CPA-IAC Overview

Terminology

The contents of the cerebellopontine angle (CPA) and internal auditory canal (IAC) cisterns include the facial nerve (CN7), the vestibulocochlear nerve (CN8), and the anterior inferior cerebellar artery (AICA) loop. The bony IAC, its fundal crests (vertical and horizontal), and its opening in the porus acusticus are also included as part of this discussion.

Embryology

The temporal bone forms as 3 distinct embryological events: (1) The external and middle ear, (2) the inner ear, and (3) the IAC. The practical implications of these 3 related but separate embryological events is that the presence or absence of the IAC is independent of the development of the inner, middle, or external ear.

The IAC develops in response to formation and migration of the facial and vestibulocochlear nerves through this area. IAC size depends on the number of migrating nerve bundles. The fewer the nerve bundles, the smaller the IAC. If the IAC is very small and only 1 nerve is seen, it is usually the facial nerve.