Motor nerve sprouting in human orbicularis muscle after botulinum A injection.

The paralytic properties of botulinum A toxin have led to its use in humans in the treatment of strabismus and facial dystonias such as essential blepharospasm. Examination of orbicularis muscle from 10 patients with essential blepharospasm who received 2-18 injections of botulinum toxin 6 weeks to 3 years prior to surgery revealed characteristic nodal, terminal and ultraterminal "sprouting" of the motor axons. Orbicularis muscle from five individuals never exposed to botulinum failed to demonstrate these changes. The significance of persistent motor nerve sprouting in response to botulinum exposure remains to be elucidated.