Chapter 12: Translabyrinthine Approach to the Internal Auditory Meatus

Topography

The internal auditory canal (IAC) is a bony canal approximately 1 cm in length, extending medially to laterally from the cerebellopontine angle through the petrous bone. Its anterior-posterior orientation aligns roughly from the external genu to the sinodural angle. The long axis of the IAC is parallel and superior to the external auditory canal, with the canal’s most lateral extent located superficially, deep to the medial wall of the vestibule and the ampullae of the three semicircular canals. The medial extent, termed the porus, is deeply situated at the junction of the middle and posterior fossae. The meatus of the IAC, located on the posterior petrous bone surface, features an acute posterior border and a flat anterior border.

The ampulla of the superior semicircular canal (SCC) represents the most superior and lateral aspect of the IAC. The labyrinthine segment of the facial nerve runs immediately anterior and slightly superior to the IAC. The subarcuate artery, traversing the superior semicircular canal’s arch within the petromastoid canal, serves as an additional superior landmark.

The medial wall of the vestibule forms the lateral boundary of the IAC fundus, where nerves enter inner ear structures. The horizontal crest divides the fundus into a smaller upper zone and a larger lower zone. The upper zone houses the vertical crest (“Bill’s bar”), separating the anterior foramen for the facial nerve from the posterior foramen for the superior vestibular nerve. The cochlear nerve passes anteriorly through a central canal surrounded by multiple foramina, while the inferior vestibular nerve exits posteriorly. The canal also contains the internal auditory artery and vein, with a possible loop of the anterior inferior cerebellar artery.

Axial views reveal a triangular bony wedge to be removed for IAC exposure. One side represents the IAC roof, the second is the floor of the labyrinthectomy dissection, and the third is the posterior fossa dura extending between the other two sides. Inferior boundaries include the jugular bulb and cochlear aqueduct.

Indications

The translabyrinthine approach is indicated in the following cases:

• Vestibular neurectomy: For vestibular vertigo in patients with non-serviceable hearing.
• Cerebellopontine angle tumor removal:
  • Acoustic neuromas: Indicated for tumors >1.5 cm in extrameatal diameter or in cases with unserviceable preoperative hearing. The approach also facilitates safe removal of giant tumors.
  • Meningiomas: Posterior or centered on the IAC with non-serviceable hearing. For tumors anterior to the canal, a transapical extension may be needed, and large petroclival tumors may require a modified transcochlear approach.
  • Other tumors: Epidermoids, dermoids, and similar lesions with non-serviceable hearing.

Landmarks

At the Beginning:

• Tegmen
• Sinodural angle
• Vestibule
• Ampullae of the superior and posterior semicircular canals
• Posterior fossa plate
• Sigmoid sinus
• Superior petrosal sinus
• Facial nerve

During the Procedure:

• Tegmen
• Sinodural angle
• Vestibule
• Posterior fossa plate
• Sigmoid sinus
• Superior petrosal sinus
• Facial nerve
• Internal auditory meatus
• Cochlear aqueduct
• Jugular bulb

At the End:

• Tegmen
• Sinodural angle
• Vestibule
• Posterior fossa plate
• Sigmoid sinus
• Superior petrosal sinus
• Facial nerve
• Internal auditory canal
• Cochlear aqueduct
• Jugular bulb

Procedure

Preparation and Initial Steps

The translabyrinthine approach typically spares the posterior bony wall of the external auditory canal and the tympanic membrane. A posterior tympanotomy may be performed, with the incus removed if necessary. After completing the labyrinthectomy, connective tissue is used to close off openings into the middle ear cavity to prevent cerebrospinal fluid (CSF) leakage.

For live surgeries, thorough removal of mastoid cells is critical to avoid postoperative CSF leakage. Retrofacial air cells are obliterated using a diamond drill, and bone dust is driven into cell tracts before opening the dura.

Step 1: Identification of Landmarks

1. Semicircular Canals: Confirm identification of all semicircular canals.
2. Ampulla of the Superior SCC: This landmark aids in locating the superior vestibular nerve, Bill’s bar, and the facial canal.
3. Vestibule: Identified after removing remnants of the semicircular canals.
4. Mike’s Dot: Marks the passageway for the superior vestibular nerve fibers.
5. Subarcuate Foramina: Found inferior to the subarcuate artery.
6. Superior Petrosal Sinus: Blue-lined with a diamond burr.
7. Jugular Bulb: Identified by skeletonizing and possibly lowering in cases of high jugular bulb.

Step 2: Exposure of the Internal Auditory Meatus

Extensive drilling exposes 270° of the IAC’s circumference. The fundus lies immediately under the vestibule, requiring removal of over 1 cm of bone for porus exposure. The bone’s posterior portion is often dense, making removal challenging.

• Outside-in Approach: Skeletonize the medial vestibule wall and thin the dura covering the IAC. Create superior and inferior troughs along the IAC’s borders. Reduce bone thickness by semicircular drilling from the IAC’s superior border to its inferior border.
• Inside-out Approach: Drill presigmoid posterior fossa dura to identify porus dura. Create troughs for 270° exposure and proceed anteriorly.

Step 3: Opening the Internal Auditory Meatus

Thin the bone overlying the IAC dura with a dissector and incise the dura:

• Inferior Opening: Incise the IAC dura along the inferior border to avoid facial nerve injury. Detach superior and inferior ampullary nerves from their canals.
• Superior Opening: Identify the vertical crest (Bill’s bar) and superior vestibular nerve. Open the dura to expose vestibular and facial nerves.

Step 4: Identification of Nerves

• Facial Nerve: Anterior superior to the vestibular nerves.
• Superior Vestibular Nerve: Posterior to the facial nerve, terminating at the superior ampulla.
• Inferior Vestibular Nerve: Found in the fundus.
• Cochlear Nerve: Anterior inferior in the IAC.

Tips and Tricks

1. Use small diamond burrs for fine bone removal.
2. Preserve an eggshell-thin layer of bone over critical structures during initial drilling.
3. Ensure 270° exposure of the IAC for safe manipulation of nerves and vessels.
4. Avoid drilling below the cochlear aqueduct level to prevent lower cranial nerve injury.
5. Maintain patience during posterior drilling for adequate exposure.

Dangers

1. Limited exposure increases the risk of blind dissection and nerve/vessel injury.
2. Improper handling of the jugular bulb can result in severe bleeding.
3. The facial nerve is particularly at risk during vestibular nerve detachment.

Anatomical Variations

The jugular bulb’s position varies, occasionally extending into the hypotympanum. Such variations necessitate careful planning and execution to prevent complications.