Which set of structures correctly lists the contents of the internal acoustic meatus?

Option C is correct because the internal acoustic meatus transmits the facial nerve (CN VII), the vestibulocochlear nerve (CN VIII), the nervus intermedius, and the labyrinthine artery, which is the arterial supply to the inner ear.

Why option C lists every true content of the meatus

The internal acoustic meatus, also called the internal auditory canal, is a bony canal in the posterior surface of the petrous part of the temporal bone. It runs laterally for approximately 8 to 10 mm from the posterior cranial fossa to the fundus, where the facial nerve enters the facial canal and the cochlear and vestibular nerves reach the inner ear structures. Four structures pass through it as a fixed set: the facial nerve (CN VII), the nervus intermedius (the sensory and parasympathetic root of the facial nerve), the vestibulocochlear nerve (CN VIII) with its cochlear and vestibular divisions, and the labyrinthine artery. The labyrinthine artery is usually a branch of the anterior inferior cerebellar artery, though it can arise directly from the basilar artery. It travels with CN VIII to supply the cochlea and vestibular apparatus. No other cranial nerve, no dural venous sinus, and no other named artery passes through this canal. Option C is the only choice that includes all four of these structures and nothing that does not belong. Every other option either inserts a nerve that runs elsewhere or omits one of the four true contents.

Anatomy of the meatus and spatial arrangement of its contents

The internal acoustic meatus is divided into superior and inferior compartments by a horizontal ridge called the transverse crest, or falciform crest. A vertical bony ridge, the vertical crest or Bill bar, further subdivides the superior compartment. This spatial arrangement is surgically relevant during a retrosigmoid or middle fossa approach to the canal. The superior compartment anterior to Bill bar carries the facial nerve. The superior compartment posterior to Bill bar carries the superior vestibular nerve. The inferior compartment anterior to the transverse crest carries the cochlear nerve. The inferior compartment posterior to the transverse crest carries the inferior vestibular nerve. The nervus intermedius runs between the facial nerve and the superior vestibular nerve in the superior compartment, which is why it can be compressed by even small vestibular schwannomas. The labyrinthine artery and its venae comitantes run inferior to the vestibulocochlear nerve within the canal. The fundus of the meatus is the lateral boundary where the facial nerve enters the fallopian canal at the meatal foramen, the narrowest segment and a common site of nerve compression. The cochlear nerve passes through the tractus spiralis foraminosus at the fundus to reach the modiolus of the cochlea.

Clinical application: vestibular schwannoma and meatal expansion

Vestibular schwannomas arise from Schwann cells of the vestibular division of CN VIII, most commonly from the inferior vestibular nerve near the glial-Schwann cell junction within the internal acoustic meatus. As the tumor grows, it expands the meatus, which is visible on high resolution CT of the temporal bone as widening of the canal beyond 2 mm asymmetry compared to the contralateral side. On T1 weighted MRI with gadolinium, the tumor enhances brightly and often extends into the cerebellopontine angle. The classic clinical triad is unilateral sensorineural hearing loss, tinnitus, and vestibular dysfunction, though hearing loss is the earliest and most consistent finding. Facial nerve weakness appears later because the facial nerve is more resistant to compression at the fundus, but the nervus intermedius can be affected early, causing altered lacrimation or taste. The House-Brackmann scale grades facial nerve function from I to VI. The Koos classification grades vestibular schwannomas from I (purely intrameatal) to IV (large with brainstem compression). Treatment options include microsurgical resection via retrosigmoid, middle fossa, or translabyrinthine approaches, or stereotactic radiosurgery for tumors under 3 cm. The labyrinthine artery must be preserved during surgery to avoid hearing loss, though sacrifice is sometimes unavoidable.

How this question is asked and what the exam tests

The internal acoustic meatus is a high yield topic because it tests both pure anatomy and clinical reasoning in a single question. The exam frequently asks which structures pass through the meatus, which nerve is most commonly affected by a tumor in the meatus, or which artery supplies the inner ear. A common variant asks the student to identify the structure that does NOT pass through the meatus, with the abducens nerve or the inferior petrosal sinus as the distractor. Another variant pairs the meatus with the jugular foramen and asks the student to match each cranial nerve to its correct foramen. The nervus intermedius is a favorite detail because many students forget it is a distinct structure that travels with CN VII. The labyrinthine artery is tested by asking its parent vessel, which is the anterior inferior cerebellar artery in the majority of cases. The fundus and the meatal foramen are tested in questions about the narrowest point of the facial canal and the site of idiopathic facial nerve compression in Bell palsy. Students who memorize the four contents of the meatus as a fixed set will answer these questions correctly regardless of how the options are rearranged.

