Inhibition of motor nerve regeneration in a rabbit facial nerve model

OBJECTIVES/HYPOTHESIS: We hypothesized that intramuscular injection of vincristine into the upper lip selectively prevents reinnervation of the target muscle (quadratus labii superioris) after facial nerve transection. STUDY DESIGN: Prospective controlled experiment. METHODS: Transection injuries with primary neurorrhaphy were performed just proximal to the buccal and zygomatic branches of both facial nerves in 20 rabbits. The left (experimental) quadratus labii superioris muscle was injected with 200 microg vincristine 72 hours after injury; the right (control) side was injected with saline. Functional, electrophysiological and histological studies were performed 6 weeks after surgery. RESULTS: After 6 weeks, the control side had a mean axon count (distal buccal branch) of 1160, conduction velocity of 50.4 m/s, reduction of compound action potential amplitude of 44%, and functional index of 1.67 units. The experimental side had a mean axon count of 265, conduction velocity of 16.5 m/s, reduction of compound action potential amplitude of 88%, and a functional index of 0.176 units. All parameters were significantly reduced by vincristine blockade (P <.05). The zygomatic division (not exposed to vincristine) displayed a trend toward increased axon counts, but this was not statistically significant (P =.131). CONCLUSIONS: These results demonstrate that reinnervation of a selected muscle can be prevented by injection of vincristine. Enhanced reinnervation of adjacent muscle groups may also occur. Thus, nerve blockade by vincristine may be useful for the prevention of synkinesis.