Keratoconus is a progressive eye disease in which the cornea thins and bulges into a cone-like shape, losing its roundness. The eventual cone shape deflects light which enters the eye towards the light-sensitive retina. The result is distorted vision.
Keratoconus can occur in one or both eyes. Keratoconus is relatively rare. If it does occur the onset usually begins in the teens or early twenties.
Symptoms of Keratoconus
Keratoconus may be difficult to detect and it typically develops slowly with few cases proceeding rapidly. As the cornea gradually becomes irregular in shape, progressively nearsightedness and irregular astigmatism increase. This creates problems such as distorted and blurry vision. Glare and light sensitivity as well. Keratoconic patients often need prescription changes every time they visit their eye doctor.
Causes of Keratoconus
The weakening of the corneal tissue which leads to keratoconus appears to be from an imbalance of enzymes in the cornea. The enzyme imbalance makes the cornea susceptible to oxidative damage from free radicals, causing weakness and corneal bulge.
Risk factors for this type of oxidative damage and weakening of the cornea include genetic predisposition, which explaining why keratoconus often affects multiple members of the same family. Keratoconus is also connected to ultraviolet (sun) overexposure, excessive eye rubbing, a history of poorly fit contact lenses along with chronic eye irritation.
For mild forms, eyeglasses or soft contact lenses help. As the severity of the disease progresses and the cornea thins and increasingly distorted shape, glasses or soft contacts will no longer provide adequate vision correction.
Treatments for moderate to advanced keratoconus include:
Gas permeable contact lenses
If eyeglasses or soft contact lenses cannot control keratoconus, then gas permeable (GP) contact lenses are usually the effective. Rigid materials enable the GP lenses to dome over the cornea, replacing the irregular shape with a smooth, uniform refracting surface thus improving vision.
There is a comfort cost though because GP contact lenses can be less comfortable to wear compared to soft lenses. Fitting of the contact lenses on keratoconic corneas are also challenging and more time-consuming. Expect frequent office visits for fine-tuning and fitting of the prescription, especially as the keratoconus continues to progress.
Some practitioners advocate “piggybacking” two different types of contact lenses on the same eye to better fit the gas permeable contact lens over the cone-shaped cornea. Some patients may find this a bit uncomfortable. The GP lens is fitted on top of the soft contact lens which sits on the eye. The approach is thought to increase patient comfort as the soft lens acts as a cushion under the rigid second lens, the GP lens.
Hybrid contact lenses
Hybrid contact lenses are a relatively new design combining highly oxygen-permeable rigid center with a soft peripheral lens “skirt.” Manufacturers claim their hybrid contacts provide crisp optics of GP lenses alongside wear- comfort rivaling soft contact lenses. Hybrid lenses are available in a wide variety of parameters to provide optimal fit which conforms best to the irregular shape of a keratoconic eye.
Scleral and semi-scleral lenses
These are gas permeable contact lenses which have a large diameter allowing the edge of the lenses to rest on the white part of the eye -the sclera. These lenses will also dome over the irregularly shaped cornea, allowing for a most comfortable fit. They will move less during eye blinks. Scleral lenses cover a larger portion of the sclera, whereas semi-scleral lenses cover a smaller white (eye) area.
These are tiny plastic inserts which are surgically inserted just underneath the eye surface in the periphery of the cornea helping to reshape the cornea. The result, clearer vision. Intacs may be advised when keratoconus patients no longer can obtain functional vision with contact lenses or eyeglasses.
Studies show that Intacs can improve the spectacle-corrected visual acuity (BSCVA) of a keratoconic eye by an average of two additional lines on a standard eye chart. The implants have the additional advantage of being removable and changeable. The surgical procedure takes 10 minutes. Intacs might delay but will not prevent the required corneal transplant, if keratoconus progresses.
This procedure, “CXL” for short, strengthens corneal tissue slowing or preventing the bulging of the eye surface. In turn this procedure can reduce the need to undergo a corneal transplant.
There are two types of corneal crosslinking: epithelium-off and epithelium-on. Epithelium-off crosslinking is where the outer portion of the cornea (epithelium) is removed to allow entry of riboflavin, a B vitamin, to the cornea. Once administered, the riboflavin is activated with UV light. With the epithelium-on method (transepithelial crosslinking), the corneal surface is left intact.
Neither procedure is FDA-approved. However, multiple clinical trials are currently underway. Although cross-linking may already be common in some countries — few doctors in the United States will perform the procedure until it is FDA-approved. For that reason, it’s also not covered by insurance. The procedure costs $2,500 per eye, not including the contacts or eye drops.
Some people with keratoconus cannot tolerate a rigid contact lens, or they are beyond the point of contact lenses or known therapies providing acceptable vision. The last resort remedy may be a corneal transplant, also called a penetrating keratoplasty (PK or PKP). Note that after successfully completing a cornea transplant, most keratoconic patients will still need glasses or contact lenses for clear vision.