Effect of variable corneal polarisation compensation on retinal fiber layer thickness measurements by means of scanning laser polarimetry after laser assisted in situ keratomileusis.

Abstract

Abstract: : Purpose: to investigate the corneal polarimetric properties changes after laser assisted in–situ keratomileusis (LASIK) and their effect on peripapillary retinal fiber layer (RNFL) thickness measurements by means of scanning laser polarimetry (SLP) with variable corneal polarisation compensator. Methods: SLP was performed on 32 randomly selected eyes of 32 consecutive healthy subjects who underwent LASIK for ametropia correction. SLP was performed after variable corneal polarisation compensation (VCC) before LASIK and 8 days after LASIK using both the previous VCC and a new VCC. Corneal polarisation properties changes, thickness data changes and their correlations with corneal ablation parameters were analysed using the Wilcoxon and Spearman's Rho tests as appropriate. Results: mean spherical equivalent (SEQ) was –4,50±3,98 and –0,25±0,77 dioptres respectively before and after LASIK. Mean ablation depth was 84,86±38,69 microns. Corneal polarisation axis but not corneal polarisation magnitude significantly changed after LASIK (p=0,0001). Average and Superior RNFL thickness showed a significant change after LASIK using both the previous and the new VCC (Average: +6,4%, p=0,0003 and +3,5%, p=0,003; Superior: –1,2%, p=0,04 and +1,9%, p=0.04). NFI (a parameter showing the probability of having glaucoma) significantly changed only after the new VCC (–7%, p=0,01). Inferior RNFL thickness, the most stable parameter, did not show significant changes after LASIK neither with the previous nor with the new VCC. Corneal polarisation axis changes showed a moderate to good linear correlation with the ablation depth (Rho=–0,5; p=0,03), with the SEQ changes (Rho=–0.39; p=0.02) and with superior RNFL thickness changes after LASIK using the old VCC (Rho=0,4; p=0.02). Ablation depth showed a moderate correlation with average RNFL thickness changes only with the previous VCC (Rho=0,38; p=0.03). Conclusions: LASIK induces changes in RNFL average and superior thickness measurements which are statistically significant but clinically irrelevant. These changes are probably related to a shift of the corneal polarisation axis. A new corneal polarisation compensation after LASIK, allows to reduce these changes without restoring the pre–LASIK thickness values.

Keywords: nerve fiber layer • refractive surgery • imaging/image analysis: clinical