Anatomy, ultrastructure, electrophysiology, and clinical functions of the nervus intermediu. Usefulness in cerebellopontine pathology

Background: 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 anatomical, ultrastructural, and electrophysiological characteristics of the nervus intermedius.

Objective: In this study, we analyzed the intraoperative electrophysiological response obtained during vestibular schwannoma surgery. Cadaveric anatomical dissections were performed to compare the results. Ultrastructure was studied using electron microscopy.

Materials and Methods: 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.

Results: 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.

Conclusions: The ultrastructural, clinical and electrophysiological findings of this study demonstrated repeatable and unmistakable motor function patterns of the perioral and perinasal muscles. This knowledge represents a solid intraoperative landmark for the successful microsurgery of cerebellopontine angle tumors.