Morphological and Functional Factors of Corneal Nerves Following Refractive Surgery

Abstract

In addition to their sensory role, corneal nerves serve as a protective and trophic function in the cornea. Any alterations to this structure, such as those occurring during refractive surgery, may have significant implications for the maintenance of a healthy cornea. This study aimed to investigate 1) the morphological and functional changes in the cornea at two different locations (central and temporal), 2) symptomatology and 3) the possible relationship between specific neuroproteins detected in tears, and changes in structure and functional factors of the corneal nerves after various refractive surgery techniques including LASIK Mechanical Microkeratome (MM), LASIK FemtoSecond (FS), Photorefractive Keratectomy. Comparisons were made with age and gender matched controls. All subjects were followed for up to 3 months. Corneal nerve morphology and its function were severely altered after the surgery and these changes were evident in both central and temporal regions of the cornea. In all the surgical groups, corneal mechanical sensitivity recovered to pre-operative levels after three months whereas corneal nerve morphology did not return to its baseline values. This may suggest a degree of redundancy in the corneal sub epithelial nerve plexus. There was no difference between LASIK with MM and LASIK with FS in terms of the amount of nerve regeneration and its function. Most of the subjects exhibited increased symptoms of discomfort at three months after the surgery and there was an association between the signs and symptoms of ocular discomfort. In addition, differences were demonstrated in the clinical signs of ocular discomfort among various surgical groups, with patients undergoing PRK exhibiting fewer clinical signs of discomfort. This difference might be attributed in part to the higher expression of NGF levels measured after PRK in this study. All together, the evidence obtained during the course of this study suggests that there is a connection between alterations in the corneal nerve structure, persisting corneal staining, increased neurotrophic effect of NGF, and increased ocular discomfort three months after surgery. The processes underlying the increased symptoms of discomfort following refractive surgery might be influenced by changes in tear film such as modulation of NGF, changes in corneal epithelial integrity and altered nerve fibre density.