The internal auditory canal (IAC), also referred to as the internal acoustic meatus lies in the temporal bone and exists between the inner ear and posterior cranial fossa. It includes the vestibulocochlear nerve (CN VIII), facial nerve (CN VII), the labyrinthine artery, and the vestibular ganglion. Knowledge of the anatomy and relationship of these structures plays a vital role during the evaluation and management of diseases involving the internal auditory canal.
Structure and Function
The internal auditory canal begins in the temporal bone within the cranial cavity at an oval-shaped opening called the porus acusticus internus. The canal runs through the petrous segment of the temporal bone, which is located between the inner ear and posterior cranial fossa. It is lined by dura and filled with spinal fluid. The rounded and smooth canal is on average 8.5mm (5.5 to 10.mm) in length and about 4mm in diameter.[rx]
The canal narrows as it moves towards the fundus, a thin cribriform plate of bone that marks the lateral boundary of the canal. The fundus separates the internal auditory canal from the cochlea and vestibule which are located in close proximity.[rx] It is divided into superior and inferior segments by the transverse crest (also called falciform crest). The superior half is further divided into anterior and inferior segments by Bill’s bar, a vertical crest of bone named after otologist Dr. William House. These two spines form three distinct osseous structures through which the facial and vestibulocochlear nerves branches can be found in a consistent pattern, represented by figure 1. The posterior portion of the fundus is filled with a macula crista, which is a series of very small openings that the vestibular nerves pass to reach the superior and inferior semicircular canals.
The vestibulocochlear nerve runs most often posteriorly to the facial nerve in the internal auditory canal. In the lateral segment of the internal auditory canal, about 3 to -4mm from the fundus, the cochlear and vestibular nerves join to form one common nerve. The vestibular portion is further segmented into the superior and inferior portions at the fundus.[rx] The superior portion of the vestibular nerve innervates three structures- the superior and lateral semicircular canals, and the utricle. The inferior portion innervates the remaining vestibular structures- the posterior semicircular canal and the saccule. The afferent projections from both the superior and inferior vestibular nerves join in the IAC at the vestibular ganglion, often called the Scarpa ganglion. Interestingly, the vestibular ganglion is one of the first ganglions to reach full mature size, at as early as the first week of postnatal life.[rx]
The singular canal is a significant landmark during surgery of the internal auditory canal and labyrinth. It carries the posterior ampullary nerves (PAN) and enters the internal auditory canal in the postero-inferior aspect near the fundus.[rx] The PAN innervates the ampulla of the posterior semicircular canal and joins the saccular nerve within the internal auditory canal to form the inferior vestibular nerve.
The facial nerve is found most often in the antero-superior quadrant of the internal auditory canal. The facial nerve exits the internal auditory canal at the meatal foramen and continues towards the geniculate ganglion as the labyrinthine segment. The nervus intermedius is a branch of the facial nerve located posteriorly to the facial nerve and anterior to the superior vestibular nerve in the internal auditory canal.[rx] It carries the sensory and parasympathetic fibers of the facial nerve and joins the main motor root of the facial nerve at the geniculate ganglion.
Blood Supply and Lymphatics
The entire inner ear receives vascular supply from the labyrinthine artery (LA), also called the internal auditory artery, which is most often a branch of the anterior inferior cerebellar artery (AICA) but can also branch from the basilar artery in a minority of cases.[rx] The LA enters the IAC medially in the anteroinferior portion of the porus acusticus internus and courses between the cochlear and facial nerve. It divides into its two terminal main branches at the fundus, the anterior vestibular artery, and the common cochlear artery.
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