Surgical Neurotology

 

Background

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. [1Although 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.

Indications

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. [16This 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. [5This 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. [46]

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, [14particularly 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. [10The 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. [13However, 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.

Contraindications

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.


Surgical Neurotology Technique


Approach Considerations

From the varied names and descriptions of the many approaches to the lateral skull base, it is clear that most named approaches share commonalities in regional dissections with other adjacent approaches. The approach chosen for a given tumor can be tailored very specifically to the anatomic regions encompassed by that tumor, and intimate knowledge of the surrounding cranial base can ensure that proper techniques are used. Equally importantly, if a chosen approach ends up being inadequate for management of a given tumor, knowledge of the lateral skull base allows one to modify and expand an existing approach to provide more effective exposure for successful tumor extirpation.

Transtemporal Approaches

Lateral Temporal Bone Resection

A postauricular incision is made and carried forward to transect the external auditory meatus. In cases in which partial or total auriculectomy are to be performed, the corresponding degrees of auricular resection are included within the incision. If a meatus is to be preserved, the meatal skin is everted and oversewn in 2 layers to create a blind meatal pouch.

A wide mastoidectomy is performed. The lateral sinus is skeletonized. The posterior wall of the external auditory canal is skeletonized in preparation for later resection. The incus is identified within the fossa incudis. An extended facial recess is performed, from the fossa incudis downward, transecting the iter chordae posterius and into the tympanic ring inferiorly. The incudostapedial joint is carefully separated to minimize vibrational trauma to the intact stapes, and the incus is removed.

The inferior tympanic ring is resected anterior to the fallopian canal and followed forward to the anterior canal wall. Dissection in this area is performed slowly and only using diamond burrs, as the caroticojugular spine and great vessels are directly medial to the dissection plane along the inferior tympanic ring. The dissection plane along the inferior tympanic ring must be maintained medial to the tympanic membrane to provide complete resection of the external auditory canal.

The superior limit of the mastoidectomy is extended anteriorly to resect the root of the zygomatic arch, and the superior cortex and air cells are extirpated to fully expose the epitympanic space. The cog is resected, and the supratubal air cell exposed. The dissection is carried forward from the supratubal air cell until the glenoid fossa is entered. The anterosuperior canal bone is completely separated from the root of the zygoma along the plane of the glenoid fossa. Care should be taken during drilling of the superior cuts because the petrous carotid artery lies medial to the supratubal air cell and Eustachian tube.

Once the superior and inferior cuts are accomplished, the external auditory canal is attached to the temporal bone medially only by the anterior bony annulus. This thin bony attachment is readily fractured by placing firm digital pressure in an anterior direction along the back side of the external auditory canal, which has now been essentially circumferentially skeletonized. After the canal is freed of all bony attachments, the soft tissue attachments anteriorly along the glenoid fossa and parotidomasseteric fascia are carefully incised and the external auditory canal removed en bloc. The extratemporal facial nerve should be identified prior to these soft tissue incisions to prevent injury.

If superficial or total parotidectomy is to be performed as well, the external auditory canal can be kept attached to the parotidomasseteric fascia, with the parotidectomy performed en bloc with the lateral temporal bone resection.

The external auditory meatus is overclosed. A temporalis muscle flap or other local subcutaneous rotation flap can be used to fill in the bony defect, [15followed by closure of the postauricular incision in standard fashion (see images below).

Lateral temporal bone resection, schematic, axial.Lateral temporal bone resection, schematic, axial.
Lateral temporal bone resection, schematic, coronaLateral temporal bone resection, schematic, coronal.

Subtotal Temporal Bone Resection

The medial limit of dissection in a lateral temporal bone resection is the incudostapedial joint, lateral to which all tissues are resected.

The subtotal temporal bone dissection encompasses all the components of the lateral temporal bone resection with further resection of the bony otic capsule. The remaining anterior petrous apex is preserved. As the lateral temporal bone resection is removed separately from bony extirpation of the otic capsule, the subtotal temporal bone resection is, by necessity, a form of piecemeal oncologic resection (see images below).

