Photochemically-induced ischemic injury of the rat sciatic nerve: A light-and electron microscopic study

Lesion of presumably ischemic origin of the rat sciatic nerve was induced p hotochemically by laser irradiation combined with systemic administration o f a photosensitizing organic dye, erythrosin B. We have studied the patholo gic features of the nerve after the photochemical insult with light- and el ectronmicroscopy and related them to behavioral signs of neuropathic pain. At the irradiated nerve site, occlusion of blood vessels was seen and the v essels were packed with aggregated thrombocytes, fibrins and deformed eryth rocytes, supporting the notion that photochemical reaction caused intraneur al ischemia. The degree of the nerve injury at the center of irradiation wa s related to the duration of the laser exposure. Brief irradiation (30 seco nds) only caused identifiable injury to myelinated fibers, whereas longer i rradiation (2 minutes) caused greater injury to myelinated and unmyelinated fibers, characterized by extensive axonal degeneration and demyelination. The rats irradiated for 2 minutes, but not 30 seconds, exhibited neuropathi c pain-like behaviors, expressed as mechanical and cold allodynia. The nerv e injury was most severe 7 days after ischemia and regeneration of both mye linated and unmyelinated fibers was observed 3 months later. The nerve caud al to the irradiation exhibited Wallerian degeneration 7 days after the ins ult, whereas at 10 mm proximal to the irradiation the nerve was largely nor mal. It is thus concluded that photochemically induced intraneural ischemia caused injury to both myelinated and unmyelinated fibers, with myelinated fibers being more susceptible. However, the development of neuropathic pain -like behaviors may require injury to the unmyelinated fibers.