Corneal Dystrophies

Eyesight depends on a complex series of interactions between the different parts of the eye. (Read about "The Eye") The cornea is the clear covering of the front of the eye. A corneal dystrophy is a condition in which one or more parts of the cornea lose their normal clarity due to a buildup of cloudy material. There are over 20 corneal dystrophies that affect all parts of the cornea. The National Eye Institute (NEI) says these diseases share many traits:

• they are usually inherited
• they affect the right and left eyes equally
• they are not caused by outside factors, such as injury or diet
• most progress gradually
• most usually begin in one of the five corneal layers and may later spread to nearby layers
• most do not affect other parts of the body, nor are they related to diseases affecting other parts of the eye or body
• most can occur in otherwise totally healthy people, male or female

Corneal dystrophies affect vision in widely differing ways. Some cause severe visual impairment, while a few cause no vision problems and are discovered during a routine eye examination. Other dystrophies may cause repeated episodes of pain without leading to permanent loss of vision.

Some of the most common corneal dystrophies include Fuchs' dystrophy, keratoconus, lattice dystrophy and map-dot-fingerprint dystrophy. See more on each below.

Fuchs' Dystrophy

Fuchs' dystrophy is a slowly progressing disease that usually affects both eyes and is slightly more common in women than in men, according to the National Eye Institute (NEI). Although doctors can often see early signs of Fuchs' dystrophy in people in their 30s and 40s, the disease rarely affects vision until people reach their 50s and 60s.

Fuchs' dystrophy occurs when endothelial cells gradually deteriorate without any apparent reason. As more endothelial cells are lost over the years, the endothelium becomes less efficient at pumping water out of the stroma. This causes the cornea to swell and distort vision. Eventually, the epithelium (the cornea's outer layer) also takes on water, resulting in pain and severe visual impairment.

Epithelial swelling damages vision by changing the cornea's normal curvature and causing a sight-impairing haze to appear in the tissue. Epithelial swelling will also produce tiny blisters on the corneal surface. When these blisters burst, they are extremely painful.

At first, a person with Fuchs' dystrophy will awaken with blurred vision that will gradually clear during the day. This occurs because the cornea is normally thicker in the morning; it retains fluids during sleep that evaporate in the tear film while we are awake. As the disease worsens, this swelling will remain constant and reduce vision throughout the day.

When treating the disease, doctors will try first to reduce the swelling with drops, ointments or soft contact lenses. They also may instruct a person to use a hair dryer, held at arm's length or directed across the face, to dry out the epithelial blisters. This can be done two or three times a day.

NEI says that when the disease interferes with daily activities, a person may need to consider having a corneal transplant to restore sight. (Read about "Transplants") The short-term success rate of corneal transplantation is quite good for people with Fuchs' dystrophy. However, some studies suggest that the long-term survival of the new cornea can be a problem.

Keratoconus

This disorder, a progressive thinning of the cornea, is the most common corneal dystrophy in the U.S., affecting one in every 2000 Americans. It is more prevalent in teenagers and adults in their 20s, according to the National Eye Institute (NEI). Keratoconus arises when the middle of the cornea thins and gradually bulges outward, forming a rounded cone shape. This abnormal curvature changes the cornea's refractive power, producing moderate to severe distortion (astigmatism) and blurriness (nearsightedness) of vision. (Read about astigmatism and nearsightedness in "Refractive Errors") Keratoconus may also cause swelling and a sight-impairing scarring of the tissue.

Studies indicate that keratoconus stems from one of several possible causes:

• An inherited corneal abnormality - About seven percent of those with the condition have a family history of keratoconus. (Read about "Family Health History")
• An eye injury - excessive eye rubbing or wearing hard contact lenses for many years, as well as direct injury to the eye. (Read about "Eye Injury")
• Certain eye diseases - such as retinitis pigmentosa, retinopathy of prematurity and vernal keratoconjunctivitis. (Read about "Retinitis Pigmentosa" "Retinopathy of Prematurity")
• Systemic diseases - such as Leber's congenital amaurosis, Ehlers-Danlos syndrome, Down syndrome (Read about "Down Syndrome") and osteogenesis imperfecta.

Keratoconus usually affects both eyes. At first, people can correct their vision with eyeglasses. But as the astigmatism worsens, they must rely on specially fitted contact lenses to reduce the distortion and provide better vision. Although finding a comfortable contact lens can be an extremely frustrating and difficult process, it is crucial because a poorly fitting lens could further damage the cornea and make wearing a contact lens intolerable.

