Intraoperative neuromonitoring in surgery of supratentorial brain tumors. Part 1. Assessment of motor conductivity

Abstract

Monitoring of motor evoked potentials in surgery of supratentorial tumors estimates integrity of cortical motor centers and subcortical pathways. Violation of motor evoked potentials takes place in mechanical injury or ischemia of motor neurons. Decrease of amplitude of motor evoked potentials more than 50 % is predictor of permanent neurological deficit.Cortical mapping gives a possibility to discover eloquent brain areas before their resection. To reveal motor centers activating stimulation is applied, to find out speech or sensory areas – the inhibiting one. Positive brain mapping allows to exclude technical fault in selection of stimulation threshold but it demands a wide craniotomy. Negative mapping is more widespread, gives opportunity to use tailored craniotomy that reduces surgical injury and duration of operation. One of the most valuable factors in cortical and subcortical brain mapping is the stimulation threshold. With monopolar «train» stimulation current 1 mA spreads into approximately 1 mm. The safe value of current intensity during tumor resection in eloquent areas is 3–5 mA.Monopolar stimulation demands less time for location of eloquent brain areas, it is as accurate as the bipolar mapping and more rarely leads to intraoperative seizures. Combination of monopolar stimulator with aspirator gives opportunity to continuously allocate pyramidal tract in tumor resection.