Objective
To know if changes in compound muscle action potential (CMAP) of human extensor digitorum brevis (EDB) muscle can be enhanced by increase in dilution volume of botulinum toxin type A.

Method
In 11 healthy volunteers, 2.5 U of botulinum toxin (Botox^Ⱂ, Allergen Inc.) in a volume of 0.1 ml normal saline was injected in EDB muscle and the same dose with the dilution volume of 0.5 ml was injected in the opposite side. We measured the pre- and post-injection M-wave amplitude and area of EDB muscle.

Results
The mean post/pre injection amplitude ratio of CMAP were 0.49⁑0.21 with the 0.1 ml of dilution volume and 0.40⁑0.12 with the 0.5 ml of dilution volume. The mean post/pre injection area ratio of CMAP were 0.51⁑0.18 with the 0.1 ml of dilution volume and 0.44⁑0.15 with the 0.5 ml of dilution volume. There was no significant difference between the two dilution volumes.

Conclusion
Five fold increase in dilution volume did not enhance the paralyzing effect of botulinum toxin type A in human EDB muscles.