Surgeon: LASIK involves far greater risk of postop ectasia than does PRK

Ectasia after PRK presents later than ectasia after LASIK, increasing the likelihood of a missed early diagnosis after surface ablation.

Marguerite B. McDonald

Even though PRK poses a significantly lower risk of corneal ectasia than does LASIK, some corneas are too thin to undergo either procedure, according to a surgeon.

Despite a lack of criteria for identifying and reporting postoperative ectasia, clinicians know the risks of ectasia associated with LASIK and PRK.

“The true incidence of post-LASIK and post-PRK ectasia remains unknown,” Marguerite B. McDonald, MD, said at Refractive Subspecialty Day preceding the joint meeting of the American Academy of Ophthalmology and the Middle East Africa Council of Ophthalmology in Chicago.

“There is no standardized definition of ectasia and no mandatory reporting system. Many laser patients are not followed by their surgeons, and the onset is very different. Nevertheless, we do know which procedure holds the greatest risk,” she said, referring to LASIK.

Dr. McDonald said she prefers a preoperative corneal thickness of at least 500 µm before performing either LASIK or PRK.

Late onset of ectasia after PRK

Depending on which procedure is performed, there is a difference in the timing of onset of postoperative ectasia, typically 3 to 5 years after PRK but only 6 to 18 months after LASIK, Dr. McDonald said.

“The onset of post-PRK ectasia is so late that it is possible to miss the diagnosis,” she said. “For example, increased myopia can be mistaken for cataractous changes.” However, preoperative and postoperative topographies, especially with difference maps, are helpful in making the diagnosis and in tracking the response to treatment such as cross-linking.

In one case of ectasia after LASIK treated with cross-linking, corneal topography appeared unchanged between preoperative examination and 1-month postoperative follow-up, but the topography difference map showed the typical early central steepening that normally follows cross-linking. The change was so subtle that it might have remained undetected with the “eyeballing” method, even by a trained observer. However, a second difference map comparing the preoperative and 6-month postoperative examinations showed definite signs of central corneal flattening, the expected final result from the cross-linking procedure.

“Once again, the subtle but clinically significant central corneal changes were most reliably detected with the topography difference map,” Dr. McDonald said.

According to Dr. McDonald, a global literature review revealed only 32 reported cases of ectasia occurring after surface ablation but showed that the number of cases was increasing over the last 3 years, possibly because of the very late onset of ectasia after PRK.

Corneal topography critical

The decision to perform PRK hinges on corneal topography, Dr. McDonald said.

“Studies suggest that the incidence [of ectasia] is lower for PRK even though PRK is reserved by most surgeons predominantly for thin, high-risk corneas,” she said. “From this we can infer that there is less risk of ectasia when performing PRK on a thin cornea vs. risk with LASIK. What we should not infer, however, is that it is always safe to do PRK on thin corneas instead of LASIK.”

To minimize the risk of postoperative corneal ectasia, the surgeon should perform PRK only if the corneal topography is normal, Dr. McDonald said.

“If the preop thickness is thin, go back and look again at the preop topographies — check that the manifest refraction is –8 D or less,” she said. “Only one exception: Perform PRK to spare tissue in an otherwise completely normal eye with a thin cornea.”

One world literature review showed that, for both LASIK and PRK, abnormal topography was the most significant factor that distinguished ectasia cases from controls, followed by residual stromal bed thickness, age and preoperative corneal thickness.

“For both LASIK and PRK, two of the top four risk factors for ectasia are related to corneal thickness,” Dr. McDonald said.

A study from Emory University, a tertiary referral center, showed that the incidence of ectasia cases following LASIK that were referred for treatment was 36 times higher than that of referred cases of ectasia after PRK.

Legal considerations

A 20-year summary of laser vision-related malpractice suits released by the Ophthalmic Mutual Insurance Company showed 196 LASIK-related cases and 16 surface ablation-related cases.

“Though it changes a bit from year to year, roughly seven times as much LASIK is performed in the U.S. as PRK,” she said.

After adjusting for the relative differences in case volumes, there is still a two times greater chance of a U.S. surgeon being sued after LASIK than after surface ablation, Dr. McDonald said.

“The primary cause for a suit after LASIK is ectasia,” she said, adding that for PRK lawsuits, ectasia is farther down the list. – by Matt Hasson

Reference:

• Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115(1):37-50.

• Marguerite B. McDonald, MD, can be reached at Ophthalmic Consultants of Long Island, 360 Merrick Road, Lynbrook, NY 11563; 516-766-2519; e-mail: margueritemcdmd@aol.com.
• Disclosure: No products or companies are mentioned that would require financial disclosure.