The nervus intermedius as a variable landmark and critical structure in cerebellopontine angle surgery: an anatomical study and classification.

BACKGROUND

An understanding of the normal topography during cerebellopontine angle surgery is necessary to obviate the anatomical distortions caused by tumors.

OBJECTIVE

The aim of this study was to analyze the morphological features of the nervus intermedius (NI) and its related structures in the cerebellopontine angle (CPA).

METHODS

Forty-three isolated human brainstems were examined to collect comprehensive morphometric and topographical data of the NI in its course from the brainstem to the ganglion geniculi, and discover its anatomical relationship with the other neurovascular structures in the CPA as well as within the meatus acusticus internus.

RESULTS

A total of 84 NI were analyzed. The number of bundles comprising the NI varied from one to five. The mean length of the cisternal segment of the NI was 11.47 mm. In most cases, a vein between the root entry/exit zones of the facial and the vestibulocochlear nerve (VN) was documented. In all cases the NI joined the facial nerve, typically (85 %) distally to the the porus within the meatus acusticus internus. The entry/exit zone of the NI can be categorized into four types: in type A, they arise directly from the brainstem; in type B, they arise solely from the facial nerve; in type C solely from the VN; and in type D, where the bundle or bundles arise from both the brainstem and the VN or the facial nerve.

CONCLUSION

The anatomical features of the NI can provide an additional variable landmark and critical structure during cerebellopontine microsurgery. Our study of the nerve's anatomy and topographical relations may contribute to preventing intraoperative nerve injuries.