Objective To determine and compare the diagnostic accuracy of electrically elicited multiplet discharges (MDs) and fasciculation potentials (FPs) in motor neuron disease (MND).

Methods Patients were eligible when they had MND in their differential diagnosis and were referred for electromyogram (EMG). Stimulated high-density surface EMG of the thenar muscles was performed on the same day as standard EMG examination. High-density recordings were analysed for presence of MDs and needle EMG of any muscle investigated in the cervical region for presence of FPs.

Results Of the 61 patients enrolled in this diagnostic study, 24 patients were clinically diagnosed with amyotrophic lateral sclerosis (ALS) and 11 patients with progressive muscular atrophy (PMA). Another diagnosis was made in 26 patients. Sixteen patients in whom MDs were detected were diagnosed with either ALS (n=11) or PMA (n=5; sensitivity=47.1%, PPV=94.1%). MDs were detected in only one patient initially diagnosed with PMA, but in whom later on, multifocal motor neuropathy could not be excluded (specificity=96.2%). Electrically elicited MDs had a higher specificity than FPs (96.2% vs 53.9%, p<0.001, n=26) and lower sensitivity (47.1% vs 85.3%, p=0.002, n=34). When considering presence of MDs in MND as neurogenic EMG abnormality, lower motor neuron involvement of ≥1 EMG region increased from 50% to 73.5% (p=0.008, n=34).

Conclusions Electrically evoked MDs are highly specific for ALS and PMA and are an early sign of lower motor neuron dysfunction.