A corneal dystrophy (dis-truh-fee) is the gradual deteriorations of one or more layers of the cornea, the dome-shaped, clear surface of the eye, located in front of the iris. The cornea is the first surface light strikes on its way through the ocular structures to the retina, in the back of the eye. It is the primary refractive surface, which means it is responsible for most of the focusing of light entering the eye.
There are over many different types of corneal dystrophies, but they share some common diagnostic clues. First, they affect both eyes and are not caused by infections, trauma or dietary issues. They do not affect other areas of the body, nor do other diseases or conditions cause them; they can occur in people who are quite healthy otherwise. They progress, but do so gradually.
Symptoms include pain or irritation, worse in either the morning or evening that can be dependent on episodes of disease activity, and varying levels of vision disruption.
Dystrophies are most often inherited and can occur at an early age, even in infancy. There are a variety of dystrophies, which affect different layers of the cornea. The two most common are epithelial basement membrane dystrophy and endothelial cell dystrophy.
Epithelial Basement Membrane Dystrophy
The epithelium is the top layer of the cornea, which is made up of several layers of flat, specialized skin cells that are normally of uniform size and arranged regularly across the surface. The basement membrane forms a barrier between the epithelium and the corneal stroma (see illustration). It is normally smooth and flat, with an even thickness throughout.
Sometimes referred to as map-dot or fingerprint dystrophy, it generally occurs in adults after the age of 40, although it can appear as late as age 70. The name of this dystrophy comes from the appearance of the cornea under high magnification in the biomicroscope, a precision optical instrument. In this form of dystrophy, the basement membrane becomes thicker and irregular in shape, so that the cells begin to break apart and disintegrate. As the membrane begins to fall apart, light can no longer be transported through the cornea to a sharp focus.
This dystrophy causes redness of the eye and discomfort that can range from mild irritation to severe pain, with these symptoms being worse at the beginning of the day, because sleep does not allow blinking, which keeps the surface of the cornea lubricated with tears.
Treatment starts with efforts to supplement the natural lubricants with artificial tears, used periodically during the day and just before sleep. At night, an ointment or gel-type lubricant may be recommended for better morning comfort; these are not typically used during the day because they blur the vision. Patients should follow their eyecare practitioner’s advice for the exact type of eye drops to use, because different formulations are appropriate for treatment, while others may do more harm than good. A vaporizer to increase the humidity in the home or workplace gives some relief, as does an eye patch; special contacts called bandage lenses may be recommended to help heal the membrane and restore its natural smooth character. In more advanced or severe cases, surgical debridement (removal of cells) of the corneal epithelium and basement membrane can be helpful to re-establish healthy tissue.
The refractive surgery called PRK (photorefractive keratectomy, called phototherapeutic keratectomy in this setting) can also be used to remove the affected cells. (The two different labels are used to differentiate between PRK performed for the reduction of nearsightedness, which is a cosmetic procedure, and PRK performed for treatment of a medical condition. The latter are usually covered by medical insurance, while cosmetic procedures are not.)
Endothelial Cell Dystrophy
This dystrophy affects the inner layer of the cornea and is sometimes called Fuch’s Dystrophy (Few-k’s). It is most common in adults of middle-age and affects women more often than men. It is also thought to be hereditary, but can occur without a previous family history, leaving the cause unknown.
The endothelium is responsible for removing water from the corneal tissues, which is the main factor in maintaining its transparency; when fluid accumulates, the cornea becomes cloudy, affecting vision. The blurred vision is usually worse in the morning, because fluid cannot evaporate away from the ocular surface as it does during the day.
As the dystrophy progresses, corneal swelling due to excess moisture can cause blister-like lesions on the front of the cornea, which are known as bullae (bull-eye); when this occurs it is referred to as bullous keratopathy.
Treatment of this particular dystrophy is aimed at removing moisture from the eyes and is done with the use of specific ointments and hypertonic eye drops. Patients should not use eye drops other than those specifically recommended by their eyecare practitioner, as most eye drops and lubricants add moisture to the eyes, not remove it.
In some patients, Fuch’s Dystrophy may occur along with glaucoma, an eye disease that is characterized by increased pressure within the eye. Increased pressure makes the endothelium less efficient, so medications aimed at reducing intraocular pressure (IOP) may be prescribed.
If cornea bullae are present and begin to rupture, this will cause increased pain and significant vision loss. In this case, a corneal transplant may be needed, although bandage lenses to help relieve the irritation and pain are sometimes helpful in the short term.
As in map-dot dystrophy (above), an alternate treatment with a laser may be recommended, this time called deep lamellar endothelial keratoplasty (DLEK), which affects the inner layers of the cornea, leaving the other areas untouched. Recently, an advanced form of DLEK using a femtosecond laser has shown encouraging results. (The femtosecond laser is being introduced into other types of ocular surgery as well, such as cataract extraction.)
Unfortunately, most corneal dystrophies are progressive and affect both eyes, but new strategies and treatments using laser-assisted surgery have shown promise for treatment.