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Feature
The Thick and Thin of Ectasia
By Miriam Karmel, Contributing Writer
 
 

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.

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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.”

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ECTASIA REGISTRY

A registry for reporting cases of ectasia after LASIK had its debut recently. The purpose of the registry “is to identify risk factors that are not currently known and to serve as a basis for clinical trials in the future,” said Dr. Stulting, who is directing the project.

There are two anticipated phases to the project. The first phase will establish a database for submission of information on patients who developed ectasia after LASIK. These cases will be evaluated against a control group of LASIK patients who did not develop ectasia, in an effort to validate known risk factors and discover new ones. Phase two will include prospective clinical trials of LASIK in cases involving unproven risk factors.

Ophthalmologists who care for patients with ectasia are encouraged to participate in the online registry by entering data on their patients at www.ectasiaregistry.com.

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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.”

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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.”

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TREATING ECTASIA

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:

  • Intacs. Intrasomal corneal ring segments can be inserted into the thinned cornea of contact lens–intolerant patients to serve as a “crutch.” They flatten the central area of the cornea and correct myopic refractive error. Intacs may halt progression, make patients more tolerant to contact lenses and, hopefully, obviate the need for a corneal transplant.
  • Collagen cross-linking. This potential treatment, for which clinical trials should soon begin, promotes the integration of the natural anchors within the cornea. Collagen cross-linking inhibits the cornea from bulging out and becoming steep and irregular. This treatment avoids ablation or cutting across the visual axis. Early reports suggest that it may halt the progression of keratoconus and that it also causes keratoconic corneas to assume a more normal shape, with consequent improvements in visual acuity.

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ECTASIA RISK ASSESSMENT

Researchers at Emory University have come up with a risk factor stratification scale intended to help prevent ectasia after LASIK.

After conducting a meta-analysis of published results from 1998 to 2005 related to post-LASIK ectasia, the researchers found that, compared with controls, ectasia cases had abnormal preoperative topographies (35.7 percent vs. 0 percent); were significantly younger (34.4 years vs. 40); were more myopic (–8.53 vs. –5.09 D); had thinner corneas before surgery (521.0 vs. 546.5 µm); and had less residual stromal bed thickness (256.3 vs. 317.3µm).1

Point system. After analyzing the data, they assigned numerical scores to the various risk factors, which included topography pattern, residual stromal bed thickness, age, preoperative corneal thickness and preoperative spherical equivalent manifest refraction. In the topography category, for example, they assigned four points to forme fruste keratoconus; three points to inferior steepening/skewed radial axis; one point to asymmetric bowtie. Zero points were assigned to normal/symmetric bowtie. By adding up the points for all the risk categories, a surgeon should have a better sense of whether the patient has a low, moderate or high risk for ectasia.

“Our current paper was written to put some science behind the anecdotes of what may or may not be risk factors,” said Dr. Randleman, lead author of the study. “We developed the ectasia scale using the literature that was available. Then we followed that up with another study where we validated the risk factors.”

The scoring system, which identified more than 90 percent of abnormal patients, was replicated using a separate population. Dr. Randleman noted that there have been numerous proposed contraindications to LASIK, including a residual stromal bed thickness less than 250 µm, a preoperative corneal thickness less than 500 µm, keratometry greater than 47 D and an Orbscan posterior float value greater than 50 µm. On the other hand, he said that “there were actually very few ‘absolute’ cut-off values.”

All about thresholds. “However, when there are too many abnormalities in combination, then the patient should be excluded from LASIK,” added Dr. Randleman. The scoring system shows that “you can’t look at things in isolation.”
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1 Randleman, B. J. et al. Ophthalmology 2008;115:in press at the time EyeNet went to print. Also published online July 12, 2007.

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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.”
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1 Binder, P. S. et al. J Cataract Refract Surg 2005;31:2035–2038.
2 Curr Opin Ophthalmol 2007;18(4):284–289.
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MEET THE EXPERTS

Douglas D. Koch, MD Professor of ophthalmology, Baylor College of Medicine, Houston. Financial disclosure: Consultant with Alcon and AMO.

Yaron S. Rabinowitz, MD Director of eye research at Cedars-Sinai Medical Center, Los Angeles, and clinical professor of ophthalmology, University of California, Los Angeles. Financial disclosure: Consultant for Wavelight and Intralase.

J. Bradley Randleman, MD Assistant professor of ophthalmology, Emory University. Financial disclosure: None.

R. Doyle Stulting, MD, PhD Professor of ophthalmology, and director of the cornea service, Emory University. Financial disclosure: None.

William B. Trattler, MD In private practice at the Center for Excellence in Eye Care, Miami. Financial disclosure: None.

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