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Low-Threshold Monopolar Motor Mapping for Resection of Primary Motor Cortex Tumors

Seidel, Kathleen MD; Beck, Jürgen MD, PhD; Stieglitz, Lennart MD; Schucht, Philippe MD; Raabe, Andreas MD, PhD

doi: 10.1227/NEU.0b013e31824c02a0
Technique Assessment

BACKGROUND: Microsurgery within eloquent cortex is a controversial approach because of the high risk of permanent neurological deficit. Few data exist showing the relationship between the mapping stimulation intensity required for eliciting a muscle motor evoked potential and the distance to the motor neurons; furthermore, the motor threshold at which no deficit occurs remains to be defined.

OBJECTIVE: To evaluate the safety of low threshold motor evoked potential mapping for tumor resection close to the primary motor cortex.

METHODS: Fourteen patients undergoing tumor surgery were included. Motor threshold was defined as the stimulation intensity that elicited motor evoked potentials from target muscles (amplitude > 30 μV). Monopolar high-frequency motor mapping with train-of-5 stimuli (HF-TOF; pulse duration = 500 microseconds; interstimulus interval = 4.0 milliseconds; frequency = 250 Hz) was used to determine motor response--negative sites where incision and dissection could be performed. At sites negative to 3-mA HF-TOF stimulation, the tumor was resected.

RESULTS: HF-TOF mapping localized the motor neurons within the precentral gyrus by using variable, low-stimulation intensities. The lowest motor thresholds after final resection ranged from 3 to 6 mA, indicating close proximity of motor neurons. Postoperatively, 12 patients had no new motor deficit, 1 patient had a minor new temporary deficit (M4+, National Institutes of Health Stroke Scale 1), and another patient had a minor new permanent deficit (M4+, National Institutes of Health Stroke Scale 2). Thirteen patients had complete or gross total resection.

CONCLUSION: These preliminary data demonstrate that a monopolar HF-TOF threshold > 3 mA was not associated with a significant new motor deficit.

ABBREVIATIONS: CST, corticospinal tract

fMRI, functional magnetic resonance imaging

HF-TOF, high-frequency train-of-5

MEP, motor evoked potential

NIHSS, National Institutes of Health Stroke Scale

Department of Neurosurgery, Inselspital, Bern University Hospital, Bern, Switzerland

Correspondence: Kathleen Seidel, MD, Bern University Hospital, Inselspital, Department of Neurosurgery, Freiburgstrasse 4, Bern, Switzerland 3010. E-mail:

Received March 13, 2011

Accepted January 4, 2012

Copyright © by the Congress of Neurological Surgeons