Time course of recovery of juvenile skeletal muscle after botulinum toxin A injection - An animal model study.

Objective: To study the effects of neuromuscular blockade using botulinum toxin A on juvenile muscles at a dosage of 6 units/kg body weight in a rat model. Design: A total of 34 male Sprague-Dawley rats (1-mo old) were used. A small incision was made along the posterior aspect of the left hind leg with the exposure of the gastrocnemius. Botulinum toxin A was injected at a dosage of 6 units/kg body weight in the medial and lateral heads of the muscle. An equivalent volume of saline were injected into the right gastrocnemius (control). Motor evoked action potentials, muscle force generation, and muscle mass and neuromuscular junction morphometry were analyzed at different time intervals up to 1 yr after toxin injection. Results: During the 1-2 wks after botulinum toxin A injection, muscle mass, electrophysiologic variables, and muscle force generation were significantly reduced but returned to nearly normal at 6 mos postinjection. In the study group, neuromuscular junction gutter depth became significantly shallower 2 mos after injection, then normalized at 1 yr. There was a nonsignificant trend toward larger neuromuscular junctions from the gastrocnemius injected with botulinum toxin A. Conclusion: Our findings provide scientific evidence to support the clinical situation in which the interinjection interval of 3-6 mos of botulinum toxin A at a similar dosage is used.