Intraoperative electrocortical stimulation of Brodman area 4: a 10-year analysis of 255 cases

BACKGROUND: Brain tumor surgery is limited by the risk of postoperative neurological deficits. Intraoperative neurophysiological examination techniques, which are based on the electrical excitability of the human brain cortex, are thus still indispensable for surgery in eloquent areas such as the primary motor cortex (Brodman Area 4). METHODS: This study analyzed the data obtained from a total of 255 cerebral interventions for lesions with direct contact to (121) or immediately adjacent to (134) Brodman Area 4 in order to optimize stimulation parameters and to search for direct correlation between intraoperative potential changes and specific surgical maneuvers when using monopolar cortex stimulation (MCS) for electrocortical mapping and continuous intraoperative neurophysiological monitoring. RESULTS: Compound muscle action potentials (CMAPs) were recorded from the thenar muscles and forearm flexors in accordance with the large representational area of the hand and forearm in Brodman Area 4. By optimizing the stimulation parameters in two steps (step 1: stimulation frequency and step 2: train sequence) MCS was successful in 91% (232/255) of the cases. Statistical analysis of the parameters latency, potential width and amplitude showed spontaneous latency prolongations and abrupt amplitude reductions as a reliable warning signal for direct involvement of the motor cortex or motor pathways. CONCLUSION: MCS must be considered a stimulation technique that enables reliable qualitative analysis of the recorded potentials, which may thus be regarded as directly predictive. Nevertheless, like other intraoperative neurophysiological examination techniques, MCS has technical, anatomical and neurophysiological limitations. A variety of surgical and non-surgical influences can be reason for false positive or false negative measurements

Experimental Study of the Course of Threshold Current, Voltage and Electrode Impedance During Stepwise Stimulation From the Skin Surface to the Human Cortex

<p>Background: Transcranial electric stimulation as used during intraoperative neurostimulation is dependent on electrode and skull impedances.</p><p>Objective: Threshold currents, voltages and electrode impedances were evaluated with electrical stimulation at 8 successive layers between the skin and the cerebral cortex.</p><p>Patients and Methods: Data of 10 patients (6f, 53 +/- 11 years) were analyzed. Motor evoked potentials were elicited by constant current stimulation with corkscrew type electrodes (CS) at C3 and C4 in line with standard transcranial electric stimulation. A monopolar anodal ball tip shaped probe was used for all other measurements being performed at the level of the skin, dura and cortex, as well as within the skull by stepwise performed burr holes close to C3 resp. C4.</p><p>Results: Average stimulation intensity, corresponding voltage and impedance for muscle MEPs at current motor threshold (CMT) were recorded: CS 54 +/- 23 mA (mean +/- SD), 38 +/- 21 V. 686 +/- 146 Omega; with the monopolar probe on skin 55 +/- 28 mA, 100 +/- 44 V. 1911 +/- 683 Omega and scalp 59 +/- 32 mA, 56 +/- 28 V. 1010 +/- 402 Omega; within the skull bone: outer compact layer 33 23 mA, 91 +/- 53 V. 3734 +/- 2793 Omega; spongiform layer 33 +/- 23 mA, 70 +/- 44 V.2347 +/- 1327 Omega; inner compact layer (ICL) 28 +/- 19 mA, 48 +/- 23 V. 2103 +/- 14980; on dura 25 +/- 12 mA, 17 +/- 12 V.643 +/- 244 Omega and cortex 14 +/- 6 mA, 11 +/- 5 V.859 +/- 300 Omega. CMTs were only significantly different for CS (P = 0.02) and for the monopolar probe between the cortex and ICL (P = 0.03), scalp (P = 0.01) or skin (P = 0.01) and between ICL and CS (P</p><p>Conclusion: The mean stimulation current of the CMT along the extracranial to intracranial anodal trajectory followed a stepwise reduction. VMT was strongly dependent on electrode impedance. CMT within the skull layers was noted to have relative strong shunting currents in scalp layers. (C) 2013 Elsevier Inc. All rights reserved.</p>