Prolonged Impairment in Corneal Innervation Following Sulfur Mustard Exposure Contributes to the Development of Partial Limbal Defficiency

Abstract

Abstract: : Purpose: Ocular injuries induced by sulfur mustard (SM) are characterized by acute corneal erosions, anterior segment inflammation and delayed keratopathy, leading to irreversible visual deficits. Corneal nerves were shown to play an important role in the maintenance of corneal metabolism and healthy ocular surface. The present study focused on the role of corneal nerves in the pathogenesis and healing processes of SM ocular injuries. Methods: Animal Care and Use Committee approval at IIBR was obtained. Rabbit eyes were exposed to SM vapor for a period of four minutes. A clinical follow–up was carried out for one month. Corneal sensitivity was determined using the Cochet–Bonnet esthesiometer. Animals were euthanized at different time points, eyes enucleated and processed for histochemistry and histology. The effect of SM on the structure and density of nerves was studied in whole mount corneas, stained for cholinesterase (ChE) activity, using both light microscopy and morphometric analysis. Corneal sections were stained with H&E and PAS for general morphology and with the monoclonal antibodies AE1 and AE3 for keratin phenotype. Results: Typical signs of SM toxicity were observed within 3–4 hrs after exposure associated with photophobia, inflammation and corneal erosions. The erosions healed spontaneously within one week. Following an asymptomatic latent period 50–90% of corneas displayed clinical signs, characterized by neovascularization and recurrent erosions. The regenerating epithelium was abnormal in respect to adhesion and corneal phenotype. Corneal nerves displayed a typical Wallerian degeneration beginning few hours after exposure and lasting for weeks associated with alterations in corneal sensitivity. Only partial regeneration was noted. Thus, two simultaneous opposing processes were taking place at the cornea for weeks after SM exposure: healing of the epithelium, concomitant with degeneration of corneal nerves and a significant decline in their density. Conclusions: The improper regeneration of corneal epithelium and the delayed clinical signs are typical for partial limbal deficiency (PLD). The prolonged damage of corneal nerves may induce deficit of factors such as SP and CGRP that are essential for normal function of corneal epithelium and for limbal stromal microenvironment, contributing to loss of limbal stem cells and development of PLD. A supplement of trophic factors may be beneficial.

Keywords: cornea: epithelium • innervation: sensation • regeneration