Comparison Between Monopolar and Bipolar Electrical Stimulation of the Motor Cortex

Summary

¶ Intra-operative neurophysiological techniques allow reliable identification of the sensorimotor region and make their anatomical and functional preservation feasible. Monopolar cortical stimulation has recently been described as a new map** technique. In the present study this method was compared to the “traditional” technique of bipolar stimulation.

 Functional map** of the motor cortex was performed in 35 patients during surgery in the central region. The central sulcus (CS) was identified by somatosensory evoked potential (SEP) phase reversal. Cortical motor map** was 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 sec train duration). Surgery was performed under general anaesthesia without muscle relaxants. Of 280 motor responses elicited by bipolar cortical stimulation, 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 cortical 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 overlap** of primary motor units over a cortical area much broader than the “classical” narrow motor strip along the CS. Bipolar cortical stimulation is more sensitive than monopolar for map** motor function in the premotor frontal cortex. Both methods are equally sensitive for map** the primary motor cortex.