Large fasciculation can clinically manifest as spinal myoclonus; electromyographic and dynamic echomyographic studies of four cases with motor neuron disease.

Objective: Patients with motor neuron disease rarely present with fasciculation which is large enough to be clinically recognized as myoclonus. This study is aimed at elucidating the features of large fasciculation manifesting as myoclonus by using surface electromyography (surface EMG) and dynamic echomyography (dynamic Echo).

Methods: Four patients with amyotrophic lateral sclerosis, two of whom clinically presented with both fasciculation and myoclonus, were studied by using the surface EMG and the dynamic Echo.

Results: At rest, all patients had fasciculation in atrophic muscles, and the surface EMG showed occasional discharges of different waveforms corresponding to fasciculation. During voluntary gentle muscle contraction, the surface EMG showed repetitive discharges in the contracting muscle, which were constant in size and waveform within each muscle. The muscle Echo at rest revealed occasional contractions of a small number of muscle fibers corresponding to fasciculation. During voluntary muscle contraction, the number of muscle fibers involved in the involuntary motor phenomena was larger in the patients who clinically presented with myoclonus compared with other patients who clinically presented only with fasciculation. In a patient who presented with myoclonus, there was no contraction in the antagonist muscle.

Conclusions: Fasciculation involving a large number of muscle fibers clinically manifests as spinal myoclonus.

Significance: Fasciculation involving a large number of muscle fibers can be a cause of spinal myoclonus.