Keratoconus (KC) is a disorder of the human eye that rarely causes blindness but can significantly interfere with vision. Sight is our most precious sense, and any diminution of it is regrettable. Keratoconus, or conical cornea, is a condition in which the normally round shape of the CORNEA is distorted and a cone-like bulge develops, resulting in significant distortion and visual impairment. This distortion has been compared to viewing a street sigh through your car windshield during a driving rain storm. Its progression is generally slow and can stop at any stage from mild to severe. As the KC progresses, the cornea bulges and thins, becoming irregular and sometimes forming scars.
|To understand keratoconus, we must first understand how the eye enables us to see and what role the cornea plays in this process.|
What is the cornea?
The eye is enclosed by a tough white sac, the sclera. The cornea is the transparent window in this white sac which allows light waves to carry the image of an object into the interior of the eye.
Light enters the eye through the surface of the cornea, which is the clear dome covering the colored part of the eye (iris). It is the part of the eye on which one puts a contact lens. The cornea's mission is to gather and focus visual images. Because it is out front, like the windshield of an automobile, it is subject to considerable abuse from the outside world. Particles of dust and grit inevitably find their way onto our outer eyes and irritate them, thereby stimulating the production of tears to wash foreign material away.
The cornea is masterfully engineered so that only the most expensive manmade lenses can match its precision. The smoothness and shape of the cornea is as vitally important to its proper functioning as is its transparency. If either the surface smoothness or the clarity of the cornea suffers, vision will become distorted.
Although appearing to be one clear membrane, the cornea is actually composed of five distinct layers of tissue, each with its respective function. The epithelium, thin outer layer, is a reliable barrier against corneal infection and usually must be damaged before an infectious agent can get a start in the stroma, middle layer of the cornea, which is made of collagen, connective tissue.
Although we can tolerate very large scars on our bodies, even a minor scar on the cornea can impair one's vision. No matter how well the rest of the eye is functioning, if the cornea is scarred, clouded, or distorted, vision will be affected.
What causes keratoconus?
Keratoconus (KC) is characterized by a bulging of the normally rounded cornea, causing distorted vision. KC has been known for at least two centuries, although it was not adequately described and distinguished from other similar corneal problems until the middle of the last century.
The causes of this affliction are still unknown despite our long familiarity with it. There has been no shortage of speculation or study, and numerous theories have been proposed.
One scientific view is that keratoconus is developmental (i.e., genetic or heredity) in origin. This suggests that it is the consequence of an abnormality of growth, essentially a congenital defect. Another theory is that KC represents a degenerative condition. Still a third postulation is that KC is secondary to some disease process. This idea sprang from the fact that it can occur in children who are not robust, and it usually affects both eyes. Another less widely held hypothesis suggests that the endocrine system may be involved. This idea gained some credence from the usual appearance of the disease at puberty. At the end of the day, no one really knows the exact cause of KC.
Heredity influences in KC are suggested by studies which show that about 7% of patients have other family members with the disease. Unless you can find evidence of KC in successive generations of your family, there is only a 1 in 10 chance of your having an offspring with some form of keratoconus.
|Bulging cornea due to thinning causes distortion that can be usually corrected with a rigid gas permeable contact lens.|
Who gets keratoconus?
The actual incidence of KC is not known. It is not one of the most common of eye diseases but it is by no means rare. It has been estimated to occur in 1 out of every 2,000 persons in the general population. The disease usually shows up in young people at puberty or in their later teen years. It is found in all parts of the United States and the rest of the world with no known significant geographic, cultural, or social pattern.
What are the signs and symptoms?
The first indication of KC is a blurring or distortion of vision. In the early stages it may be corrected with glasses; frequent changes in prescription may be required. The continued thinning of the cornea usually progresses slowly for 5 to 10 years and then tends to stop. Occasionally, it progresses rapidly; and, in the advanced stage, the patient may experience a sudden clouding of vision in one eye that clears over a period of weeks or months. This is called "acute hydrops" and is due to the sudden infusion of fluid into the stretched cornea. In advanced cases superficial scars form at the apex of the corneal bulge resulting in more vision impairment.
