Role for IOM
Although it is limited compared with modalities used for supratentorial or spinal cord surgery, IOM has a central role in the resection of brainstem tumors. Its use enables the mapping of cranial nerve nuclei and thereby facilitates the identification of “safe” entry zones to the brainstem intraoperatively.
Anesthesia in the Setting of Brainstem IOM
As with other forms of IOM, the modalities being followed are critically sensitive to the effects of anesthesia. It is important to maintain adequate communication with the anesthesia team throughout surgery to ensure adequate feedback from IOM. Basic principles include:
- Ultra-short-acting paralytics limited to induction: The use of paralytics during intubation is acceptable but should be avoided intraoperatively due to adverse effects on MEPs.
- Constant infusion IV agents for steady state of anesthesia: A continuous infusion IV anesthetic agent (e.g., propofol, fentanyl) is preferable.
- Knowledge and avoidance of agents that degrade IOM: Hypothermia, inhaled halogenated anesthetic agents, and intermittent injection of intravenous anesthetics should be avoided.
Brainstem mapping is a technique for mapping cranial nerve nuclei on the floor of the fourth ventricle. Brainstem mapping is useful for surgical approaches to the pons in the midline or when addressing tumors extending into the fourth ventricle. Brainstem mapping is not useful for ventrally or laterally located lesions.
- Localization of motor cranial nerves: Brainstem mapping allows for intraoperative localization of motor pathways of CN VII (orbicularis oris and oculi), IX (posterior pharyngeal wall), XI (cricothyroid or vocalis muscle), and XII (intrinsic tongue muscles) (37, 38, 98). Starting with a current of 2.0 mA, a monopolar or bipolar probe is applied to an area of interest for no longer than 5 seconds and is moved at 1-mm intervals with successively decreased current levels to create a functional map of the floor of the fourth ventricle.
- Distortion of anatomy by tumor mapped: Stereotypic distortions of the nucleus of CN VII have been described for tumors involving the pons (98). This nucleus can be shifted rostrally or caudally depending on its anatomic relationship to the tumor.
- Update map during resection: Repeated mapping can be used to assess changes in relative location of critical nuclei as the surgery progresses.
- Limitations must be kept in mind: Limitations of brainstem mapping include disruption to the flow of surgery, as resections must be stopped to remap, and the inability to assess afferent pathways associated with a given cranial nerve.
Corticospinal and Corticobulbar Tract Monitoring
Cranial nerves can be monitored: Corticobulbar tract or cranial nerves are directly stimulated and the muscles innervated by the motor branches of cranial nerves VII, IX, X, and XII. The goal of corticobulbar tract monitoring is through the multi-synaptic pathway. Care must be taken, however, as the muscles can be stimulated through either direct cranial nerve stimulation or the multi-synaptic pathway.
- Deterioration in cranial nerve and extremities MEPs correlates with postoperative paresis: Within the limitations of the technique, MEPs of the corticospinal tract can be used for monitoring descending motor tracts during resections of brainstem tumors. Instead, both reversible and irreversible changes in corticospinal tracts and corticobulbar tracts can predict new postoperative motor deficits (39).
Somatosensory Evoked Potentials and Brainstem Auditory Evoked Potentials
- SSEPs and BAEPs of limited use in monitoring: Combined, SSEPs and BAEPs monitor only about 20% of the brainstem and therefore have a limited role in surgery for intrinsic brainstem lesions (40). SSEPs can be normal throughout an entire procedure despite the occurrence of brainstem injury and postoperative deficit (39). Additionally, the information is not in real-time given that the acquisition of the number of potentials needed for averaging can result in up to a minute of delay.
- Useful for understanding surgery’s impact on sensory function: Despite defined disadvantages, SSEPs and BAEPs are easily obtained, providing additional data points reflective of brainstem function.