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Keratoconus

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What Is Keratoconus?

Keratoconus ("ker-ah-toe-KOH-nus") is a deformity of the clear front surface of the eye (cornea) in which the center of the cornea bulges or becomes cone-shaped. This condition can cause significant vision loss that cannot be corrected with glasses.

The term keratoconus derives from the Greek and Latin word roots "kerato" (cornea) and "conus" (cone-shaped), or cone-shaped cornea.

The cornea accounts for about 70 percent of the focusing power of the eye. The curvature of the cornea, when properly combined with the focusing power of the lens inside the eye, allows for images to appear clearly on the retina. If the corneal shape changes, such as occurs during keratoconus, then the image cannot be properly focused.

How is Cornea Constructed?

The cornea is made of a series of interwoven layers of tissue. These layers are held together by collagen. Inside the eye there is constant pressure, similar to blood pressure throughout the rest of the body. The collagen provides the strength of the cornea, allowing the eye to maintain a constant curved shape despite the pressure in the eye. If the collagen becomes weakened, then the pressure in the eye can cause the eye to bulge.

What Causes It?

The exact cause of keratoconus is unknown, but genetics appear to play a role.

Another theory is that it is caused by defects in the collagen, or destruction of collagen fibers or bonds in the cornea due to abnormal enzyme activity.

Who Is at Risk?

The prevalence of keratoconus worldwide is estimated to be 50 to 230 cases per 100,000 people (or affecting approximately one person in a thousand).

People with a family history of the condition are at greater risk, with research showing that 6 to 8 percent of keratoconus patients have a positive family history for the condition.

Atopic diseases, including allergies, asthma and eczema, also have been associated with an increased risk of keratoconus. And some studies suggest vigorous rubbing of the eyes contributes to the progression of the disease.

People affected by Down syndrome also are at greater risk of keratoconus, with studies estimating that up to 15 percent of people with Down syndrome have the eye disease.

LASIK and Keratoconus

Corneal ectasia ("ek-TAY-see-ah"), a bulging and deformation of the cornea that closely resembles keratoconus, is an infrequent but serious complication of LASIK surgery. This condition is thought to be caused by weakening of the inner layers of the cornea during refractive surgery. With a weakened cornea, the corneal surface cannot properly withstand the internal pressure of the eye and the pressure causes the corneal surface to bulge outward in an irregular shape, simulating keratoconus-like symptoms.

Keratoconus Symptoms

Keratoconus symptoms usually begin around puberty, and include:

  • Increasing nearsightedness and astigmatism
  • Blurred vision that cannot be fully corrected with eyeglasses
  • Glare or sensitivity to light
  • Ghost images or double vision
  • Streaks or cone-shaped halos around lights
  • Night vision problems
  • Itchy eyes

Keratoconus generally progresses slowly over a number of years. But the course and severity of keratoconus can vary significantly.

The condition usually occurs in both eyes, but it often is significantly worse in one eye than the other.

Detecting Keratoconus

The only way to know for sure if you have keratoconus is to undergo a comprehensive eye exam by an optometrist or ophthalmologist.

During the exam, your eye doctor or a trained assistant will measure the curvature of your cornea with an instrument called a keratometer. If your keratometry readings are abnormally steep, this may indicate keratoconus or a mild, non-progressive variety of the condition called forme fruste ("form-froost") keratoconus.

Another, more advanced device called a corneal topographer might also be used to measure your cornea. Corneal topography provides a highly detailed map of the shape of the front surface of your eye to help your eye doctor detect even the earliest signs of keratoconus.

Your eye doctor also will examine your cornea under high magnification with a table-mounted biomicroscope (or "slit lamp"), which can project a thin slit of light onto the eye and reveal thinning and bulging of the central cornea and other signs of keratoconus.

Significant changes to your eyeglasses prescription (a significant increase in nearsightedness and astigmatism) and a decrease in your best-corrected visual acuity on an eye chart also can suggest keratoconus.

Also, because keratoconus causes thinning of your cornea, your eye doctor might use an automated instrument to measure corneal thickness, a procedure called pachymetry ("pah-KIM-eh-tree").

How Is Keratoconus Treated?

When keratoconus progresses to the stage that eyeglasses no longer can adequately correct your vision, treatment usually begins with fitting of specially designed rigid gas permeable (RGP or GP) contact lenses or hybrid contact lenses, which are larger lenses with a GP center surrounded by a soft lens "skirt."

(Your eye doctor may recommend GP or hybrid contacts even before significant vision changes occur to attempt to prevent early keratoconus from worsening, by preventing the corneal surface from bulging outward,)

The rigid optical zones of GP and hybrid lenses lay over the deformed corneal tissue of a keratoconic eye to create a smooth, symmetric optical surface to better focus light into the eye. Even significant cases of keratoconus often can be successfully treated with GP or hybrid contact lenses that are specially designed for irregular corneas.

When contact lenses cannot sufficiently correct vision problems caused by keratoconus, one of a variety of surgical options usually is recommended. Options include implantation of intrastromal corneal ring segments (e.g. Intacs), corneal cross-linking procedures, or a partial or full-thickness corneal transplant.

Can Keratoconus Be Prevented?

There is no known way to prevent keratoconus.

However, if you have allergies and itchy eyes, you should avoid rubbing your eyes vigorously, as this has been associated with the development and progression of keratoconus symptoms in some individuals.

If you wear contact lenses, be sure to have regular exams to make sure your lenses fit you properly and are not causing too much stress on your corneas.

Resources:
1. A locus for autosomal dominant keratoconus: linkage to 16q22.3-q23.1 in Finnish families. Investigative Ophthalmology & Visual Science. October 2002.
2. Bilateral recurrent self-induced keratoconus. Eye & Contact Lens. November 2008.
3. Health care management of adults with Down syndrome. American Family Physician. September 2001.
4. "Non-inflammatory Ectactic Disorders." Cornea (chapter 78). Mosby. New York. 2005.
5. Keratoconus: diagnosis and treatment. International Ophthalmology Clinics. Winter 1997.
6. Corneal topography of early keratoconus. American Journal of Ophthalmology. August 1989.
7. Baseline findings in the Collaborative Longitudinal Evaluation of keratoconus (CLEK) Study. Investigative Ophthalmology & Visual Science. December 1998.

Note: This information is for general education purposes only. It is not to be used as a substitute for medical advice from your eye doctor or refractive surgeon.

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