Effects of transcranial stimulating electrode montages over the head for lower-extremity transcranial motor evoked potential monitoring.
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.
Department of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
Ryosuke
R
Tomio
Department of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
Takenori
T
Akiyama
Department of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
Takayuki
T
Ohira
Department of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
Kazunari
K
Yoshida
D004566:Electrodes; D019054:Evoked Potentials, Motor; D006257:Head; D006801:Humans; D035002:Lower Extremity; D065908:Transcranial Direct Current Stimulation
CMAP = compound muscle action potential; LE = lower extremity; MEP = motor evoked potential; TES = transcranial electrical stimulation; UE-tMEP = upper-extremity tMEP; cMEP = cortical MEP; diagnostic and operative techniques; electrode montage; lower extremity; neurosurgery; tMEP = transcranial MEP; transcranial electric stimulation; transcranial motor evoked potential