Internal acoustic meatus !

The internal acoustic meatus is a canal on the inner side of the temporal bone; it carries the facial nerve and the nerve for hearing and balance.

See in 3D →

Opens in the Durgical app, no sign-up needed.

What it is

A canal in the inner surface of the petrous temporal bone.

What it does

Carries the facial and vestibulocochlear nerves.

Key facts

Where: Posterior surface of the petrous temporal bone
Function: Transmits the facial (VII) and vestibulocochlear (VIII) nerves and the labyrinthine artery

Relations

Contents: Facial nerve (VII) with its sensory root (nervus intermedius), the vestibulocochlear nerve (VIII) as cochlear and superior and inferior vestibular nerves, and the labyrinthine artery and vein
Course: From its opening (porus) on the posterior surface of the petrous temporal bone it runs laterally for about a centimetre to a blind end (fundus), a thin plate pierced by the nerves to the inner ear
Lateral: At the fundus, the cochlea in front and the vestibule and semicircular canals behind; the facial nerve leaves the upper front corner to enter the facial canal
Medial: Cerebellopontine angle: the pons, the flocculus of the cerebellum and the anterior inferior cerebellar artery, which often loops into the opening

How to find it

It cannot be felt. In a dry skull it is the oval opening on the posterior surface of the petrous ridge, seen from inside the posterior cranial fossa above the jugular foramen. On axial CT or MRI it lies at the level of the inner ear, roughly in line with the external acoustic meatus.

Examination

Tuning fork tests: with a lesion here the loss is sensorineural, so Rinne stays positive (air better than bone) and Weber lateralises to the better ear
Pure-tone and speech audiometry: one-sided sensorineural loss, with speech understanding worse than the tone loss would predict, suggests a lesion beyond the cochlea
Corneal reflex: reduced on the side of a large tumour that has reached the trigeminal nerve, often before facial weakness appears

On imaging

MRI with gadolinium, or thin heavily T2-weighted slices, is the standard to find a vestibular schwannoma: an enhancing mass in the canal that may bulge into the angle like an «ice cream cone». CT with bone windows shows the canal widened or eroded.

Clinical notes

Facial weakness is late and uncommon in vestibular schwannoma even when VII is stretched; early facial palsy suggests another lesion, such as a facial nerve schwannoma or a meningioma
Schwannomas of the vestibular nerve on both sides are the hallmark of neurofibromatosis type 2
The labyrinthine artery, usually from the anterior inferior cerebellar artery, is an end artery: its occlusion can cause sudden deafness and vertigo
Surgery for tumours in the canal (translabyrinthine, retrosigmoid or middle fossa approaches) is planned around preserving hearing and facial function

Memory aid

At the fundus the meatus splits into four areas: facial nerve (VII) upper front, cochlear nerve lower front, the two vestibular nerves behind.

Vestibular schwannoma (acoustic neuroma)

Test yourself

Think of the answer, then tap the question to see it.

Which structures pass through the internal acoustic meatus?

The facial nerve (VII) with the nervus intermedius, the vestibulocochlear nerve (VIII) and the labyrinthine artery and vein.

What tumour widens the internal acoustic meatus, and what is its usual first sign?

Vestibular schwannoma (acoustic neuroma), from the vestibular nerve; the usual first sign is one-sided hearing loss with tinnitus.

How are the nerves arranged at the fundus of the internal acoustic meatus?

Facial nerve upper front, cochlear nerve lower front, the two vestibular nerves behind; split by the horizontal crest and Bill bar.

What do the Rinne and Weber tests show with a lesion in the internal acoustic meatus?

Sensorineural loss: Rinne stays positive (air better than bone) and Weber lateralises to the better ear.

What do bilateral vestibular schwannomas in the internal acoustic meatus indicate?

Neurofibromatosis type 2; they are its hallmark.

Add to my cards (5)

Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): meatus acusticus internus TA2 671
Official Spanish (DeCS): Meato acústico interno
English (TA2): Internal acoustic meatus
Also called: internal auditory canal

Sources: Terminologia Anatomica (TA2); Anatomy, Head and Neck, Skull Foramen; Neuroanatomy, Cranial Nerve 8 (Vestibulocochlear).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.