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Facial muscles displayed dose-dependent response to onabotulinum toxin A
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Similar dose-dependent reduction in motor unit and maximal voluntary amplitudes were experienced in upper and lower facial muscles of patients receiving onabotulinum toxin A, according to recent study results.
Researchers studied 26 patients (mean age, 42.4 years; 24 women) recruited between Feb. 1, 2009, and April 1, 2009, from a private US plastic surgery practice. None of the patients had received previous onabotulinum toxin A treatment. Electromyography (EMG) data were collected before, at 2 to 4 weeks and at 3 months after 8-U onabotulinum toxin A injections to each corrugator muscle and 2-U injections to each depressor anguli oris (DAO) muscle.
Durations of mean motor unit (MU) and maximal amplitudes at baseline, 2 to 4 weeks and 3 months after treatment were the main outcome measures. At baseline, corrugator and DAO muscles displayed similar mean MU amplitudes. Seventy-eight percent MU and 64% maximal amplitude reduction for the corrugator muscle were recorded at 2 to 4 weeks after injection (P=2.7 x 10-8), compared with 54% MU and 18% maximal amplitude reduction for the DAO muscles (P=1.3 x 10-14).
EMG at 3 months showed that corrugator muscles still had lower MU (69.99 mcV vs. 81.81 mcV; P=.20) and maximal amplitude (5.49 ms vs. 7.22 ms; P=2.6 x 10-6) than DAO muscles. At 3 months, both muscle groups had partially recovered function.
“The dose disparity appears to result from differences in the amount of paralysis required to achieve desirable aesthetic results,” the researchers concluded. “Amplitude and duration of MU at baseline were not significantly different for the muscle groups; however, the decrease in amplitude in the higher-dosed corrugator muscles was significantly greater than that in the lower-dosed DAO muscles at 2 to 4 weeks after injection, suggesting a dose response.
“Electromyography may prove to be a valuable tool for obtaining objective electrophysiological data when evaluating new chemodenervation medications in the asesthetic and functional arenas.”
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