Skull Base Tumors: Evaluation and Microsurgery

Imaging

In most patients, magnetic resonance imaging is performed preoperatively to determine the suspected diagnosis, extent of disease, and status of surrounding anatomic structures. CT with bone windows is often useful for visualizing the extent of bony skull base involvement by tumor. Major arterial involvement may necessitate diagnostic cerebral angiography with possible test occlusion. For internal carotid artery (ICA) involvement, the contralateral ICA is injected, as are the vertebral arteries, to assess collateral supply. For venous sinus involvement, the pattern of venous drainage and patency of venous structures need to be determined, including any collateralization. The presence of nearby draining veins (eg, a vein of Trolard adjacent to a parasagittal meningioma) should be carefully evaluated and must be preserved during the tumor resection. If there is arterial involvement with poor collateral or a failed occlusion test, preoperative evaluation of donor vessels for possible bypass needs to be performed. Finally, preoperative angiography can be performed to embolize feeding vessels, particularly those that are not amenable to early interruption during the surgical approach.

Selection of Approach

Several factors are considered when selecting the ideal skull base approach. The tumor type and origin must be considered. In the case of meningiomas, most tend to be low grade (ie, World Health Organization [WHO] grade I) and restrict themselves to within the anatomic limits of a single approach. For skull base meningiomas, the “goal” of the approach is to turn it into a convexity meningioma, meaning in ideal circumstances a circumferential exposure of the tumor and affected dura, with the ability to interrupt the tumor blood supply early in the case and proceed with early debulking, without the need to manipulate the brain and cranial nerves. Some meningiomas span multiple anatomic compartments along the skull base, either with gross tumor or with dural or bony disease. Examples include spheno-orbito-cavernous meningiomas or petroclival meningiomas with middle and posterior cranial fossa disease. In these cases, maximal access is achieved by extending the skull base approach or using more than one approach in combination.

Other tumors, particularly malignant sinonasal tumors or locally aggressive bony tumors such as chordomas, often violate anatomic boundaries and affect the bony skull base, dura, paranasal sinuses, and intradural space. This in addition to achieving tumor-free surgical margins will influence the type of surgical approach. One is more likely to require multiple approaches in these instances. Examples include combining a transcranial with a transnasal approach (open or endoscopic) for sinonasal malignancies traversing from the nasal cavity across the cribriform plates and dura into the brain, or chordomas with significant lateral extension requiring a combination of midline and lateral skull base approaches. The need for adequate reconstruction to avoid postoperative CSF leaks and other wound-related complications must be addressed—for example, harvesting a pericranial flap for reconstruction of anterior cranial fossa dural defects following tumor resection. This is particularly important in the setting of malignancy, as most likely the patient will be undergoing adjuvant radiotherapy in the weeks following surgery.

Anesthesia and Intraoperative Monitoring

For cranial base operations, the neuroanesthesiologist maintains blood pressure and oxygenation within normal limits. Arterial line, central venous catheter, and, in some cases, jugular bulb oximetry are utilized. Brain relaxation is achieved with modest hyperventilation, diuretics, and, in some cases, lumbar CSF drainage or ventriculostomy. For prolonged skull base tumor operations, infusions of tranexamic or aminocaproic acid can be run to promote hemostasis.

The indication for and types of intraoperative monitoring (IOM) implemented will depend on what is at risk with the tumor resection. If IOM is being used, intravenous or volatile anesthesia is usually required, with avoidance of neuromuscular blockers at the phases of the operation when IOM is required. In most cases, transcranial cortically evoked motor-evoked potentials (MEPs) and somatosensory-evoked potentials (SSEPs) provide information regarding the integrity of the long tracts during the procedure. If needed, electrodes can be placed in the target muscles of cranial nerves III, VI (typically by the surgeon under the periorbita adjacent to the superior and lateral rectus muscles, respectively), V, VII, IX, X (using either electrodes or specialized endotracheal tube), XI, and XII for intraoperative mapping using a nerve stimulator. For the facial nerve, transcranial electrical stimulation can provide facial nerve cortically evoked motor action potentials and provide additional data regarding functional continuity throughout the operation. Auditory-evoked potentials can be recorded when hearing is at risk—for example, during acoustic neuroma surgery or during approaches requiring temporal bone drilling. These techniques include brainstem auditory-evoked potentials, cochlear nerve action potentials, and electrocochleography.