The spectrum of fiber loss in a model of neuropathic pain in the rat: an electron microscopic study

Recently, Bennett and Xie reported that when the sciatic nerve of the rat is ligated loosely, the rat develops a pain syndrome with many features similar to those observed in neuropathic pain states in man. Anatomical and physiological studies to date indicate that the major pathology is a loss of large diameter myelinated fibers distal to the ligatures, with more subtle changes in small myelinated fibers. With a view to evaluating possible changes in the unmyelinated fibers, we have performed an electron microscopic analysis of the sciatic nerve 2 weeks after four ligatures were applied, at which time the animals displayed profound hyperalgesia and mechanical and thermal allodynia. Cross-sectional photomontages of regions proximal and distal to the ligatures were studied. Consistent with light microscopic and electrophysiological studies, we found a near complete loss of large myelinated fibers distal to the ligatures. Phagocytosis of large fibers was common. There was also considerable variation in the damage to small myelinated fibers. In some fascicles many small (less than 3 microns) myelinated axons remained; in other fascicles none could be detected. Importantly, we also found significant changes in the unmyelinated fiber spectrum. Counts of unmyelinated axons revealed a 34% and 71% decrease in the distal compared to the proximal nerve, in the two rats studied. The large clusters of unmyelinated axons that characterize normal nerve (and the nerve proximal to the ligatures) were rarely found distally. Rather, many of the unmyelinated axons coursed singly or in very loose bundles. Many of the surviving axons were shrunken and distorted, although still in contact with Schwann cells.(ABSTRACT TRUNCATED AT 250 WORDS)