The relationship between nervus intermedius anatomy, ultrastructure, electrophysiology, and clinical function. Usefulness in cerebellopontine microsurgery

Abstract

Although previous studies have described the clinical features of the nervus intermedius (NI), no attempt has yet been made to describe the relationship between the ultrastructural and electrophysiological characteristics of the nervus intermedius and its motor competence. In this study, we analyzed the intraoperative electrophysiological response obtained during vestibular schwannoma surgery. The ultrastructure was studied using electron microscopy. Thirty-six consecutive patients underwent microsurgery for vestibular schwannoma with cerebellopontine angle tumors. The patients were extensively monitored intraoperatively. Selective stimulation of the nervus intermedius was attempted in all cases. The patients were then examined postoperatively and followed for a minimum of 1 year. Forty-three isolated human brainstems were analyzed to collect the ultrastructural NI data. We found a correlation between the NI motor responses in the perinasal and perioral regions and the ultrastructure characteristics, with few (0.5 %) but large myelinated motor fibers (diameters >12 μm). Both characteristics are consistent with the clinical observation of transient weakness of the levator anguli oris muscle. These observations indicate a relationship between the intraoperative electrophysiological identification of the NI nervus intermedius and its clinical and ultrastructural characteristics. Identifying the NI in the deformed anatomy of tumors could provide a fixed landmark during cerebellopontine surgery and help prevent damage of the facial nerve.