In spite of the advances in functional neuro-imaging (fMRI, tractography), neuro-navigation and neuro-intensive care, brainstem surgery remains challenging due to the high concentration of critical neural structures within a small volume. Accordingly, even minor injury could result in severe and sometime life-threatening complications for the patient.
Over the past two decades, intraoperative Neurophysiology (ION) has progressively emerged as an extremely valuable discipline to warrant the functional information that anatomy itself fails to provide. Neurophysiological mapping techniques are used for both intraoperative identification of key structures, such as the motor nuclei of the cranial nerves (particularly the VII, IX, X, XI and XII) on the floor of the fourth ventricle and the corticospinal bundle in the cerebral peduncle. Therefore, these techniques help to determine safe entry corridors for intrinsic brainstem tumors or to decide when to stop resection during the removal of dorsally exophytic tumors or fourth-ventricle tumors invading the floor of the ventricle itself.
Monitoring techniques, vice versa, are not aimed to identify ambiguous neural structures but rather to assess sub-continuously the functional integrity of neural pathways. It should be considered that traditional ION techniques such as SSEPs and BAEPs together could assess no more than 20% of the brainstem area, suggesting that significant brainstem injury could occur in the absence of SSEP and BAEP changes. With the advent of muscle motor evoked potential (mMEPs), ION became way more reliable thanks to the possibility to specifically monitor both corticospinal and corticobulbar motor pathways, these latter being of paramount importance during brainstem surgery. While in supratentorial surgery and spinal cord surgery, warning criteria for mMEP changes have been quite well established these are less defined during brainstem surgery.
The so-called corticobulbar MEPs have been introduced in the mid 2000s as a new technique which essentially allowed to extend to motor cranial nerves the principle of MEP monitoring, as a valid alternative to the recording of spontaneous electromyographic (EMG) activity.
Corticobulbar MEPs are elicited through transcranial electrical stimulation with recording electrodes inserted in the muscles innervated by motor cranial nerves, typically orbicularis oris and oculi (VII), posterior wall of the pharynx or vocal cords (IX/X), trapezius (XI) and tongue muscles (XII).
There are no standard warning criteria for corticobulbar MEP interpretation, but there is general consensus that irreversible MEP loss is a poor prognostic sign, correlating with severe and long- lasting cranial nerve palsy, while the preservation of MEPs usually predicts no deficits or only minor and transient deficits. Significant (50%-80%) amplitude drops are indicative of at least transient deficits.
Not all functional pathways are monitorable. Monitoring of oculomotion is still limited by the lack of techniques to monitor the fasciculus longitudinalis medialis. Techniques to monitor the afferent (sensory) pathways for the lower cranial nerve-mediated reflexes such as swallowing and coughing are still lacking. In the past few years, yet, monitoring of reflex circuits within the brainstem, such as the laryngeal abductor reflex, has emerged as a new strategy to indirectly assess the functional integrity of pathways for which there were no ION techniques in the past.
