Objective: To illustrate the electrophysiological and clinical findings in a patient with functional truncal orthostatic-dependent tremor.

Background: Typical orthostatic tremor is a unique type of oscillation with frequency up to 16 Hz, in which the shaking is confined mostly to the legs and can be alleviated by walking, sitting, or leaning against a wall. It is unusual to encounter orthostatic-dependent tremor of the trunk, and the pivotal features are worthy of elucidation.

Method: The 34-year-old woman developed truncal bouncing oscillation for 2 months. Her shaking was noted when sitting, standing, or walking. The tremor would disappear when lying flat or leaning her trunk against a wall or chair back. The symptom may cause her to ambulate with a tendency of toe-walking. She was detected to have hyperthyroidism with a positive antithyroglobulin antibody initially. However, the shaking was not improved in accordance with the thyroid function improvement. Multiple-channel surface EMG recordings of bilateral L2, T7, T8 paraspinal muscles, left deltoid muscle, bilateral tibialis anterior, and right first dorsal interosseous were performed. Distraction maneuvers, including ballistic hand tracking, toe-tapping, heel tapping, and finger tapping in frequencies different from the truncal tremor rhythm, were attempted to exclude the possibility of a functional disorder.

Results: Surface EMG power spectrum analysis disclosed a 4-5 Hz tremor activity that is present in all of the truncal leads, corresponding to the involuntary truncal movement. The truncal tremor was abolished by toe-tapping, and the frequency can be entrained by heel-tapping. Differently, the truncal shaking was not perturbed by distraction maneuvers of the upper limbs. The tremor activities would disappear on leaning her trunk against the chair back.

Conclusion: The peculiar clinical features observed in this patient do not fit with descriptions of previously known truncal movement disorders such as diaphragm flutter, propriospinal myoclonus, or belly dancer dystonic movement. Surface EMG recording illustrated the tremor frequency being different from that of classical OT and the partially distractible tremor of the current case suggesting a possible psychogenic nature. It is intriguing to notice that the tremor can only be abolished or entrained by distraction maneuvers by moving the lower limbs, and the reason for this phenomenon warrants future elucidation.

References: 1. Benito-León, J., & Domingo-Santos, Á. (2016). Orthostatic Tremor: An Update on a Rare Entity. Tremor and other hyperkinetic movements (New York, N.Y.), 6, 411. https://doi.org/10.7916/D81N81BT 2. Hassan, A., & van Gerpen, J. A. (2016). Orthostatic Tremor and Orthostatic Myoclonus: Weight-bearing Hyperkinetic Disorders: A Systematic Review, New Insights, and Unresolved Questions. Tremor and other hyperkinetic movements (New York, N.Y.), 6, 417. https://doi.org/10.7916/D84X584K 3. Erro, R., Bhatia, K. P., & Cordivari, C. (2014). Shaking on Standing: A Critical Review. Movement disorders clinical practice, 1(3), 173–179. https://doi.org/10.1002/mdc3.12053

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MDS Abstracts - https://www.mdsabstracts.org/abstract/functional-truncal-orthostatic-dependent-tremor-in-a-patient-with-autoimmune-thyroid-disorder/