Revisiting somatosensory evoked potentials in motor neuron diseases: neurophysiological insights from a large cohort

To systematically investigate Somatosensory Evoked Potential (SEP) abnormalities in a large cohort of patients with Motor Neuron Disease (MND), and to explore their relationship with Motor Evoked Potentials (MEPs) and clinical phenotypes.

We retrospectively analyzed 267 patients with confirmed MND who underwent standardized SEPs and transcranial magnetic stimulation. Patients were divided into pure/predominant Upper Motor Neuron (UMN) and pure/predominant Lower Motor Neuron/Amyotrophic Lateral Sclerosis (LMN/ALS) groups. SEP abnormalities were assessed using internal normative data, including prolonged latencies, reduced amplitudes, and increased N20-P25 amplitudes. MEPs were classified semi-quantitatively as normal or abnormal by independent raters.

At least one SEP abnormality was detected in 75 % of patients, with no significant differences between the UMN and LMN/ALS groups. Increased N20-P25 amplitudes were observed in both phenotypes, suggesting widespread sensory cortical hyperexcitability across the MND spectrum. In contrast, abnormal MEPs were significantly more frequent in UMN patients (p < 0.001). No significant association was found between SEP abnormalities and MEP findings. Upper- and lower-limb SEP latencies were strongly correlated (all p < 0.001), whereas increased SEP amplitudes did not correlate with latency abnormalities.

SEP abnormalities are highly prevalent in MND and appear largely independent from corticospinal dysfunction. Increased SEP amplitudes likely reflect primary cortical sensory hyperexcitability rather than impaired sensory conduction.

These findings support the concept of MND as a multisystem network disorder that involves sensory cortical circuits and highlight the role of SEPs in the diagnostic workup.