Corneal collagen cross-linking (CXL) is an investigational procedure that is used to arrest the progression of corneal ectasia secondary to keratoconus, pellucid marginal degeneration and/or LASIK surgery. Currently, CXL is not approved by the FDA; however, eye care specialists outside the United States have been performing the procedure successfully for more than a decade.
The fundamental purpose of CXL is to increase corneal rigidity, making it less prone to thinning and deformation. This is achieved by saturating the cornea with riboflavin, and then exposing it to ultraviolet-A (UVA) light. The UVA light interacts with the riboflavin to form chemical bonds both between and within the corneal collagen fibrils, which induces stiffening. The procedure preserves corneal transparency and, in essence, accelerates the aging of the cornea. As the collagen fibrils become compacted, mild corneal flattening and thinning may result.
There are two CXL techniques under investigation in the United States: epithelium-removed (epi-off) and epithelium-on. Initially, most clinicians preferred epi-off CXL, because it was believed that the patient would experience better riboflavin absorption. However, several studies currently are evaluating the clinical efficacy of epithelium-on CXL, which requires a shorter healing time and less intensive postoperative management.
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Nonetheless, at this time, the epi-off technique generally is considered more effective.
In either technique, the cornea is soaked in riboflavin until it can be detected in the anterior chamber via a flare check (usually about 30 minutes). If the corneal pachymetry measurement is thinner than 400μm following riboflavin saturation, the application of UVA light could potentially damage the endothelial cells. In this instance, neither form of CXL likely should be performed. And, because approximately 50μm of epithelial tissue is removed during epi-off CXL, patients with a corneal thickness of less than 350μm definitely are not considered candidates for the procedure.
There are several items for the comanaging O.D. to consider. Individuals who undergo epi-off CXL should be managed similarly to a post-PRK patient (e.g., monitored for complications associated with infection, delayed wound healing and prolonged pain). Additionally, all patients may develop mild haze or opaque streaking in the stroma after the procedure. Typically, haze is temporary and rarely visually significant. No additional management steps are required for these patients, unless the haze is severe and visually obstructive.
At around one year post-treatment, most patients will experience easier contact lens fittings and better vision (both corrected and uncorrected). The European success rate in halting ectasia is 92%. But if continued ectasia is documented after one year, a second CXL treatment may be nescessary. Keep in mind that CXL does not actually reverse ectasia; it simply helps to curtail progression.
