In our clinical experience, some patients indicate that their spasticity improves on the day of botulinum injection. Previous studies have indicated that compound motor action potentials (CMAPs) show a decrease as early as 2 days post-injection. However, no studies have assessed if neurophysiological changes take effect earlier than 48hours post-injection.

Fifteen healthy control subjects (10 F; mean age 41±11 years) participated in this randomized double-blind study. We injected 10 units each of onabotulinumtoxinA and incobotulinumtoxinA in the extensor digitorum brevis (EDB) muscle, one in each foot. We performed serial CMAP assessments using nerve conduction study of the peroneal nerves at following time intervals after botulinum injection - 24hours and 6 days post-injection. We used an ANOVA with repeated measures separately for each type of toxin.

The mean EDB amplitudes differed statistically significantly between the tested time points in both the incobotulinumtoxinA group [F(2, 28)=157.329, P<0.0001] and onabotulinumtoxinA group [F(2, 28)=66.464, P<0.0001]. Post-hoc tests using the Bonferroni correction revealed that each type of toxin resulted in a statistically significant decrease in the EDB amplitude from pre-injection time point to 24hours and 6 days post-injection (P<0.0001). In the incobotulinumtoxinA group (34% reduction at 24hours and 68% reduction at 6 days), and onabotulinumtoxinA group (31% reduction at 24hours and 61% reduction at 6 days).

We conclude that both types of toxins tested in this study exert a significant decrease in EDB CMAP amplitude at 24hours post-injection.