Prolonged vastus lateralis denervation after botulinum toxin type A injection.

Intramuscular injection of botulinum toxin (BoNT) produces reversible blockade of neuromuscular transmission. In animal experimental models, recovery begins within four weeks and is usually complete by twelve weeks. We present evidence of prolonged denervation following BoNT injection of the vastus lateralis (VL) muscle to correct quadriceps muscle imbalance in patients with chronic anterior knee pain. Needle electromyography data were obtained from 10 subjects who had received a single BoNT treatment 5 to 19 months earlier as part of a clinical trial. Insertional and spontaneous activity, recruitment, and motor unit action potentials were examined. Clear differences between the injected and non-injected VL muscles, which correlated with the time since injection, were identified in all subjects. All 10 subjects studied with needle EMG showed evidence of persisting denervation in the BoNT-A injected VL muscle beyond the period of neuromotor recovery expected from animal experimental studies.