Abstract
purpose. Nerve fibers in the cornea are disrupted by photorefractive procedures. In this study, the denervation and reinnervation of human central corneas were evaluated by sequential, quantitative measurements of nerves viewed by confocal microscopy in vivo during the first year after LASIK.
methods. Seventeen eyes were studied of 11 patients who had undergone LASIK to correct myopia from −2.0 D to −11.0 D. Eyes were treated with an excimer laser with a planned 180-μm flap. Central corneas were scanned throughout their full thicknesses by confocal microscopy before and at 1 week and 1, 3, 6, and 12 months after LASIK. Nerve fiber bundles appeared as bright, well-defined, linear structures that were sometimes branched and usually appeared in several consecutive frames. The number of nerve fiber bundles per scan in two to eight scans per eye per visit was determined in the subbasal region, the full-thickness stroma, the stromal flap (layer between the most anterior keratocyte and the flap interface), and the stromal bed (layer between the flap interface and the endothelium).
results. In the subbasal region, the number of nerve fiber bundles decreased by more than 90% 1 week after LASIK and was significantly lower at all times after surgery than it was before surgery (P < 0.001). It increased 6 and 12 months after LASIK, but remained less than half of the preoperative value. In the stromal flap, the number of nerves at all times after surgery was also significantly less than before surgery (P < 0.001) and did not increase significantly by 1 year. In the stromal bed, there were no significant differences among any of the nerve measurements before and after LASIK (P = 0.24).
conclusions. In the corneal flap, the number of subbasal and stromal nerve fiber bundles decreases by 90% immediately after LASIK. During the first year after LASIK, subbasal nerve fiber bundles gradually return, although by 1 year their number remains less than half of that before LASIK.
The cornea is richly innervated by nerve fibers of the ophthalmic division of the trigeminal nerve.
1 Owing to this dense sensory innervation, the cornea is extremely sensitive to external stimuli, a property that is very important for stimulating tear secretion, maintaining normal physiologic balance, and initiating aversion reflexes that serve to protect the eye.
2
Keratorefractive surgery disrupts the integrity of corneal nerves. In photorefractive keratectomy (PRK) the epithelium and anterior stroma, which contain most of the nerve fibers, are removed. In laser in situ keratomileusis (LASIK), which is now the more widely used procedure for correcting myopia, the microkeratome cuts the subbasal nerve bundles and the superficial stromal nerve bundles in the flap interface, although nerves that enter the flap through the hinge region are spared. The nerves of the anterior stromal bed, which is subsequently ablated, are destroyed by the excimer laser treatment. Nerve fibers gradually reinnervate the cornea and sensation slowly returns,
3 4 5 6 although it is not known how quickly the nerves return or whether their number is as high as it was before treatment.
Subbasal and stromal nerves have been studied histologically in postmortem specimens obtained after LASIK in rabbits
7 and in specimens obtained after LASIK in humans.
8 9 Clinical confocal microscopy has provided a means of repeated noninvasive examination of corneal nerves.
10 11 The clinical confocal microscope has been used to examine corneal nerves after LASIK procedures in humans in both cross-sectional
3 12 and longitudinal
13 studies. None of the investigations, however, attempted to quantify the number of nerves or their density. In this longitudinal study, we used confocal microscopy to measure sequential changes in subbasal and stromal nerve density for 12 months after LASIK.
The study included 17 eyes of 11 patients (the remaining 5 eyes in these patients had reoperations for undercorrections and were excluded). There were 10 women and 1 man, aged 20 to 46 years (mean age ± SD, 32.2 ± 9.2). All eyes had normal anterior segments, intraocular pressures (≤22 mm Hg), and fundi. Contact lens wear was discontinued 2 weeks (soft lenses) or 3 weeks (hard lenses) before the operation. Patients who had diabetes mellitus or glaucoma or were currently using any topical ocular medication were excluded from the study. The mean preoperative spheroequivalent of refraction was −6.56 ± 2.44 D (range, −2.00 to −11.00 D). No patients wore contact lenses after LASIK. The Mayo Clinic Institutional Review Board approved the study, which conformed to the principles of the Declaration of Helsinki for research involving human subjects. All patients provided informed consent after the nature and possible consequences of the study were explained to them.
Myopia or myopic astigmatism was corrected by LASIK performed with an excimer laser (Star; VISX, Santa Ana, CA). A flap was created with a planned thickness of 180 μm by using a microkeratome (Hansatome; Chiron Vision Corp., Claremont, CA). Patients fixated on a target during the ablation. The mean planned ablation depth was 62.8 ± 26.2 μm (range, 18–110). The stromal bed was irrigated with balanced salt solution at room temperature before and after flap replacement to eliminate residual debris. At the end of the operation, the flap was allowed to dry in place for at least 3 minutes to facilitate adhesion.
After LASIK, topical medications consisted of fluorometholone 0.1% (FLM; Allergan Inc., Irvine, CA) four times per day for 1 week and tapered over 2 weeks, and ofloxacin 0.3% (Ocuflox; Allergan Inc., Irvine, CA) four times per day for 5 days.
The postoperative nerve fiber bundle data were not normally distributed, primarily because of the large number of scans without visible nerves. Therefore, they were summarized by using medians and interquartile ranges (25th and 75th percentiles, Q25 and Q75, respectively). Median numbers of nerve fiber bundles in each region at each observation time were compared by using the Friedman test, a nonparametric version of the repeated measures analysis of variance. Significant differences were investigated after adjusting for multiple comparisons by using the Student-Newman-Keuls procedure. P < 0.05 was considered statistically significant. Generalized estimating equation (GEE) models were completed to account for any potential correlation between the two eyes of an individual. In all cases, results of the GEE model were similar to results of the Friedman test, and only the results of the Friedman test are presented.
The numbers of nerve fiber bundles in each region of the cornea at each observation time are given in
Table 1 and
Figure 3 . Of the approximately 6000 nerve fiber bundles identified in this study, only 43 were less than 200 μm in visible length. They were all in subbasal nerves from 1 to 12 months after LASIK. Before LASIK, the subbasal region contained the greatest number of visible nerve fiber bundles. The number decreased in deeper layers and was lowest in the layers destined to become the stromal bed.
After LASIK, the number of nerve fiber bundles in the subbasal region decreased significantly (P < 0.001) by more than 90%, compared with the number before LASIK. All estimates in this region after LASIK were significantly different from each other, except between 1 week and 1 month and between 1 and 3 months. There was no significant correlation between the intended ablation depth and the change in subbasal nerve fiber bundles from 3 to 12 months after LASIK (r = −0.22, P = 0.39). In the full-thickness stroma and in the stromal flap, the numbers of nerves identified at all times after LASIK were significantly less than estimates before (P < 0.02 and P < 0.001, respectively), and post-LASIK estimates were not significantly different from each other. In the stromal bed there were no significant differences among any estimates before or after LASIK (P = 0.24).