The middle fossa approach has been traditionally used for resection of small acoustic neuromas and meningiomas along the lip of the internal auditory canal (IAC). The natural extension of this approach has been the anterior petrosal approach (anterior petrosectomy) that involves expanded bony resection at the petrous apex to expose the upper petroclival region and ventrolateral brainstem.

Transpetrosal approaches have evolved to offer advantages over traditional operative corridors, such as retromastoid, middle fossa, and pterional trajectories for exposing lesions within the petroclival area. The terminology used to describe transpetrosal approaches is very nonspecific and sometimes confusing because different nomenclature has been used to report their modifications. However, if we systematically analyze all modified transpetrosal procedures, we find that they can be classified into two types:

1. Anterior petrosectomy: removal of the petrous apex
2. Posterior petrosectomy: removal of the petrous pyramid

More radical or extensive transpetrosal osteotomies that have been described in the literature are often combined approaches comprised of conventional craniotomies in conjunction with an anterior or posterior petrosectomy.

The petrous bone contains many critical structures including the cranial nerves VII/VIII complex, internal carotid artery, and labyrinth. Anterior and posterior petrosectomies aim to preserve the integrity of the labyrinth. A total petrosectomy will sacrifice the labyrinth; I believe this procedure is rarely, if ever, necessary.

In this chapter, I attempt to review the technical nuances for a standard anterior petrosal osteotomy and describe the operative maneuvers to minimize the need for additional bone removal when resecting lesions along the upper third of the clivus and ventrolateral midbrain and upper pons. The tenets for the middle fossa approach to vestibular schwannomas are discussed in another chapter.

The most common indications for an anterior petrosectomy include:

• Intrapetrous lesions, with or without epidural extension, including:
  • Anterior petrosal cysts (epidermoid or cholesteatomas)
  • Chordomas and chondrosarcomas (endoscopic transnasal route preferred)
• Upper petroclival and Meckel’s cave tumors, including:
  • Meningiomas
  • Trigeminal schwannomas
• Intracisternal dermoid and epidermoid cysts extending across midline
• Lateral and ventral midbrain and pontine intra-axial lesions (cavernous malformations and tumors)
• Posterior circulation aneurysms (“low lying” basilar apex and basilar trunk aneurysms)

Some targets that are not suitable for this approach are:

• Purely clival tumors (originating medial to the groove of the inferior petrosal sinus or petroclival sulcus)
• Tumors extending caudal or originating caudal and/or posterior to the IAC
• Extensions of the tumor into the cranial nerves’ foramina
• Purely lower pontine and medullary lesions

As a general rule, the anterior petrosectomy should be preferred over conventional corridors for:

• Tumors entirely medial to and above the IAC
• Tumors that span the middle and posterior fossae
• Younger patients for whom more aggressive resection is desired

Magnetic resonance (MR) imaging reveals the limits of the tumor and defines the candidacy of the tumor for this approach. Because of the limited working space offered via this route and its inflexibility for expansion intraoperatively, inappropriate patient selection leads to disappointing results and subtotal tumor removal. The need for combined operative corridors should be planned preoperatively.

A high-resolution CT scan of the temporal bone can provide additional information about the degree of pneumatization of the petrous apex. This imaging modality is especially beneficial for intraosseus lesions.

Some degree of temporal lobe retraction is required and the use of a lumbar drain is highly advised, especially for lesions on the dominant side. Intraoperative neurophysiologic monitoring of the facial nerve (electromyography) and brainstem auditory evoked responses (BAERs) is very helpful for localizing the facial nerve and warning the surgeon of maneuvers that may place the brainstem at risk.

The ultimate goal of the anterior petrosal approach is to expose the ventral and ventrolateral aspects of the upper third of the brainstem. With this goal in mind, the importance of the steps involved in this skull base approach can be more readily rationalized. Tumors extending caudal to the IAC are not suitable for this approach, and a posterior petrosectomy should be considered to access them.

Before positioning the patient, I place a lumbar drain. Cerebrospinal fluid (CSF) drainage through the lumbar drain is imperative in avoiding temporal lobe injury during the lobe’s extradural retraction. Often 40 to 60cc of CSF is drained gradually (in 10cc aliquots) until the lobe is completely mobilized for adequate exposure of the middle fossa floor. I also prepare the lower quadrant of the patient's abdomen for harvesting fat grafts used for dural closure.

The patient is positioned supine on the operating table. A shoulder roll is placed under the patient's contralateral shoulder and the head rotated until the sagittal suture is parallel to or minimally angled from the floor. If the patient has a supple neck, he or she can tolerate up to 70 degrees of rotation. If the patient has a more rigid neck, the size of the shoulder roll can be increased to compensate for the lack of rotation.

For obese patients with less mobile necks, the lateral position must be used to avoid interruption of adequate venous return. I have a low threshold for using the lateral position because petrosectomy operations are often prolonged; physiologically neutral body positions (including neck posture) are associated with less discomfort after surgery.

The surgeon positions the skull clamp so the pins stay out of the operative working zone. My preference is to place the single pin anteriorly over the frontal area and the other two pins low on the occipital bone. A line connecting the single pin with the midpoint between the opposite two pins (swivel rocker arm) must always cross the equator of the patient's head to prevent skull clamp fixation failure during surgery.

