Short-term effects of botulinum toxin on the lateral rectus muscle of the cat

Botulinum toxin type A (BTX) is often used as an alternative to surgery for the treatment of strabismus and many other motor or cosmetic problems. Although numerous studies established BTX as a powerful transmission-blocking agent at the neuromuscular junction, no evaluation of extraocular muscle (EOM) contractile properties after administration of BTX exists. Some anatomical studies on EOM fiber types suggested a long-term preferential effect of BTX on orbital layer, singly innervated muscle fibers. In this study, we examined the short-term effects of BTX on the contractile properties of normal lateral rectus muscle to determine the functional effect of BTX on muscle-force output over time. Measurements of muscle tension and the corresponding EMG evoked by stimulation of nerve VI were made hourly for up to 18 h following BTX administration. An intramuscular BTX injection of 2 U caused a dramatic decrease in maximum twitch and tetanic tension of the muscle in response to different frequencies of stimulation. This suppression developed gradually over time, with a concomitant reduction of EMG amplitude. No significant changes in muscle-speed-related characteristics (e.g., twitch contraction time, fusion frequency) were found. The results suggest a functional effect of BTX on all muscle fiber types, although, with the dose used, we did not observe complete muscle paralysis within the time of recording. The time course of muscle tension suppression by BTX also was frequency dependent, with the lower stimulation frequencies being more affected, suggesting that implementation of higher frequencies could still produce adequate eye movements.