Subtotal temporal bone resection, schematic, axialSubtotal temporal bone resection, schematic, axial.
Subtotal temporal bone resection, schematic, coronSubtotal temporal bone resection, schematic, coronal.

Total Temporal Bone Resection

The total temporal bone resection involves exactly its namesake: the entire temporal bone is resected. An oncologically sound resection is attempted, but the nature of the temporal bone anatomy makes piecemeal resection the standard. Intratemporal neural and vascular structures are resected when oncologically indicated, often including the facial nerve, cochleovestibular nerve, and lateral sinus-/jugular bulb (see images below).

Total temporal bone resection, schematic, axial. Total temporal bone resection, schematic, axial.
Total temporal bone resection, schematic, coronal.Total temporal bone resection, schematic, coronal.

Translabyrinthine Approach

The translabyrinthine approach to the lateral skull base is considered the workhorse of surgical approaches to the cerebellopontine angle used by neurotologists. It provides excellent visualization of the facial nerve along its entire course, shortened depth of tumor dissection from the outer cortex, complete visualization of the internal auditory canal, wide access to the cerebellopontine angle, and minimal cerebellar retraction (see images below). The anatomy of the mastoid and posterior otic capsule are familiar, and the approach is a natural extension of the commonly dissected anatomic spaces within the temporal bone. The translabyrinthine approach is described in detail in the Medscape Reference topic Acoustic Neuroma.

Translabyrinthine approach, schematic, axial. Translabyrinthine approach, schematic, axial.
Translabyrinthine approach, schematic, coronal. Translabyrinthine approach, schematic, coronal.

Modified Translabyrinthine Approach

The advantages of the translabyrinthine approach are tempered by the singular disadvantage of postoperative anacusis. To try and evade this outcome, the labyrinth can be resected in a modified fashion in an attempt to preserve hearing. [23]

The standard translabyrinthine approach is performed laterally, with wide excision of the mastoid air cells and resection of the tegmen mastoideum, tegmen tympani, and posterior bony skeleton overlying the sigmoid sinus and posterior fossa dura. Mastoid air cells and bony spiculations overlying the otic capsule bone are removed to delineate the contour of the posterior otic capsule.

The labyrinthectomy is then performed in an abbreviated fashion to avoid injury to the membranous labyrinth. The semicircular canals are bluelined as far anteriorly, posteriorly, and inferiorly as possible without entering the lumina. Using diamond burrs, each limb of each semicircular canal is transected and then plugged immediately with bone wax. The horizontal semicircular canal is used as a starting point, with its anterior and posterior limbs transected and plugged. The posterior semicircular canal is followed inferiorly to the ampullated end and transected, followed by transection and plugging of the crus communis. Finally the ampullated end of the superior semicircular canal is transected and plugged.

The central otic capsule bone overlying the vestibule is gradually thinned until attenuated to near eggshell thickness. Previous reports of translabyrinthine modification described opening and then plugging the vestibule with bone wax. [2However, the rate of success in hearing preservation appears to be greater when the vestibule is not violated at all. [4]

The remaining otic capsule bone posterior to the vestibule is resected posteriorly down to the posterior fossa plate and medially to the porus acusticus. The internal auditory canal is skeletonized as laterally as possible, without violating the vestibule. The posterior fossa and medial internal auditory canal dura are opened, and the exposed tumor is resected in the usual fashion (see the images below).

Because of the limitation in anatomic dissection by preservation of the vestibule, only the medial half of the internal auditory canal is directly visible via this approach. In addition, resection of all the otic capsule bone posterior to the vestibule necessarily transects the vestibular aqueduct and endolymphatic duct, placing the patient at risk for latent endolymphatic hydrops, especially in the “fortunate” case of those patients whose endolymphatic function and hearing are preserved following surgery.

Modified translabyrinthine approach, schematic, axModified translabyrinthine approach, schematic, axial.
Modified translabyrinthine approach, schematic, coModified translabyrinthine approach, schematic, coronal.

