Effects of transcranial stimulating electrode montages over the head for lower-extremity transcranial motor evoked potential monitoring

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OBJECTIVE

The aim of this study was to determine the most effective electrode montage to elicit lower-extremity transcranial motor evoked potentials (LE-tMEPs) using a minimum stimulation current.

METHODS

A realistic 3D head model was created from T1-weighted images. Finite element methods were used to visualize the electric field in the brain, which was generated by transcranial electrical stimulation via 4 electrode montage models. The stimulation threshold level of LE-tMEPs in 52 patients was also studied in a practical clinical setting to determine the effects of each electrode montage.

RESULTS

The electric field in the brain radially diffused from the brain surface at a maximum just below the electrodes in the finite element models. The Cz-inion electrode montage generated a centrally distributed high electric field with a current direction longitudinal and parallel to most of the pyramidal tract fibers of the lower extremity. These features seemed to be effective in igniting LE-tMEPs.

Threshold level recordings of LE-tMEPs revealed that the Cz-inion electrode montage had a lower threshold on average than the C3–C4 montage, 76.5 ± 20.6 mA and 86.2 ± 20.6 mA, respectively (31 patients, t = 4.045, p < 0.001, paired t-test). In 23 (74.2%) of 31 cases, the Cz-inion montage could elicit LE-tMEPs at a lower threshold than C3–C4.

CONCLUSIONS

The C3–C4 and C1–C2 electrode montages are the standard for tMEP monitoring in neurosurgery, but the Cz-inion montage showed lower thresholds for the generation of LE-tMEPs. The Cz-inion electrode montage should be a good alternative for LE-tMEP monitoring when the C3–C4 has trouble igniting LE-tMEPs.

ABBREVIATIONS CMAP = compound muscle action potential; cMEP = cortical MEP; LE = lower extremity; MEP = motor evoked potential; TES = transcranial electrical stimulation; tMEP = transcranial MEP; UE-tMEP = upper-extremity tMEP.
Article Information

Contributor Notes

INCLUDE WHEN CITING Published online September 23, 2016; DOI: 10.3171/2016.7.JNS16643.Correspondence Ryosuke Tomio, Department of Neurosurgery, Keio University School of Medicine, 35 Shinanomachi, Shinjuku, Tokyo 160-8582, Japan. email: tomy0807@hotmail.com.

© AANS, except where prohibited by US copyright law.

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