brain. Worse yet, because these sick capillaries tend to fall apart, they often leak blood into the retina and this blocks the light from getting to adjacent healthy retinal cells.

Neovascularization
As the blood vessels slowly die, they release chemicals that have the ability to stimulate the growth of new blood vessels, a phenomenon known as neovascularization. But this does not supply the tissues with replacement capillaries, because these new vessels are not healthy and do not function correctly. In fact, they leak and bleed even more than the sick original vessels. Worse yet, the leaked blood products clot and form scar tissue. This scar tissue tends to contract, and it can pull on whatever ocular tissue to which it attaches. If this ocular tissue is the retina, it can pull the retina off of the back of the eye, a phenomenon known as tractional retinal detachment. Neovascularization will continue unchecked so long as there are starving blood vessels to produce the growth factor that stimulates it.

Clinically Significant Macular Edema
When sick or new blood vessels leak in the retina, the fluid tends to accumulate in the region known as the macula, the portion of the retina used for fine vision. This condition is called edema. Fluid in the eye can be absorbed by healthy capillaries but because the center of the macula (the fovea) normally has no capillaries, the retina has great difficulty removing fluid from this region and vision can be lost. In addition, as the blood vessels surrounding the macula die, the area from which no fluid can be absorbed increases in size. Clinically significant macular edema is a condition in which sufficient fluid has accumulated in the macula to threaten or actually diminish vision. It is often treatable with laser therapy.

Other Ocular Problems of Diabetes
Diabetes also damages large blood vessels and can increase a patient's chance of sustaining a blockage of their retinal vessels, a stroke in the eye. When a vein is occluded, a great deal of hemorrhage can occur in the eye. And although many eyes survive this event, many others lose a great deal of tissue. In these latter cases, the dying tissues release the growth factors for abnormal new blood vessels and neovascularization will result. Neovascularization can affect parts of the eye other than the retina, because the growth factor flows in the liquid media throughout the eye. When this growth factor is deposited upon the iris, abnormal new vessels can grow there and cause the iris to scar down to the lens. This can trap the aqueous humor (the fluid produced behind the iris) and lead to dangerously elevated pressures in the eye (glaucoma). In addition, the growth factor can deposit in the trabecular meshwork where the aqueous humor drains from the front of the eye and cause new vessels to block the trabecular meshwork. This will prevent fluid from draining away from the eye and can also lead to glaucoma. Because diabetes makes blood vessels throughout the entire body sick, it can lead to strokes in the tiny vessels that supply the nerves controlling the eye muscles. When this occurs, a patient will complain of double vision (diplopia) and often eye pain as well. Finally, diabetes can also decrease vision by speeding the formation of cataracts.

Laser Therapy for Diabetics
When your retina specialist sees some of the problems caused by diabetes in the eye, they can often offer laser therapy to help stem the progression of the disease. First, the specialist must determine whether abnormal blood vessels are growing, and will often use a special photographic technique called fluorescein angiography to map these vessels. The specialist can then use a laser to seal the leaking vessels. In clinically significant macular edema, this will enable the healthy vessels to catch up with the leakage and adsorb the fluid; vision can improve significantly in this case. When the retinologist sees neovascularization in the retina, iris or trabecular meshwork, they know that the retina has many dying cells and he will use the laser to kill the dying cells more quickly, leaving more food and oxygen for the remaining cells and thereby removing the impetus for the retina to secrete the abnormal vessel growth factor.