Ultrastructure of mepivacaine-induced damage and regeneration in rat extraocular muscle.

Adult rats were given a single retrobulbar injection of 50 microliters of 2.0% mepivacaine and the lateral rectus muscles were examined ultrastructurally at intervals from 15 min to 30 days post-injection. There were three purposes of the study: (1) to determine the extent of muscle fiber damage caused by the anesthetic; (2) to document the subsequent course of muscle fiber regeneration; and (3) to relate these findings to clinical data on possible adverse effects of local anesthetics on human extraocular muscle function. The lateral rectus muscle was massively damaged by exposure to the anesthetic, with membrane lesions seen as early as 15 min after the injection. Intracellular damage was followed by the phagocytic removal of the remnants of the damaged muscle fibers. The activation of satellite cells to myoblasts began during the phase of phagocytosis, and between 3 and 4 days after injection multinucleated myotubes actively forming sarcomeres appeared. Even during later stages of muscle fiber regeneration, evidence of damage was seen in muscle fibers that were not destroyed during the first 2 days post-injection. The results of this experiment show (1) that the vast majority of lateral rectus muscle fibers are rapidly broken down by the anesthetic, but that the destroyed muscle fibers are replaced by regenerating ones; and (2) that the ultrastructure of regeneration of extraocular muscle fibers differs little from the regeneration of somatic muscle fibers. The myotoxic effects of retrobulbarly applied local anesthetics in rats seem to be much greater than they are in primates.