Ultrastructural findings in the peripheral nerves in idiopathic hemifacial spasm and trigeminal neuralgia were similar. At an early stage, microneuroma of the nerve fibers was characteristic, and the thickening of the myelin sheath was slight. The myelin sheath markedly thickened after repeated attacks.
Segmental demyelination or thinning of the myelin sheath seemed to be related to paroxysm. It is postulated that this is because of the formation of a short circuit between the afferent and efferent fibers.
Kumagami H. Neuropathological Findings of Hemifacial Spasm and Trigeminal Neuralgia. Arch Otolaryngol. 1974;99(3):160–164. doi:10.1001/archotol.1974.00780030168002