DIABETIC EYE DISEASE

by Prema Abraham, MD

Dr. Abraham is director of vitreoretinal and retinovascular services, at Black Hills Regional Eye Institute, in Rapid City, South Dakota.

Introduction

Diabetes is a common medical problem which affects the body's ability to regulate blood sugar levels. Over a period of years, high blood sugar will damage small blood vessels in the body and often cause problems such as kidney failure, sensory abnormalities in the hands and feet, and eye problems. The focus of this presentation is to describe how diabetes affects vision, what specific factors contribute to visual loss, and the technological advances that are available today to evaluate and manage diabetic eye disease. Although in the worst cases an individual may suffer permanent loss of vision in one or both eyes, it is important to remember that most diabetics who carefully control their diabetes and get good eye care can prevent many of the visual complications of the disease.

Diabetes is present in approximately three percent of Americans, however individuals in special populations such as Native Americans and the elderly are much more likely to suffer from the disease. For example, nearly 50% of some Native American groups are affected, and among the nation's elderly, about 15% are affected. Because one form of diabetes (i.e., adult-onset, or type II diabetes) may be present in a person for several years before the diagnosis is established, some of the preventable complications may already be in the early or moderate stages when the diagnosis is finally made and treatment initiated.

What is Diabetes?

Diabetes is much more than having an elevated blood sugar level. It is a complex endocrine disorder which can affect many aspects of the body's metabolism which in turn have detrimental effects on a variety of vital organs. Left unchecked, diabetes may result in complete kidney failure and require the use of hemodialysis. The disease also causes heart disease and is a potent risk factor for heart attacks. Nerve endings can also be severely damaged, leaving the patient with a painful burning sensation in the hands and feet or causing numbness in the same areas. This loss of sensation places the diabetic at increased risk of hurting him/herself without being aware of the damage. The numbness, along with the detrimental effects that diabetes may have on the immune system, increases the risk of serious infection. Eye damage from diabetes may include any one or a combination of problems such as cataracts, glaucoma, loss of night vision, double vision, eye infections, fluctuating vision, and retinopathy, a broad category of problems affecting the retina.

There are two categories of diabetes, one which affects individuals at a younger age (juvenile onset, or type I) and the other which affects middle-aged and elderly adults (adult onset, or type II). Both types are associated with elevated blood sugars but the complications of the disease may vary in severity and rate of onset and progression depending on the type.

How Does the Eye Work?

To understand how diabetes affects the eye it is important to know how the normal eye functions. The eye works very much like a camera, with a focusing lens in the front and the film in the back. In the eye, the retina plays the role of a camera's film, receiving the image of the object at which the camera is focused. The retina, which is actually a direct extension of brain tissue, transmits the visual information through the optic nerve to areas of the brain which process it into vision. In a camera, no matter how clear or strong the lens and how perfectly focused the image may be, if the film is not working well the camera will not take good pictures. Similarly in the eye, if the retina is diseased vision will be impaired no matter how clear and strong the lens may be. Furthermore, if the space between the lens and the retina is obscured with blood or other material, vision will be impaired.

When a diabetic goes to the ophthalmologist, the doctor looks carefully at the retina. With special instruments the blood vessels which normally travel through the optic nerve can be viewed as they branch out and nourish the retina. One small but particularly important area of the retina, called the macula, is sometimes referred to as the "sweet spot" of the retina. This "sweet spot" is the area responsible for detailed vision such as is required for reading, driving, needle threading and other similarly detailed visual functions. The remainder of the retina is for side vision -or peripheral vision- and not useful for fine vision. In order to maintain meaningful vision for activities such as reading it is essential that the macula remain healthy and unobstructed. If the macula does become damaged or covered with blood the eye does not necessarily become blind. It is possible that the side vision will remain fully functional but reading and identification of faces will be extremely difficult or impossible. For example, it may be possible to perceive that someone is approaching but without the "sweet spot" it will be impossible to visually identify that person.

What is Retinopathy?

Retinopathy is the general name given to diseases of the retina. In diabetics, it can take a variety of forms and may affect both type I and type II diabetics. Type I diabetics are usually free of retinopathy for the first five years after diagnosis because there is rarely much time between disease onset and diagnosis. On the other hand, because they may have had the disease for several years prior to diagnosis, type II diabetics may already have diabetic eye problems at the time of diagnosis or shortly thereafter.

The principal problem of the retina caused by diabetes involves the very fine blood vessels which nourish the nerve tissue. High blood sugar causes these vessels to become damaged and then leak fluid and fatty material into the nerve tissue of the retina. The retina becomes swollen and does not function normally. This form of retinopathy is called background or non-proliferative retinopathy. The medical term for swelling is edema; when this process involves the "sweet spot" of the retina, it is called macular edema.

Another, more serious form of diabetic retinopathy, is called proliferative retinopathy. Like the non-proliferative form, this form is initiated by high blood sugars over a period of years. However, in the proliferative form the damage inflicted on the small retinal blood vessels results in impaired blood flow to the retinal tissue. Some vessels actually close off completely and deprive an area of the retina of much needed oxygen and other blood-borne nutrients. Consequently, the oxygen-deprived tissue sends out a signal which stimulates the growth of new blood vessels. Unfortunately the new vessels are distinctly abnormal and are the source of many serious diabetic eye complications.

