Comparison between monopolar and bipolar electrical stimulation of the motor cortex

Intra-operative neurophysiological techniques allow reliable identification of the sensorimotor region and make their anatomical and functional preser vation feasible. Monopolar cortical stimulation has recently been described as a new mapping technique. In the present study this method was compared to the "traditional'' technique of bipolar stimulation. Functional mapping of the motor cortex was performed in 35 patients during surgery in the central region. The central sulcus (CS) was identified by so matosensory evoked potential (SEP) phase reversal. Cortical motor mapping w as first performed by monopolar anodal stimulation with a train of 500 Hz ( 7-10 pulses) followed by bipolar stimulation (pulses at 60 Hz with max. 4 s ec train duration). Surgery was performed under general anaesthesia without muscle relaxants. Of 280 motor responses elicited by bipolar cortical stim ulation, 54.28% [152] were located in the primary motor cortex (PMC). 37.85 % [106] outside the motor strip in the secondary motor cortex (SMC), and 8% [22] posterior to the CS. Of 175 motor responses elicited by monopolar cor tical stimulation 68.57% [120] were located in the SMC. 23.42% [41] in the SMC and 8% [14] posterior to the CS. Contrary to the general clinical view, there is considerable overlapping of primary motor units over a cortical area much broader than the "classical" narrow motor strip along the CS. Bipolar cortical stimulation is more sens itive than monopolar for mapping motor function in the premotor frontal cor tex. Both methods are equally sensitive for mapping the primary motor corte x.