In most cases says NEI, the cornea will stabilize after a few years without ever causing severe vision problems. But in about 10 to 20 percent of people with keratoconus, the cornea will eventually become too scarred or will not tolerate a contact lens. If either of these problems occurs, a corneal transplant may be needed. This operation is successful in more than 90 percent of those with advanced keratoconus. Several studies have also reported that 80 percent or more of these patients have 20/40 vision or better after the operation. (Read about "Clinical Studies")

Lattice Dystrophy

Lattice dystrophy gets its name from an accumulation of amyloid deposits, or abnormal protein fibers, throughout the middle and anterior stroma. During an eye examination (Read about "Eye Exams"), the doctor sees these deposits in the stroma as clear, comma-shaped overlapping dots and branching filaments, creating a lattice effect. Over time, the lattice lines will grow opaque and involve more of the stroma. They will also gradually converge, giving the cornea a cloudiness that may also reduce vision.

In some people, these abnormal protein fibers can accumulate under the cornea's outer layer - the epithelium. This can cause erosion of the epithelium. This condition is known as recurrent epithelial erosion. The National Eye Institute (NEI) says these erosions:

• alter the cornea's normal curvature, resulting in temporary vision problems
• expose the nerves that line the cornea, causing severe pain. Even the involuntary act of blinking can be painful.

To ease this pain, a doctor may prescribe eye drops and ointments to reduce the friction on the eroded cornea. In some cases, an eye patch may be used to immobilize the eyelids. With effective care, these erosions usually heal within three days, although occasional sensations of pain may occur for the next six-to-eight weeks.

By about age 40, some people with lattice dystrophy will have scarring under the epithelium, resulting in a haze on the cornea that can greatly obscure vision. In this case, a corneal transplant may be needed. Although people with lattice dystrophy have an excellent chance for a successful transplant, the disease may also arise in the donor cornea in as little as three years. NEI says in one study, about half of the transplant patients with lattice dystrophy had a recurrence of the disease from between two to 26 years after the operation. Of these, 15 percent required a second corneal transplant. Early lattice and recurrent lattice arising in the donor cornea responds well to treatment with the excimer laser.

Although lattice dystrophy can occur at any time in life, the condition usually arises in children between the ages of two and seven.

Map-Dot-Fingerprint Dystrophy

This dystrophy occurs when the epithelium's (the cornea's outer layer) basement membrane develops abnormally (the basement membrane serves as the foundation on which the epithelial cells, which absorb nutrients from tears, anchor and organize themselves). When the basement membrane develops abnormally, the epithelial cells cannot properly adhere to it. This, in turn, causes recurrent epithelial erosions, in which the epithelium's outermost layer rises slightly, exposing a small gap between the outermost layer and the rest of the cornea.

Epithelial erosions can be a chronic problem. They may alter the cornea's normal curvature, causing periodic blurred vision. They may also expose the nerve endings that line the tissue, resulting in moderate to severe pain lasting as long as several days. Generally, the pain will be worse on awakening in the morning. Other symptoms include sensitivity to light, excessive tearing and foreign body sensation in the eye.

Map-dot-fingerprint dystrophy, which tends to occur in both eyes, usually affects adults between the ages of 40 and 70, although it can develop earlier in life. Also known as epithelial basement membrane dystrophy, map-dot-fingerprint dystrophy gets its name from the unusual appearance of the cornea during an eye examination. Most often, the affected epithelium will have a map-like appearance, i.e., large, slightly gray outlines that look like a continent on a map. There may also be clusters of opaque dots underneath or close to the map-like patches. Less frequently, the irregular basement membrane will form concentric lines in the central cornea that resemble small fingerprints.

The National Eye Institute (NEI) says typically, map-dot-fingerprint dystrophy will flare up occasionally for a few years and then go away on its own, with no lasting loss of vision. Most people never know that they have map-dot-fingerprint dystrophy, since they do not have any pain or vision loss. However, if treatment is needed, doctors will try to control the pain associated with the epithelial erosions. They may patch the eye to immobilize it or prescribe lubricating eye drops and ointments. With treatment, these erosions usually heal within three days, although periodic flashes of pain may occur for several weeks thereafter. Other treatments include anterior corneal punctures to allow better adherence of cells; corneal scraping to remove eroded areas of the cornea and allow regeneration of healthy epithelial tissue; and use of the excimer laser to remove surface irregularities.