40 - Neuromonitoring in Endoscopic Skull Base Surgery

Abstract

Endoscopic skull base surgery has revolutionized treatment of tumors and other lesions at the cranial base. However, care must be taken to avoid damage to neurovascular structures located at the cranial base. A wide assortment of intraoperative neuromonitoring techniques can be utilized to locate and determine the functionality of neural structures during surgery. While these are frequently used in endoscopic skull base surgery, they are generally not amenable to endoscopic orbital or trans-orbital surgery due to inability to place leads in the surgical field. Here we discuss the most commonly used intraoperative neuromonitoring techniques including somatosensory and motor evoked potentials (SSEP/MEP), brainstem auditory evoked potentials (BAEP), visual evoked potentials (VEP), and the main focus of the chapter, electromyography (EMG). Each technique has its own unique technique, indications, and pitfalls. EMG of the extraocular muscles is particularly useful for determining the position and functionality of cranial nerves III, IV, and VI for lesion in the cavernous sinus and superior orbital fissure. We describe the technique for free running and triggered EMG of the extra-ocular muscles. We discuss utility, clinical evidence, and case examples. VEPs may be used when performing surgery on the cisternal or intracanalicular segment of the optic nerve, but not in intra-orbital surgery. VEPs are relatively new compared to other techniques and are still undergoing refinements and, as of now, not widely adopted due to important limitations.