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Petrosal Approach
Petroclival tumors arise from or involve the petroclival junction cephalad from the jugular tubercle, medial to the trigeminal nerve, and anteromedial to the internal auditory canal (IAC). , Tumors of the petroclival area are a particular challenge because they often involve the middle and posterior cranial fossae, cause significant brainstem compression, invade the cavernous sinus, and abut or surround the upper cranial nerves and basilar artery. This complex anatomy requires an individualized surgical approach for each patient based on tumor origin, area of tumor extension, and preoperative neural function.
The term combined petrosal approach actually describes numerous surgical approaches to the petroclival area. The term combined refers to the fact that this is a combination of the middle fossa approach with any variation of a posterior fossa skull base approach. The combination of these approaches allows for excellent exposure of the medial petrous bone and clivus from the cavernous sinus to the foramen magnum. The combined petrosal approach is a synonymous with the terms transpetrosal approach, transpetrous approach, and infratemporal presigmoid approach.
First described by Decker and Malis, the combination middle fossa and posterior fossa exposure allows the surgeon to take advantage of the strengths of each approach while minimizing brain retraction. The middle fossa approach gives the surgeon good exposure of the petrous bone and clivus superior to the IAC, whereas the posterior approach provides surgical exposure inferior to the IAC. A tumor that is superior to the IAC can be accessed by a middle fossa approach alone. A tumor that is inferior to the IAC and tentorium cerebelli can be addressed with a posterior approach alone. It stands to reason that tumors involving both areas are best treated with a combination of these surgical procedures.
The most common tumor of the petroclival area is the meningioma. The less common lesions include chordomas, chondrosarcomas, epidermoids, and vascular abnormalities such as aneurysms and arteriovenous malformations of the basilar system. Tumors that remain intracranial can be treated with a combined petrosal approach, while tumors that extend into the infratemporal fossa require a lateral approach.
Combined Petrosal Variations
All of the combined petrosal approaches include similar middle fossa exposure as a component of the procedure. The variations of the combined petrosal approach are classified by the type of posterior approach used. Traditionally, three variations of the combined petrosal approach were described: the retrolabyrinthine, translabyrinthine, and transcochlear approaches. , A fourth variation described by Sekhar et al., which provides exposure in between the retrolabyrinthine approach and the translabyrinthine approach, uses a partial labyrinthectomy for the posterior approach. This approach removes most of the labyrinth, while preserving functional hearing in 80% of patients. These four approaches give different degrees of anterior petrous bone exposure, which change the level of brainstem and clivus exposure. The narrow angle between the anterior brainstem and clivus is the major factor influencing the level of visualization across both of these structures. Generally, greater anterior removal of the otic capsule allows a more direct angle across the clivus, allowing improved medial exposure to the brainstem, basilar artery, and central clival depression. More aggressive anterior otic capsule removal secondarily provides improved access to the ipsilateral medial petrous bone as well ( Fig. 51.1A ).
The choice of posterior approach depends on tumor location and size, preoperative hearing level, and extent of brainstem compression. The translabyrinthine and transcochlear approaches sacrifice residual hearing. The transcochlear approach also includes posterior facial nerve transposition, leading to a temporary facial paralysis and a risk of permanent injury. The translabyrinthine approach is used when preoperative hearing is poor, and the transcochlear approach is reserved for the largest of tumors that cross the midline of the clivus. Tumor effect on the brainstem is also an important consideration. Sometimes tumors with a larger anterior-posterior diameter necessitate a less radical posterior approach because the posterior compression of the brainstem opens the angle between the clivus and brainstem. The tumor in Fig. 51.1B was addressed with a translabyrinthine variation of the combined petrosal approach, despite involvement of the contralateral clivus. The posterior displacement and contralateral displacement of the brainstem kept the angle between the tumor-involved clivus and brainstem open, obviating the need for a transcochlear approach.