Retrolabyrinthine Approach

As described, the retrolabyrinthine approach is an even more restricted version of the translabyrinthine approach in which the entire posterior otic capsule is functionally preserved. Limits of dissection continue to be the middle fossa dura and tentorium cerebelli superiorly, the jugular bulb inferiorly, and the posterior fossa dura and sigmoid sinus posteriorly. Anteriorly, rather than traverse the posterior otic capsule, the semicircular canals are bluelined posteriorly to maximize anterior exposure without opening the lumina. Trautmann’s triangle of bone overlying the posterior fossa dura is fully resected between the sigmoid and otic capsule, and the sigmoid itself can be completely decompressed to effect additional retraction and exposure posteriorly (see the images below).

Retrolabyrinthine approach, schematic, axial. Retrolabyrinthine approach, schematic, axial.
Retrolabyrinthine approach, schematic, coronal. Retrolabyrinthine approach, schematic, coronal.

Transotic Approach

The transotic approach follows in similar fashion to the translabyrinthine approach, with wide and complete mastoidectomy and skeletonization of the middle and posterior fossa petrous dura. Departing from the standard translabyrinthine technique, the external auditory meatus is transected and overclosed, the canal wall is taken down, and the external auditory canal, tympanic membrane, and lateral ossicles resected. This lateral temporal bone resection is described in greater detail at the beginning of the section.

The anterior mesotympanum is now directly accessible. The cochlea is then fully resected to expose the anterior petrous apex, and a variable amount of resection of the anterior apex can be performed depending on the size and location of tumor. The anterior face of the internal auditory canal is skeletonized, and the intracranial vault is accessed through the dura overlying the petrous apex to expose the prepontine cistern. If a posterior labyrinthectomy is performed also, the entire internal auditory canal can be skeletonized and exposed.

The transotic approach makes use of lateral anterior exposure by means of external auditory canal resection and overclosure to provide access to the anterior petrous apex medially. The facial nerve is left in situ throughout its course, from within the internal auditory canal, through the temporal bone, and out the stylomastoid foramen (see images below). If additional exposure is necessary, a transcochlear approach is used.

Transotic approach, schematic, axial. Transotic approach, schematic, axial.
Transotic approach, schematic, coronal. Transotic approach, schematic, coronal.

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.

The transotic approach is performed as described above. Following labyrinthectomy, the fallopian canal is skeletonized, and the facial nerve is unroofed from the meatal foramen to the stylomastoid foramen. The internal auditory canal is opened from fundus to porus, and the posterior fossa dura is opened to expose the cerebellum and brainstem. The facial nerve is elevated completely out of the bony fallopian canal, fully mobilized from the brainstem to the stylomastoid foramen, and transposed posteriorly to rest on the cerebellum.

With the facial nerve completely transposed posteriorly, the remaining cochlea can be resected, if not already done so. The anterior petrous apex to the clivus can be resected, and the anterior limit of dissection becomes the petrosal internal carotid artery. Through this approach, the entire lateral and ventral aspects of the brainstem, the clivus, and the prepontine cistern are directly visible with minimal or no cerebellar retraction (see images below).

Transcochlear approach, schematic, axial. Transcochlear approach, schematic, axial.
Transcochlear approach, schematic, coronal. Transcochlear approach, schematic, coronal.

Infratemporal Fossa Type A Approach

A wide mastoidectomy is performed. The posterior wall of the external auditory canal is skeletonized in preparation for later resection. The lateral sinus is skeletonized from its transition from the transverse sinus superiorly to the jugular bulb inferiorly. This requires complete resection of the mastoid tip in order to expose the jugular vein’s exit out the skull base inferiorly. The exit of the facial nerve through the stylomastoid foramen is in close proximity here; therefore,* the facial nerve must be identified and preserved prior to skeletonization of the jugular bulb and foramen.

