MOTOR-NEURON DISEASE WITH PYRAMIDAL TRACT DYSFUNCTION INVOLVES THE CORTICAL GENERATORS OF THE EARLY SOMATOSENSORY-EVOKED POTENTIAL TO TIBIAL NERVE-STIMULATION

We evaluated somatosensory evoked potentials (SEPs) to tibial nerve st imulation in 39 patients with sporadic motor neuron disease using mult iple scalp derivations (earlobe reference). SEPs were altered in 22 of 29 amyotrophic lateral sclerosis (ALS) patients, whereas they were un affected in 10 progressive muscular atrophy (PMA) patients. The main c hanges involved the amplitude and the field distribution of the early P40 and N37 cortical potentials with different modalities varying from a selective loss of the P40 potential (33% of tested sides) to absenc e of all early cortical SEPs (22% of tested sides). The later componen ts following N50 were generally spared. The commonly used Cz-Fz montag e was inadequate for detecting these alterations. Central afferent con duction was slightly affected. The selective loss of cortical SEPs and their close correlation with clinicoelectrophysiologic evidence of ce ntral motor system involvement strongly support a cortical origin of t he SEP alterations in ALS. We suggest that neuronal loss in the somato sensory cortex may selectively affect the generator sites of the corti cal SEPs to lower limb stimulation.