SSEM (SSEP) and TceMEP Neuromonitoring in Neurosurgery: Technological Advancement with Navigation

Introduction: Intraoperative neurophysiological monitoring (IONM) has been used to minimize neurosurgical morbidity during operative manipulations to prevent irreversible damage by detecting changes in the brain, spinal cord, cranial or peripheral nerves. Changes must be discovered prior to structural injury to prevent iatrogenic injury. For the changes to have meaningful sensitivity and specificity and alter the course of surgery, their morphology must be of appropriate quality; their importance in cranial surgery when monitoring motor function, depends on them providing reliable quality-assured consistent information. This technique must be standardized with minimal variance and technical differences in electrode placement relative to underlying anatomy, thereby optimizing the signal-to-noise ratio.

Transcranial electrode placement has been crucial when localizing the motor cortex to prevent contralateral motor deficits. Generally, the Transcranial Electrical Motor Evoked Potential (TceMEP) is evoked by stimulating at the scalp at conventional sites of the 10–20 System or slightly anterior sites modified to M1-M2 and M3-M4 per MacDonald’s nomenclature.

Using craniometric points, close approximation of the location of the precentral (PCS) and central sulcus (CS) can be accomplished. However, during TceMEP stimulation, depending on the electrode placement selection noted, we may be close to the PCS or primary motor area but not exactly over the sulci (CSF) requiring greater current concentration. This, in addition to individual variations, can create a significant increase in the noise relative to valuable signal.

To obtain anatomically-accurate stimulation, precise placement of electrodes can greatly decrease the electrical intensity needed for an evoked response along the corridor of the sulcus, leading to more accurate results with fewer false positives. With recent advances of pre-operative planning with co-registered CT/CTA/MRI sequences and intraoperative neuronavigation, more accurate and precise patient-specific placement of electrodes can be implemented.

Objective: This study focuses on the use of navigation to more precisely identify the precentral sulcus, superior, inferior frontal sulcus and central sulcus, for the purposes of both scalp and cortical electrode placement in intracranial surgery.

Materials and Methods: Pre-operative and intraoperative neuromonitoring data were collected from a group of 20 patients undergoing intracranial surgery. Purpose-built 3D rendering software for cortical and subcortical mapping with segmentation of critical pathways for somatosensory, motor and cranial nerves was used to co-locate these anatomic regions and guide the optimum position of respective electrode placement. In particular, frequency of TceMEP stimulation, threshold, amplitude, and latency were measured and recorded. Data was analyzed and compared with data collected from cases not implementing the use of intraoperative navigation. Video recordings of electrode placement were also obtained.

Results and Conclusions: Using the aforementioned methods, we have been able more accurately place electrodes over the sulcus, Precentral sulcus, Superior and inferior frontal sulcus leading to a decreased overall electrical threshold required for stimulation, thereby monitoring only those structures that are clinically relevant during the surgery. This results in much lower stimulation intensities with the benefit of significantly less patient movement in addition to optimizing the signal-to-noise ratio yielding more robust data for actionable decision making to alter the course of surgery.