People with keratoconus develop an irregular, cone-shaped cornea — the clear window that covers the eye's iris and pupil. This change in the cornea's structure blurs vision and makes people more nearsighted over time. Changes usually begin in the late teens and early 20s, then progress for about 10 to 20 years. No one knows the cause, but keratoconus runs in families. Most people with keratoconus can wear rigid gas permeable (RGP) contact lenses to correct vision problems. About 10% of people must have a corneal transplant to improve their vision. This operation is succesful in more than 90% of people. After a transplant, about 60% of people must still wear RGP contact lenses to correct their vision.
What is keratoconus?
Normally, the cornea — a clear window at the front of the eye that covers the iris and pupil — is round like a dome. In people with keratoconus, the centre of the cornea thins and develops an irregular, cone-shaped bulge. The word keratoconus comes from the Greek words karato (cornea) and konos (cone).
Changes in the cornea’s shape affect the way people see. In people with keratoconus, vision becomes blurred and distorted — like trying to see a road sign through a car windshield in a downpour.
Keratoconus usually begins in the late teens or early twenties. It progresses over time and usually affects both eyes. Changes tend to occur over 10 to 20 years, then stabilize on their own.
Keratoconus affects everyone’s eyes differently. Its effects on vision range from mild to severe. In the early stages, keratoconus causes:
- slight blurring
- visual distortion
- light or glare sensitivity
- tearful eyes
As keratoconus progresses, the cornea bulges more. Visual distortion and blurriness worsen. People become increasingly nearsighted (have more difficulty seeing distant objects).
In a small number of people, keratoconus causes sudden swelling, which leads to rapid changes in vision. Swelling occurs when tiny cracks develop in a steep, cone-shaped bulge in the central cornea.
Keratoconus is a treatable disease. While it affects the quality of vision, it does not usually cause vision loss.
No one knows what causes keratoconus, but it tends to run in families. About 7% of people have a relative with keratoconus. It often occurs in people with other diseases, e.g., Marfan’s syndrome, atopic disease, Down’s syndrome, mitral valve prolapse and other eye diseases, such as retinitis pigmentosa and retinopathy of prematurity.
Research into the causes of keratoconus is active and controversial. Some eye doctors speculate that keratoconus is a birth defect. Others believe it is a degenerative condition. Since keratoconus occurs after puberty, some eye doctors think the endocrine system is involved. Then again, it could be caused by a specific disease process. No one knows for sure.
Most keratoconus appears to occur randomly. No one knows exactly how often it happens. Estimates range from 50 to 230 cases per 100,000 people. People with no family history of keratoconus have a 1 in 10 chance of having a child who will develop keratoconus.
In the early stages, eyeglasses or contact lenses may improve the vision of someone with keratoconus. Rigid glass permeable (RGP) contact lenses may correct blurred vision (astigmatism) and neutralize nearsightedness (myopia).
Since keratoconus often stabilizes after a few years, for most people, RGP contact lenses may be a permanent remedy for vision problems. Under these rigid lenses, tears smooth out the cornea’s irregular surface. A good fit is important, because poorly fit RGP lenses may aggravate keratoconus. Corneal topography may help the eye doctor order RGP lenses that fit properly.
In about 10% of people, the cone-shaped cornea becomes too steep to fit contacts comfortably. Scarred corneal tissue may cause enough discomfort to prevent vision correction with RGP lenses.
At this stage, a corneal transplant is often used to correct keratoconus. During eye surgery, the cone-shaped cornea is removed and replaced with a donated cornea. The success rate of this operation is higher than other types of transplants because the cornea has no blood supply.
Corneal transplantation is a common operation. Eye banks in major cities collect healthy corneas from donors and supply eye surgeons with corneal tissue on request.
More than 90% of people have successful corneal transplants. After the operation, about 60% of people still need to wear contacts or eyeglasses to correct nearsightedness (myopia).
In epikeratophakia, an eye surgeon grafts a layer of corneal epithelial cells ("skin" cells from the cornea’s upper layers) around the central cone to flatten the cornea. This operation has a similar success rate to corneal transplantation.
The answer to this question is unknown, but some people believe that firmly rubbing your eyes often may contribute to keratoconus — an irregularly, cone-shaped cornea that distorts vision. Eye researchers have reported vigorous eye rubbing in 66% and 73% of people with keratoconus. These people used their knuckles to rub their eyes.
Itchy eyes may be caused by a number of changes within corneal tissue, so researchers do not yet know whether eye rubbing causes or is an effect of keratoconus. Until they find out, it may be wise to minimize eye rubbing.
A visual acuity exam, which tests the eye’s ability to see object details and shapes at a specific distance, may spot the first signs of keratoconus. During this exam, a person with keratoconus may see slightly warped or distorted images (irregular astigmatism).
People who develop keratoconus often visit their eye doctor because they are unsatisfied with their eyeglasses. Their prescription may change several times — from 0.75 to 1.5 diopters over 12 to 18 months. They often have to squint to see better. Halos appear around street lights at night. They may see ghost or multiple images.
A biomicroscope or slit lamp can detect corneal changes in moderate to severe keratoconus. This exam may detect a brownish ring-shaped deposit (Fleischer’s ring) at the base of the abnormal zone.
In severe keratoconus, the slit lamp will detect fine white lines (Vogt’s striae) deep within the middle layer of the cornea. In some people, the eye doctor may find corneal swelling or scars.
To track the progress of keratoconus, it is helpful to know the exact shape of the cornea. To do so, your eye doctor may use one of two methods: keratometry or corneal topography.
A keratometer is an instrument that assesses the curvature of the cornea. It projects circles of light (mires) onto the central corneal, stretching from two points about 3 mm apart. Circles that aren’t perfectly round are a sign of keratoconus. The usefulness of keratometry is limited, because irregularities may occur outside the central cornea, where they can’t be detected by this method.
Corneal topography uses computer technology to measure many points on the cornea’s surface. From these measurements, the computer plots a map of the cornea’s contours — much like a relief map of the earth. The procedure is fast and painless — like taking a photograph of the eye. With this map, the eye doctor can order special contact lenses to correct the nearsightedness related to keratoconus.
For more information
National Keratoconus Foundation
http://www.djo.harvard.edu/meei/PI/Keratoconus.html
http://www.opt.indiana.edu/lowther/keratoconus
http://www.intelihealth.com
http://www.opt.indiana.edu/lowther/html/topog.htm
