Transcochlear and Transotic Approaches to the Cerebellopontine Angle

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The transcochlear (TC) approach was first described by William House and William Hitselberger in 1976 to treat midline clival lesions and cerebellopontine angle (CPA) masses arising anterior to the internal auditory canal (IAC) ( Fig. 46.1 ). Before the development of the transcochlear approach, these lesions were often considered inoperable due to reduced surgical visibility caused by the cerebellum and brainstem. In 1980, Herman Jenkins and Ugo Fisch introduced the transotic (TO) approach, with the objective of obtaining a direct lateral, and the widest possible, exposure of the CPA, from the superior petrosal sinus to the jugular bulb, and from the internal carotid artery to the sigmoid sinus ( Fig. 46.2 ).

Suboccipital approaches have been attempted for anterior CPA and clival lesions, but total lesion removal is often impossible due to the interposition of the cerebellum and the brainstem. ^, The transpalatal-transclival approach was attempted in the early 1970s for intradural midline lesions with little success. This approach had several shortcomings: exposure was often inadequate, the operative field was relatively far from the surgeon, the lateral blood supply was not visible to the surgeon, and there was an increased risk of intracranial complications from oral contamination. The retrolabyrinthine approach is limited anteriorly by the posterior semicircular canal, while the translabyrinthine approach is restricted by the facial nerve in accessing tumors anterior to the IAC, around the intrapetrous carotid artery, or anterior to the brainstem. Although the extended middle fossa and the combined transpetrous approaches enable the complete removal of petroclival meningiomas in patients with serviceable hearing, their primary limitation is poor access to tumors with inferior or midline extensions. ^, The endoscopic-assisted transsphenoidal approach has been gaining acceptance as an alternative approach to access midline intracranial lesions arising from the clivus and petrous apex.

The TC and TO approaches are similar in their goal to provide improved access to the anterior CPA as extensions of the translabyrinthine approach by removal of the cochlea and petrous apex ( Fig. 46.3 ). Although they were designed for the resection of vestibular schwannomas, they have been adapted for petroclival meningiomas, intradural clival lesions, chordomas, petrous apex cholesteatomas, and intradural epidermoids. Originally, the two approaches differed in that 1) the TC approach transposes the facial nerve posteriorly while the TO approach transposes the facial nerve anteriorly, partially into the parotid gland; and 2) the TC approach preserves the canal wall, the tympanic membrane, and lateral structures, while the TO approach removes the tympanic ring and closes the ear canal in a blind pouch. The TC approach was later modified to include the tympanic ring removal and blind pouch canal skin closure, as advocated by Derald Brackmann (personal communication, 2013), ^, while the TO approach was modified to preserve the fallopian canal during subtotal petrosectomy to decrease postoperative facial paralysis.

Indications

The TO approach was originally designed to address tumors of 2.5 cm or less in their mediolateral extent, but both the TC and TO approaches are currently used for tumors of all sizes. As mentioned, besides vestibular schwannomas, other pathologies such as meningiomas, epidermoid tumors, destructive cholesteatomas, large cholesterol granulomas, chordomas, paragangliomas, jugular foramen schwannomas, and less common lesions can all be reached in this fashion. The added exposure through the removal of the cochlea is ideal for those tumors with a larger than usual anterior extension toward the petroclival junction, the petrous apex, the hypotympanum, the anterior jugular foramen, or into the cochlea. More extensive lesions may require facial nerve translocation, as described in the infratemporal fossa type A approach for the jugular foramen and the upper cervical region.

Advantages

The TC and TO approaches afford wide intradural exposure of the anterior CPA without brain retraction, allowing for the visualization of the bilateral cranial nerves (CN) V, VI, VII, VIII, IX, X, XI, bilateral CPA, and the basilar and vertebral arteries. Exposure of the petrous apex and the clivus is possible along with visualization of the entire base of anterior CPA tumors, allowing the blood supply of midline lesions (especially from the internal carotid artery) to be accessed and controlled. This is particularly important for meningiomas arising from the petroclival region, which can be significantly devascularized through the bone removal required for these approaches. ^, ^,

Drawbacks

The main disadvantages of the TC and TO approaches include the sacrifice of hearing in the operated ear and the risk of temporary or permanent facial paralysis. Therefore, these approaches are typically reserved for patients without serviceable hearing in the involved ear or when the tumor is too far medial or inferior for the extended middle fossa or transpetrous approaches. With the use of continuous facial nerve monitoring and preservation of the fallopian canal when possible, the incidence of permanent facial paralysis can be minimized.

