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Surgeons should be vigilant for cases of pellucid marginal degeneration (PMD), which is difficult to detect and may result in ectasia after LASIK. Individuals with PMD who seek refractive surgery are at high risk for ectasia if not detected in preoperative screening.
Individuals with PMD often seek refractive surgery because they have had unsatisfactory results with glasses or contact lenses, but their condition places them at high risk for ectasia if not detected in preoperative screening, said Dr. Rabinowitz, director of ophthalmology research, Cedars-Sinai Medical Center, Los Angeles.
PMD is a devastating complication of LASIK or PRK, likely to result in an extremely unhappy patient and possibly in litigation, he continued. Therefore, clinicians should become familiar with the subtle topographic and clinical changes that occur in PMD so that they can be recognized during surgical prescreening.
Other clinical features of PMD include against-the-rule astigmatism with flattening in the area above the thinning, prominent lymphatics at the inferior limbus, scissoring on dilated retinoscopy, and an older age at onset. Keratoconus typically begins shortly after puberty and stops in the mid-30s, whereas PMD more commonly commences in the early 30s and continues to progress.
Value of videokeratography
Videokeratography is extremely useful in differentiating the two conditions, Dr. Rabinowitz said. The view in cases of keratoconus is an asymmetric bowtie witha skewed radial axis pattern. In PMD, videokeratography will show a crab-claw appearance with inferior flattening.
Moderate to advanced cases of PMD are relatively easily detected during a slit-lamp examination. Early forms, however, can be very subtle and often difficult to recognize; often these patients can have a refraction of 20/20, Dr. Rabinowitz said.
Pachymetry results may provide a clue to a suspected case of PMD, even if the eye appears clinically normal. Because the cornea typically gets thicker from the center to the periphery, results showing very little difference between central and inferior pachymetry warrant closer investigation.
In more advanced cases, the typical crab-claw topography will be a prominent indicator of PMD, but in the early stages, inferior flattening characterized by an isolated inferior blue spot may be the only suggestion of the disease, Dr. Rabinowitz explained.
He reviewed the outcomes of five patients suspected to have PMD who had refractive surgery (three LASIK and two PRK) to suggest the risks of performing refractive surgery in this group.
"Every single one of them ended up being very unhappy with their refractive outcome," Dr. Rabinowitz said. "Two required grafts, and three of them required rigid gas-permeable contact lenses."
Postoperative acuity ranged from 20/70 to 20/400, and patients experienced side effects such as severe central corneal scarring, haze, and glare.
"It's really important to screen carefully to prevent ectasia," Dr. Rabinowitz emphasized. "Do a careful slit-lamp evaluation. Include [suspected PMD] in your differential diagnosis. Atypical topography and normal central pachymetry may fool the unsuspecting refractive surgeon."
Treatment options for PMD include glasses, contact lenses, crescentic lamellar keratoplasty, or combined lamellar keratoplasty and penetrating keratoplasty. Placement of intrastromal corneal implants (Intacs, Addition Technology Inc.) with a femtosecond laser (IntraLase, Advanced Medical Optics) is a new, good, and safe reversible treatment option for these patients, Dr. Rabinowitz said.