- Mental Health
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 article, we focus on the translabyrinthine and transcochlear approaches.
Planning and positioning
• Management of some cerebellopontine angle lesions is best accomplished between interaction of the neurosurgeon and the neuroot1ologist.
• The role of each surgeon in the procedure, potential complications, and realistic postoperative goals are discussed preoperatively with the patient.
• The extension of the surgical approach is determined preoperatively based on lesion location, tumor size, preoperative facial nerve function, and serviceable hearing.
• Serviceable hearing includes a pure tone average threshold better than 50 dB, speech discrimination greater than 50%, or both (50/50 rule).
• The patient lies supine, with the head at the end of the table and rotated to the contralateral side.
• The patient is strapped to the table to allow tilting of the table safely during the procedure.
• Facial nerve monitoring electrodes are placed in the orbicularis oris and oculi muscles.
• 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.
• 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.
• The rationale for this approach includes exposure of the posterior fossa and 320-degree exposure of the IAC circumference while sacrificing any residual hearing.
• Indications include removal of cerebellopontine angle lesions with preoperative unserviceable hearing, regardless of lesion size (e.g., vestibular schwannoma, meningioma, epidermoid, dermoid).
• Lesions extending anteriorly to prepontine cistern
• Ipsilateral chronic otitis media (relative)
• Only hearing ear
Translabyrinthine 7-4: Because the translabyrinthine approach is an extension of the retrolabyrinthine approach, the same steps are used to identify the facial nerve and lateral and posterior semicircular canals and to skeletonize the posterior and middle fossa dura (see Procedure 20). A labyrinthectomy is then performed. It is started by blue lining and opening the lateral semicircular canal from anterior to posterior. The posterior semicircular canal is located posterior and perpendicular to the lateral canal. Attention must be paid to the facial nerve as this dissection is performed because it lies parallel to the lateral canal in the tympanic segment and parallel to the posterior canal in its vertical segment.
Translabyrinthine 7-5: The lumen of the posterior semicircular canal is followed superiorly to its junction with the superior semicircular canal at the common crus. The superior canal is opened toward its ampulla anteriorly. The subarcuate artery is identified in the center of the arch of this canal and can be cauterized as the dissection is carried medially. Leave the superior ampulla unopened because it serves as a valuable landmark for the lateral-superior limit of the IAC fundus.
Translabyrinthine 7-6: The common crus is followed until the vestibule is opened. The facial nerve is skeletonized further at the second genu to widen access to the vestibule. The spherical recess of the saccule is localized in the anterior portion of the vestibule, and the elliptic recess of the utricle is localized in the posterior portion. The ampullated end of the superior canal and the ampullated end of the posterior canal provide the expected superior and inferior limits of the IAC.
Translabyrinthine 7-7: The IAC dissection is started in the mid-portion and extended posteriorly toward the porus. Superior and inferior troughs are created and deepened parallel to the identified path of the IAC dura. Bone is removed from two thirds of the circumference of the IAC using a diamond bur. All presigmoid bone is removed from underlying posterior fossa dura. The jugular bulb is defined medial to the facial nerve and serves as the inferior limit of the labyrinthine portion of the dissection. Wider access to larger lesions in the cerebellopontine angle require complete removal of bone inferior and superior to the IAC with extensions anterior to the porus acusticus. The middle fossa dura and superior petrosal sinus should also be cleared of bone to facilitate this access.