UC San Diego

Objective Local injections such as bupivacaine are used to treat strabismus, although the mechanism of its clinical effects is unknown. The purpose of this study is to analyze the effects of bupivacaine on extraocular muscle regeneration in a pre-clinical model. Design Laboratory study. Subjects, Participants, and/or Controls Three-month-old male New Zealand white rabbits with age- and sex-matched saline injection controls and un-injected controls. Methods One superior rectus per animal was injected with either 3% bupivacaine or saline control and harvested at 1 week, 1 month, or 3 months after treatment. Histomorphometric analysis was used to determine the effects of bupivacaine on extraocular muscles. Data were analyzed for the global and orbital layer separately, as well as the entire muscle cross-sectional area. Main Outcome Measures. Muscle morphology, markers of muscle regeneration, muscle size. Results 3% bupivacaine treatment has a myodestructive effect on the injected extraocular muscle with disruption of myofiber organization and areas of missing myofibers. However, even one week after injection, we observed significantly more embryonic myosin heavy chain positive myofibers, which signify newly regenerating myofibers, in the bupivacaine group compared to the saline group. Embryonic myosin heavy chain is still significantly expressed at 1 month after bupivacaine injection in extraocular muscles, in stark contrast to skeletal muscles elsewhere in the body where embryonic myosin heavy chain decreases 1-2 weeks after injury. There is a significant increase in the density of muscle stem cells and myofibers with centralized nuclei 3 months after bupivacaine injection compared to saline in both the orbital layer and global layer. Muscle size and extracellular matrix components did not increase at 3 months after bupivacaine injection compared to saline. Conclusions Bupivacaine’s effects on extraocular muscles may be explained in part by bupivacaine triggering a regenerative response, initially forming new myofibers and increasing the number of muscle stem cells to exert its effects as a treatment for strabismus.