Surgical Neurotology

The lateral skull base is an anatomical area shared by both the neurotology and neurosurgery teams. Both teams have at their disposal numerous approaches for traversing the lateral skull base and accessing the intracranial vault. ^[1] Although many lesions in this area are considered benign (60–75%) or neurogenic (20%) in origin, they can result in local destruction and functional deficits. Extensive familiarity with the challenging anatomy in this region is crucial to successful surgery since extensive surgical approaches are often needed to access and remove disease. The discussion of approaches is divided, somewhat arbitrarily, into 3 general categories: transtemporal, suboccipital, and subtemporal.

The transtemporal approaches encompass techniques in which the temporal bone is the primary target through which dissection and intracranial access are achieved. The suboccipital approaches include those procedures where variations in suboccipital craniotomies provide access to the posterior fossa. The subtemporal approaches constitute procedures where access to varying regions of the middle or posterior fossae are achieved caudal to the temporal lobe.

Finally, combined middle fossa- posterior fossa approaches are discussed. As will be readily demonstrated, many approaches combine elements of subtemporal, suboccipital, and transtemporal vectors and can be considered in multiple ways.

A surgical team’s approach will depend on both disease location and etiology. Completely removing disease or obtaining adequate oncologic margins must be weighed against morbidity of the resection. Etiology is equally important; while surgical excision is the only curative option, the national trend towards treatment of paragangliomas in this area is observation, and non-surgical therapy such as radiation. This is partly due to the relatively slow growth of parangliomas – an average of 0.8mm/year.

Continuing advances in the field of skull base surgery aim to decrease patient morbidity while improving access to disease location. While further developments and studies are needed to draw conclusions, some surgeons have found success using endoscopic techniques for approaching the lateral skull base.

Lateral temporal bone resection

The lateral temporal bone resection is a procedure that can be performed for direct therapeutic effect in resecting tumors of the external auditory canal. These tumors typically arise from the skin of the auricle or ear canal itself. Additionally, tumors can extend from the nearby parotid gland. The most common is histopathology squamous cell carcinoma. Depending on the size, location, invasion, and nature of tumor, the lateral temporal bone resection can be combined with partial or total auriculectomy, parotidectomy and / or neck dissections to provide appropriate complete surgical extirpation of neoplastic disease. ^[16] This resection is performed when the tumor is involving the external auditory canal and extending to but not beyond the tympanic membrane. In cases in which the tumor has extended beyond the bony confines of the external auditory canal or through the tympanic membrane and is involving the mesotympanum or mastoid air cells, a subtotal or total temporal bone resection is indicated.

In addition to its use in the definitive treatment of neoplastic disease in the external auditory canal, the lateral temporal bone resection serves as the basis for lateral temporal bone resection involving other transtemporal approaches to the skull base, including the transotic, transcochlear, and infratemporal fossa type A approaches, which are described later in the section.

Subtotal temporal bone resection

When tumor has extended into the middle ear cleft, a subtotal temporal bone dissection is warranted. However, recurrence rates and long-term mortality are significantly increased when tumors have breached this plane and entered the medial temporal bone .

Total temporal bone resection

When neoplastic disease has invaded the medial temporal bone and resection beyond the otic capsule is necessary to achieve total tumor resection, a total temporal bone resection is performed. Despite the aggressive nature of the procedure, long-term and disease-free survival following this resection is dismally poor.

When the petrous carotid artery is resected, the procedure is sometimes termed a radical temporal bone resection. Involvement of the intrapetrous carotid artery by neoplastic disease portends poor short-term survival.

Modified translabyrinthine approach

The modified translabyrinthine technique can be considered in a similar light as the posterior semicircular canal occlusion procedure for intractable benign paroxysmal positional vertigo, but the modified translabyrinthine approach involves occlusion of all three semicircular canals instead of just one.

While selective occlusion of the posterior semicircular canal for intractable vertigo allows for excellent postoperative hearing outcomes, the vast majority of surgical recipients maintain normal hearing. ^[5] This is unfortunately not the case for the modified translabyrinthine approach. Occlusion of all three canals and skeletonization of the vestibule, in conjunction with intracanalicular tumor dissection performed in the modified translabyrinthine approach, yields postoperative preservation of serviceable hearing in at best 40–50% of patients. ^{[4, 6]}

Retrolabyrinthine approach

Despite the potential to preserve hearing, the retrolabyrinthine approach has limited application due to its extremely narrow window of intracranial exposure.

Occasionally, a small tumor of the cerebellopontine angle has little or no extension into the internal auditory canal, or a tumor of the mastoid has extended through the posterior fossa dural plate into the cerebellopontine angle. In such cases where transmastoid access to the cerebellopontine angle can be significantly restricted without impeding tumor removal, a strictly retrolabyrinthine dissection can be performed.