How is Retinopathy Treated?

As with many chronic conditions, the best management strategy is based on prevention. For the diabetic this means careful and consistent blood sugar control. Equally important to the diabetic individual is frequent eye evaluations which may permit early detection and treatment of retinopathy. When the eye doctor examines the retina of a diabetic patient and evidence of retinopathy is discovered the next step usually involves obtaining special photographs of the retinal blood vessels. This process, called fluorescein angiography, utilizes a fluorescent dye injected into an arm or hand vein which then circulates throughout the body. When this dye flows through the retinal blood vessels, a series of photographs can be taken with specialized camera equipment. The photos help locate areas of abnormal blood vessels and guide the treatment process.

One straightforward method for managing leaking blood vessels is to seal them with a highly focused beam of laser energy. In effect, zapping leaking retinal vessels stops further leaking of material from the vessels thereby limiting the detrimental effect on vision. When the angiogram identifies discrete areas of leaking vessels the laser treatments can be confined to these specific areas. This is called focal treatment. When the leaking is not due to one or two specific areas a more broad treatment is required.

When the problem of leaking vessels is widespread and excess fluid accumulates in the retinal tissue it becomes necessary to apply laser treatment scattered over a larger area. This is called grid treatment, since the laser spots are applied in a grid pattern. This method, like the focal treatment described above, is useful in the treatment of macular edema.

In either situation the goal of treatment is to stop the vessels damaged by the diabetes from continuing to leak and to stop the progression of the visual impairment. Unfortunately this treatment cannot restore vision that is already lost.

The other, more serious form of diabetic retinopathy, proliferative retinopathy, is also treated with a laser. However, whereas the treatments in the non-proliferative form are directed at sealing leaking vessels and not destroying tissue, the objective in the treatment of proliferative retinopathy is to actually destroy unhealthy, damaged retinal nerve tissue. To accomplish this, a deeper laser burn, of larger size, is used. This may seem counterproductive to the preservation of vision, but the outcome of these destructive treatments does ultimately minimize the cumulative loss of vision. Remember, it is the blood and oxygen deprived retinal tissue that stimulates the growth of abnormal vessels which never provide any benefit to the retina.

Destroying the unhealthy retinal tissue eliminates the stimulus for the growth of new, abnormal vessels. These abnormal vessels are fragile and can bleed into the interior of the eye, obscuring vision. Diabetics with proliferative retinopathy may awaken in the morning with new floaters or other visual changes because of bleeding which occurred during the night. Because of rapid eye movements associated with dreaming, diabetics are particularly vulnerable to the breaking of the abnormal vessels while sleeping.

Treatment of proliferative retinopathy is accomplished by applying hundreds of destructive spots of laser across much of the peripheral retina. Even though there is relatively widespread treatment there is very little, if any, sensation of fine visual loss because only the side viewing retina is treated. Furthermore, the area of the retina being treated is already diseased due to the diabetes. Patients may notice a reduction in side vision or night vision and difficulty with light/dark adaptation following this type of treatment. This treatment is called panretinal photocoagulation or PRP, and usually requires multiple treatment sessions to complete. As mentioned above, laser treatment of proliferative retinopathy is directed at preventing progression of vision loss and does not result in recovery of vision already lost.

Today, new multi-wavelength lasers are available which permit the surgeon to select the most appropriate type of laser beam for the specific problem the patient may have. This flexibility helps the surgeon maximize the beneficial effects while minimizing the detrimental effects of laser treatment.

Another complication of proliferative retinopathy is the formation of scar tissue in the eye associated with the growth of the abnormal vessels. As the scar tissue grows it may pull on the retina and tear it off of the back of the eye. This is called a traction retinal detachment and can be quite severe. A traction retinal detachment may cause severe loss of vision. In some cases it ultimately results in loss of the eye altogether. Panretinal photocoagulation, by destroying blood and oxygen deprived retinal tissue, reduces the stimulus for abnormal blood vessels growth thereby limits the development of scar tissue. In turn, less scar tissue translates into reduced risk of retinal detachment.

What Other Treatments are Available?

In some cases bleeding into the eye may occur despite laser treatment. If the amount of bleeding is significant it can obscure vision. When this happens the blood can be surgically removed in the operating room by a retinal surgeon. In addition to removing the blood from within the eye, this surgery has the added benefit of removing the entire jelly-like substance which normally occupies the space in the eye chamber. By removing this substance, the structure into which the abnormal vessels grow is eliminated, and therefore future abnormal growth is quite rare. The jelly-like substance is called the vitreous and the procedure is called a vitrectomy. Vitrectomy combined with other microsurgical techniques is also the procedure used to repair traction retinal detachments.

Conclusion

Diabetic eye disease is an important cause of severe vision loss in patients under age 60. However, with regular eye exams problems can be detected in their early stages. Early detection means that treatment can be started before serious visual loss occurs. All patients with diabetes should have their eyes examined at least once each year, and individuals with more advanced disease may benefit from more frequent evaluations. These exams should be performed by an experienced eye care professional who will dilate the pupils with special eye drops so that the entire retina can be more easily viewed. If laser treatment or other surgery is required, then one may be referred to a retinal specialist.

Through education and cooperation with medical and eye care professionals, the diabetic patient can remain optimistic about successfully managing the disease and its ocular complications.