The inferior facial nerve can be easily localized by 2 means. The facial nerve can be skeletonized along its vertical or transmastoid course from the second genu superiorly down to the stylomastoid foramen inferiorly. Alternatively, the digastric ridge can be identified within the mastoid tip laterally and followed medially to the stylomastoid foramen.

The facial nerve is skeletonized from the second genu to the stylomastoid foramen, and an extended facial recess is performed. The lateral temporal bone is then removed, as described above in the section for Lateral Temporal Bone Resection.

Following removal of the lateral temporal bone, the entire course of the fallopian canal is now visible, from the tympanic segment to the stylomastoid foramen. The entire bony fallopian canal is skeletonized and then gently dissected away from the facial nerve. The facial nerve is then lifted away from the remaining trough of the fallopian canal and transposed anteriorly, taking as its points of rotation the geniculate ganglion and the stylomastoid foramen. The infratemporal facial nerve can be dissected further along the main trunk to provide additional mobility anteriorly.

The postauricular incision is carried inferiorly into the upper neck, which is dissected to expose and provide control over the carotid artery and jugular vein. With the facial nerve out of the way, the jugular bulb can be dissected completely of its bony skeleton within the temporal bone. The lateral sinus is either extraluminally packed or suture ligated above the level of the jugular bulb tumor, and the jugular vein is suture ligated and transected in the upper neck. The tumor is then resected between the 2 ligated ends.

The tumor residual within the jugular bulb itself is resected last because removal at this site causes brisk bleeding from the inferior petrosal sinus, which (because of its medial location within the cranial base) cannot be controlled prior to tumor resection. Control of the inferior petrosal sinus is only accomplished following complete tumor resection within the jugular bulb, when the lumen of the inferior petrosal sinus can be identified and packed off with oxidized cellulose polymer or other hemostatic packing agent (see images below).

Considerable care must be taking during tumor dissection and packing of the jugular bulb and foramen because the lower cranial nerves exit the medial wall of the jugular foramen. Excessive pressure on the lower nerves, even if they are maintained anatomically intact, can easily lead to dysphagia, dysphonia, vocal cord paralysis, and shoulder weakness.

If the tumor extends intracranially, the posterior fossa dura can be incised and resected in continuity with the jugular bulb, and the intracranial portion of the tumor can be removed through this dural opening.

Following completion of tumor resection, the external auditory meatus is overclosed as in the procedure for lateral temporal bone resection. The facial nerve is replaced to a neutral, anatomic position, and the bony defect can be filled with a temporalis muscle flap or other local rotation flap.

Infratemporal fossa type A approach, schematic, axInfratemporal fossa type A approach, schematic, axial.
Infratemporal fossa type A approach, schematic, coInfratemporal fossa type A approach, schematic, coronal.

Fallopian Bridge Technique

The fallopian canal is completely skeletonized along its vertical or mastoid component. Rather than remove the bone and transpose the facial nerve, the nerve is kept intact within the canal and wide dissection is performed anteriorly within the tympanic ring and posteriorly within the retrofacial air cell tract.

The vertical facial nerve is kept in situ, and tumor dissection medial to the fallopian bridge is carried out in piecemeal fashion, accessing the tumor anterior and posterior to the bridge. This technique is most effective for small glomus jugulare tumors and in temporal bones with very well developed air cell tracts (see images below).

Fallopian bridge technique, schematic, axial. Fallopian bridge technique, schematic, axial.
Fallopian bridge technique, schematic, coronal. Fallopian bridge technique, schematic, coronal.

Transjugular Approach

The transjugular approach begins with a wide mastoidectomy and complete skeletonization of the lateral sinus from the transverse- sigmoid junction superiorly to the jugular bulb inferiorly. Trautmann’s triangle of bone is removed to fully expose the presigmoid posterior fossa dura. The posterior mastoid is widely resected to expose the retrosigmoid posterior fossa dura. The mastoid tip is resected, and the jugular foramen is exposed. The vertical facial nerve is delineated and skeletonized to the stylomastoid foramen. The retrofacial air cell tract is extirpated, as is the inferior tympanic ring, with the facial nerve left in place to form the fallopian bridge.

