SCIENTIFIC CORRESPONDENCE Influence of post-LASIK corneal healing on scanning laser polarimetric measurement of the retinal nerve fibre layer thickness

Aim: To investigate the influence of laser assisted in situ keratomileusis (LASIK) on the values for retinal nerve fibre layer thickness (RNFLT) as measured with scanning laser polarimetry (SLP) during the healing process of the cornea after LASIK. Methods: SLP with the GDx instrument was performed on 20 consecutive healthy subjects without any eye disease undergoing LASIK for ametropia correction. The SLP measurements were performed before the surgery, and at 1 and 3 days, as well as at 3 months, after LASIK. Thickness data from images of one randomly selected eye per subject were analysed using the ANOVA and Duncan multiple comparison tests. Correlation coefficients between RNFLT data and the treatment parameters were also calculated. Results: Somewhat similar results were found for the different retinal areas. The measured values for superior average RNFLT decreased significantly at all time points compared to the preoperative baseline (p<0.003, Duncan test), but increased significantly between postoperative day 1 and the final visit at 3 months (p=0.025, Duncan test). Inferior average RNFLT in the early postoperative days was significantly smaller than at 3 months after LASIK (p<0.05, Duncan test), and tended to be smaller than at baseline. The thickness values before surgery and at the final visit, however, showed no significant difference (p=0.698, Duncan test) in this region. Ellipse average RNFLT was significantly smaller in the early postoperative days than the baseline value before LASIK. However, the measured value had significantly increased again by the time of the final visit (p<0.02, Duncan test). This value at the final visit showed no difference from the baseline value (p=0.46, Duncan test). The changes in the nasal average and temporal average RNFLT were not statistically significant. No correlation was found between the change in the SLP measured thickness values and central corneal thickness at baseline and its change after surgery, nor with the change in cylindrical correction due to LASIK, or the length of the suction time during surgery (p>0.05 for all correlations). Conclusion: The SLP technique is sensitive to the corneal optical properties, and RNFLT as measured with SLP shows changes after LASIK. Most of these changes, however, diminish with time after surgery, and the values tend to return to the preoperative results during the first 3 months of corneal healing following uncomplicated LASIK. It appears that in uncomplicated cases the transient RNFLT changes are artefacts and do not imply pathological thickness alterations due to LASIK. Scanning laser polarimetry (SLP) of the retinal nerve fibre layer has become a widely used technique for glaucoma diagnosis and, more recently, for follow up monitoring. The measurement principle is based on the optical retardation caused by the parallel retinal nerve fibres around the optic nerve head in the illuminating laser beam, which is reflected from the eye back to the detector. The linear relation between the thickness of the retinal nerve fibre layer and the measured optical retardation is used for the software provided calculation of the retinal nerve fibre layer thickness (RNFLT) in each pixel of the image. These measurements provide the basis for the calculation of the RNFLT for the different retinal sectors, and for the diagnostic algorithm for the detection of glaucoma. Since glaucoma is defined as a progressive loss of the retinal ganglion cells and their axons, 13 SLP provides a non-invasive technique which is, in principle, specific for the diagnosis and follow up of glaucoma and other optic neuropathies. Optical retardation, however, is caused not only by the axons of the retinal ganglion cells. The cornea is another important source of retardation, and this fact influences the diagnostic sensitivity of SLP. 15 The commercially available instrument for SLP, the GDx nerve fibre analyser (Laser Diagnostic Technologies, Inc, San Diego, CA, USA, Software Version 1.0.16) contains a built-in compensator, which compensates for the average corneal polarisation axis; but it cannot be individualised for the particular eye. Since the axis of corneal polarisation has been found to be stable over a period of at least 1 year, this would not seem to affect the longitudinal follow up with SLP, provided that the cornea remains intact. However, refractive corneal surgery, such as photorefractive keratectomy and laser in situ keratomileusis (LASIK), does influence the status of the cornea. Since tonometry after LASIK and photorefractive keratectomy underestimates the true intraocular pressure due to the decrease of the central cornea thickness, SLP will in future probably attain even greater importance in glaucoma diagnosis and follow up after refractive corneal surgery. It has been demonstrated by our group, and confirmed by other authors that SLP measurements and the reproducibility of the SLP measurement are not influenced by photorefractive keratectomy. In contrast with this a decrease of polarimetric RNFLT has been reported in the early period following LASIK surgery. However, at the same time the RNFLT as measured with scanning laser tomography and optical coherence tomography remained stable. This