Botulinum toxin paralysis of the orbicularis oculi muscle. Types and time course of alterations in muscle structure, physiology and lid kinematics.

In chronically prepared guinea pigs, we investigated the time course of botulinum toxin A's (Bot A) effect on the blink reflex by monitoring lid movements and EMG activity prior to and after Bot A injection into the orbicularis oculi muscle (OOemg), or after nerve crush of the zygomatic nerve. We correlated these alterations with the morphological changes of the orbicularis oculi (lid-closing) muscles of the same animals. After Bot A treatment there was a profound reduction of OOemg activity and blink amplitudes as well as a slowing of maximum blink down-phase velocity. Blink up-phases, however, remained unchanged. Gradual recovery of OOemg magnitude and blink amplitude started around day 6; a functioning blink reflex appeared on day 21, and full recovery of blink amplitude occurred by day 42. Crushing the zygomatic branch of the facial nerve produced similar changes in blink parameters, but recovery was much more rapid (15 days) than for Bot A-treated guinea pigs. The morphological analysis demonstrated that Bot A produced a denervation-like atrophy in the orbicularis oculi. No fiber type-specific alterations were noted, and all muscle fiber types ultimately recovered, with no longstanding consequences of the transient denervation. Our findings support the notion that functional recovery was the result of preterminal and terminal axonal sprouting that subsequently re-established functional innervation. Moreover, differences between the present findings and those seen after injection of Bot A into the extraocular muscles strongly support the hypothesis that the composition in terms of muscle fiber type and the properties of the motor control system of a given muscle greatly influence both how the particular muscle responds to toxin injection, and how effective the toxin is in resolution of neuromuscular disorders that affect a particular muscle. The present findings were consistent with clinical observations that Bot A produces only temporary relief in patients with essential blepharospasm. It is likely that the efficacy of Bot A in treatment of blepharospasm could be improved by using agents that suppress terminal sprouting. The close correspondence of the changes in blink physiology between human patients and guinea pigs after Bot A treatment demonstrate that the guinea pig is an excellent model system for testing strategies to prolong the beneficial effects of Bot A treatment in relieving lid spasms in human subjects.