The upper neck is dissected to identify and control the carotid artery and jugular vein and locate and preserve the lower cranial nerves. The jugular vein is ligated, divided, and dissected superiorly to the jugular foramen. The lateral sinus is then ligated and transected in continuity with the surrounding posterior fossa dura anteriorly and posteriorly (see images below).

The transected section of lateral sinus and jugular bulb is then resected in similar fashion to the infratemporal fossa A approach, but with more posterior retraction, dural resection, and wider exposure of the intracranial vault. As this is a presigmoid-postsigmoid approach, access to the jugular bulb, jugular foramen, and intracranial vault can be achieved from a more posterior vector, and the facial nerve does not need to be mobilized and transposed anteriorly as is done in the infratemporal fossa A approach. In addition, the external auditory canal does not need to be resected, thereby preserving the native middle ear mechanics and conductive hearing normally lost in the infratemporal fossa A approach.

Transjugular approach, schematic, axial. Transjugular approach, schematic, axial.
Transjugular approach, schematic, coronal. Transjugular approach, schematic, coronal.

Suboccipital Approaches

Retrosigmoid Approach

The retrosigmoid approach (see images below) is described in detail in the Medscape Reference topic Acoustic Neuroma

Retrosigmoid approach, schematic, axial. Retrosigmoid approach, schematic, axial.
Retrosigmoid approach, schematic, coronal. Retrosigmoid approach, schematic, coronal.

Far Lateral Approach/Transcondylar Approach

The far lateral approach, also called the transcondylar approach when modified in such a manner, is an extension of the retrosigmoid approach caudally to the foramen magnum. The standard retrosigmoid approach exposes the posterior fossa at its superior and lateral limits, along the transverse and lateral sinuses, respectively.

The far lateral approach extends this craniotomy inferiorly to encompass the foramen magnum. The occipital bone alone can be removed, but if inadequate for tumor exposure, the condyle can be partially resected in continuity with this craniotomy to extend exposure anteriorly. [9]

A C-shaped or S-shaped incision is made approximately midway between the postauricular crease and the occipital midline. The occipital attachments of the sternocleidomastoid and posterior occipital musculature are released. The C1 lamina is palpated, adjacent to which is the vertebral artery. A suboccipital craniotomy is performed and extended to the foramen magnum and to partially resect the posterior third to half of the occipital condyle, with care taken around the vertebral artery. If more than half of the occipital condyle is resected, an occipitocervical fusion must be performed to avoid the risk of craniocervical instability (see images below).

Transcondylar approach (far lateral), schematic, aTranscondylar approach (far lateral), schematic, axial.
Transcondylar approach (far lateral), schematic, cTranscondylar approach (far lateral), schematic, coronal.

Extreme Lateral Approach

The extreme lateral approach can be thought of as an extension of the transcondylar approach laterally, ie, traversing the lateral sinus, which denotes the lateral limit of dissection in the transcondylar approach. It can also be thought of as an extension of the transjugular approach inferiorly, with resection of the lower lateral sinus superiorly and extension of intracranial access through the foramen magnum and as low as C1 or C2 inferiorly (see images below). [11]

Extreme lateral approach, schematic, axial. Extreme lateral approach, schematic, axial.
Extreme lateral approach, schematic, coronal. Extreme lateral approach, schematic, coronal.

Subtemporal Approaches

Postauricular Subtemporal Approach/*Posterior Petrosectomy/Middle Fossa Approach

The standard middle fossa approach used in neurotology to access the internal auditory canal via subtemporal craniotomy is often termed a posterior subtemporal approach or posterior petrosectomy in the neurosurgical literature (see the images below).

Middle fossa approach (postauricular subtemporal),Middle fossa approach (postauricular subtemporal), schematic, axial.
Middle fossa approach (postauricular subtemporal),Middle fossa approach (postauricular subtemporal), schematic, coronal.

