A novel technique for monitoring somatosensory evoked potentials during brainstem surgery: an illustrative case series

The surgical management of brainstem tumors and cavernous malformations represents one of the most demanding challenges in neurosurgery, given the critical concentration of eloquent neural structures in this region. These include nuclei, cranial nerve roots and their corticonuclear tracts, corticospinal and spinothalamic pathways. Intraoperative injury to these critical structures may result in permanent neurological deficits, including progressive sensory deterioration during the postoperative course. Therefore, continuous monitoring of the functional state of the spinothalamic tracts during surgery is essential. Peripheral somatosensory evoked potentials (pSEP) have some disadvantages and limitations during such procedures. The aim of this study was to evaluate the feasibility of a novel technique for monitoring SEPs during brainstem surgery, termed brain stimulus-induced somatosensory evoked potential (bsiSEP). Continuous bsiSEP monitoring by direct stimulation of the floor of the fourth ventricle and recording of cortical responses from the scalp was performed in five consecutive patients with brainstem lesions (3 medulla oblongata tumors, 1 tumor and 1 cavernous malformation of the pontine tegmentum) undergoing microsurgical resection. Preoperative and postoperative neurological sensory deficits, neurophysiological data, and surgical outcomes were analyzed. Continuous bsiSEP monitoring was available and reproducible in all cases, although 3 patients had varying degrees of preoperative sensory neurological impairment. One patient experienced a worsening of sensory symptoms. The results of the bsiSEPs were consistent with the neurological status of the patients after surgery in all cases. Peripheral SEPs were available in four cases, but all showed false-positive results. Continuous monitoring of bsiSEP is a reproducible technique and can predict outcome during brainstem surgery. It may improve the monitoring of spinothalamic tracts and overcome the limitations of pSEPs. This technique needs further refinement, but could be used during brainstem surgery to reduce postoperative sensory deficits.