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.

Ahead of Print: nTMS Influence on Surgical Planning

Background: Brain tumor surgery near the motor cortex requires careful planning to achieve the optimal balance between completeness of tumor resection and preservation of motor function. Navigated transcranial magnetic stimulation (nTMS) can be used to map functionally essential motor areas pre-operatively.

Objective: To evaluate how much influence, benefit, and impact nTMS has on the surgical planning for tumors near the motor cortex.

Methods: This study reviewed the records of 73 patients with brain tumors in or near the motor cortex, mapped pre-operatively with nTMS. The surgical team prospectively classified how much influence the nTMS results had on the surgical planning. Stepwise regression analysis was used to explore which factors predict the amount of influence, benefit, and impact nTMS has on the surgical planning.

Ahead of Print: Safe Resection of Arteriovenous Malformations in Eloquent Motor Areas Aided by Functional Imaging and Intraoperative Monitoring

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BACKGROUND: Arteriovenous malformations (AVMs) proximal to motor cortical areas or motor projection systems are challenging to manage due to the risk of severe sensory and motor impairment. Surgical indication in these cases therefore remains controversial.

OBJECTIVE: To propose a standardized approach for centrally situated AVMs, based on functional imaging and intraoperative electrophysiological evaluation.

METHODS: We conducted a retrospective analysis of 15 patients who underwent surgical treatment for AVMs in motor cortical areas or proximal to motor projections. Preoperative assessment included fMRI and 3D-tractography. Operations were performed under continuous electrophysiological monitoring aided by direct brain stimulation. We identified critical bloody supply to the motor areas by temporary occluding the feeding vessels under electrophysiological monitorization. Clinical outcome was evaluated using the modified Rankin Scale.

Editor Choice: Navigated Transcranial Magnetic Stimulation and Functional Magnetic Resonance Imaging: Advanced Adjuncts in Preoperative Planning for Central Region Tumors

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BACKGROUND: Tumor resection in the vicinity of the motor cortex poses a challenge to all neurosurgeons. For preoperative assessment of eloquent cortical areas, functional magnetic resonance imaging (fMRI) is used, whereas intraoperatively, direct cortical stimulation (DCS) is performed. Navigated transcranial magnetic stimulation (nTMS) is comparable to DCS in activating cortical pyramidal neurons.

OBJECTIVE: To evaluate the reliability of nTMS compared with fMRI and DCS for preoperative resection planning of centrally located tumors.

METHODS: In a prospective series, 11 patients (ages, 20-63 years; mean, 41.9 ± 14.9 years, 2 women) with tumors located in or adjacent to the motor cortex were evaluated for surgery. fMRI and nTMS were applied for preoperative assessment of the extent of tumor resection. A 3-dimensional anatomic data set with superimposed fMRI data was integrated in the eXimia Navigated Brain Stimulation station for ensuing motor cortex mapping by nTMS. Responses from nTMS were evaluated by electromyographic response. During surgery, the coordinates of each DCS site were unambiguously defined and integrated into neuronavigation. A post hoc comparison of the coordinates of nTMS, fMRI, and DCS was performed.

Ahead of Print: Preoperative Functional Mapping for Rolandic Brain Tumor Surgery: Comparison of Navigated Transcranial Magnetic Stimulation to Direct Cortical Stimulation

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BACKGROUND: Transcranial magnetic stimulation (TMS) is the only non-invasive method for presurgical stimulation mapping of cortical function. Recent technical advancements have significantly increased the focality and usability of the method.

OBJECTIVE: To compare the accuracy of a 3D-MRI navigated TMS system (nTMS) to the gold standard of direct cortical stimulation (DCS).

METHODS: The primary motor areas of 20 patients with rolandic tumors were mapped preoperatively with nTMS at 110% of the individual resting motor threshold. Intraoperative direct cortical stimulation was available from 17 patients. The stimulus locations eliciting the largest EMG response in the target muscles (“hotspots”) were determined for both methods.