Objective: The aims of this study were to present rare findings of motor evoked potentials (MEPs) in 3 patients with spastic paraparesis and to show that careful interpretation is indispensable in experiments done with very high intensity stimulation.
Methods: The conduction along several segments of the descending tracts was studied by our previously published method in 3 patients with spastic paraparesis.
Results: The threshold for activation of descending tracts was markedly increased in all the patients. In one patient, both transcranial electrical and magnetic cortical stimulation elicited responses with 4 different latencies. They were compatible with the latencies of I1-, D(D1)-, D2- or D3-waves. Very high intensity stimulation elicited D2 waves (activation around the cerebral peduncle) or D3 waves (activation at the foramen magnum level). In the other two patients, unexpectedly, the latency of responses to foramen magnum level stimulation was longer than the cortical latency. Foramen magnum and spinal cord stimulation could not excite the corticospinal tract but activated other slowly conducting descending tracts (about 20 m/s), whereas cortical stimulation activated the corticospinal tract.
Conclusions: The site of activation following cortical stimulation was variable when very high intensity stimulation is used. The descending tracts that contribute to the onset of electromyographic (EMG) responses may not be the same after cortical and spinal stimulation in patients with severely affected corticospinal tract, especially when using very high intensities of stimulation. Such factors complicate the interpretation of EMG responses obtained in patients with severely affected corticospinal tracts.