Electrophysiological Evidence of Contralateral Neuromuscular Effects Following Long-Term Botulinum Toxin Therapy in Hemifacial Spasm.

Hemifacial spasm (HFS) is a cranial nerve disorder characterized by involuntary contractions of muscles innervated by the facial nerve. Botulinum toxin type A (BoNT-A) is widely used for symptom control. Although local diffusion is well established, the extent and clinical relevance of BoNT-A spread to contralateral muscles remain unclear. This study aimed to investigate the contralateral neuromuscular effects of BoNT-A in patients undergoing long-term treatment with BoNT-A. This retrospective cross-sectional study included 39 patients with HFS (mean age, 58.6 ± 8.5 years). Bilateral compound muscle action potentials (CMAPs) were recorded before and four weeks after the BoNT-A injection. Single-fiber electromyography (SFEMG) jitter and mean consecutive difference (MCD) were evaluated contralaterally using concentric needle electrodes. Patients were categorized as first-time (<i>n</i> = 10) or long-term (<i>n</i> = 29; treatment duration: 1-20 years) BoNT-A recipients. Contralateral CMAP amplitudes decreased by 21.1% post-injection (<i>p</i> < 0.001). MCD increased from 33.2 ± 5.6 to 37.0 ± 5.3 µs (<i>p</i> < 0.001), and jitter rose by 81%, from 7.9 ± 6.2 to 14.3 ± 8.1 µs (<i>p</i> < 0.001). The percentage increase in MCD was significantly higher in long-term versus first-time patients (12.7% vs. 7.5%; <i>p</i> = 0.039), suggesting a cumulative neuromuscular effect. Spontaneous myokymia or fasciculations were clinically observed in four long-term patients. These findings provide electrophysiological evidence that unilateral BoNT-A injections may induce neuromuscular transmission abnormalities in the contralateral facial muscles. This effect appears more pronounced in chronically treated individuals, highlighting the need for awareness of potential bilateral spread when planning long-term therapy.