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LASIK for moderate to high myopia does not appear to be a cause of retinal nerve fibre layer damage in otherwise healthy, nonglaucomatous eyes, according to a study measuring retinal nerve fibre layer thickness using scanning laser polarimetry (SLP). Speaking at the annual meeting of the American Academy of Ophthalmology, Neil T. Choplin MD presented the findings of a study that used SLP (GDx VCC, Laser Diagnostic Technologies Inc) to measure retinal nerve fibre layer thickness in 57 eyes prior to LASIK and twice at one month after the refractive surgery. The researchers measured once using the preoperative variable corneal compensation settings and another time using new customised variable corneal compensation settings obtained after surgery. All eyes had more than -5.0 D of myopia and the average SE for the group was -7.5 D. The average amount of corneal stroma ablated was 101 microns. The results showed LASIK changed the anterior segment birefringence, which was not unexpected since LASIK changes structure of the cornea. When that change was not accounted for, all of the retinal nerve fibre layer values appeared to worsen significantly. However, none of the SLP measurements obtained with the new postoperative compensation settings were significantly different from their preoperative values. "Use of the microkeratome during LASIK has raised concern that the application of high vacuum and transiently high IOP could damage the nerve fibre layer, and there are conflicting reports to date about that risk, with some studies showing there is no effect, especially in low myopes, whereas other studies of moderate to high myopes indicate reductions in retinal nerve fibre layer thickness. However, those changes were not confirmed with other imaging technologies," said Dr. Choplin. "Using variable custom corneal compensation to correct for LASIK-induced anterior segment birefringence changes, our study shows that this surgery is not associated with a significant change in retinal nerve fibre layer thickness. However, any effect associated with LASIK-associated microkeratome application in eyes with pre-existing optic nerve or retinal nerve fibre layer damage that are at greater risk for injury from even a transient IOP elevation is yet to be determined."An experienced operator, Qienyuan Zhou, PhD, an employee of Laser Diagnostic Technologies, performed all the SLP measurements for the study. The LASIK procedures were performed at the Naval Medical Center in San Diego , California . Comparison of the preoperative and postoperative corneal compensation values showed that LASIK resulted in a significant shift in the slow polarisation axis of anterior segment birefringence, from a mean of 29.3 degrees nasally downward to 13.9 degrees nasally downward. However, the magnitude of anterior segment birefringence was not significantly changed, with the mean pre-and post-operative values being 37.1 nm and 39.5 nm, respectively. Preoperative to postoperative changes in the 12 nerve fibre layer output measures obtained with SLP were tested for statistical significance using methods that corrected for multiple comparisons. When the comparisons were made using the postoperative data obtained with the preoperative variable corneal compensation settings, 11 of the 12 SLP parameters indicated significant worsening after surgery. Only symmetry was not significantly changed. However, with the appropriate correction for the LASIK-induced change in corneal birefringence, none of the SLP parameters showed a statistically significant change after the refractive surgery. Dr. Choplin explained that while SLP is designed to measure the thickness of the retinal nerve fibre layer as a function of its birefringence, the signal it detects represents the birefringence of the entire eye, including the anterior segment. The contribution of the retinal nerve fibre layer is isolated using a proprietary compensator that removes the anterior segment portion of the signal, the majority of which is accounted for by the cornea. The early versions of the commercially available scanning laser polarimeter incorporated a fixed compensator. That technology was based on the erroneous assumption that all corneas were the same with a 60 nm magnitude and slow polarisation axis of 15 degrees nasally downward. "However, direct measurements of anterior segment birefringence show that up to 30% of eyes have a slow polarisation axis more than 10º away from the assumed value and there is wide variation in magnitudes, which range from 0 to more than 120 nm. Therefore, the majority of patients being evaluated with the early iterations of the GDx had anterior segment values that did not match the fixed compensator, and consequently, the machine had fairly poor sensitivity and specificity," Dr. Choplin said. The customised corneal compensator uses a scan from the macula to determine the amount and orientation of anterior segment birefringence, and it can be varied in magnitude and orientation to fit most corneas.
Neil Choplin MD
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