The Internal Auditory Meatus (IAM): It’s neuroanatomy & comment

by Frank E. Musiek

Introduction

The internal auditory meatus (IAM) is a canal in the temporal bone that extends from the bony cochlea medially to an opening in the posterior aspect of the petrous portion of the temporal bone. This structure is germane to audiologists because it contains three nerves of interest to audiologists: 1- the auditory nerve, 2- the vestibular nerves, and 3- the facial nerve. These actual compose two of the cranial nerves – number 7 (facial) and 8 (auditory and vestibular).

Many audiologists either directly or indirectly assess these cranial nerves in their daily activities as clinicians.

Functional Anatomy

In adult humans, the IAM is just under a centimeter (cm) in length and about 4 millimeters (mm) in diameter. The facial, auditory and vestibular nerves course its length before exiting the temporal bone and projecting across the cerebellopontine angle into the lateral aspect of the brainstem at the ponto-medullary junction. These cranial nerves are longer than the IAM as they extend past the ends of the IAM. The position of these nerves is of import. The facial nerve is located superior and anterior within the IAM. Within the IAM the facial can be observed to have two segments the facial nerve proper and the nervous intermedius. The facial nerve proper carries mostly motor fibers and the nervous intermedius is composed of mostly sensory fibers. The auditory nerve is positioned beneath the facial, hence it is anterior and inferiorly located in the IAM. The auditory nerve is about 22 mm in length and as it courses through the IAM it twists clockwise slightly before entering the brainstem. In the posterior aspect of the IAM is the vestibular nerves. These can be divided into superior and inferior segments. (Musiek and Baran, 2007; Fatterpekar et al.,1999).

One of the best and most common ways to view the IAM and associated nerves in cross section is by a Stenver’s view. This a lateral radiologic view across the head so the image aimed at the left side of the head would be of the right IAM. This would show a cross section of the right  IAM (see figure 1). In examining a Stenver’s view of the IAM the nerve groupings can be placed into four quadrants (canals) following the location scheme mentioned earlier. The diameter of the 4 canals (approximately) making up the IAM are a shade under one millimeter with the exception of the canal of the auditory nerve which is larger (~2 mm). These measurements though approximated here, are critical to determination of constriction of these nerves from congenital conditions (Fatterpekar et al. 1999).

Of interest is a bony structure that actually divides the IAM in half along the horizontal plane. This bony eminence is called Bill’s bar named after William (Bill) House, the famous otologic surgeon from the House Otologic  Group. To my best recollection William House named the structure. Though these three groups of nerves are designated into four quadrants and four canals they are not always easily identified. Sometimes they seem to “run together” and it is a difficult process to determine the boundaries of these individual nerves. This is where intraoperative monitoring can play a useful role.

Vascular Anatomy

The vascular anatomy of the IAM is of course critical to its function. The IAM’s blood supply is from the vertebra-basilar system. The labyrinthine or Internal auditory artery (IAA) branches from the basilar artery or the anterior inferior cerebellar artery in the ventral brainstem (pons).

The IAA generally enters the medial aspect of the IAM. Then there are 3 branches from the IAA, 1- the anterior vestibular artery, (AVA), 2- the cochlear artery, (CA), 3- the vestibular- cochlear  artery, (VCA) Sometimes there are two IAAs, when this is the case there is usually a vestibular and cochlear artery from each (Brunsteins and Ferreri, 1995). It is of interest that in some research on vasculature of the IAM that the “common cochlear artery” is not in their scheme (Brunsteins and Ferreri, 1995). It is this writer’s view that perhaps the differential between the common cochlear artery and the IAA is difficult and  in some cases may be the same artery.

Two Key Disorders of the IAM

Though there are a number of disorders localized to the IAM two key ones will be mentioned here. One of course is the vestibular schwannoma (better known as an acoustic tumor) and the vascular loop. Approximately 90 per cent of the time the vestibular schwannomas arise from the vestibular nerves within the IAM and will often expand into the cerebellopontine angle. It will compress all nerves in the IAM with the auditory showing the most effect. The vascular loop syndrome is characterized by the anterior inferior cerebellar artery looping into or near the opening of the IAM. This also can disturb function of all the nerves in the IAM. These two disorders are and should be of interest to audiologists and affect the natural anatomy of the IAM.

References

1. Fatterpekar GM, Mukherji SK, Lin Y, Alley JG, Stone JA, Castillo M. (1999) Normal canals at the fundus of the internal auditory canal: CT evaluation. J Comput Assist Tomogr., Sep-Oct;23(5):776-80.
2. Musiek, F. and Baran, J. (2007) The Auditory System: Anatomy, Physiology, and Clinical Correlates, Plural Pub., San Diego
3. Brunsteins, D. and Ferreri, A. (1995)  Microsurgical anatomy of arteries related to the internal auditory meatus. Acta Anatomica, 152, 143 – 150