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Keratoconus

Keratoconus, or "conical cornea" (from kerato- cornea and conus cone), is an eye condition or disease in which the cornea thins and changes shape to be more like a cone than a parabola.

Symptoms and diagnosis

Keratoconus can cause the vision to become distorted quite badly, with "ghosting", "streaking", and light sensitivity all often reported. The exact nature of the visual distortion introduced by kerataconus is most clearly seen with a point of light on a dark background – instead of seeing one moon the person may see over 100 images spread out in a fairly complex and random pattern. The pattern does not change from day to day, but over the seasons it often takes on new forms. Doctors tend not to ask exactly what the patient sees, but rather test the vision on a standard Snellen chart of progressively smaller letters. Definitive diagnosis is obtained using corneal topography, a non-invasive visualization of the shape of the surface of the cornea.

The visual distortion comes from two sources, one being the irregular deformation and many bumps on the surface of the cornea; the other being scarring that occurs on the exposed highpoints. Although some think the scarring is from wear on abrasion by contact lenses, the phenomena appears in those who do not use lenses and as such appears to be an aspect of the corneal degradation.

Keratoconus

Keratoconus and the associated vision loss, if in both eyes, can affect the person's ability to legally drive a car and function normally. Corrective lenses though, in most cases, allow the person to still drive a car.

Prevalence and cause

Keratoconus is a rare disease – it is estimated that 1 person out of 1,500 is affected – and seems to have genetic components. It is sometimes called the "academic's eye disease" because two-thirds of the people afflicted with it have IQs above 130. Keratoconus is also diagnosed more often in people with Down Syndrome, though the reasons for this link have not yet been determined. Keratoconus is a member of the family of atopic diseases, which include asthma, allergies, and eczema, and it is not uncommon for several or all of these diseases to affect one person. Science has no definitive explanation of what causes kerataconus.

Most people who go on to develop keratoconus will have normal vision or mild astigmatism at the onset of puberty, but will be diagnosed as having the disease in the late teenager years or early 20s. Vision will seem to fluctuate over a period of months, driving patients to change spectacle prescriptions frequently but as the condition worsens, contact lenses are required.

Contact lenses

Contact lenses improve vision by means of tear fluid filling the gap between the irregular corneal surface and the smooth regular inner surface of the lens, thereby creating the effect of a smoother cornea. Traditionally, these lenses have been the "hard" or gas-permeable contact lens variety, although recently somewhat effective "soft" or hydrophilic lenses have become available. Also, recently developed are hybrid lenses which are hard in the centre and soft around the edges. These soft or hybrid lenses do not work for everyone.

Many special types of gas permeable contact lenses have been developed for keratoconus, and affected people should seek out both doctors specialized in conditions of the cornea, and contact-lens fitters who have experience with patients with keratoconus. One popular brand is the Boston Gas Permeable lens.

Some patients also find good vision correction and comfort with a "piggyback" lens combination, where gas permeable "hard" lenses are worn over soft lenses, both providing a degree of vision correction. Understandably, fitting this lens combination requires experience on the part of the lens fitter, and patience on the part of the keratoconic patient. Scleral lenses are also sometimes used; these lenses cover a larger portion of the corneal surface.

Corneal transplant

In the worst cases, where vision correction is not possible, thinning of the cornea has become advanced, or scarring as a result of contact lens wear causes problems of its own, a corneal transplantation or 'penetrating keratoplasty' is required. The corneal transplant surgery uses a special device called a trephine which is like a corneal cookie-cutter. It cuts a button of tissue from the eye. The corneal surgeon then sews the donor cornea to the existing eye tissue, usually using a combination of running and individual sutures.

The cornea does not have a direct blood supply, so donor tissue does not have to be blood type matched. Eye banks check the donor corneas for any disease or cellular irregularities.

The acute recovery period can take four to six weeks and full post-operative vision stabilization often takes a year or more but most transplants are very stable in the long term. The sutures used usually dissolve over a period of three to five years but individual sutures can be removed during the healing process if they are causing irritation to the patient.

Cornea transplants are usually performed under sedation as outpatient surgery, and require careful follow-up with an eye surgeon for a number of years. Frequently, vision is greatly improved after the surgery, but even if the actual visual acuity does not improve, because the cornea is a more normal shape after the healing is completed, patients can more easily be fitted with corrective lenses.

Complications of corneal transplants are mostly related to vascularization of the corneal tissue and rejection of the donor cornea. Vision loss is rare, though difficult-to-correct vision is possible. When rejection is severe, repeat transplants are often attempted, and are frequently successful. Long-term outlook for corneal transplants is usually favorable once the initial healing period is completed and a few years have elapsed without problems.