Keratectasia may lack risk factors

TAMPA – One researcher has identified age as a strong risk factor for keratectasia, while another points to a lack of scientific research to identify any risk factors.

Michael Twa, OD, PhD, FAAO, of the University of Houston, discussed here at the Optometric Council on Refractive Technology meeting how keratectasia did not begin with LASIK. It has occurred with every type of refractive procedure, including radial keratotomy, PRK, LASIK and anterior lamellar keratoplasty, he said.

Dr. Twa outlined three proposed mechanisms for the condition: exacerbation of a pre-existing keratectasia, tissue removal beyond the limits of the cornea’s structural integrity and aberrant wound healing.

“Several authors have suggested that perhaps we have this other category of ectasia that arises for unknown reasons,” Dr. Twa said. “You can’t see it coming; it’s not a pre-existing condition. We need more research in this area to understand these surprise cases.”

Dr. Twa reviewed some recent research on risk factors for keratectasia. Randleman, in a paper published earlier this year in Ophthalmology, “looked at many cases and identified five key factors in developing keratectasia after refractive surgery. He analyzed topographic pattern, residual stromal bed thickness, age, preoperative thickness and degree of myopia. Randleman showed that age was a strong risk factor. “Younger patients between the age of 18 and 21 years had the highest risk,” Dr. Twa said.

He added that forme fruste keratoconus and myopia above 8 D were also strong risk factors.

Perry Binder, MD, described some individual preoperative and operative factors that increased the risk of ectasia, Dr. Twa continued, including unknown factors, unmeasured factors and assumed risk factors.

According to Dr. Binder in an article this year in Cataract and Refractive Surgery Today: “If one eliminates cases with preoperative ectatic conditions and performs LASIK and leaves residual corneal bed of more than 300 to 400 micrometers, one can expect to reduce the incidence … and until scientific studies prove otherwise, one may perform LASIK or PRK on eyes with assumed risk factors as long as all aforementioned screening was appropriately performed and appropriate informed consent was obtained.”

“His argument is to go ahead and do these cases,” Dr. Twa stated. “There’s not really any good evidence to support these risk factors for ectasia. You have his permission to do these cases.”

Dr. Twa also reviewed several new diagnostic instruments for detecting corneal ectasia. Reichert’s Ocular Response Analyzer (ORA) measures elasticity and viscosity and reports them as a single compound index as corneal hysteresis and corneal response factor. Another OCRT lecturer, Marc Bloomenstein, OD, asked about the importance of corneal hysteresis.

“Corneal hysteresis and corneal response factor are giving you two pieces of information combined,” Dr. Twa responded. “They’re not easily decoupled. I don’t think these compound indices are where we’re going to end up as the most useful output from this instrument. It’s still in evolution.”

He predicted in his presentation that the use of this instrument in the future may allow clinicians “to identify keratoectasia before anterior or posterior corneal manifestation become apparent by videokeratography. We need to get more sophisticated about how we use the information coming from the ORA,” he added.

Other new instrumentation includes the Pentacam (Oculus) and Gallilei corneal imaging systems, which Dr. Twa referred to as the current challengers to Bausch & Lomb’s Orbscan.

Strengths of the Pentacam include centrally weighted measurement, simplified peripheral reconstruction and better coverage of the entire anterior segment.

Dr. Twa referred to the Gallilei as the first rotating dual Scheimpflug analzyer, which addresses the problem of lateral misalignments that can handicap the Pentacam. The Gallilei’s software also incorporates the latest technology, he added, including novel surface reconstruction algorithms and keratoconus detection methods.