Hemifacial spasm due to posterior fossa tumors: the impact of tumor location on electrophysiological findings.

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.