Electromyographic differences between normal upper and lower facial muscles and the influence of onabotulinum toxin a

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.

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.

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.

A private oculofacial plastic surgery practice.

Twenty-six subjects recruited from February 1 through April 1, 2009.

Electromyography recordings and cosmetic onabotulinum toxin A injections.

Mean motor unit (MU) durations and maximal amplitudes at baseline and 2 to 4 weeks and 3 months after injection.

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.

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.

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