Transotic approach

On the opposite end of the spectrum, the transotic and transcochlear approaches expand rather than restrict the limits of dissection. ^[7]  When the tumor has significant anterior petrous apex or cerebellopontine angle extension, the translabyrinthine approach may be extended anteriorly.

Transcochlear approach

When wide anterior petrous apex exposure is necessary, the facial nerve can be removed from its entire course within the bony fallopian canal and transposed posteriorly. This anterior petrous apex exposure is termed the transcochlear approach.

Infratemporal fossa type A approach

When wide exposure along the posterior petrous apex and jugular bulb are required, the infratemporal fossa type A approach is used. This approach is commonly used when resecting tumors of the jugular bulb, such as glomus jugulare tumors.

Fallopian bridge technique

A significant morbidity to the infratemporal fossa A approach is the necessity for facial nerve transposition. Facial nerve transposition causes a complete facial paralysis acutely. Long-term facial function rarely improves beyond a House Brackmann grade 3, since long-term facial synkinesis from bulk movement of the nerve almost always occurs.

An alternative approach to achieving lateral exposure of the jugular bulb is to perform the so-called fallopian bridge technique.

Transjugular approach

The transjugular approach is used to expose the jugular bulb, jugular foramen, and cerebellopontine angle for resection of combined extracranial - intracranial jugular foramen tumors. ^[8]

Retrosigmoid approach

This approach is useful forthe following tumors larger than 3–4 cm or with significant inferior or superior extension and patients with a high-riding  jugular bulb, constricting inferior access to the cerebellopontine angle through a translabyrinthine craniotomy. The retrosigmoid approach is also used when hearing conservation is desired, ^[14] particularly in tumors with significant cisternal extension not amenable to middle fossa approach. The retrosigmoid approach is described in detail in the Medscape Reference topic Acoustic Neuroma.

Far lateral approach/transcondylar approach

The transcondylar approach offers extended access to the posterior fossa from the tentorium to the cervicomedullary junction.

Extreme lateral approach

The transcondylar approach can be extended to encompass the lateral sinus, jugular bulb, and jugular foramen. This approach is termed the extreme lateral approach. Lateral extension provides access to the ventral cervicomedullary junction* or to the jugular foramen itself. ^[10] The approach is often combined with a retrolabyrinthine extension superiorly into the mastoid cavity, which is used when the target of dissection is the jugular bulb and jugular foramen as opposed to the ventral cervicomedullary junction.

Postauricular subtemporal approach/posterior petrosectomy/middle fossa approach

The middle fossa approach is applicable for tumors that are intracanalicular. The great advantage to the middle fossa approach is the ability to visualize the entire internal auditory canal from fundus to porus and resect tumors completely with high rate of hearing preservation. However, this approach, without modification, does not provide adequate posterior fossa exposure and hence is not appropriate for tumors extending more than 3–5 mm beyond the porus acusticus. The middle fossa approach is described in detail in the Medscape Reference topic Acoustic Neuroma.

Infratemporal fossa type C approach

The infratemporal fossa type C approach is used for tumors along the anteriormost extent of the petrous bone and clivus but with more significant extension subcranially within the infratemporal fossa and pterygomaxillary fissure.

Extended middle fossa approach

When additional exposure is needed within the cerebellopontine angle through a subtemporal craniotomy, the middle fossa or postauricular subtemporal approach can be combined with the preauricular subtemporal approach to provide such exposure. This extended middle fossa approach provides adequate access to the cerebellopontine angle to allow for tumor removal for intracanalicular tumors with significant cisternal component. ^[13] However, this approach is of limited use for tumors with significant brainstem compression.

Combined middle fossa-posterior fossa approaches

Large intracranial tumors that traverse both the middle and posterior fossae often cannot be fully accessed by a subtemporal or suboccipital approach alone. Combining subtemporal and suboccipital approaches allows for wide exposure of the prepontine cistern, clivus, and petroclival junction, where such tumors present.

Subtemporal-translabyrinthine petrosal approach

The subtemporal- translabyrinthine petrosal craniotomy provides even greater anterior exposure, not just to the petroclival region, but a complete view of the internal auditory canal as well. The advantage of greater anterior exposure is tempered by the postoperative anacusis induced through this approach. A variation of this approach, the transcrusal approach, attempts to maintain neurosensory hearing function by limiting translabyrinthine resection to just the superior and posterior semicircular canals.

Anterior Transpetrosal Approach-Total Petrosectomy Approach

This temporal bone dissection, under either name, is not commonly performed. The entire anterior petrous apex does not need to be resected to visualize the clivus and prepontine cistern: this can be partially accessed with a subtemporal- retrolabyrinthine approach or completely accessed with a subtemporal- transcochlear approach without brain retraction. Total petrosectomy for advanced, invasive malignant disease causes significant perioperative morbidity and possibly mortality yet will do little to improve prognosis.