Keratoconus is a degenerative eye disease that affects the cornea—the clear front surface of the eye.
The cornea is the primary refracting (light-focusing) structure of the eye, accounting for roughly 70 percent of the eye's total focusing power. Together, the cornea and the lens inside the eye focus light onto the retina at the back of the eye to enable clear vision.
In a normal eye, the cornea has a smooth, mildly prolate shape—meaning it is nearly spherical but is steepest at the center (apex), and gradually gets flatter toward the periphery.
Keratoconus is a degenerative thinning and weakening of the corneal tissue, causing the central cornea to bulge forward in a distorted and unpredictable fashion. As keratoconus progresses, vision problems caused by loss of the cornea's normal profile cannot be corrected with eyeglasses.
People with advanced keratoconus can lose their ability to perform common visual tasks that we all take for granted—such as driving a car, watching TV, reading a book or computer screen, etc.
Thankfully, keratoconus usually can be successfully treated.
Treatment of keratoconus is designed to:
- Slow or stop the thinning and bulging of the cornea and provide a new refracting surface for the eye; or
- Normalize the shape of the cornea by surgical means.
Generally, treatment options for keratoconus include contact lenses, corneal collagen cross-linking, intrastromal corneal ring segments (ICRS) and cornea transplantation.
In most cases, the preferred treatment for keratoconus is fitting the eye with rigid gas permeable (GP) contact lenses.
Though certain soft contact lenses sometimes can correct vision problems caused by mild keratoconus, soft lenses typically are thin and pliable and "drape" over the irregularly shaped cornea of a keratoconic eye. Therefore, soft lenses cannot fully correct all distortions of vision caused by keratoconus because they conform somewhat to the irregular shape of the cornea.
GP contact lenses, on the other hand, are made of rigid oxygen-permeable plastic materials, which when laid on the cornea, cause the front surface of the cornea to take a more regular shape, even when the cornea has a highly irregular surface.
By maintaining their shape on the eye, GP lenses essentially replace the distorted optics of a keratoconic cornea with a smooth, regular-shaped refracting surface and thereby restore good vision.
The design of GP lenses for keratoconus treatment depends on the degree and type of corneal distortion. In many cases, lenses for keratoconus are significantly larger in size than conventional GP lenses used for the correction of nearsightedness, farsightedness and/or astigmatism in a healthy eye.
These larger-than-normal GP lenses are called corneo-scleral or scleral contacts and are designed to vault over the entire cornea to correct visual aberrations caused by keratoconus and provide an acceptable fit.
Sometimes, to improve the comfort of GP lenses for keratoconus treatment, a soft contact lens is fitted under the GP lens to provide a cushioning effect. This fitting technique is called piggybacking.
An alternative to piggybacking soft and GP lenses is to fit the eye with hybrid contact lenses specially designed for keratoconus treatment. These lenses have a rigid GP center, surrounded by a "skirt" of soft lens material to provide greater wearing comfort than an all-GP lens.
In many cases, people with mild to moderate keratoconus do very well with contact lenses and are able to achieve clear, comfortable vision without the need for eye surgery.
Corneal Collagen Cross-Linking
If contact lenses fail to provide adequate vision correction for keratoconus or cannot be worn comfortably, surgical treatment usually is required.
A procedure that is becoming popular for keratoconus treatment in recent years is corneal collagen cross-linking (CXL).
In a healthy eye, the shape of the cornea is stabilized by cross-links that exist between parallel strands of connective tissue (collagen) in the cornea that are responsible for its strength and transparency. In keratoconus, these cross-links are compromised, causing the cornea to weaken and bulge forward, resulting in distorted vision.
In the corneal collagen cross-linking procedure, the eye is treated with a topical application of riboflavin (vitamin B2), followed by exposure to controlled ultraviolet light to strengthen the collagen cross-links and stabilize the cornea.
Basically, the CXL procedure consists of these steps:
- Numbing drops are applied to your eye for comfort.
- The thin surface layer of the cornea (epithelium) may or may not be removed to prepare the eye for treatment.
- Riboflavin drops are applied to your eye.
- You will be asked to look directly at a special UV light for 30 minutes. (Riboflavin is reapplied every 5 minutes during the irradiation period.)
The entire procedure takes approximately one hour (per eye) and is performed on an outpatient basis.
Several research studies have shown that corneal cross-linking surgery prevents further vision loss from keratoconus in greater than 95 percent of patients and improves vision in up to 80 percent.
Researchers also have reported using a combined procedure of CXL followed by topography-guided PRK for keratoconus treatment, achieving excellent results.
Though CXL has been approved by all nations in the European Union since 2006, it still is considered an investigational procedure in the U.S. and therefore is not covered by medical insurance when performed in the United States.
Corneal cross-linking also has been shown to be effective for the treatment of post-LASIK ectasia, which shares many of the characteristics of keratoconus.
Your eye doctor can help you find an ophthalmologist near you who performs the CXL procedure for more information.
Intrastromal Corneal Ring Segments
Another surgical procedure used for keratoconus treatment is intrastromal corneal ring segment (ICRS) implantation. The most popular corneal ring segments used by eye surgeons in the United States are called Intacs.
Intacs are small, clear arc-shaped pieces of medical-grade plastic that are inserted into the periphery of the cornea, in tiny channels created with a manual surgical tool or a femtosecond laser.
When in place, they reduce the curvature of the central cornea to reduce myopia, astigmatism and the other aberrations caused by keratoconus.
ICRS implantation sometimes is combined with CXL to produce better refractive outcomes than what either procedure is capable of producing alone, and to strengthen the central cornea for greater stability.
Intacs and other ICRS procedures also can be used to correct corneal ectasia after LASIK surgery.
In cases of progressive and advanced keratoconus, the only viable treatment may be a full-thickness corneal transplant (also called penetrating keratoplasty or PK).
According to the National Keratoconus Foundation, only 15 to 20 percent of patients with keratoconus require cornea transplant surgery.
The basic steps of a penetrating keratoplasty surgery for keratoconus treatment are:
- A central full-thickness "button" of the diseased cornea is removed from the eye.
- A healthy donor button of the exact same size is positioned on the eye.
- The donor cornea is sutured in place.
Recently, surgeons have begun using femtosecond lasers for creating the corneal buttons for PK, which has resulted in greater accuracy for matching the size of the donor button to the removed keratoconic tissue.
Healing after a cornea transplant takes several months, but studies have shown encouraging results, with greater than 85 percent of patients undergoing PK for keratoconus treatment achieving best-corrected visual acuity of 20/40 or better 18 months after surgery.
After a cornea transplant, steroid eye drops are used for several months to prevent rejection of the donor cornea. Sometimes use of these drops must be continued indefinitely, and other eye medications may be needed as well.
For these reasons, it is essential to have routine eye exams as directed by your surgeon after a cornea transplant surgery for keratoconus treatment.
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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.