Chronic myopathy induced by repeated bupivacaine injections.

The effect of recurrent cycles of muscle fiber degeneration-regeneration was studied by repeated bupivacaine injections into the rat anterior tibial muscle. Injections of 0.6 ml of 0.75% bupivacaine were performed weekly for 6 months. The rats were allowed to recover for another 2 months and then killed. Histological and histochemical stains showed striking changes, including marked variability in fiber size, numerous internal nuclei, extensive fiber splitting and many whorled fibers. Combined staining for end-plate cholinesterase and terminal axons showed a markedly enlarged zone of terminal innervation. These findings suggest that the observed morphological changes usually attributed to a primary myopathic process may instead be the manifestations of impaired and incomplete regeneration occurring after cycles of degeneration-regeneration.