The internal acoustic meatus and its meningeal layers: a microanatomical study

Abstract

Object. The authors studied the cadaveric heads of 22 adults to describe the internal acoustic meatus (IAM) and its contents. Special attention was paid to the length of the arachnoidal and dural sheaths surrounding the neural structures, including the vestibular ganglion. An additional goal of this study was to verify anatomically the concept of arachnoidal duplication, which is reputedly induced by medial growth of vestibular neuromas and helpful in atraumatic dissection. Methods. Twelve cadaveric heads (24 IAMs) were injected with colored latex and fixed in formalin. Cautious removal of the skull vault and the brain or the skull base allowed superior and anteroinferior views of the IAM, respectively. Photographs were obtained after removal of the bone canal and dissection of the meninges with the aid of optic magnification. Ten IAMs were prepared for histological study and the osteological anatomy of the fundus was endoscopically described for the remaining 10. The dura mater covered the bone structures of the IAM, and the arachnoidal membrane of the cerebellopontine cistern invaginated into this dural cul-de-sac as a “muff.” The entire neurovascular content of the IAM, including the vestibular ganglion, was surrounded by this arachnoidal sheath in which cerebrospinal fluid circulated. The length of this arachnoidal sheath was the same ventrally and dorsally and, in all specimens, the entrance of the cochleovestibulofacial complex into the subarachnoid space was located at the fundus level. Conclusions. In this study the authors demonstrated the existence of an acousticofacial cistern containing every nerve of the vestibulocochleofacial complex, including the vestibular ganglion from which acoustic neuromas develop. These findings clearly contradict the theory of the duplication of arachnoidal layers during medial growth of vestibular neuromas and may explain some of the intraoperative difficulties encountered in the atraumatic dissection of these tumors.