Dose-response curve of human extensor digitorum brevis muscle function to intramuscularly injected botulinum toxin type A.

To develop a human model for measuring the effect of intramuscularly injected botulinum toxin, we injected both extensor digitorum brevis (EDB) muscles in 13 healthy volunteers with seven varying doses of botulinum toxin type A. We measured, pre- and postinjection, EDB M-wave amplitude, area, and mean rectified voltage (MRV) (obtained during maximal voluntary muscle activation). There was a logarithmic-appearing dose-response relationship between increasing doses of botulinum toxin and decline in EDB M-wave amplitude, area, and MRV. The decline was incrementally less at higher doses of toxin and appeared to level off at a maximal effect of 85 to 90% decrement from baseline (85 to 90% "paralysis") at 15 to 20 units. The peak toxin effect was present on day 6 postinjection. Measurement of EDB M-wave amplitude, area, and MRV is a simple objective method for quantifying the onset and degree of human muscle "paralysis" following botulinum-toxin injection.