neurosurgery and ENT

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.