The Study Of Corneal Nerve Injury And Recovery After Excimer Laser Refractive Surgery

Objective To investigate morphological and functional changes in corneal nerves after laser in situ keratomileusis (LASIK) and epipolis laser in situ keratomileusis (epi-LASIK). Methods 18 healthy New Zealand white rabbits underwent LASIK on one eye and epi-LASIK on the contralateral eye randomizedly. 4 eyes of 2 normal rabbits were used as the normal control. The excimer laser photoablation procedures of LASIK and epi-LASIK were the same. The diameter of optical ablation zone was 6mm and the ablation depth was 81μm (-5. 0 diopters). The rabbits were killed at 1 day, 3 days, 1 week, 1 month, 3 months and 6 months after surgery. Corneal nerves were stained with the gold chloride impregnation method. Corneal sensitivity was examined using a Cochet-Bonnet esthesiometer preoperatively and at 1 day, 3 days, 1 week, 1 month, 3 months and 6 months after surgery. Results After epi-LASIK, superficial corneal nerves within the photoablation area disappeared. After LASIK, except the hinge of the flap, corneal nerves (including deep stromal nerves) within the flap disappeared. After epi-LASIK, a few regenerative nerve fibers emerged within the photoablation area at 1 day, and many long regenerative nerve fibers emerged with subepithelial nerve bundles rebuilded within a few areas at 3 days and 1 week. After LASIK, a few regenerative nerve fibers emerged at the edge of the flap and passed into the stroma of the flap at 3 days and 1 week, but no subepithelial nerve bundles were found. The recovery time of epithelial nerves in the center of the cornea was 1 to 3 months after epi-LASIK and 6 months after LASIK. After epi-LASIK, the superficial stromal nerve plexus within the photoablation area formed with abnormal morphology in the early period and rebuilded at 6 months. After LASIK, the superficial stromal nerve plexus within the flap formed with normal morphology at 6 months. Cornealsensitivity greatly reduced after LASIK and recovered to normal level at 6 months. Corneal sensitivity slightly reduced after epi-LASIK and recovered to normal level at 3 months. Corneal sensitivity was lower in LASIK group than epi-LASIK group at the same time-point postoperatively (t=16. 11, 18.50, 16.92, 5.20; P<0. 01). Conclusion Corneal nerves are injured more severely and recover slowlier after LASIK than epi-LASIK.Part IIThe experimental study of Changes of nerve growth factorafter LASIK and epi-LASIKObjective To detect the changes of nerve growth factor (NGF) in cornea during the corneal wound healing after LASIK and epi-LASIK. Methods 24 healthy New Zealand white rabbits underwent LASIK on one eye and epi-LASIK on the contralateral eye randomizedly. 8 eyes of 4 rabbits were used as the normal control. The excimer laser photoablation procedures of LASIK and epi-LASIK were the same. The diameter of optical ablation zone was 6mm and the ablation depth was 81μm (-5. 0 diopters). The rabbits were killed at 1 day, 3 days, 1 week and 1 month after surgery. The central part of cornea with 6mm diameter was cut for detecting. NGF protein expression in cornea was detected by Immunohistochemistry and western blot. NGF mRNA expression in corneal epithelium and stroma was detected by Reverse transcription (RT)-polymerase chain reaction (PCR). Results Immunohistochemical staining for NGF was very weak in the normal control and LASIK group. Immunohistochemical staining for NGF increased in the epithelial layer at 1 and 3 days after epi-LASIK, especially at 1 day. Comparing with the normal control, NGF protein expression reduced at 1 and 3 days (t=-12. 52, -9. 95; P=0. 001) after LASIK and recovered at 1 week (P>0.05). No differences were found in NGF protein expression between epi-LASIK group and the normal control (P>0.05). At 1 and 3 days after surgery, NGF protein expression was less in LASIK group than epi-LASIK group (t=-6.50, -5.85; P=0.023, 0.028). Comparing with the normal control, NGF mRNA expression in corneal epithelium was upregulated at 1 and 3 days after epi-LASIK with the highest level at 1 day (t=9. 88, 3.96; P=0.001, 0.021). No differences were found in NGF mRNA expressionin corneal epithelium between LASIK group and the normal control (P>0. 05). At 1 and 3 days after surgery, NGF mRNA expression in corneal epithelium was more in epi-LASIK group than LASIK group (t=6. 86, 6.82; P=0. 021, 0. 021). Comparing with the normal control, NGF mRNA expression in corneal stroma was upregulated at 1 day after epi-LASIK (t=5.25, P=0. 006). No differences were found in NGF mRNA expression in corneal stroma between LASIK group and the normal control (P>0. 05). At 1 day after surgery, NGF mRNA expression in corneal stroma was more in epi-LASIK group than LASIK group (t=26. 1, P=0.001). Conclusion Upregulated expression of NGF mRNA and protein in corneal epithelium during the early period after epi-LASIK may benefit corneal nerve regeneration and corneal sensitivity. The reduction of NGF protein expression in cornea during the early period after LASIK may be caused by extensive loss of corneal nerves. The reducton of NGF protein may be related to the great loss of corneal sensitivity and slow corneal nerve regeneration during the early period after LASIK.Part IIIThe clinical study of corneal sensitivity recoveryafter LASIK and LASEKObjective To compare the recovery of corneal sensitivity after LASIK and laser-assisted subepithelial keratectomy (LASEK). Methods Central corneal sensitivity (CCS) was measured with a Cochet-Bonnet esthesiometer in 59 patients before and 2 weeks, 1, 3 and 6 months after myopic correction by LASIK or LASEK. 50 patients were treated with LASEK and 9 patients were treated with LASIK. Results CCSs reduced in the early period after LASEK and LASIK, but more reduction was seen after LASIK. At every observing spot, the CCS after LASEK was better than LASIK (at 2 weeks, 1 and 3 months P<0. 01, at 6 months P=0. 031). In myopic correction of less than 6 diopters (D), the CCS recovered at 3 months after LASEK (P>0.05), but it didn't recover at 6 months after LASIK (P=0.023). In myopic correction of more than 6 D, the recovery of CCS was seen at 6 months after LASEK (P>0. 05). Correlations between CCS and ablation depth were found at 1 3 and 6 months after LASEK (r=-0. 419,-0.433, -0.413; P<0.01), but no correlation was found after LASIK (P>0. 05). Conclusion Recovery of corneal sensitivity is faster after LASEK than LASIK. The recovery of corneal sensitivity after LASEK is related to ablation depth.