Functional end-plate recovery in long-term botulinum toxin therapy of hemifacial spasm: a nerve conduction study

Botulinum toxin type-A is currently thought to be effective and safe for hemifacial spasm (HFS). The pre-synaptic block of acetylcholine release at the neuromuscular junction induces depression of orbicularis oculi muscle compound motor action potential (CMAP). The aim of our study was to evaluate at what extent end-plate functional recovery is possible even in botulinum toxin treatments lasting up to 15 years. We examined 81 outpatients with primary HFS (mean treatment duration = 7.2 ± 4.2 years) who underwent neurophysiologic study, once clinical effect of the previous treatment had vanished. The mean CMAP amplitude, mean rectified amplitude of response 1 (R1) of the blink reflex and area of response 2 (R2) of treated orbicularis oculi muscle were measured in comparison to the controlateral side. Mean amplitude of the above mentioned parameters was slightly lower (about 20%; p < 0.001) in the treated side at the end of the follow-up period (4.7 ± 1.7 months). The CMAP amplitude reduction weakly correlated with the interval from last treatment, while other neurophysiologic parameters did not change due to treatment duration or total toxin amount. Our study demonstrates that botulinum toxin affects compound motor action potential and blink-reflex responses for at least 4-5 months in HFS patients. The residual block is slight and does not increase with repeated injections after several years of treatment. Our study, beside confirming the long-term efficacy of botulinum toxin treatment for HFS, provides neurophysiologic evidence that therapeutic effect may be obtained without hindering the regenerative potential of the nerve-muscle complex.