A case of low-frequency myokymia visualized by simultaneous EMG-ultrasound recording

Spontaneous muscle twitching observed on clinical examination can be challenging to distinguish between myokymia and fasciculation, particularly in peripheral nerve hyperexcitability syndromes where both phenomena may coexist. Because both fasciculations and myokymia may appear in Isaacs syndrome, ultrasonography alone can be misleading when discharge frequency is low, and correlation with needle EMG is essential for accurate differentiation.

We report a 53-year-old man with an 11-year history of progressive lower-limb twitching, cramps, and nocturnal muscle pain, in whom superficial muscle contractions were difficult to classify visually. Neuromuscular ultrasound revealed spontaneous contractions in multiple muscles, but their appearance overlapped with isolated fasciculations. Simultaneous needle electromyography and ultrasound of the left vastus medialis demonstrated low-amplitude grouped discharges recurring at intervals of approximately 4-9 s, most frequently 6-8 s, with burst durations of 60-200 ms and amplitudes generally below 200 μV, findings consistent with very low-frequency myokymic discharges. Needle EMG also showed widespread fibrillation potentials, positive sharp waves, and fasciculation potentials. Based on these clinical, electrophysiological, and imaging findings, the patient met the diagnostic criteria for probable Isaacs syndrome. Treatment with intravenous methylprednisolone followed by prednisolone 10 mg/day led to improvement in muscle cramps.

This case demonstrates that visually similar ultrasonographic muscle contractions may correspond to fundamentally different electrophysiological phenomena on needle EMG, depending on the underlying pathophysiology, such as low-frequency myokymic discharges. Recognition of this distinction may help refine the interpretation of spontaneous muscle activity observed on ultrasonography and avoid oversimplified labeling as fasciculations.