Cortical mapping, subcortical mapping and motor evoked potential monitoring using the monopolar short train technique: advantages and limitations.

Introduction: Cortical and subcortical neurophysiological mapping techniques based on the original Penfield's work are still widely used nowadays. Yet this bipolar 60Hz technique has some drawbacks related to the risk of intraoperative seizures, the limited feasibility in young children and the fact that, due to stimulation parameters, continuous MEP monitoring cannot be done. We reviewed our 6-year experience of combined cortical and subcortical mapping, and muscle motor evoked potentials (mMEP) monitoring during surgery in 121 brain lesions involving motor areas, using a high frequency monopolar short train of stimuli technique. Materials and methods: The somatosensory evoked potential phase reversal technique was used to identify the central sulcus. We used direct cortical stimulation to identify cortical motor areas and subcortical stimulation to follow subcortical white matter motor tracts during tumor removal. This was done by using short trains of 5 to 7 monopolar square-wave stimuli of 500 μs duration, interstimulus interval of 4 ms, repetition of 1-2 Hz, and intensity up to 20 mA; motor responses were recorded from needle electrodes inserted in controlateral muscles. During tumor removal, continuous mMEP monitoring was performed from a strip cortical electrode and/or transcranially. Motor outcome was assessed on admission, at discharge from hospital and, in a subset of patients, at a 4-month follow-up, using the Medical Research Council scale. We used association in two-way tables with chi-square test of significance and α = 0.05, as well as the analysis of variance (ANOVA) to detect correlations between neurophysiological and outcome-related variables. Results: The monitorability rate for different neurophysiological techniques ranged between 90 and 97%. At discharge from hospital, 18% of patients improved their motor grade when compared to pre-operatively, 61% remained unchanged, 18% presented with a 1-grade worsening and 6% with more than 1 grade worsening. Most of these deficits recovered over time. In the subgroup of 39 patients with a 4-month follow-up, none presented with a worsening of more than 1 grade and all, except 2, had a good motor function (grade 4 or 5). Lower subcortical stimulation thresholds correlated with an aggravated post-operative outcome (p-value = 0.04). Patients who presented at the end of surgery a more than 75% drop in MEP amplitude had a significantly higher incidence of post-operative motor deficit at discharge from hospital (p-value = 0.008). However, these two prognostic variables were not related (p-value > 0.05). Conclusion: Muscle MEP amplitude mostly correlates with post-operative outcome in brain surgery but criteria for MEP interpretation are far less established than for spinal cord surgery, and this may account for some discrepancy between intraoperative findings and post-operative outcome. Nevertheless, there is evidence suggesting that MEP monitoring adds to the established value of cortical and subcortical mapping, ultimately offering a better chance to identify an impending injury to the motor system. Our experience confirms that the use of intraoperative neurophysiological techniques minimizes the risk of permanent severe neurological deficits when operating in critical areas, ultimately encouraging a more aggressive surgical removal even in these locations.