経頭蓋磁気刺激による顔面神経刺激部位の同定—動物実験による検討

Transcranial Magnetic Stimulation of the Facial Nerve : Identification of the Actual Excitation Site in the Cat

　経頭蓋磁気刺激による頭蓋内顔面神経の刺激部位に関しては，様々な議論がされている。今回，成猫を用い側頭骨内顔面神経を露出し直接電気刺激を行い誘発筋電図を記録し，その潜時を磁気刺激誘発筋電図と比較した。さらに顔面神経を内耳孔より末梢に順次切断し磁気刺激筋電図を記録し，刺激部位の同定を試みた。その結果，磁気刺激誘発筋電図の潜時は顔面神経を内耳道内で刺激した筋電図の潜時と一致した。顔面神経の切断実験では，内耳道内末梢部での切断で磁気刺激筋電図が消失した。以上より，経頭蓋磁気刺激による顔面神経の被刺激部位は内耳道内末梢部であることが確認された。また，8字型コイルではコイルの位置よりもコイルの方向により筋電図波形の変化が大きいことより，この刺激機序に神経の走行方向が関与していることが示唆された。

The site where transcranial magnetic stimulation excites the facial nerve was studied in 6 cats. Trans-cranial magnetic stimulation of the facial nerve was recorded from the left mentalis muscle. A figure-of -eight shaped magnetic coil was used, and coil induction direction had more influence on the facial nerve evoked compound muscle action potentials (CMAPs) than the coil position.

No change could be detected in the CMAPs before and after craniotomy, after cerebellar lobectomy and after exposure of the facial nerve in the facial canal. The facial nerve was stimulated electrically at the porus, meatal portion, geniculum and horizon-tal portion. The latencies of the CMAPs for each portion were measured and compared with the magnetic response, which was coincidental with that of the meatal portion.

The facial nerve was then transected distally from the porus, and CMAPs following magnetic stimulation were recorded at each step. The CMAPs disappeared when the nerve was transected at the fundus.

The results of both approaches in this study led to the conclusion that transcranial magnetic stimula-tion excites the facial nerve at the meatal portion.