Motor neuron disease (MND) involves the degeneration of variable proportions of upper and lower motor neurons (UMNs, LMNs). Electrophysiological tests for LMN degeneration are well established but an analogous biomarker of UMN damage is lacking. We are investigating 15–30 Hz intermuscular coherence (IMC) as a candidate; such coherence is prominent in normal humans and macaques during certain motor tasks. Selective corticospinal tract (CST) ablation in macaques caused loss of significant 15–30 Hz IMC on the side of the lesion, which persisted despite functional recovery. In patients with primary lateral sclerosis (PLS) no significant 15–30 Hz IMC was detected; by contrast, patients with progressive muscular atrophy (PMA) retained significant 15–30 Hz IMC. Similarly, in MND-mimicking conditions 15–30 Hz IMC was typically only diminished where the condition involved UMN damage. In hereditary spastic paraparesis, lower limb 15–30 Hz IMC was diminished but remained statistically significant in 50% of cases. Patients with neuropathies or myopathies typically retained significant 15–30 Hz IMC unless afferent pathways were involved, reflecting the role of afferent systems in generating 15–30 Hz IMC. Two prospectively assessed MND patients (ALS phenotype) showed significantly decreased 15–30 Hz IMC at least 8 months before a definitive diagnosis. Hence, 15–30 Hz IMC is a potential biomarker of UMN dysfunction in MND.
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