Objective: Examine the kinematic properties of the myorhythmia with quantitative motion analysis.

Background: Myorhythmia is a rare form of tremor that is defined by a uniquely low frequency of less than 4 Hz. This oscillatory movement disorder has been variably described as synchronous or asynchronous between body parts and as jerky, rhythmic and pseudorhythmic in appearance, but these kinematic properties have not been examined with quantitative motion analysis.

Method: We studied disabling myorhythmia in the head and upper limbs (right>left) of a 51-year-old woman with multiple sclerosis. Her myorhythmia developed approximately three months after an episode of acute demyelination in her brainstem and cerebellum. Head and hand tremors at rest and during posture were recorded with inertial measurement units containing triaxial accelerometers and gyroscopic transducers. Synchrony among the hands and head was examined with time-frequency coherence analysis. Half-power spectral bandwidth and interquartile range of cycle-to-cycle frequency change were computed as measures of rhythmicity. Waveform deviation from sinusoidality was quantified as total harmonic distortion.

Results: Head and hand oscillations occurred at 2.5-3.2 Hz. Tremor amplitude fluctuated greatly during the 30-60 second recordings, and oscillation was only intermittently present in the left hand at rest. Narrow frequency bandwidths (≤0.21 Hz) and interquartile frequency changes (≤0.38 Hz) were indicative of marked rhythmicity. Her head and hand oscillations were intermittently synchronous (coherence = 0.8-1.0). Waveform was not perfectly sinusoidal. The harmonic distortion in acceleration recordings (mean, SD: 0.46, 0.25) was significantly greater than in gyroscopic recordings (mean, SD: 0.24, 0.10; t = -3.172, df = 8, p = 0.013). However, harmonic distortion was much less in displacement (mean, SD: 0.09, 0.04) and rotation (mean, SD: 0.12, 0.04) and was comparable to that of a triangular wave.

Conclusion: This is the first quantitative demonstration of very high rhythmicity and nearly perfect coherence (synchrony) of myorhythmia between different body parts. Tremor rhythmicity in our patient was comparable to the most rhythmic essential and Parkinson tremors. The jerky appearance of our patient’s myorhythmia was largely, if not entirely, due to large amplitude fluctuations, not harmonic waveform distortion.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/myorhythmia-a-quantitative-study-of-synchrony-and-rhythmicity-between-the-head-and-upper-limbs/