Objective: To evaluate in a randomized, double-blind, sham-controlled, crossover trial the efficacy of non-invasive trans-spinal direct current stimulation (tsDCS) for managing primary orthostatic tremor (POT).

Background: Intractable POT is a rare paradigmatic disorder as the disease is defined by the sensation of postural instability with little or no therapeutic resources. The electrophysiological hallmark is the presence of a fast tremor (13-18Hz) in the lower limbs and trunk, which only appears in the upright position. The pathophysiology of the POT is still unclear. Invasive spinal cord stimulation has been proposed as an alternative treatment with limited success. We hypothesize that the tremor may be related to a deficit of proprioceptive feedback. To address this question, we tested the effects of a single-session tsDCS, a novel and non-invasive tool to modulate spinal cord excitability.

Methods: Fifteen patients (aged 62±7 years) with a diagnose of intractable POT were enrolled. Maximal time in upright position (mean of 3 trials) and EMG activity of tibialis anterior muscles were recorded before and after a single-session of tsDCS. The instruction given to the patients was to stand up, with arms by their sides, and to stay in upright position until reaching a feeling of instability. Patients underwent two visits, with a washout period of at least 7 days. During each visit, patient received two stimulations, sham tsDCS (2.5mA, 90sec) and active, either cathodal or anodal tsDCS (2.5mA, 20min) at the 11th thoracic vertebra level. The order of the active stimulations was randomized. Sham condition was always administered prior to the active one.

Results: With respect to baseline, sham tsDCS significantly improved the time in upright position by 22% (IQR 4-47%) when followed by anodal tsDCS and by 42% (IQR 28-89%) when followed by cathodal tsDCS. Both anodal and cathodal tsDCS significantly improved the time in upright position with respect to sham (p<0.05). In addition, only active conditions reduced the amplitude of tremor (p<0.05). Interestingly, clinical global impression (CGI) scores correlated with the time in upright position (Spearman: 0.59

Conclusions: A single-session of tsDCS can reduce the amplitude of tremor and instability in POT. This opens a new vista to experimental treatment options using spinal DC stimulation in neurological disorders.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/trans-spinal-direct-current-stimulation-for-managing-primary-orthostatic-tremor/