The Best of Ophthalmology.....EyeWorld News Magazine

Cutting to the truth about corneal ectasia

by Maxine Lipner Contributing Editor

One of the most serious LASIK complications a patient can encounter is corneal ectasia, a condition that can lead to progressive myopia and astigmatism, loss of uncorrected acuity, and loss of best-corrected acuity. Unfortunately for the physician, it can be difficult to predict which patients are likely to develop ectasia. However, investigators now find that several risk factors, including high myopia, forme fruste keratoconus, optical zone size, and high intraocular pressure, likely play an important role. Indeed, one study indicates that 88% of patients who developed ectasia had forme fruste keratoconus.

Studying patient risk factors

R. Doyle Stulting, MD, PhD, professor of ophthalmology at Emory University, Atlanta, recently took a closer look at contributory factors for ectasia in a retrospective observational study. Stulting surveyed patients from his contact lens practice and identified 13 who had corneal ectasia to try to determine predictive factors for the condition. "Six of these patients were excluded from the analysis because of poor preoperative data, or a defined cause of ectasia, leaving us with 10 eyes of seven patients," Stulting said.

A typical patient in his study was a 44-year-old woman, with preoperative acuity of -8.5 D, and pachymetries of about 500, who underwent LASIK. The flap cut was made with the Hansatome microkeratome with a 180 plate. "Usually we get about 150mm average with our microkeratomes, so that is a very conservative estimate of the amount of tissue that we removed," Stulting said. By three months postoperatively, the patient's Orbscan showed some inferior steepening and inferior thinning. By six months, it was apparent that the patient had corneal ectasia.

When Stulting reviewed the data from the seven patients, some commonalities emerged. Patients who developed ectasia tended to be highly myopic. The mean spherical equivalent in this population was 9 D of myopia. By comparison, Stulting's typical LASIK patient population at Emory Vision at the time had only about 4.5 D to 5 D of myopia. "So these ectasia patients, we believe, were significantly more myopic," Stulting said.

Cases of forme fruste keratoconus, defined as topographic abnormalities without clinical signs of keratoconus, were also much more evident in this population. "Preoperative topographies showed forme fruste keratoconus in seven of eight eyes or 88%," Stulting said. Another key factor considered was stromal bed thickness, which Stulting calculated using preoperative and postoperative pachymetry. "There were seven eyes with a residual stromal bed of less than 250 mm and four with a residual stromal bed of less than 200 mm," Stulting said. "All of our patients had risk factors of one sort or another, and all of them had at least one risk factor other than myopia greater than 8 D." Most patients had two or three risk factors, investigators found. "We found that 88% of patients had forme fruste keratoconus, 50% had myopia over 8 D and 70% had residual stromal beds of less than 250 mm," Stulting said.

Investigators compared these results to 100 consecutive cases that involved patients with more than 8 D of myopia and one year of follow up preoperative and postoperative data. "Forme fruste keratoconus was the most significant discriminator," Stulting said. "Thin residual stromal beds were significant as well, but not nearly to the extent of forme fruste keratoconus for the development of postoperative ectasia."

Possible LASIK factors

Factors such as optical zone size, ablation depth, and intraocular pressure also may play a pivotal role in the development of ectasia, believes Maria Jose Cosentino, MD, associate director, Instituto de la VisiÃ³n, Hospital de ClÃnicas, School of Medicine, University of Buenos Aires, Buenos Aires, Argentina. Cosentino, together with Carlos J. Argento, MD, director, Instituto de la VisiÃ³n, recently analyzed the influence of these factors in corneal ectasia following LASIK. "Our purpose was to analyze the influence of some of the different factors involved - the residual stromal bed depth, the optical zone size, and the intraocular pressure," Cosentino said.

The investigation used a computer simulation to cross these three. By leaving the same residual bed, but using different optical zones and different IOPs, the investigators were able to simulate conditions under which ectasia was likely to occur. For example, with a residual stromal bed of 370 mm with an optical zone of 5 mm and an IOP of 21-mm-Hg corneal displacement was almost 30 mm. When the optical zone was brought out to 7 mms with an IOP of 15-mm-Hg, corneal displacement went down to 16 mm. With a constant ablation depth, the corneal displacement that occurred with simulated smaller optical zones at 18.39 mm was greater than that resulting from larger optical zones, at 15.28 mm, Argento and Cosentino found.

The simulation included residual beds of 420, 370, 320, and 270 mm. There also was a correlation to the amount of tissue left in the stromal bed. "With the thinnest residual stromal bed we had a major corneal displacement that reached almost 80 mm - so at this level it appears as clinically significant corneal ectasia," Cosentino said. Investigators found that there was an important collective interplay between the optical zone size, ablation depth and intraocular pressure. The thin residual bed, the small optical zone (5 mm optical zone) and the high intraocular pressure produced more corneal ectasia, and the thick residual bed, the large optical zone (8 mm/9 mm) and the low intraocular pressure produced less corneal ectasia, Cosentino said.

While there is no information in the literature on the role of intraocular pressure and optical zone size in corneal ectasia after LASIK, Cosentino believes that the evidence suggests that this is potentially an important contributory factor and should be kept in mind. "We ask you to consider the use of the ocular hypotensive drugs in cases of corneal ectasia after LASIK," Cosentino said.

Contact Information
Cosentino: Fax +54-11-48-22-83-74
Stulting: 404-250-9700, fax 404-250-9066

Key risk factors such as forme fruste keratoconus, high myopia, high intraocular pressure, optical zone size have been fingered in the development of corneal ectasia.