Presigmoid Approaches to Posterior Fossa: Translabyrinthine and Transcochlear

There are multiple variations of the presigmoid approach to the posterior fossa: retrolabyrinthine, transcrusal, translabyrinthine, transotic, and transcochlear. Each variation increases the amount of temporal bone resected, which increases the surgical freedom at the expense of increased surgical morbidity of cranial nerves VII and VIII. In this chapter, we focus on the translabyrinthine and transcochlear approaches (retrolabyrinthine is described in Procedure 20 ).

Planning and positioning

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Management of some cerebellopontine angle lesions is best accomplished between interaction of the neurosurgeon and the neurootologist.
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The role of each surgeon in the procedure, potential complications, and realistic postoperative goals are discussed preoperatively with the patient.
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The extension of the surgical approach is determined preoperatively based on lesion location, tumor size, preoperative facial nerve function, and serviceable hearing.
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Serviceable hearing includes a pure tone average threshold better than 50 dB, speech discrimination greater than 50%, or both (50/50 rule).
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The patient lies supine, with the head at the end of the table and rotated to the contralateral side.
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The patient is strapped to the table to allow tilting of the table safely during the procedure.
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Facial nerve monitoring electrodes are placed in the orbicularis oris and oculi muscles.
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In hearing preservation cases, auditory brainstem responses are monitored by placing an acoustic ear insert in the external auditory canal, a recording electrode on the vertex, a reference electrode in the ipsilateral ear lobule, and a ground electrode.
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Preoperative steroids and antibiotics are used. Before opening the dura, mannitol (0.5 to 1 g/kg) is given.

Figure 7-1:

Contrast-enhanced T1-weighted magnetic resonance image of a large vestibular schwannoma shows a straight route to access the posterior fossa.

Figure 7-2:

The retrolabyrinthine (RL) approach provides access to the presigmoid posterior fossa dura between the sigmoid sinus and the labyrinth. The translabyrinthine (TL) approach entails sacrificing the labyrinth to give direct access to the internal auditory canal (IAC) and cerebellopontine angle without cerebellar retraction. Bone drilling occurs extradurally, limiting subarachnoid exposure to bone dust and associated headache. The transcochlear (TC) approach extends the translabyrinthine approach anteriorly, by sacrificing the entire inner ear and rerouting the facial nerve to provide access to the anterior cerebellopontine angle, petrous apex, and ventral brainstem.

Figure 7-3:

Proper positioning for the translabyrinthine/transcochlear approach.

Translabyrinthine approach

Indications

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The rationale for this approach includes exposure of the posterior fossa and 320-degree exposure of the IAC circumference while sacrificing any residual hearing.
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Indications include removal of cerebellopontine angle lesions with preoperative unserviceable hearing, regardless of lesion size (e.g., vestibular schwannoma, meningioma, epidermoid, dermoid).