Electrophysiologic investigation during facial motor neuron suppression in patients with hemifacial spasm: possible pathophysiology of hemifacial spasm: a pilot study

Abstract

Objective: To evaluate the pathophysiological mechanism of hemifacial spasm (HFS), we performed electrophysiological examinations, such as supraorbital nerve stimulation with orbicularis oris muscle recording and lateral spread tests, after suppressing the patient's central nervous system by administering intravenous diazepam.

Methods: Six patients with HFS were recruited. Supraorbital nerve stimulation with orbicularis oris muscle recording and the lateral spread test were performed, followed by intravenous application of 10 mg diazepam to achieve facial motor neuron suppression. Subsequently, we repeated the two electrophysiological experiments mentioned above at 10 and 20 minutes after the patients had received the diazepam intravenously.

Results: Orbicularis oris muscle responses were observed in all patients after supraorbital nerve stimulation and lateral spread tests. After the diazepam injection, no orbicularis oris muscle response to supraorbital nerve stimulation was observed in one patient, and the latencies of this response were evident as a slowing tendency with time in the remaining five patients. However, the latencies of the orbicularis oris muscle responses were observed consistently in all patients in the lateral spread test.

Conclusion: Our results suggest that ectopic excitation/ephaptic transmission contributes to the pathophysiological mechanisms of HFS. This is because the latencies of the orbicularis oris muscle responses in the lateral spread test were observed consistently in the suppressed motor neuron in our patients.

Keywords: Blinking; Diazepam; Electromyography; Hemifacial spasm; Pathophysiology.

Fig. 1

(A) Electrodiagnostic findings of the lateral spread test and (B) orbicularis oris muscle response to supraorbital nerve stimulation before diazepam injection in a patient with hemifacial spasms. In the lateral spread test, the direct response was recorded in the orbicularis oculi muscle, and the indirect response was recorded in the orbicularis oris muscle. R1 and R2 responses were the orbicularis oculi muscle (O. oculi) responses that were elicited by supraorbital nerve stimulation, and early and late responses were orbicularis oris muscle (O. oris) responses that were elicited by supraorbital nerve stimulation. We identified two different components in the orbicularis oris muscle responses to supraorbital nerve stimulation: an early response and a late response, the latter of which showed a longer latency.

Fig. 2

Results of a lateral spread test (A, B) and orbicularis oculi muscle and orbicularis oris muscle responses to supraorbital stimulation (C, D). In the simplest mechanism of supraorbital nerve stimulation, the electrical stimulus would activate the supraorbital nerve afferents that reach the facial motor neurons of the orbicularis oculi muscles. Therefore, early and late orbicularis oris muscle responses were passed through the facial motor neuron after supraorbital stimulation, and the responses would have been caused by either facial motor neuronal hyperexcitability or ephaptic transmission and ectopic excitation. Hence, the responses were gradually delayed under conditions in which the facial motor neurons were suppressed by diazepam. However, the results of the lateral spread test, demonstrated consistent latencies both before and after diazepam injection.