Corneal Cross Linking

Corneal cross linking (CXL) is a treatment for people who suffer from a cornea that becomes unstable and weak. The cornea begins to steepen or protrude, causing blurred and distorted vision, sometimes greatly impacting quality of life. Corneal cross linking is a method used to slow the progression of corneal changes. Corneal cross linking uses UV light to strengthen chemical bonds inside the cornea. As of February 2012, corneal cross linking is not an FDA-approved procedure and is not commonly-performed in the U.S. as of yet.

What Causes an Unstable Cornea?

The two most common causes of corneal instability are cornea ectasia and keratoconus. They are the second most frequent cause for corneal transplant surgery, and together account for 15% of the corneal transplant surgeries performed in the United States.

  • Corneal ectasia: Corneal ectasia is a condition in which the cornea loses its ability to keep a regular, normal shape, usually from the cornea becoming too thin. The cornea becomes so thin that the internal eye pressure can cause the cornea to become distended or expanded. Because the cornea contributes a lot to the overall power of the eye, vision becomes distorted and extremely blurry. Traditional glasses and contact lenses do not always correct this distorted vision. Corneal ectasia is most commonly caused by refractive surgery, such as LASIK or radial keratotomy. Ectasia is not a normal occurrence after refractive surgery but can occur in people who were not the best candidates for the procedure. Some people may have had an underlying, sometimes undetectable corneal dystrophy that was difficult to measure prior to surgery. Ectasia can also occur in a condition called pellucid marginal degeneration that is often lumped in with keratoconus.
  • Keratoconus: Keratoconus is a disorder of the cornea. In keratoconus, the cornea thins and bulges outward like a cone, resulting in distorted vision. As the shape of the cornea changes, nearsightedness and astigmatism may develop. The corneal changes of keratoconus usually occur very slowly. Although keratoconus does not cause blindness, it can reduce the ability to focus without eyeglasses or contact lenses and may severely impact one's quality of life.
  • Pellucid Marginal Degeneration: Pellucid Marginal Degeneration is thought to be somewhat of a subset of keratoconus. In the past, many doctors mistakenly diagnosed Pellucid as keratoconus. It is also characterized by thinning of the corneain the inferior and peripheral part of the cornea. It many cases it affects only one eye or one eye much worse than the other.

Corneal Cross Linking Procedure

Corneal cross linking attempts to strengthen the bonds within the cornea to create stability. Your doctor will first instill topical anesthetic eye drops. Then, your doctor will remove the top layer of your cornea, or epithelial cells, to expose the middle layers.

The exposed cornea will then be bathed with a sterile riboflavin solution for 30 minutes. Riboflavin drops are then allowed to saturate the cornea for another 30 minutes while being exposed to a careful dose of ultraviolet (UVA) light. The UVA light causes a chemical reaction with the riboflavin that creates link and bonds to form in the collagen within the cornea to make it stiffer. Antibiotic eye drops or ointment is then applied to the eye. Some doctors will instill a bandage contact lens until the epithelial cells grow back, which may take 2-4 days. The procedure attempts to make the cornea stronger in hopes to prevent the condition from worsening and in some cases, causes the cornea to return somewhat to its natural curved shape.

Corneal Cross Linking Recovery

A few days after undergoing corneal cross linking, you may have some mild corneal swelling. You may feel some minor irritation, burning or foreign body sensation until the cornea totally heals. Antibiotic drops are usually prescribed for a few days. Although most people have no problems after surgery, some are at risk for developing a mild clouding of the cornea that could possibly reduce vision slightly.

Over the next six months, your doctor will see you often to measure your vision (refraction) and perform measurements, such as corneal thickness measurements (pachymetry) or cornea mapping (corneal topography) until your cornea stabilizes. It is important to note that CXL does not magically restore your vision in any way. It is designed to strengthen the cornea to slow or stop progression and make the cornea more receptive to contact lens wear or another mode of vision correction.

What You Should Know About Corneal Cross Linking

CXL is being investigated to move into a "front-line" treatment option as opposed to a later treatment for keratoconus to prevent severe progression and the need for a corneal transplant. Researchers are also looking into CXL as a possible treatment for serious eye infections. Severe corneal infections can cause scarring and even melting of the cornea. CXL has been shown to treat these infections when antibiotics do not work well. The CXL process interrupts degrading enzymes that bacteria produce and also inhibits the growth of the bacteria themselves.

Studies are also investigating whether CXL could be performed as a part of certain LASIK procedures to strengthen the cornea and reduce the risk of surgically-induced corneal ectasia.

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Article Sources

  • Kabat, Alan G and Joseph W Sowka. CXL for Infectious Disease? Review of Optometry. December 2011.
  • Karpecki, Paul M and Diana Shechtman. A look at corneal crosslinking. Review of Optometry, August 2011.