Preauricular Subtemporal Approach/Anterior Petrosectomy

The middle fossa approach is sometimes termed the posterior or postauricular subtemporal approach in the neurosurgical literature. By contrast, the preauricular subtemporal approach or anterior petrosectomy takes the internal auditory canal as its posterior limit of dissection rather than as the center of dissection.

Anterior petrosectomy is often aided first by additional lateral exposure by means of osteotomy and downward mobilization of the zygomatic arch, allowing for lower subtemporal craniotomy and wider exposure for Kawase’s triangle.

The anterior petrous apex bone between Meckel’s cave and the internal auditory canal is then removed. The limits of this anterior petrosectomy, dubbed Kawase’s triangle, or more appropriately Kawase’s quadrangle, are the Gasserian ganglion and mandibular nerve anteriorly, the confluence of the superior petrosal sinus and anterior dural edge of the internal auditory canal medially and posteromedially, the cochlea posterolaterally, and the line parallel to the horizontal petrous carotid artery that passes from the geniculate ganglion to the Gasserian ganglion laterally. [12]

Removal of the bone delineated within this quadrangle exposes the posterior fossa dural lining of the anterior petrous apex, beyond which provides access to the prepontine cistern and clivus (see images below).

Anterior petrosectomy (preauricular subtemporal), Anterior petrosectomy (preauricular subtemporal), schematic, axial.
Anterior petrosectomy (preauricular subtemporal), Anterior petrosectomy (preauricular subtemporal), schematic, coronal.

Infratemporal Fossa Type B Approach

If additional anterior extension is desired, the mandibular nerve is transected proximal to foramen ovale and separated from the Gasserian ganglion. This provides unimpeded access to the anterolateral triangle of the middle fossa floor and the lateral sphenoid wing. For tumors basal tumors with infratemporal fossa extension, the greater wing of the sphenoid can be resected and the infratemporal fossa opened. This exposure of the anterior petrous apex extending to the foramen ovale, transecting the mandibular nerve, with access to the infratemporal fossa via a subtemporal approach is termed the infratemporal fossa B approach. As originally described, the infratemporal fossa B approach used a postauricular incision, ear canal overclosure and lateral temporal bone resection for exposure of the Eustachian tube and petrous apex in conjunction with the subtemporal craniotomy (see the images below).

Infratemporal fossa type B approach, schematic, axInfratemporal fossa type B approach, schematic, axial.
Infratemporal fossa type B approach, schematic, coInfratemporal fossa type B approach, schematic, coronal.

Infratemporal Fossa Type C Approach

The infratemporal fossa type C approach continues the shift of transbasal exposure anteriorly, from infratemporal fossa type A where exposure is centered along the jugular bulb, to the infratemporal fossa type B, where exposure is centered along the petrous carotid artery.

The infratemporal fossa type C approach is an extended anterior petrosectomy approach with a preauricular incision and posterior limit of dissection, thereby preserving the external auditory canal and middle ear mechanics. Anteriorly, dissection traverses the mandibular and maxillary nerves to encompass a greater degree of the sphenoid greater wing, including the anterolateral and anteromedial triangles. Zygomatic osteotomy and downfracture are again performed to allow unimpeded access to resection of the lateral sphenoid wing and exposure of the infratemporal fossa (see images below).

Infratemporal fossa type C approach, schematic, axInfratemporal fossa type C approach, schematic, axial.
Infratemporal fossa type C approach, schematic, coInfratemporal fossa type C approach, schematic, coronal.

Extended Middle Fossa Approach

The standard middle fossa approach is performed. Dissection is carried to the petrous ridge and superior petrosal sinus posteriorly. Anteriorly, dissection is carried beyond the middle meningeal artery, which is ligated and transected at the foramen spinosum. Anterior dissection along the middle fossa floor is extended to Meckel’s cave, and the 3 branches of the trigeminal nerve are delineated and preserved. Anterior petrosectomy is then performed as described above, with extirpation of the bone within Kawase’s triangle or quadrangle.

