Objective: To study the clinical and electrophysiological characteristics of series of patients with slow orthostatic tremor (sOt)

Background: sOT is characterized by tremor <13Hz affecting the lower limbs during standing. Two thirds are secondary to another disorder[1] but it may develop alongside ET in some [2]. Functional neurological disorders (FNDs) mimic other movement disorders but demonstrate internal inconsistency [3]. Orthostatic hyperkinesias, i.e. OT and orthostatic myoclonus (OM), are considered weight-bearing abnormal movements. Surface EMG (sEMG) is essential in clarifying the nature of sOT [4].

Method: We examined the clinical and electrophysiological records of patients with a measurable tremor detectable with sEMG in the lower limbs during standing.

Results: 16 patients (4 male, 12 female) were included. 3 patients (19%) had a previous diagnosis of a different movement disorder (1 drug-induced parkinsonism, 1 essential tremor, 1 spinal myoclonus). 5 had a pre-existing gait disorder. None had FND. Most frequent complaints were tremor alone (44%), tremor and imbalance (19%) and imbalance alone (25%). 86.7% had a visible tremor on standing. 80% had another action tremor which was bimanual in 11/12. In sEMG: mean tremor frequency was 6.7±1.9 Hz (range 4-10), 40% of tremors were distractible, disappearing in sEMG during mental or manual tasks but none entrained. 80% of registries attenuated with trunk leaning but migration to the upper limbs was only detected in 33.3%.1 displayed OM, progressively structured as a sOT. 1 showed a sOT that subsided after 3 seconds. 4 patients underwent a DaTSCAN, 1 of which was pathological. 6 patients were diagnosed with FND, 2 with secondary sOT (1 drug-induced, resolved after SSRI was stopped; 1 due to PD). 7 patients received a diagnosis of idiopathic sOT, one in combination with orthostatic myoclonus and associated to an polyneuropathy, in our view insufficient to establish causality.1 patient received a descriptive diagnosis of “transient tremor of unsteadiness” given its unclear nature.

Conclusion: Functional slow orthostatic tremors might be frequently overlooked, being the second most frequent sOT in our case series. The diagnosis can be supported through demonstration of internal inconsistency during sEMG. Idiopathic forms of sOT may be more frequent than previously reported.

References: 1. Hassan A, Caviness J. Slow Orthostatic Tremor: Review of the Current Evidence. Tremor Other Hyperkinet Mov (N Y). 2019 Nov 26;9. doi: 10.7916/tohm.v0.721. PMID: 31832265; PMCID: PMC6886496.
2. Wee AS, Subramony SH, Currier RD. “Orthostatic tremor’ in familial-essential tremor. Neurology. 1986 Sep;36(9):1241-5. doi: 10.1212/wnl.36.9.1241. PMID: 3748391.
3. Aybek, S. & Perez, D. L. Diagnosis and management of functional neurological disorder. BMJ o64 (2022) doi:10.1136/bmj.o64.
4. Hassan A, van Gerpen JA. Orthostatic Tremor and Orthostatic Myoclonus: Weight-bearing Hyperkinetic Disorders: A Systematic Review, New Insights, and Unresolved Questions. Tremor Other Hyperkinet Mov (N Y). 2016 Nov 18;6:417. doi: 10.7916/D84X584K. PMID: 28105385; PMCID: PMC5233784.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/functional-neurological-disorders-might-be-a-frequent-cause-of-slow-orthostatic-tremor-insights-from-a-case-series/