Debate: Best appraoch to thin cornea

When a patient seeks laser vision correction surgery but has a thin cornea, refractive surgeons face a decision-making dilemma.

Presuming there are no other risk factors for postLASIK ectasia, surface ablation still is preferred over a flap-based procedure, according to Dr Marguerite B. McDonald. "However, review of relevant studies with data on safety, predictability and efficacy outcomes of thin-flap LASIK support its use in this situation," said Dr Jan A. Venter.

"Published studies suggest that the incidence of ectasia is lower after PRK even though it is used predominantly in high-risk, thin corneas, and from this information, we can infer there is less risk of ectasia when performing PRK versus LASIK in a thin cornea," said Dr McDonald, clinical professor of ophthalmology, NYU Langone Medical Center, New York, USA. "However, one should not infer from this that PRK is always safe.

"Remember that a special informed consent will not protect you from a lawsuit," she continued. "If the preoperative corneal thickness is thin, go back and look again at the preoperative topography because the likelihood of a corneal dystrophy is increased in eyes with thinner corneas, and don't operate if the refractive error is more than –8 D."

Incidence of ectasia

Although the true incidence of postLASIK ectasia and postPRK ectasia remain unknown, there is evidence that the ectasia risk is greater after LASIK versus PRK. Dr McDonald noted that a review of the world literature found only 13 papers reporting ectasia following surface ablation and a total of only 32 eyes. In addition, in a 2008 paper on risk assessment for ectasia after corneal refractive surgery, Randleman et al. reviewed the world's English-language literature and found 23 times as many cases of postLASIK ectasia as postPRK ectasia. In the same year, Woodward et al. reported that among eyes with ectasia presenting for treatment to their tertiary referral centre, there were 36 times as many LASIK cases compared with PRK.

"These relative differences occurred in spite of the fact that, according to market data, surgeons most often reserve surface ablation for eyes with the thinnest corneas," Dr McDonald said.

Ophthalmic Mutual Insurance Co. data on refractive surgery-related lawsuits also highlight a difference in ectasia risk between LASIK and surface ablation. In a review spanning 20 years of malpractice coverage for refractive surgery, Menke et al. reported almost 200 LASIK-related lawsuits versus only 16 litigation cases related to surface ablation. "Even after adjusting for the almost 7-fold difference between LASIK and surface ablation in case volume in the US, there remained a 2-fold greater chance of being sued for LASIK. Furthermore, ectasia was the primary cause for a LASIK lawsuit, whereas it was much further down on the list of causes for PRK litigation," explained Dr McDonald.

She observed there are several factors contributing to the lack of information on the true incidences of ectasia after both PRK and LASIK. They include the absence of any standardized definitions and mandatory reporting systems along with the fact that many refractive surgery patients are not followed by their primary surgeon. In addition, postPRK ectasia may have a delayed onset.

"Ectasia postLASIK can present 6 to 18 months after surgery but may not appear for 3 to 5 years after PRK," she said. "With the late onset, postPRK ectasia may be misdiagnosed, and the associated increase in myopia may be mistaken for nuclear sclerotic changes.

"In fact, literature reports of ectasia after surface ablation seem to be growing in number, perhaps reflecting the late onset of these events," Dr McDonald added.