Preoperative Evaluation
Most patients who present to a neurotologist with a petroclival tumor have had either a computed tomography (CT) scan or magnetic resonance imaging (MRI) of the brain for evaluation of cranial nerve findings or nonspecific complaints. MRI and CT imaging modalities are complementary and obtained for every patient with these complex lesions. The MRI must be a contrast-enhanced study, but the CT scan does not have to be. MRI is superior for determining the extent and character of the tumor, whereas CT scan gives the bony detail necessary for surgical planning. The surgeon determines from these studies whether the combined petrosal approach is appropriate and which variation thereof is needed. High-quality images are required with thin cuts (3 mm for MRI and 1 mm for CT) through the skull base. It is preferable that the CT images are acquired with bony windowing and not just reformatted to bony windows after image acquisition. All images are reviewed with a neuroradiologist and the operating neurosurgeon before any planned intervention.
Important temporal bone arterial and venous variations can often be detected from MRI; however, angiography with a venous phase should be considered for each patient in whom a combined petrosal approach is being contemplated. Conventional angiography may be used, but advancements with MR angiography/venography and CT angiography/venography have made them appealing alternatives with lower risks to the patient. Important venous anomalies exist that may result in catastrophe if not recognized before surgery. The main venous drainage of the temporal lobe is through the vein of Labbé, a single tributary that runs along the inferior surface of the temporal lobe and typically anastomoses into the transverse sinus. The vein of Labbé may run through the tentorium, however, and insert into the superior petrosal sinus, rather than the transverse sinus. Because the superior petrosal vein and tentorium transection is a key component of the combined petrosal approach, the vein of Labbé would be at risk if this anatomical variation is present. Accidental transection can lead to venous infarction of the temporal lobe, with particularly grave consequences for the patient, especially if it occurs in the dominant hemisphere. A dominant sigmoid sinus should also be recognized before surgery. The surgeon must exercise extra caution not to injure a dominant sigmoid sinus because the contralateral venous drainage may be insufficient or absent, leading to a high risk of venous infarction.
A final concerning anatomical variation is a transverse sinus that does not connect at the torcular Herophili. This situation leads to one side of the venous system draining the sagittal sinus system. The sigmoid on the side draining the sagittal system must be preserved to prevent stroke. Angiography detects these variations so that the surgeon can optimize the treatment plan. Angiography also gives the surgeon the option of preoperative embolization for particularly vascular tumors.
Preoperative Preparation
A neurosurgeon is part of the surgical team in all cases. Patients are given preoperative antibiotics to help prevent postoperative wound infections. A first-generation cephalosporin such as cefazolin is used for non-penicillin-allergic patients, and clindamycin is given for patients who have a known penicillin allergy. The patient is anesthetized with the head on the foot of the bed to allow adequate leg clearance for the surgeon. The anesthesiologist is instructed to use short-acting or no paralysis so that detection of facial nerve stimulation and somatosensory evoked potentials are not compromised during the procedure. Because these procedures often take many hours, extra care is taken to pad the patient and bed appropriately to prevent pressure points that can lead to skin breakdown or peripheral neuropathy. The arms are crossed over the patient’s chest, which is a more relaxed arm position than is tucking the arms at the side and prevents pressure at the medial epicondyle of the humerus, a common point of ulnar nerve injury in prolonged cases.
An arterial line with double venous access is placed by the anesthesiologist. The blood pressure cuff is preferably placed on the upper extremity opposite the tumor side so that cuff inflation does not affect the surgeon’s hands during the procedure. The patient is either double or triple strapped to the bed because significant bed tilting is necessary at times. The patient is kept in a supine position with the head turned so that the operative side is up and parallel to the floor. If there is limitation to neck rotation, a shoulder roll may be placed under the shoulder to roll the torso away. The head is placed in a Mayfield head holder and secured to the bed. All retractors are secured to the head holder. The bed is turned 180 degrees away from the anesthesiologist to allow adequate room for the scrub nurse, surgeon, and microscope. A wide head shave is performed with an electric razor, and a small amount of alcohol is used to remove oil from the skin, which is allowed to dry. Benzoin is placed, and clear adhesive drapes are placed around the periphery of the planned surgical site. The abdomen should be prepared so that a fat graft can be harvested.
Cranial nerves IV, VI, VII, IX, X, XI, and XII are monitored during the procedure. For large tumors with brainstem compression, somatosensory evoked potentials are also monitored. For patients in whom hearing preservation is being attempted, auditory brainstem responses are monitored. We have found that contralateral monitoring of CN VII is helpful as well. This allows differentiation of facial nerve irritation caused from surgical trauma versus light anesthesia and the occasional external electrical artifact.
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