PubMed · 10171546
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Y Young, R M Jones. Monitoring neuromuscular function.. https://pubmed.ncbi.nlm.nih.gov/10171546/
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We performed transcranial magnetic stimulation (TMS) to elucidate the D- and I-wave components comprising the motor evoked potentials (MEPs) elicited from the leg motor area, especially at near-threshold intensity. Recordings were made from the tibialis anterior muscle using needle electrodes. A figure-of-eight coil was placed so as to induce current in the brain in eight different directions, starting from the posterior-to-anterior direction and rotating it in 45 degrees steps. The latencies were compared with those evoked by transcranial electrical stimulation (TES) and TMS using a double cone coil. Although the latencies of MEPs ranged from D to I3 waves, the most prominent component evoked by TMS at near-threshold intensity represented the I1 wave. With the double cone coil, the elicited peaks always represented I1 waves, and D waves were evoked only at very high stimulus intensities, suggesting a high effectiveness of this coil in inducing I1 waves. Using the figure-of-eight coil, current flowing anteriorly or toward the hemisphere contralateral to the recorded muscle was more effective in eliciting large responses than current flowing posteriorly or toward the ipsilateral hemisphere. The effective directions induced I1 waves with the lowest threshold, whereas the less effective directions elicited I1 and I2 waves with a similar frequency. Higher stimulus intensities resulted in concomitant activation of D through I3 waves with increasing amount of D waves, but still the predominance of I1 waves was apparent. The amount of I waves, especially of I1 waves, was greater than predicted by the hypothesis that TMS over the leg motor area activates the output cells directly, but rather suggests predominant transsynaptic activation. The results accord with those of recent human epidural recordings.
The authors assessed the efficacy, safety, and tolerability of vagus nerve stimulation (VNS) for refractory epilepsy in 45 adults 50 years of age and older. They determined seizure frequency, adverse effects, and quality of life. At 3 months, 12 patients had a >50% decrease in seizure frequency; at 1 year, 21 of 31 studied individuals had a >50% seizure decrease. Side effects were mild and transient. Quality of life scores improved significantly with time.
Time-varying magnetic fields induce electric fields that can cause physiological stimulation. Stimulation has been empirically characterized as a function of dB/dt and duration based on experiments using trapezoidal and sinusoidal gradient waveforms with constant ramp time, amplitude, and direction. For two-dimensional (2D) spiral scans, the readout gradient waveforms are frequency- and amplitude-modulated sinusoids on two orthogonal axes in quadrature. The readout gradient waveform therefore rotates with amplitude and angular velocity that are generally not constant. It does not automatically follow that spiral stimulation thresholds can be predicted using available stimulation models. We scanned 18 normal volunteers with a 2D spiral scan and measured global thresholds for axial, sagittal, and coronal planes. We concluded that the stimulation model evaluated accurately predicts slew rate-limited spiral mean stimulation thresholds, if the effective ramp time is chosen to be the half-period at the end of the spiral readout.