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 stimulation in 39 patients with sporadic motor neuron disease using multiple scalp derivations (earlobe reference). SEPs were altered in 22 of 29 amyotrophic lateral sclerosis (ALS) patients, whereas they were unaffected in 10 progressive muscular atrophy (PMA) patients. The main changes 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 absence of all early cortical SEPs (22% of tested sides). The later components following N50 were generally spared. The commonly used Cz-Fz montage was inadequate for detecting these alterations. Central afferent conduction was slightly affected. The selective loss of cortical SEPs and their close correlation with clinicoelectrophysiologic evidence of central motor system involvement strongly support a cortical origin of the SEP alterations in ALS. We suggest that neuronal loss in the somatosensory cortex may selectively affect the generator sites of the cortical SEPs to lower limb stimulation.