WFG offers therapeutic option for postRK eyes

March 7, 2012

Wavefront-guided ablation using either LASIK or PRK is a safe and effective therapeutic option for improving visual problems in patients with a history of radial keratotomy, according to Mounir A. Khalifa, MD.

Alexandria, Egypt-Wavefront-guided (WFG) ablation using either LASIK or PRK is a safe and effective therapeutic option for improving visual problems in patients with a history of radial keratotomy (RK), according to Mounir A. Khalifa, MD.

“Many patients who have undergone incisional refractive surgery experience loss of best spectacle-corrected visual acuity (BSCVA) and complain about halos, glare, starburst, and reduced contrast sensitivity,” said Dr. Khalifa, professor and chairman, Department of Ophthalmology, Tanta University, Alexandria, Egypt.

“Management of these problems is complicated by a number of challenges, including difficulty with accurate refraction, presence of irregular astigmatism, increased ocular aberrations, and risks of scarring and haze with laser vision correction,” he said. “While there are some disadvantages for using either LASIK or PRK in eyes with previous RK, our experience shows that if a reliable wavefront map can be obtained, these eyes can benefit from WFG laser vision correction.

Dr. Khalifa added several caveats, however. The choice between LASIK and PRK should take into account the number of RK incisions and time since RK. Due to the possibility for RK scars to give way with LASIK flap creation or elevation, LASIK is considered for eyes with eight incisions or less and if RK was performed at least 10 years earlier. In addition, it should be performed using a thick flap and a waiting period of 3 months between flap creation and aberrometry/ablation to avoid the Arabian tent effect.

PRK is performed in eyes with more than eight incisions or that are less than 10 years postRK, but an Amoils brush should not be used for epithelial removal, and mitomycin-C 0.02% is applied for 40 to 60 seconds intraoperatively to minimize the risk of haze.

“If one cannot obtain a reliable wavefront map, which is defined as having reproducible results in three consecutive captures, we recommend the ablation be performed using a topography-guided technique,” Dr. Khalifa said.

He presented results from a series of 39 postRK eyes that were treated with a WFG ablation at the Horus Vision Correction Center, Alexandria, Egypt, of which 21 were treated with LASIK and 18 with PRK. LASIK was performed using an ophthalmic diagnostic instrument (WaveScan, Abbott Medical Optics) for aberrometry and a microkeratome (M2, Moria) to create a 130-μm flap.

In the LASIK group, mean age was 41.5 years, mean manifest sphere was –3.73 D, and mean cylinder was –1.9 D. The mean age of the PRK patients was 10 years younger, 32.5 years, and they had a mean manifest sphere of –2.92 D and mean cylinder of –1.9 D. For the entire cohort, mean follow-up after WFG laser vision correction was 3.5 months with a range between 1 and 12 months.

There was no significant difference between groups in the efficacy index, but it was <80% in both groups (LASIK 71%, PRK 77%) since correction of all preoperative manifest refraction was limited by RK scars and the difference in ablation efficacy in the scarred tissue, Dr. Khalifa noted.

At 3 months after the surgery, MRSE was similar in the two groups and <1.5 D in all eyes. Almost 90% of eyes in both groups had <1 D of residual SE, and MRSE was <0.5 D in just over half of the eyes in each group.

The safety index exceeded 100% in both groups. However, the results favored LASIK over PRK (172% versus 148%) because BSCVA losses of 1 and 2 lines occurred in two PRK eyes as a result of haze.

“There were no cases of BSCVA loss after LASIK, and BSCVA improved by 1 or 2 lines in about 40% of eyes in both the LASIK and PRK,” Dr. Khalifa said. “These gains in BSCVA can be attributed to improved irregular astigmatism and reductions in higher-order aberrations (HOAs).”

Mean RMS values for total HOAs, coma, trefoil, and spherical aberration significantly improved in both groups. Mean spherical aberration RMS improved significantly more after LASIK than after PRK, with a shift from positive to negative in the LASIK eyes, while coma and trefoil improved significantly more after PRK than after LASIK.

“Intersection of the RK cuts with LASIK flap creation can induce new surface irregularities that can lead to refractive errors and new HOAs,” he said. “This issue is avoided with PRK, but corneal haze is a risk after PRK and patients also experience more postoperative discomfort and delayed visual recovery.”

The LASIK-treated eyes also benefited with a significant improvement in mesopic contrast sensitivity, while there was no change in this quality of vision measure after PRK.

“The improvement in contrast sensitivity after LASIK as well as in spherical aberration can be explained by the fact that the LASIK eyes had fewer RK scars,” Dr. Khalifa concluded.

Dr. Khalifa has no financial interest in the subject matter.

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