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Editors Selection IGR 9-4

Visual function: Retinal nerve fibre layer and LASIK

Comment by David Greenfield on:

11721 Retinal nerve fiber layer measurements do not change after LASIK for high myopia as measured by scanning laser polarimetry with custom compensation, Choplin NT; Schallhorn SC; Sinai M et al., Ophthalmology, 2005; 112: 92-97

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12045 Scanning laser polarimetry retinal nerve fiber layer thickness measurements after LASIK, Zangwill LM; Abunto T; Bowd C et al., Ophthalmology, 2005; 112: 200-207

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Corneal ablation following LASIK has been demonstrated to affect intraocular pressure (IOP) measurements obtained with Goldmann applanation tonometry, and clinicians must consider this in the post-operative monitoring of glaucomatous eyes. LASIK also produces significant intra-operative IOP elevation for a duration of approximately 1 minute, prompting many investigators to appropriately investigate the hypothesis that pressure-induced damage to the optic nerve and retinal nerve fiber layer (RNFL) may develop. The studies by Zangwill et al. (164) and Choplin et al. (152) nicely complement the existing body of literature on this topic. Both studies evaluate the short-term (1-3 month) effect of LASIK on corneal birefringence and RNFL thickness among a series of myopic, non-glaucomatous eyes. Similar conclusions were identified: corneal birefringence is altered during LASIK and RNFL assessment using birefringence-based technologies (e.g. scanning laser polarimetry) may be subject to artifact unless custom corneal compensation strategies are employed such as those in the current commercial polarimeter (GDx-VCC, Carl Zeiss Meditec, Dublin, CA). Two points are worthy of consideration. Recent data¹ demonstrates that atypical birefringence images exist using GDx-VCC and are more common in myopic eyes of older patients. The patients in the series independently reported by Zangwill et al. and Choplin et al. concern eyes with moderate to severe myopia and many may have had atypical birefringence images. A support vector machine (SVM) score (0-100) has been shown to be well correlated with the magnitude of atypical birefringence and should be obtained in all subjects with exclusion of eyes with a score ≤ 60. Secondly, the natural history of RGC/RNFL injury leading to cell death and tissue atrophy sufficient to be detectable with posterior segment imaging technologies has not yet been determined. Thus, it remains unclear whether LASIK-induced IOP elevation actually produces in-vivo RNFL loss or simply no detectable short-term RNFL loss. It would be interesting to continue to follow these cohorts longitudinally to address this. These two papers represent important additions to literature and provide strong evidence that no short-term RNFL atrophy occurs following LASIK. These studies remind us that glaucoma patients undergoing refractive surgery represent a unique population that require a more thoughtful and analytical management approach. Long-term studies are necessary to substantiate whether RGC death develops in normal and glaucomatous eyes during suction-cup induced IOP elevation.

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