Multiplet discharges after electrical stimulation: New evidence for distal excitability changes in motor neuron disease

Abstract

We hypothesized that action potentials evoked by distal stimulation might trigger ectopic activity (multiplet discharges, MDs). By studying MDs, we investigated the involvement of the axonal part of the peripheral motor neuron in amyotrophic lateral sclerosis (ALS) and progressive muscular atrophy (PMA). We performed stimulated high-density surface EMG recordings of the thenar muscles in 10 ALS/PMA patients, five recordings per patient over a three-month period. Furthermore, motor unit number estimates (MUNE) and ALSFRS-R scores were obtained in sessions 1 and 5. MDs were found in all patients, in 21% of the sampled motor units, and in response to 2.4% of the stimulations. The interspike interval range of the MD components was 2.9–6.5 ms, which is compatible only with a distal MD origin. The number of MDs, as percentage of the number of applied stimuli, was correlated with a decline in ALSFRS-R (r =0.80, p =0.006) and MUNE (r =0.72, p =0.02). In conclusion, MDs can be elicited with electrical stimulation in ALS and PMA patients. Analysis of MD characteristics provides further indications for pathophysiological excitability changes in the most distal part of the motor neuron. MDs are associated with clinical deterioration.

Key words::

• ALS
• PMA
• multiplet discharges
• excitability
• fasciculations

Acknowledgements

This work was financially supported by the Netherlands Organization for Scientific Research (NWO) and Fonds NutsOhra.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.