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There are 12 pairs of cranial nerves (CN). All of them can be monitored except CN I (Olfactory). Various monitoring techniques (also known as Modalities) that can be utilized intraoperatively include spontaneous Electromyography (s-EMG), triggered Electromyography (t-EMG), Cranial Nerves Motor Evoked Potentials (CN-MEP), Nerve Action Potentials (NAP), Visual Evoked Potentials (VEP), Somatosensory Evoked Potentials (SSEP) (CN V and CN VII) and Brainstem Auditory Evoked Potentials (ECoch, AN-CAP, BAEP). CN II (Optic) is a sensory nerve that can be monitored intraoperatively by visual stimulation by flash goggles and recording from the retina (ERG) and visual cortex (VEP). CN V (Trigeminal) has a sensory division which is monitored for Trigeminal SEP (TSEP). CN VIII (VestibuloCochlear) is also a sensory nerve that can be monitored by auditory stimulation. Factors affecting BAER response may include N2O, hypothermia, and inhalational agents. Multilevel auditory responses are recorded from the tympanic membrane/cochlea (Electrocochleography: ECoch), directly from the auditory nerve (Auditory Nerve Compound Action Potentials: AN-CAP), and the brainstem (BAEP) when preservation of hearing is a high priority.
The remaining CNs are motor and can be monitored intraoperatively by spontaneous (s-EMG) and triggered EMG (t-EMG). Reasons for monitoring CNs during high-risk brain surgeries include protecting CNs from any damage during the surgical procedure. It also provides real-time feedback to the surgeons and helps in the identification and mapping of the CN course, evaluation of the CN function, and localization of the damaged nerves. EMG responses can be visualized and are audible as well. Various perioperative factors affecting CN EMG, MEP, and VEP responses may include muscle relaxants, inhalational agents, hypothermia, ischemia, etc. CN-MEP can be used to evaluate the integrity of the corticobulbar tracts. TIVA with no muscle relaxant is the ideal anesthetic regimen for monitoring surgeries involving cranial nerves. Various types of intraoperative insults include mechanical, ischemic, electrical, and chemical insults. Monopolar or bipolar stimulations can be used for nerve identification and mapping. Monopolar stimulation is more sensitive and less specific. Whereas, bipolar stimulation is more specific and less sensitive.
Multimodality intraoperative neurophysiological monitoring can be effectively utilized in preventing injuries to cranial nerves during brain surgery procedures. Optic (CN II) and Vestibulocochlear (CN VIII) nerves can be monitored by visual and auditory brainstem responses for the preservation of vision and hearing. Cranial nerves III (Oculomotor), IV (Trochlear), V (Trigeminal), VI (Abducens), VII (Facial), IX (Glossopharyngeal), X (Vagus), XI (Spinal Accessory), and XII (Hypoglossal) can be monitored intraoperatively by s-EMG and t-EMG. The surgeon can decide to limit the tumor resection to the changes in neurophysiological responses. This minimizes any post-operative neurological deficits due to surgical manipulation of the tumor involving the upper and lower cranial nerves. Intraoperative monitoring of CN II-XII can be done effectively and with high confidence without causing any risk to patient safety.
Intraoperative Neurophysiological Monitoring (IONM) helps in better patient outcomes by minimizing risks related to the functional status of the nervous system during surgical procedures. An IONM alert to the surgical team during the surgery can help them to identify the cause and take immediate corrective action.