Why the other options fail

One-glance recall table

Mnemonics

What the exam actually asks

• The four fixed contents of the internal acoustic meatus are CN VII, nervus intermedius, CN VIII (cochlear and vestibular divisions), and the labyrinthine artery. No other named structure passes through it.
• The nervus intermedius is the sensory and parasympathetic root of the facial nerve. It carries taste from the anterior two thirds of the tongue via the chorda tympani and parasympathetic fibers to the lacrimal and submandibular glands.
• The labyrinthine artery is a branch of the anterior inferior cerebellar artery in most individuals. It is the sole arterial supply to the cochlea and vestibular apparatus.
• Vestibular schwannomas arise from the vestibular division of CN VIII, most commonly the inferior vestibular nerve, and expand the meatus on imaging.
• Bill bar is the vertical bony crest at the fundus that separates the facial nerve anteriorly from the superior vestibular nerve posteriorly. It is a key landmark in middle fossa surgery.
• The meatal foramen, where the facial nerve enters the fallopian canal, is the narrowest segment of the facial canal and the site of compression in Bell palsy.

Traps that cost marks

• Confusing the greater petrosal nerve with a content of the meatus. It branches from CN VII at the geniculate ganglion and exits through the greater petrosal hiatus, a separate opening.
• Assuming the abducens nerve passes through the meatus because it runs near the petrous apex. It actually traverses Dorello canal, which is lateral and inferior to the meatus.
• Forgetting the nervus intermedius as a distinct structure. Many students list only CN VII and CN VIII and miss the nervus intermedius, which is a separate nerve bundle in the superior compartment.
• Confusing the labyrinthine artery with the anterior inferior cerebellar artery itself. The labyrinthine artery is a branch of the AICA, not the AICA proper.

Frequently asked

Does the nervus intermedius pass through the internal acoustic meatus?

Yes. The nervus intermedius is a distinct nerve bundle that runs in the superior compartment of the internal acoustic meatus, positioned between the facial nerve anteriorly and the superior vestibular nerve posteriorly. It carries general sensory fibers from the external auditory canal, taste fibers from the anterior two thirds of the tongue via the chorda tympani, and parasympathetic preganglionic fibers to the pterygopalatine and submandibular ganglia. It joins the facial nerve at the geniculate ganglion and is functionally part of CN VII, but anatomically it is a separate structure within the meatus.

What is the blood supply to the inner ear and which artery travels with CN VIII?

The inner ear receives its entire arterial supply from the labyrinthine artery, which travels with the vestibulocochlear nerve through the internal acoustic meatus. The labyrinthine artery is a branch of the anterior inferior cerebellar artery in approximately 85 percent of individuals, though it can arise directly from the basilar artery. It divides into the anterior vestibular artery, the vestibulocochlear artery, and the main cochlear artery. Because the inner ear is an end organ with no collateral supply, occlusion of the labyrinthine artery causes sudden sensorineural hearing loss and vertigo.

Why does a vestibular schwannoma cause hearing loss before facial weakness?

Vestibular schwannomas arise from the vestibular division of CN VIII, most commonly the inferior vestibular nerve near the glial-Schwann cell junction within the internal acoustic meatus. The cochlear nerve lies in the inferior compartment of the meatus and is compressed early by the expanding tumor, producing unilateral sensorineural hearing loss and tinnitus. The facial nerve lies in the superior compartment anterior to Bill bar and is more resistant to compression because of its position and thicker epineurium. Facial weakness appears later, typically when the tumor extends into the cerebellopontine angle and displaces the brainstem.

References

• Gray's Anatomy, 42nd. Chapter on the cranial cavity and temporal bone, internal acoustic meatus and facial nerve
• Snell's Clinical Anatomy by Regions, 10th. Chapter on the head and neck, cranial nerves and foramina of the skull
• Bailey and Love's Short Practice of Surgery, 28th. Chapter on the ear and lateral skull base, vestibular schwannoma
• Robbins and Cotran Pathologic Basis of Disease, 10th. Chapter on the central and peripheral nervous system, schwannoma