The warning-sign hierarchy between quantitative subcortical motor mapping and continuous motor evoked potential monitoring during resection of supratentorial brain tumors

Clinical article

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Object

Mapping and monitoring are believed to provide an early warning sign to determine when to stop tumor removal to avoid mechanical damage to the corticospinal tract (CST). The objective of this study was to systematically compare subcortical monopolar stimulation thresholds (1–20 mA) with direct cortical stimulation (DCS)–motor evoked potential (MEP) monitoring signal abnormalities and to correlate both with new postoperative motor deficits. The authors sought to define a mapping threshold and DCS-MEP monitoring signal changes indicating a minimal safe distance from the CST.

Methods

A consecutive cohort of 100 patients underwent tumor surgery adjacent to the CST while simultaneous subcortical motor mapping and DCS-MEP monitoring was used. Evaluation was done regarding the lowest subcortical mapping threshold (monopolar stimulation, train of 5 stimuli, interstimulus interval 4.0 msec, pulse duration 500 μsec) and signal changes in DCS-MEPs (same parameters, 4 contact strip electrode). Motor function was assessed 1 day after surgery, at discharge, and at 3 months postoperatively.

Results

The lowest individual motor thresholds (MTs) were as follows (MT in mA, number of patients): > 20 mA, n = 12; 11–20 mA, n = 13; 6–10 mA, n = 20; 4–5 mA, n = 30; and 1–3 mA, n = 25. Direct cortical stimulation showed stable signals in 70 patients, unspecific changes in 18, irreversible alterations in 8, and irreversible loss in 4 patients. At 3 months, 5 patients had a postoperative new or worsened motor deficit (lowest mapping MT 20 mA, 13 mA, 6 mA, 3 mA, and 1 mA). In all 5 patients DCS-MEP monitoring alterations were documented (2 sudden irreversible threshold increases and 3 sudden irreversible MEP losses). Of these 5 patients, 2 had vascular ischemic lesions (MT 20 mA, 13 mA) and 3 had mechanical CST damage (MT 1 mA, 3 mA, and 6 mA; in the latter 2 cases the resection continued after mapping and severe DCS-MEP alterations occurred thereafter). In 80% of patients with a mapping MT of 1–3 mA and in 75% of patients with a mapping MT of 1 mA, DCS-MEPs were stable or showed unspecific reversible changes, and none had a permanent motor worsening at 3 months. In contrast, 25% of patients with irreversible DCS-MEP changes and 75% of patients with irreversible DCS-MEP loss had permanent motor deficits.

Conclusions

Mapping should primarily guide tumor resection adjacent to the CST. DCS-MEP is a useful predictor of deficits, but its value as a warning sign is limited because signal alterations were reversible in only approximately 60% of the present cases and irreversibility is a post hoc definition. The true safe mapping MT is lower than previously thought. The authors postulate a mapping MT of 1 mA or less where irreversible DCS-MEP changes and motor deficits regularly occur. Therefore, they recommend stopping tumor resection at an MT of 2 mA at the latest. The limited spatial and temporal coverage of contemporary mapping may increase error and may contribute to false, higher MTs.

Abbreviations used in this paper:CST = corticospinal tract; DCS = direct cortical stimulation; DTI = diffusion tensor imaging; MEP = motor evoked potential; MRCS = Medical Research Council Scale; MT = motor threshold; TOF = train of five.

Article Information

Address correspondence to: Kathleen Seidel, M.D., Department of Neurosurgery, Inselspital, Bern University Hospital, 3010 Bern, Switzerland. email: seidelkathleen@hotmail.com.

Please include this information when citing this paper: published online November 30, 2012; DOI: 10.3171/2012.10.JNS12895.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Percentage of patients with new MRCS (MRC)-graded motor deficits postoperatively, at discharge, and at the 3-month follow-up visit.

  • View in gallery

    New postoperative motor deficits in the different lowest mapping MT groups 1 day after surgery (left bar), at discharge (center bar), and at 3 months (right bar). To further evaluate the correlation of motor deficits with mapping threshold, the 2 patients with permanent deficits after 3 months due to intraoperative vascular injury and infarction (MT 13 mA and 20 mA) were removed from the 11- to 20-mA group. For details, see text about electrophysiological findings and causes for permanent deficits in Results.

  • View in gallery

    New postoperative motor deficits according to the DCS-MEP signal alterations 1 day after surgery (left bar), at discharge (center bar), and at the 3-month visit (right bar).

  • View in gallery

    Patients with DCS-MEP monitoring changes in the different mapping threshold groups. The 2 patients with vascular injury (MT 13 and 20 mA) are not shown (see legend of Fig. 2).

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