Importance Empirically determined doses of onabotulinum toxin A for aesthetic treatments are as much as 5 times higher for the upper than for the lower facial muscles.
Objective To use electromyography (EMG) to determine objectively whether the disparity between doses is due to intrinsic differences between the muscle groups' responses to onabotulinum toxin A or to variable amounts of paralysis required to achieve the desired aesthetic outcomes.
Design We collected EMG data before and at 2 to 4 weeks and 3 months after 8- and 2-U onabotulinum toxin A injections to the corrugator and depressor anguli oris muscles, respectively.
Setting A private oculofacial plastic surgery practice.
Participants Twenty-six subjects recruited from February 1 through April 1, 2009.
Interventions Electromyography recordings and cosmetic onabotulinum toxin A injections.
Main Outcome Measures Mean motor unit (MU) durations and maximal amplitudes at baseline and 2 to 4 weeks and 3 months after injection.
Results Baseline mean MU amplitudes were similar for the corrugator and depressor anguli oris muscles. At 2 to 4 weeks after injection, 78% MU and 64% maximal amplitude reduction for the corrugator muscle were detected, but only 54% MU and 18% maximal amplitude reduction for the depressor anguli oris (P = 2.7 × 10−8 and P = 1.3 × 10−14, respectively). At 3 months, function was partially recovered for both muscle groups.
Conclusions and Relevance Onabotulinum toxin A causes a similar dose-dependent reduction in MU and maximal voluntary amplitudes for muscles of the upper and lower face. The dose disparity appears to result from differences in the amount of paralysis required to achieve desirable aesthetic results.
Level of Evidence 2.