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The Thick and Thin of Ectasia
Ectatic corneas are both a contraindication and an occasional consequence of LASIK. But how thin is too thin? Not all corneas are predictable.
“When we started doing LASIK, we didn’t really understand that ectasia was a significant issue,” said William B. Trattler, MD. It took awhile, he said, “to better understand who was at risk or to see this was really an issue.” Today, nearly 10 years after the first case was reported, iatrogenic, post-LASIK ectasia is one of the most controversial issues in refractive surgery.
The etiology is unknown, and, as Dr. Trattler pointed out, “there are significant differences in opinion” regarding the relevance of risk factors. Yet surgeons may avoid this particularly insidious complication of LASIK by heeding the possible risk factors, which include: high myopia, patient age, reduced preoperative corneal thickness, reduced residual stromal bed thickness after laser ablation and asymmetrical corneal steepening (forme fruste keratoconus, keratoconus or pellucid marginal degeneration).
Understanding a safe threshold. Still, questions abound. How thick should the cornea be to maintain structural integrity? How deep can the surgeon go? And why do some patients with abnormal topographies not develop ectasia following LASIK, while some patients with normal-looking eyes do?
“There’s still an evolution of trying to understand who is an appropriate patient for LASIK and who should not be offered LASIK,” said Dr. Trattler. Until post-LASIK ectasia is better understood, there will be unhappy patients. And there will be doctors who are wary of the medicolegal consequences.
CONSENSUS AND CONSEQUENCES
In an attempt to clarify some of the issues surrounding post-LASIK ectasia, a committee of cornea and refractive surgeons assembled by the International Society of Refractive Surgery/ American Academy of Ophthalmology and the American Society of Cataract and Refractive Surgery issued a consensus report in November 2005.1 The group was convened following a multimillion-dollar legal judgment in favor of a young man who developed ectasia following LASIK. The consensus group spelled out the known risk factors for weeding out unsuitable candidates for LASIK. The group also stated that ectasia is a known risk of laser vision correction. When complications arise, “it does not necessarily mean that the patient was a poor candidate for surgery, that the surgery was contraindicated or that there was a violation of the standard of care.”
The group described a continuum of clinical findings that ranged from “the clearly normal” to the “clearly pathologic” cornea. The difficulty of accurately predicting which patients will develop this dreaded complication of LASIK lies in the ambiguous middle.
So just how common is it? Fortunately, surgically induced ectasia is rare, though the number of cases is not known. “We don’t know the answer to what percentage of LASIK patients develops ectasia,” said J. Bradley Randleman, MD, who suspects the number is underreported. “We’ve been pretty good at screening out a lot of cases,” said Yaron S. Rabinowitz, MD, a member of the 2005 consensus committee. “I’m amazed there aren’t a lot more.”
“We estimated one in 2,500 cases, with older screening technology,” Dr. Randleman said. “I think there’s a good chance that it should be lower. With appropriate screening it will be one in 5,000 or less.”
ANTICIPATING THE MAJOR RISKS
Poor preoperative screening was the focus of two multimillion-dollar ectasia lawsuits that found in favor of the plaintiffs. In one, a 32-year-old man claimed his surgery in October 2000 never should have been performed because of keratoconus, which he said was present before the surgery or could have been anticipated. In 2005, a jury awarded him $7.25 million, nearly doubling the previous record.
As the major risk factors for ectasia become clearer, two vital considerations can help guide the refractive surgeon around those factors:
Topography. “Most people believe the most common risk factor is abnormal topography,” said Dr. Rabinowitz. “If you look at the literature and take out all the other risk factors, in the vast number of cases there was abnormal topography.” Yet clearly, he added, ectasia can occur in the presence of normal topography.
He added that there are still suspicious topographic patterns that aren’t well understood. “Some are high risk and others not high risk. That still needs to be worked out.” But, said Dr. Rabinowitz, “knowingly performing LASIK on a patient with keratoconus or pellucid marginal degeneration,” would be a deviation from the standard of care.
Dr. Randleman, who has developed a risk assessment tool (see “Ectasia Risk Assessment”) for post-LASIK ectasia, agrees. “Abnormal topography stands alone as something that can exclude people,” he said. “There are some firm patterns that we know are abnormal,” he said. “If a patient has a topographic pattern that indicates keratoconus, pellucid marginal degeneration or forme fruste keratoconus, then they should absolutely be excluded from LASIK, even if the remainder of their examination is normal.”
