Comparison of descending volleys evoked by transcranial and epidural motor cortex stimulation in a conscious patient with bulbar pain

Objective: To compare the pattern of activation of motor cortex produced by transcranial magnetic stimulation and epidural electrical stimulation. Methods: The spinal volleys evoked by transcranial magnetic stimulation and epidural electrical stimulation over the cerebral motor cortex were recorded from an electrode inserted into the cervical epidural space of one conscious subject who also had a cortical epidural electrode over the motor area. The volleys were termed D- and I-waves according to their latency. Magnetic stimulation was performed with a figure-of-eight coil and the induced current flowed either in a postero-anterior (PA) or in latero-medial (LM) direction. Results: At active motor threshold intensity LM magnetic stimulation evoked a D wave whereas PA stimulation evoked an I1 wave with later I waves being recruited at increasing stimulus intensities. Electrical epidural stimulation evoked both a D wave and I waves. However, the D wave evoked by electrical epidural stimulation had a longer latency than the LM D wave, suggesting either a more proximal site of activation of the pyramidal axon or activation of slightly faster conducting set of corticospinal fibres by LM stimulation. The I3 wave evoked by electrical epidural stimulation also had a longer latency than the PA I3-wave Conclusions: Epidural stimulation of the motor cortex can produce repetitive excitation of corticospinal neurones. The order of recruitment of the volleys, and the latency of the D and I3 waves may be slightly different to that seen after transcranial magnetic stimulation. Significance: Our findings suggest that there may be subtle differences in the populations of neurones activated by the two forms of stimulation.