Myorhythmia: A Quantitative Study of Synchrony and Rhythmicity Between the Head and Upper Limbs.

Background: Myorhythmia is a <4 Hz oscillatory movement disorder that has been variably described as synchronous or asynchronous between body parts and as jerky or rhythmic in appearance, but there is no published report of quantitative motion analysis. Methods: A 51-year-old woman developed disabling myorhythmia in the head and upper limbs (right>left) approximately three months after a relapse of multiple sclerosis in her brainstem and cerebellum. Head and bilateral hand motion was recorded at rest and during posture with triaxial accelerometers and gyroscopic transducers. Recordings were analyzed with spectral power and coherence analyses. Frequency variability was quantified as half-power spectral bandwidth and interquartile range of cycle-to-cycle frequency change. Waveform deviation from sinusoidality was quantified as total harmonic distortion. Results: The 2.5-3.2 Hz head and hand oscillations exhibited narrow frequency bandwidths (≤0.21 Hz) and interquartile frequency changes (≤0.38 Hz). Amplitude fluctuated greatly, but head and hand oscillations were intermittently synchronous (coherence 0.8-1.0). Waveform was not perfectly sinusoidal and varied with the transducer. Conclusions: This is the first quantitative demonstration of very high rhythmicity and nearly perfect coherence of myorhythmia between different body parts, consistent with the classification of myorhythmia as a form of tremor. Limitations of the quantitative methods are discussed.