Preoperative Evaluation

Individuals with tumors that require the TC or TO approach may have minimal symptoms, even with large tumors. ^, Patients with meningiomas and intradural epidermoids may be nearly symptom-free until they present with CN V findings or signs of increased intracranial pressure. ^, Their hearing and vestibular function are frequently normal, with acoustic reflex decay or abnormal auditory brainstem responses being the only anomalies. Facial twitching is commonly found in patients with petrous apex epidermoids, which manifest with unilateral hearing loss and tinnitus in 80% of the cases. Imbalance, ataxia, and parietal or vertex headaches may be the only complaints in 20% of these patients. There is a high rate of jugular foramen syndrome in patients with meningiomas. Seizures, dysarthria, and late signs of dementia from hydrocephalus were common presenting symptoms in the past.

All CNs are assessed, especially the facial nerve function, which should be documented on physical examination. Pure-tone and speech audiometry and acoustic reflex testing are performed. High-resolution computed tomography (CT) with contrast enhancement, magnetic resonance imaging (MRI), and magnetic resonance angiography (MRA) are essential for diagnosis and surgical planning. CT is performed for bony assessment of lesions within or invading the temporal bone and to determine the status of mastoid air cell pneumatization. For instance, petrous apex and intradural epidermoids are expansile, spherical, or oval lesions, with scalloped bone edges on CT. They are isodense to cerebrospinal fluid (CSF) on CT with capsular enhancement. On MRI, they are hypointense on T1-weighted images and hyperintense on T2-weighted images. Meningiomas enhance with contrast on MRI and often manifest with a “dural tail.” Evaluation of the blood supply of some tumors may also require MRA. In tumors surrounding or invading the intrapetrous carotid artery, the patency of the circle of Willis and the carotid residual pressure need to be evaluated. Preoperative balloon occlusion of the carotid artery and selective embolization are performed 1 day before surgery. Radioisotope, xenon, or positron emission tomography studies are used to assess the cerebral perfusion during occlusion studies.

The natural history of petrous apex epidermoids is one of slow growth. They may produce cranial neuropathies and may become secondarily infected. The treatment is difficult when infection occurs, which may result in meningitis, sepsis, or death. Intracranial epidermoids spread through the cisterns and subarachnoid planes to the neighboring regions, including the opposite CPA. Petrous ridge meningiomas are space-occupying lesions that can increase the intracranial pressure. After surgical removal, the intracranial pressure is often reduced, and the CN symptoms tend to improve.

The option of conservative management by close monitoring with serial MRI to follow tumor growth is presented to all patients, especially for patients with nonprogressive, long-standing symptoms (e.g., elderly patients), patients with small tumors and normal or serviceable hearing, patients with significant medical illnesses, or patients who refuse to undergo surgery. These patients are informed that rapid or eventual tumor growth may require surgery, and that their facial nerve outcomes and hearing preservation may be compromised as a result.

When discussing surgery, risks and complications include bleeding, infection, complete hearing loss, temporary or permanent facial paresis or paralysis, transient vestibular symptoms, taste disturbance, headache, CSF leak, scarring, swallowing difficulty, aspiration pneumonia, stroke, and, rarely, death.

Patients are advised to undergo neurosurgical evaluation as the neurosurgical team may be involved in surgery, especially if there is intracranial or intradural extension. The translabyrinthine and retrosigmoid approaches are the most established and popular techniques, whereas the middle fossa approach has traditionally been reserved for small tumors in patients with serviceable hearing. These approaches should also be considered in lieu of the TC and TO approaches with input from the neurosurgical team.

Surgical Anatomy

The intracranial structures that may be exposed by the TC and TO approaches include the entire lateral aspect of the pons and upper medulla, CNs V through XI, and the basilar artery. Posterior fossa exposure is extensive, except inferiorly, where it is limited in the area of the jugular foramen and foramen magnum. The degree to which the neural compartment of the jugular foramen is visible depends on the height of the jugular bulb. Modifications to the approaches permit identification of the anterior aspect of the pons and both sixth CNs and improved identification of the basilar artery and vertebrobasilar junction.

Surgical Technique

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