Corneal damage in photokeratitis--why is it so painful?

The primate cornea was examined ultrastructurally subsequent to ultraviolet radiation (UVR) (300 nm) exposures of 0.08 and 0.225 J/cm2. The lower exposure caused significant, but incomplete, destruction of the epithelium and mild edema of the posterior stroma. The higher exposure resulted in complete destruction of the epithelium, significant stromal swelling, and some decomposition of endothelial cells. Widespread intracellular edema of the endothelium was also noted with the higher exposure. The ultrastructural examination subsequent to these UVR exposures also helps to explain why UV-induced keratitis is so painful. The 0.08 J/cm2 exposure left the epithelial and subepithelial-stromal nerve plexuses intact, whereas the 0.225 J/cm2 exposure destroyed the epithelial axons but spared the subepithelial ones. These results suggest that the pain associated with UV keratitis results from the combined effect of epithelial cell loss and sparing of the subepithelial axons.