Ahead of Print: Low Threshold Mapping Motor Cortex
Background: Microsurgery within eloquent cortex is a controversial approach due to the high-risk of permanent neurological deficit. Few data exist showing the relationship between mapping stimulation intensity required for eliciting a muscle motor evoked potential (MEP) and 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 MEP 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 MEPs from target muscles (amplitude >30[mu]V). Monopolar high-frequency motor mapping with train-of-five stimuli (HF-TOF) (pulse duration=500[mu]s; interstimulus interval=4.0ms; frequency=250Hz) was used to determine motor response negative sites where incision and dissection could be performed. At sites negative to 3mA 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 3mA-6mA indicating close proximity of motor neurons. Postoperatively, twelve patients had no new motor deficit, one patient had a minor new temporary deficit (M4+, NIHSS 1), and another patient had a minor new permanent deficit (M4+, NIHSS 2). Thirteen patients had complete or gross total resection.
Conclusion: These preliminary data demonstrate that a monopolar HF-TOF threshold higher than 3mA was not associated with a significant, new motor deficit.
From: Low Threshold Monopolar Motor Mapping for Resection of Primary Motor Cortex Tumors by Seidel et al.