[Intraoperative Monitoring During Cerebellopontine Angle Tumor Removal]

Two types of intraoperative monitoring of the cranial nerve motor function have been widely used during the removal of cerebellopontine angle tumors. The first type involves anatomical mapping through directly stimulating the cranial nerve to confirm its location. The second type involves monitoring motor function preservation through direct stimulation-compound muscle action potential (Ds-CMAP), motor-evoked potential (MEP) using transcranial electrical stimulation, and free-run electromyography (EMG). Particularly for patients with vestibular schwannomas, anatomical mapping is important to confirm the location of the facial nerve, which is likely to be deviated or compressed by a tumor. Ds-CMAP monitoring adjacent to the root exit zone of the facial nerve is useful for detecting facial nerve damage. Monitoring facial MEP, induced using transcranial electrical stimulation, is also useful in predicting postoperative facial motor function. Free-run electromyography EMG provides real-time monitoring of facial motor function; however, objective evaluation is challenging intraoperatively. Brainstem auditory evoked potential monitoring has been widely used to preserve hearing during the removal of cerebellopontine angle tumors. Cochlear nerve action potentials recorded directly from the cochlear nerve provide more useful monitoring for predicting postoperative hearing function. To preserve the motor function of the glossopharyngeal and vagus nerves, both pharyngeal MEP recorded from the swallowing muscle and vagus nerve MEP recorded from the vocal cord using transcranial electrical stimulation are useful in predicting postoperative swallowing function. A clear understanding of the purposes, methods, and evaluations of various types of cranial nerve monitoring during the removal of cerebellopontine angle tumors is essential.