Serial measurements were performed on the compound muscle action potential (CMAP) amplitude and the force generated by the rat lower hind limb flexors to investigate the time course of intramuscular injections of botulinum toxin type A (BOTOX®). Thirty animals were used in this in vivo rat model. CMAP amplitude and muscle force were measured at predetermined intervals for 28 weeks. Compound muscle action potential amplitude and force declined markedly the first 5 to 7 days after injection of BOTOX but recovered in a near linear manner. The response magnitude and recovery rate were dose-dependent. Recovery of CMAP amplitude preceded recovery of muscle force. No clear evidence of a systemic effect on the untreated leg or a concentration effect was found. CMAP amplitude may be useful in determining optimal timing of repeat injections in treating neuromuscular disorders.
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