Pachymetry. Another important factor is preserving enough residual stromal bed; the question, though, is how much is that? Traditionally the accepted range has been 200 to 325 µm, with 250 µm chosen as the arbitrary cutoff. “But for each cornea it’s different. Nobody knows what the magic number is,” said Dr. Rabinowitz.
To preserve enough residual stromal bed, Dr. Trattler added, “it’s most important to measure the patient’s flaps at the time of surgery.” Yet unpublished data from 2005 survey conducted by Magill Research Center at the Medical University of South Carolina found only 34 percent of U.S. refractive surgeons routinely perform intraoperative pachymetry, he said.
Dr. Rabinowitz agreed that pachymetry during surgery is essential because keratomes produce such variation in the thickness of the flap. “You cut the flap and lift it up, then you measure it,” he said. If the measurement is too low, abort the procedure, he said. “If you don’t have enough tissue, a few months later you can do PRK.”
OTHER RED FLAGS
Age. Since 2005, doctors have become increasingly aware of the role that the patient’s age plays as a predictive factor, said R. Doyle Stulting, MD, PhD, who was a member of the consensus committee. “Early on we focused on parameters that we can measure in the clinic, like corneal curvature, corneal thickness, residual stromal bed and the degree of myopia. Then we began seeing people who did not have any of these predictive factors and they still developed ectasia,” Dr. Stulting said. “It turned out they’re significantly younger than the ones who have identifiable risk factors. We believe those are people who may have developed keratoconus or forme fruste keratoconus had they not had LASIK.”
Douglas D. Koch, MD, agrees. “Be especially wary of questionable topography in young patients who need large corrections, as this group has been shown to have a higher incidence of ectasia.”
Asymmetry. Another red flag is asymmetry between the eyes, said Dr. Trattler. He described a patient whose eyes appeared normal, but who had astigmatism at 90 degrees in one eye and at 180 degrees in the other. Asymmetry may be a sign that one eye is progressing toward keratoconus or pellucid marginal degeneration, he said.
Enhancements. Dr. Trattler also urged caution before proceeding with LASIK enhancements. Because LASIK patients who present for an enhancement with myopic astigmatism may potentially have early ectasia, he advised carefully examining the topography for asymmetry. Also, it is important to confirm that the topography and refraction are relatively stable, which means obtaining old records for LASIK procedures performed elsewhere.
In the meantime, patients who are not good candidates for LASIK need not despair. PRK is becoming a more commonly accepted alternative, Dr. Rabinowitz said.
Dr. Koch agreed. “If there is a good likelihood of leaving too little tissue, avoid LASIK and switch to PRK. In eyes with questionable topography, consider PRK or no surgery at all.”
Treatments for post-LASIK ectasia are the same as for keratoconus, said Dr. Randleman. Treating with contact lenses is most common; penetrating keratoplasty (PK) is the treatment of last resort. Even if an eye warrants PK, the prognosis is excellent, according to the 2005 consensus group, which reported graft survival rates of 97 percent and 92 percent at five and 10 years, respectively.
But in the vast majority of cases, PK can be avoided. At Emory University, where Dr. Randleman has treated some 75 ectasia cases, only about 8 percent have required a corneal transplant.
There are surgical alternatives to conventional penetrating keratoplasty, including anterior lamellar keratoplasty, which enables targeted replacement or augmentation of corneal stroma, without replacement of endothelium.2
Additional interventions designed to enhance corneal stability include:
WHAT'S IN STORE?
Today researchers are looking for the gene or genes responsible for keratoconus, which could lead to the development of blood tests that would enable surgeons to identify patients at risk of ectasia before any clinical signs are evident. Dr. Rabinowitz’s group at Cedars-Sinai Medical Center is working on one such molecular genetic test.
In the meantime, questions remain. Not known is whether currently identified risk factors are sufficient to allow the prediction of ectasia, or whether researchers ought to be looking at other factors, said Dr. Stulting. “We believe that there is an inherent instability in the corneas of people who are going to develop ectasia,” he said, adding, “We may be directly able to measure that instability.”
2 Curr Opin Ophthalmol 2007;18(4):284–289.