Numerous styles of skin incisions have been described for this approach.

For a nonlinear incision, a myocutaneous flap is mobilized in a single layer. A myofascial cuff may be left on the bone to facilitate closure. For the horseshoe incision, the flap is reflected inferiorly with fishhooks; enough muscle dissection should be completed until the posterior root of the zygomatic arch is clearly visible. The operator must take care not to dissect so low as to violate the mandibular joint or external auditory canal.

More burr holes may be used, if necessary, to preserve the integrity of the dura. Alternatively, the entire bony cut may be done using a diamond drill. Preservation of the dura’s integrity is crucial to avoid injury to the lobe during the later stages of extradural dissection along the middle fossa floor.

The origin of the GSPN can be identified posteriorly at the hiatus; the dura is then stripped anteriorly in line with the nerve to prevent dislocation of the nerve out of its groove. The second caveat to the epidural exposure is to find the “true edge” of the petrous ridge. The superior petrosal sinus forms the petrosal groove along the superior aspect of the petrous ridge. The upper edge of this groove is often mistaken for the petrous ridge, resulting in inadequate elevation of the dura. Proper elevation of the dura along the ridge allows the retractor’s blade to readily rest against the petrous ridge and optimally retract the dura without slippage. I place two retractor blades: one along the arcuate eminence, and the second just lateral to the trigeminal impression.

The next step is to identify the meatal plane—the area of bone covering the IAC. Three ways of locating the meatal plane have been described. The Garcia-Ibanez technique relies on the relationship between the GSPN and arcuate eminence, usually located at an angle of 120 degrees. Bisection of this angle provides the general location of the IAC and a starting point for drilling the petrous ridge. Similarly, the Fisch technique uses an approximately 60-degree angle between the long axis of the arcuate eminence and the meatal plane to estimate the location of the IAC.

It is important to note that the anatomy of the middle fossa floor is widely variable, and the arcuate eminence is not always a reliable locator of the superior semicircular canal. As a general rule, the IAC courses at a 45-degree angle from a line that runs from the arcuate eminence perpendicular to the petrous ridge; this is my preferred method to locate the IAC.

In most cases of petroclival meningiomas, there is no need to fully expose the overlying dura of the IAC, and only the most medial aspect of the canal may be exposed as a reference. It is advantageous to dissect the dura propria from the sheath of V3 and the lateral border of the gasserian ganglion to optimize elevation of the temporal dura.

I usually ask my otology colleagues to be involved during the resection of the petrous apex.

Dural incisions should maximize the intradural extent of exposure.

Primary dural closure is obviously impossible in this area and alternative methods are needed. Adipose tissue with its globular texture is one of the best barriers against CSF leakage. Strips of adipose tissue are placed across the dural opening to seal the dural defect. Before placement of the adipose grafts, all air cells including the ones at the petrous apex and mastoid area must be meticulously waxed.

Alternatively, a vascularized muscle flap prepared from the posterior aspect of the temporalis muscle may be rotated to fill the defect in the dura. This latter method is used during repeat operations for patients who have undergone radiation treatment. The bone flap is replaced and secured using miniplates, and the rest of closure is conducted in the standard fashion.

A lumbar drain is used to drain 8cc per hour of CSF for 48 hours after surgery. A head CT scan the morning after surgery should exclude significant pneumocephalus before the drain is utilized. Patients are mobilized as soon as possible.

If postoperative rhinorrhea is encountered, a lumbar drain is reinstituted and continued for 3-4 days. If leakage is evident after discontinuation of the lumbar drain, repeat operative intervention with rewaxing of the apical air cells and repacking of the resection cavity with fat strips is required. The lumbar drain is then continued for another 2 to 3 days.

• Patient selection is one of the key elements for operative success. Anterior petrosectomy is designed to access the ventral and ventrolateral regions of the upper brainstem or the upper third of the clivus. This corridor is not designed to reach below the IAC. This approach offers a narrow, inflexible working zone and does not allow the operator to reach large tumors. Large tumors may require a staged approach through adjuvant routes.
• A lumbar drain is indispensable to avoid retraction injury. The dura must remain intact until the operator intentionally opens the dura.
• If lumbar CSF drainage does not allow adequate temporal lobe decompression, the procedure should be abandoned and an alternative approach contemplated at a different operative session. Forceful lobar retraction is hazardous and leads to disappointing results.
• Patience is important during petrous bone drilling. Practice in the laboratory is an indispensable prerequisite for safety and efficacy of petrosectomy. Our ENT colleagues are essential members of our team during this procedure.
• Incisions within the dura must be carefully placed. Tumors displace cranial nerves and vessels, and injury to these vital structures during the dural opening is often most unexpected. It is OK to say “there it is” and be wrong a thousand times, but it is not OK to say “there it was” and be right even once.

For additional illustrations of the middle fossa-transpetrous apex approach to the anterosuperior cerebellopontine angle, please refer to the Jackler Atlas by clicking on the image below:

DOI: https://doi.org/10.18791/nsatlas.v5.ch04.3