Cryptogenic hemifacial spasm. A neurophysiological study

Whether hemifacial spasm (HFS) is due to axono-axonal ephaptic transmission or to facial nucleus abnormal hyperexcitability remains controversial. The neurophysiological hallmark of HFS is the delayed response (DR). This response has an indirect pathway and thus a long latency. It is evoked A) as a distant response in muscles innervated by a facial branch other than the one stimulated, and B) in muscles innervated by the branch stimulated. In this work, 99 single all-or-none DRs of 24 cases of cryptogenic HFS were studied by threshold stimulation of a branch, or of the trunk, of the facial nerve. A) Eighty-eight distant DRs were studied. Fifty-four of them were frequently evoked as double discharges (DDs), or sometimes as multiple discharges, with a 3 to 7 ms interval. A collision technique, using paired stimuli, showed that the second discharge of 12 out of 20 DDs was accompanied by a back-wave due to a proximal ectopic re-excitation on the axon, or to the back-firing of an alpha cell (F-response), or to both. F-waves in HFS were more frequent than in normals or in other facial pathologies. B) Eleven DRs were recorded in muscles innervated by the facial branch stimulated. Some of them persisted when, using a stronger stimulus, the same all-or-none potential was also evoked as a direct response. In other cases both the direct response and the DR were evoked with identical stimulation threshold. The direct-indirect response interval of these 11 DRs was shorter than the normal M-F interval. These findings suggest that, in the case of HFS, axons are interconnected by uni- and bidirectional ephapses. Self-sustained repetitive firing in such a group of axons apparently results from re-excitations occurring both at the ephapse site and at the cellular level (F-responses). Spasm develops when several groups fire together. The changing excitability of the alpha cells modulates the importance of the phenomenon.