Motor-evoked potentials: unusual findings

Objective: The aims of this study were to present rare findings of motor ev oked potentials (MEPs) in 3 patients with spastic paraparesis and to show t hat careful interpretation is indispensable in experiments done with very h igh intensity stimulation. Methods: The conduction along several segments of the descending tracts was studied by our previously published method in 3 patients with spastic para paresis. Results: The threshold for activation of descending tracts was markedly inc reased in all the patients. In one patient, both transcranial electrical an d magnetic cortical stimulation elicited responses with 4 different latenci es. 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). I n the other two patients, unexpectedly, the latency of responses to foramen magnum level stimulation was longer than the cortical latency. Foramen mag num and spinal cord stimulation could not excite the corticospinal tract bu t activated other slowly conducting descending tracts (about 20 m/s), where as cortical stimulation activated the corticospinal tract. Conclusions: The site of activation following cortical stimulation was vari able when very high intensity stimulation is used. The descending tracts th at contribute to the onset of electromyographic (EMG) responses may not be the same after cortical and spinal stimulation in patients with severely af fected corticospinal tract, especially when using very high intensities of stimulation. Such factors complicate the interpretation of EMG responses ob tained in patients with severely affected corticospinal tracts. (C) 1999 El sevier Science Ireland Ltd. All rights reserved.