New insights into the pathophysiology of primary hemifacial spasm

Primary hemifacial spasm (pHFS) is due to a benign compression of the facial motor nerve by an offending vessel, leading to increased nerve excitability. Facial nerve hyperexcitability presents two different aspects. First, there is a spontaneous and ectopic generation of action potentials on the incriminated nerve and then this ectopic impulse can propagate and spread “laterally” from one facial nerve branch to another. This results in spontaneous and synkinetic spasms affecting one hemiface. Although the increase in excitability certainly concerns the nucleus of the facial motor nerve in the brainstem, it seems unlikely that the primary origin of this hyperexcitability and the associated phenomenon of lateral spreading strictly originate at the nuclear level. In fact, the mechanisms causing facial nerve hyperexcitability per se remain unknown. The leading implication of a structural nerve lesion, such as segmental demyelination, induced by vessel compression, is also unconvincing. In contrast, a functional mechanical factor increasing nerve excitability is extremely probable, that it is either due to compression or stretch resulting from the neurovascular conflict. Axonal ion channel changes are obviously associated with this mechanism. Then the lateral spreading of nerve fibre hyperexcitability probably results from an ephaptic process, the “cross-talk” between axons being located in the region of the conflict or in the transition zone between central and peripheral myelin, at the end of the facial nerve root exit zone. In any event, pHFS is due to a functional increase in facial nerve excitability triggered by an offending vessel and this clearly explains the remarkable and rapid efficacy of surgical microvascular decompression.