Quantitative Subcortical Monopolar Motor Mapping Thresholds for Safe Eloquent Tumor Surgery

Keywords: surgery, complications, electrocorticography, brain tumor, technique

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     There is an increasing interest in supratentorial tumor surgery to investigate the distance of stimulation sites to the corticospinal tract (CST).
     The objective was to evaluate quantitative low monopolar stimulation thresholds in correlation to motor outcomes in surgery close to the CST.
     A analysis in a cohort of 70 patients (2009-2011) was performed regarding the lowest subcortical mapping threshold (monopolar stimulation, train of 5 stimuli, ISI of 4.0ms, impulse width of 500µs) and stable monitoring of direct cortical stimulated motor evoked potentials (DCS-MEP) via a 4-contact strip electrode.
     Lowest MT was defined as the minimum stimulation intensity which elicited MEP at =30µV amplitude under general anesthesia. Motor outcome was assessed after surgery, at day of discharge and the 3 month follow-up visit.
     Lowest individual stimulation thresholds were as follows (MT in mA, number of patients): =15mA n=11; 14-10mA n=8; 9-8mA n=3; 7-6mA n=12; 5-4mA n=21; =3mA n=15. At the 3 month follow-up two patients had a reversible (lowest MT 5mA and 4mA) and three patients a permanent motor deficit (MT 13mA, 6mA, Mapping (MT =1mA) during monitoring of stable DCS-MEP had 80% sensitivity and 98% specificity corresponding to a positive predictive value of 80% and negative predictive value of 98% for no permanent motor deficit.
     This is a retrospective study.
     A subcortical MT =1mA with stable monitoring of DCS-MEP had a minimal risk for permanent motor deficit
     That is true provided that mapping is repeated with high frequency.


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