Abnormal muscle responses in hemifacial spasm: F waves or trigeminal reflexes?

In patients with hemifacial spasm (HFS), abnormal muscle responses (AMR) are frequently present. The objective of this study was to investigate whether the afferent input of AMR is mediated by antidromic facial nerve stimulation or orthodromic trigeminal nerve stimulation.

AMR in the orbicularis oris muscle were recorded in 28 patients with HFS. When AMR were present, they were recorded after subthreshold stimulation of the facial nerve and weak stimulation delivered to the skin.

AMR were recordable in 24 (86%) of the patients, and usually consisted of the early constant component (mean onset latency, 10.0 ms) and late variable component (35.3 ms), similar to R1 and R2 of the blink reflex. The early or late components of AMR, or both, were frequently elicited after subthreshold stimulation of the facial nerve (43%) and skin stimulation (88%).

AMR are likely to be mediated by trigeminal afferent inputs, rather than antidromic activation of the facial nerve, and are a type of trigeminal reflex.