Keratoconus


Keratoconus has no known cure, and many people do not even know they have it because it begins as nearsightedness and astigmatism. It is a progressive disorder that may progress rapidly or sometimes take years to develop. It can severely affect the way we see the world, including simple tasks such as driving, watching TV, or just reading a book. Some keratoconus patients have described their vision as being “blind with light.”

Keratoconus is a non-inflammatory, self-limiting ectasia of the axial portion of the cornea. It is characterized by progressive thinning and steepening of the central cornea. As the cornea steepens and thins, the patient experiences a decrease in vision which can be mild or severe depending on the amount of corneal tissue affected.

For unknown reasons, patients with Keratoconus commonly have a combination of the following characteristics; dark haired , a history of atopic allergies and a "type A" personality.

Onset of keratoconus occurs during the teenage years with a mean age of onset of age 16 years, but onset has been reported to occur at ages as young as 6 years.  Keratoconus rarely develops after age 30 years. Keratoconus shows no gender predilection and is bilateral in over 90% of cases. In general, the disease develops asymmetrically: diagnosis of the disease in the second eye lags about five years after diagnosis in the first. The disease process is active for about five to 10 years, then it may be stable for many years. During the active stage, change may be rapid.

Typically, vision loss can be corrected early by spectacles; later, irregular astigmatism requires optical correction with rigid contact lenses. Contact lenses provide a uniform refracting surface and therefore improve vision. Contact lenses can improve vision, but they can also scar the cornea. Patients should be informed upon diagnosis that they will likely require contact lenses eventually. Although most patients can continue to read and drive, some feel quality of life is adversely affected. Patients need to know that eye examinations will be required annually or more frequently to monitor progression. About 20% of patients will eventually need a corneal transplant.

Etiology
The proposed etiology of keratoconus includes biochemical and physical corneal tissue changes, but no one theory fully explains the clinical findings and associated ocular and non-ocular disorders. It is possible that keratoconus is an end result or final common pathway of many different clinical conditions. It has been found in association with hereditary predisposition, atopic disease, certain systemic disorders, and long-term rigid contact lens wear.

Diagnosis
Identifying moderate or advanced keratoconus is fairly easy. However, diagnosing keratoconus in its early stages is more difficult, requiring a thorough case history, a search for visual and refractive clues and the use of instrumentation. Often, keratoconus patients have had several spectacle prescriptions in a short period, and none has provided satisfactory vision correction. Refractions are often variable and inconsistent. Keratoconus patients often report monocular diplopia or polyopia and complain of distortion rather than blur at both distance and near vision. Some report halos around lights and photophobia.

Many objective signs are present in keratoconus. Retinoscopy shows a scissoring reflex. Direct ophthalmoscopy may show a shadow. If the pupil is dilated and a +6.00 D lens is in the ophthalmoscopic system, the cone may appear as an oil or honey droplet when the red reflex is observed.

The keratometer also aids diagnosis. This instrument measures the curvature of the cornea by reflecting light rings off the cornea and distortion of the reflected rings can be observed by using the keratometer. The initial keratometric sign of keratoconus is absence of parallelism and inclination of the mires. These can easily be missed in mild or early cases. As the cornea advances, the mires appear smaller. To extend the range of the keratometer, an ancillary lens is placed on the front of the keratometer . If a +1.25 D lens is used, this extends the range to 60 D. To record a reading, 8 D is added to the drum reading (for example, if the drum reads 45 D, adding 8 D yields an actual reading of 53 D). A +2.25 D lens extends the range to 68 D by adding 16 D to the reading.

The photokeratoscope or topographer placido disc can provide an overview of the cornea and can show the relative steepness of any corneal area. The even separation of the rings in the spherical and the astigmatic cornea and the uneven spacing of the rings--especially inferiorly--in the keratoconic cornea should be noted. The central rings may show a tear-drop configuration termed "keratokyphosis."

Reduced visual acuity in one eye, due to the disease's asymmetry, may be a clue with the early keratoconus patient. This sign is often associated with oblique astigmatism. In early keratoconus, the patient may become less myopic six months later as the astigmatism increases.

Treatment
Treatment of keratoconus depends on the severity of the condition. Initially, eyeglasses are often successful in correcting the myopia (near sightedness) and astigmatism; however, as the disease advances vision is not adequately corrected and requires rigid contact lenses to aid in flattening the corneal surface and providing optimal visual correction. Contact lens fitting can be difficult in patients with keratoconus requiring frequent visits and lens changes but recent advances in the contact lens industry as provided more fitting options. Lastly, when good vision can no longer be attained with contact lenses or intolerance to the contact lens develops, corneal transplantation is recommended. This is only necessary in about 10% of patients with keratoconus and carries a success rate of greater than 90%, one of the highest for corneal transplantation. Although this procedure replaces the thinned central portion of the corneal, eyeglasses and contact lenses are often required for maximal visual acuity.