Piper rhythm-like electromyographical activity in muscle stiffness in sodium channel myotonia representing potassium-aggravated myotonia and myotonia permanens

A 39-years-old female with potassium-aggravated myotonia due to p.Q1633E in SCN4A experienced painful muscle stiffness triggered by exercise, potassium-rich fruits, and cold exposure, which progressed into a rigid state. Needle electromyography (EMG) during muscle stiffness showed synchronous, rhythmic, and regular activities starting at ~60 Hz and ~6 mV. A 37-year-old male with myotonia permanens due to a splicing-affecting indel variant in intron 21 of SCN4A experienced cold-induced myotonia. EMG recordings during muscle stiffness showed similar synchronous, rhythmic, and regular activities starting at ~80 Hz and 6.5 mV. In both patients, the frequencies and amplitudes were gradually decreased with relief of muscle stiffness. In either patient, single motor unit potentials by spontaneous activity were not explicitly recognized. In both patients, the activities produced a characteristic sound which, while similar in pitch to the 'dive bomber' sound of classical myotonia, lacked its typical waxing and waning quality. The activities were similar to the Piper rhythm that was originally reported in fatigued normal muscle. Visual inspection of EMG activities in the literature revealed that similar Piper rhythm-like EMG activities were presented in Satoyoshi disease, myotonia permanens, paramyotonia congenita, and a rare form of nondystrophic myotonia. In Satoyoshi disease and fatigued normal skeletal muscle, the activities during muscle stiffness were less than 60 Hz, whereas, in sodium channelopathies, they started at 60 Hz or higher, which may be a hallmark of hyperexcitability of muscle membrane in sodium channelopathies.