How is KC treated?
In the earliest stages of KC, ordinary eyeglasses may correct the mild myopia (near-sightedness) and astigmatism that is experienced. As the disease advances, rigid gas permeable contact lenses are the only way to correct vision adequately; most of the time this is a permanent remedy. These must be fitted with great care, and most KC patients need frequent checkups and frequent contact lens changes to achieve good vision and comfort. In some instances the use of a rigid gas permeable contact lens on top of a soft contact lens (so called piggy backing) provides and alternative solution. It should be noted that technological advances in both gas permeable contacts and soft contacts offer more and more possibilities for keratoconus patients. However, rigid gas permeable contact lenses still offer the best vision. There is a new lens out that is gas permeable in the center and has a soft lens "skirt" attached to the periphery. This helps those who can not tolerate the feeling of rigid gas permeable lens (RGP). Contact lens wearing is definitely the preferred method of managing KC until surgery is necessary.
|Photo on the left shows a patient with advanced keratoconus. Notice the central scarring that is severely affecting his vision. Contact lenses can no longer provide functional vision. This patient is on a waiting list for a corneal transplant.|
In only about 10% of KC cases, a corneal transplant becomes necessary. In this process much of the central cornea of the KC patient is removed and is replaced with the cornea of a recently deceased person. Eye banks in major cities collect healthy corneas from decedents and fulfill requests from eye surgeons for these corneas. This is a highly organized and sophisticated system, and corneal transplants are quite common and very successful operations. The probability of rejection is less than that of any other transplanted organ simply because the cornea has no blood supply. Rejection of other organs is usually due to incompatibility between the donor and the recipient and is mediated by blood cells.
While there is a 95% rate of success in corneal transplants for KC, this procedure, as with all operations, involves potentially serious risks. In cases where the first transplant is not successful, a second can be undertaken successfully but is extremely rare. Permanent loss of vision, though also extremely rare, can occur. Corneal transplantation is considered only in those cases in which contact lenses cannot be worn or provide inadequate vision despite the most skilled efforts.
While the surgical transplantation of a new cornea will resolve the basic problem of corneal surface irregularity, eyeglasses/contact lenses are usually needed for vision correction. In most cases RGP contact lenses will be required to correct the large amount of corneal distortion that is sometimes associated with the transplant regardless of the skill of the surgeon.
|What is involved in corneal transplant surgery?|
|Photo on right is post corneal transplant patient. Notice the black sutures from 6:00-9:00. They actually go all around to hold in the graft and are not removed for 9-12 months.|
In the late 20th century, eye surgery has reached a remarkable level of efficiency, effectiveness, and relative comfort. Corneal transplant surgery is the most successful of all transplant procedures, and techniques are improving constantly.
Either general or local anesthesia can be used in this procedure. The patient may need to spend a night or two in the hospital, will need a ride home, and, once there, will assume a quiet lifestyle for several days. Most people experience surprisingly little pain or discomfort following the surgery. Time off work will vary with individuals and the kind of work they do. Generally, with a sedentary job the patient should be back to work in a few days to a couple of weeks.
The surgeon uses a high-tech instrument to remove the distorted cornea and to cut out a similar "button" from the donor cornea. Looking through a surgical microscope, the surgeon places the donor cornea button in the round hole of the patient's cornea and stitches them together with sutures much finer than a human hair.
Bandages are usually removed about a week after the surgery, but the patient does not see clearly at that time. It takes six to nine months before vision stabilizes, and eyeglasses or contact lenses can be prescribed. Individual cases vary a great deal; therefore, timeframes mentioned above are very general.
Be alert to any changes in your eye condition or in your vision. If you experience blurring, scratchiness, irritation, watering, or any discharge, you must contact your eye care practitioner. This may signal a problem with your eyes' tolerance of your contact lenses or the need for refitting. You should, of course, take normal care of your eyes and avoid the use of any substance not prescribed by your eye care practitioner. Women should be careful about cosmetics. Everyone suffering from KC should wear goggles when swimming and safety glasses when engaged in yard work or athletics.