The internal auditory canal is a bony canal in the temporal bone that transmits the facial nerve, vestibulocochlear nerve, and other structures. It begins as an oval opening called the porus acusticus internus and runs through the petrous bone, lined with dura and filled with spinal fluid. The canal contains five main nerves - the facial nerve, nervus intermedius, cochlear nerve, superior vestibular nerve, and inferior vestibular nerve. It also contains important landmarks like the transverse and vertical crests that separate the different nerve pathways.

1. INTERNAL AUDITORY
MEATUS
Dr Sankalp C Aavarti

2. • 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

3. • AKA internal auditory canal / porus acousticus.
• The filaments of the acoustic portion of VIII penetrate tiny
openings of the lamina cribrosa of the cochlear area.
• Transverse crest: separates superior vestibular area and facial
canal (above) from the inferior vestibular area and cochlear
area (below)
• Vertical crest (AKA Bill’s bar – named after Dr. William House):
separates the meatus to the facial canal anteriorly (containing
VII and nervus intermedius) from the vestibular area posteriorly
(containing the superior division of vestibular nerve).

5. • The “5 nerves” of the IAC:
• 1. facial nerve (VII)
• 2. nervus intermedius: the somatic sensory branch of the facial nerve 3.
acoustic portion of the VIII nerve (mnemonic: “Coke down ” for cochlear
portion)
• 4. superior branch of vestibular nerve: passes through the superior
vestibular area to terminate in the utricle and in the ampulla of the
superior and lateral semicircular canals (mnemonic superior
• =LSU (Lateral & Superior semicircular canals and the Utricule))
• 5. inferior branch of vestibular nerve: passes through inferior vestibular
area to terminate in the saccule

8. • 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.
• 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.

10. • 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.
• 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.