prescribing for astigmatism
The Benefits of Corneal Cross-Linking for Keratoconus
BY VISHAKHA THAKRAR, OD, FAAO
When the buzz around corneal cross-linking (CXL) started a few years ago, there was some skepticism. Just as with any new procedure, many of us took the wait-and-see approach. It has been six years since Wollensak et al introduced the procedure, and this technique has provided new hope to patients who have keratoconus and other corneal ectatic conditions.
How and Why it Works
Keratoconus is a noninflammatory, progressive thinning disorder that is usually bilateral, but typically affects one eye more than the other. The mechanism of keratoconus involves alterations in the organization and structure of the collagen fibrils of the corneal stroma. Structural weakness of the cornea results in corneal protrusion, irregular astigmatism, corneal scarring, and loss of bestcorrected acuity. Post-LASIK ectasia can result in the same biomechanical corneal changes.
CXL was first performed on humans in 1999 in Germany. This procedure involves the use of riboflavin (vitamin B2) and ultraviolet light (UVA) to induce collagen cross-linking. Riboflavin prevents UVA from penetrating beyond the anterior 300μm of the corneal stroma, thereby protecting the posterior cornea, lens, and retina. This procedure increases collagen strength and changes the corneal structure to delay or stop the progression of corneal thinning, potentially preventing penetrating keratoplasty. Studies demonstrate an increase in collagen fiber diameter and repopulation of keratocytes in the mid and anterior stroma. Long term studies have demonstrated that CXL stops keratoconus progression.
Vinciguerra et al (2009) demonstrated statistical improvements in uncorrected and bestcorrected visual acuity, refractive results, and spherical aberration 12 months after CXL. Topographic results as well as the average keratometry measurements after 12 months demonstrated significant flattening in the flatter and steeper meridians of the cornea. The corneal curvature steepened for the first three months after the procedure and then progressively flattened until the 12-month point.
The earlier this procedure is offered, the better the prognosis. Depending on the disease severity, management options after CXL may include spectacles, soft contact lenses, and GP corneal and scleral lenses. In patients who have mild keratoconus, contact lenses may not be necessary — the cornea may flatten enough to warrant spectacle therapy or soft contact lenses. GP lenses, however, will be the more common form of treatment.
Contact lens fitting may be challenging in the first year after the procedure because of potential flattening over that period of time. As we know, keratoconus patients are highly dependent on their lenses to function on a daily basis. Some patients may choose to treat one eye at a time to avoid any time period completely without lenses. Three months after the surgery, I start comparing the topographies to check for corneal stability in order to begin contact lens refitting. For patients who insist on wearing their contact lenses, try their old lenses to determine whether the fit and vision are temporarily acceptable.
Trials Underway in the U.S.
CXL shows promise for keratoconus and ectatic patients. My patients, treated in Europe and Canada, have shown no progression of ectasia or keratoconus. Multi-center clinical trials are currently underway in the United States. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #165.
Dr. Thakrar has a specialty contact lens practice and is a clinical optometrist at TLC Laser Eye Center in Mississauga, Ontario. She is also a professional affairs consultant to Vistakon.