All bone above and behind the internal auditory canal is resected, providing a 270º dissection around the porus acusticus medially. The dural around the porus and along the face of the petrous ridge is incised and followed into the internal auditory canal, allowing access to both the internal auditory canal space as well as limited access to the cerebellopontine cistern.

If additional exposure is desired, the tentorium cerebelli can be divided superior to the middle fossa exposure, after ligation or clipping of the superior petrosal sinus. The limits of exposure in the extended middle fossa technique are the otic capsule laterally, the petrous carotid artery anteroinferiorly, Meckel’s cave and the Gasserian ganglion anteromedially, and the safe limit of temporal lobe retraction superiorly. Safe temporal lobe retraction, as with standard middle fossa approaches, is limited in part by traction on the vein of Labbé (see the images below).

Extended middle fossa approach, schematic, axial. Extended middle fossa approach, schematic, axial.
Extended middle fossa approach, schematic, coronalExtended middle fossa approach, schematic, coronal.

Combined Middle Fossa–Posterior Fossa Approaches

Subtemporal–Retrosigmoid Petrosal Approach

Combining a subtemporal with a retrosigmoid approach provides better access to the petroclival junction than either approach alone. However, a significant disadvantage lies in the integrity of the lateral sinus via this approach. This anatomic limitation forces dissection to be performed within 2 compartments, the presigmoid/subtemporal compartment and the retrosigmoid/suboccipital compartment (see the images below).

Subtemporal – retrosigmoid approach, schematic, axSubtemporal – retrosigmoid approach, schematic, axial.
Subtemporal – retrosigmoid approach, schematic, coSubtemporal – retrosigmoid approach, schematic, coronal.

Subtemporal–Retrolabyrinthine Petrosal Approach

Combinations of a subtemporal approach with a presigmoid approach are termed petrosal or transpetrosal approaches. The subtemporal component typically involves a subtemporal craniotomy without significant additional dissection along the middle fossa floor. The presigmoid component can vary based on the amount of the petrous apex is kept intact or resected. The superior petrosal sinus is ligated or clipped and transected, and the tentorium cerebelli is divided to bring the middle and posterior fossa spaces into continuity.

The subtemporal–retrolabyrinthine petrosal craniotomy provides good exposure to the petroclival region and some direct prepontine and clival exposure without sacrificing the otic capsule. Retrolabyrinthine dissection is carried to completely resect the posterior fossa dural plate. The semicircular canals are bluelined but preserved (see the images below).

Subtemporal – retrolabyrinthine approach, schematiSubtemporal – retrolabyrinthine approach, schematic, axial.
Subtemporal – retrolabyrinthine approach, schematiSubtemporal – retrolabyrinthine approach, schematic, coronal.

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. A technique similar to posterior semicircular canal occlusion or modified translabyrinthine approach is used, with semicircular canal resection and lumen packing up to the ampullae and crus communis, hence the name transcrusal (see the images below).

Subtemporal – translabyrinthine approach, schematiSubtemporal – translabyrinthine approach, schematic, axial.
Subtemporal – translabyrinthine approach, schematiSubtemporal – translabyrinthine approach, schematic, coronal.

Subtemporal-Transcochlear Petrosal Approach

The subtemporal- transcochlear approach extends transpetrosal dissection even further anteriorly, enabling direct view of the entire clivus and prepontine cistern with no brain retraction (see images below).

Subtemporal – transcochlear approach, schematic, aSubtemporal – transcochlear approach, schematic, axial.
Subtemporal – transcochlear approach, schematic, cSubtemporal – transcochlear approach, schematic, coronal.

Anterior Transpetrosal Approach/Total Petrosectomy Approach

This temporal bone dissection, under either name, is uncommonly 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 (see images below).

Total petrosectomy, schematic, axial. Total petrosectomy, schematic, axial.
Total petrosectomy, schematic, coronal. Total petrosectomy, schematic, coronal.

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