'Zapping the flap' may be valuable in re-treatments

New Orleans-Surface ablation is an important option for enhancement procedures in eyes with thin corneas or previous flap complications that are at risk for poor outcomes after LASIK treatment, said Raymond M. Stein, MD, at the International Society of Refractive Surgery subspecialty day at the American Academy of Ophthalmology meeting.

"We are now seeing enhanced outcomes with surface ablation after LASIK due to smoother ablations and improved optical and transition zones, along with use of adjunctive techniques, including cooling strategies using a frozen BSS 'popsicle' that may decrease corneal inflammation, intraoperative mitomycin-C, and perioperative vitamin C," said Dr. Stein, medical director, Bochner Eye Institute, and assistant professor of ophthalmology, University of Toronto, Ontario.

"[Previously], at the refractive surgery subspecialty day meeting in 1998, David Hardten, MD, recommended [that ophthalmologists] 'don't zap the flap' based on clinical reports showing risks of visually significant haze and scarring after such procedures," Dr. Stein said.

Indications Discussing thin corneas, Dr. Stein observed that the threshold for the minimal amount of tissue to be left in the bed has increased over time, and currently, 250 µm is the generally accepted threshold. He noted that if the surgeon decides to proceed with LASIK enhancement, the bed thickness should be measured before proceeding with the ablation.

"If residual stromal thickness is estimated to be less than 250 µm after LASIK enhancement, consider repositioning the flap and proceeding with surface ablation, possibly with adjunctive mitomycin-C," Dr. Stein said.

He added that surgeons are likely to notice significant wrinkles in Bowman's membrane in a very high percentage of those eyes. That phenomenon is probably secondary to the difference in surface area between the bed and flap but does not appear to affect BCVA.

In any eye where there has been a flap complication, recutting the flap to perform LASIK may be best avoided, based on a report showing that procedure is associated with interface slivers causing loss of BCVA. In addition, in the case of a buttonhole flap, there is significant risk for another buttonhole if a flap is recut in an eye with a steep or thin cornea.

In contrast, surface ablation may minimize the risks of corneal haze and scarring that can occur when a buttonhole is present, and intraoperative mitomycin-C may further decrease those risks.

"Although surface ablation has been reported to be performed even immediately in eyes with a buttonhole flap, most surgeons recommend waiting 1 to 2 months until the refraction is stable and there is no evidence of epithelial ingrowth," Dr. Stein said.

In eyes with an irregular or partial flap, there is a risk of recurrent problems with flap recutting, especially if risk factors for suction loss are present. Surface ablation avoids that issue and allows treatment with full optical and transition zones while also potentially reducing superficial scarring, he continued.

In eyes with epithelial basement membrane dystrophy that was not recognized at the time of the primary LASIK procedure, surface ablation is preferred for any necessary enhancement since lifting the flap poses a risk for epithelial ingrowth. Phototherapeutic keratectomy (PTK) on the surface of the flap is an option in eyes with recurrent erosion, Dr. Stein said.

As reported by Roger Steinert, MD, PTK with a broad-beam laser also offers an alternative to treat eyes with chronic striae that are refractory to other standard techniques.

In his protocol, laser pulses are applied until the epithelial fluorescence begins to disappear between the striae. Then a masking agent is used to create a smoother surface by filling in the valleys, and additional pulses are applied to reduce the higher striae peaks.

Use of PTK with that method has resulted in improved BCVA and quality of vision, Dr. Stein noted.

Results of various studies highlight the efficacy and safety now attainable when performing PRK enhancement in eyes previously treated with LASIK. In his own series of 25 eyes that underwent PRK re-treatment with adjunctive mitomycin-C, Dr. Stein reported no eye developed significant corneal haze or experienced loss of BCVA.