Ephaptic transmission is one of the electrophysiological hallmarks of
hemifacial spasm. It is generally accepted that in the majority of patients with idiopathic
hemifacial spasm, microvascular compression of the facial nerve at the site where the nerve exits the brain stem is the underlying cause. Whether the actual site of the ephapse is at the site of the lesion or at a nuclear level due to hyperexcitability of the facial motor nucleus is still controversial. Rarely,
hemifacial spasm may be due to space occupying lesions in the cerebellopontine angle or in the brain stem. We report the electrophysiological findings of four patients with
hemifacial spasm due to extra-axial
tumors in different locations of the posterior fossa. The location of the
tumor was intrameatal in one patient, in the cerebellopontine angle in two patients and in the brain stem in another patient. Facial nerve motor neurographies including
transcranial magnetic stimulation revealed abnormal findings in two patients. Selective stimulation of facial nerve branches demonstrated delayed (ephaptic) responses in all but one patient whose
hemifacial spasm had disappeared
after treatment with
carbamazepine. The latencies of the delayed responses did not correlate with the
tumor location. In sum, the site of ephaptic transmission cannot be reliably determined by latency measurements of the delayed response because of its variability which is probably caused by the different size and diameter of the axons participating in ephaptic transmission as well as by the extent of focal
demyelination at the site of the lesion. A neuroradiological work up including MR imaging should be mandatory in all patients with
hemifacial spasm because electrophysiological studies fail to differentiate between idiopathic and symptomatic
hemifacial spasm.