Orthostatic Tremor and Orthostatic Myoclonus: Weight-bearing Hyperkinetic Disorders: A Systematic Review, New Insights, and Unresolved Questions

Abstract

Background: Orthostatic tremor (OT) and orthostatic myoclonus (OM) are weight-bearing hyperkinetic movement disorders most commonly affecting older people that induce "shaky legs" upon standing. OT is divided into "classical" and "slow" forms based on tremor frequency. In this paper, the first joint review of OT and OM, we review the literature and compare and contrast their demographic, clinical, electrophysiological, neuroimaging, pathophysiological, and treatment characteristics.

Methods: A PubMed search up to July 2016 using the phrases "orthostatic tremor," "orthostatic myoclonus," "shaky legs," and "shaky legs syndrome" was performed.

Results: OT and OM should be suspected in older patients reporting unsteadiness with prolonged standing and/or who exhibit cautious, wide-based gaits. Surface electromyography (SEMG) is necessary to verify the diagnoses. Functional neuroimaging and electrophysiology suggest the generator of classical OT lies within the cerebellothalamocortical network. For OM, and possibly slow OT, the frontal, subcortical cerebrum is the most likely origin. Clonazepam is the most useful medication for classical OT, and levetiracetam for OM, although results are often disappointing. Deep brain stimulation appears promising for classical OT. Rolling walkers reliably improve gait affected by these disorders, as both OT and OM attenuate when weight is transferred from the legs to the arms.

Discussion: Orthostatic hyperkinesias are likely underdiagnosed, as SEMG is often unavailable in clinical practice, and thus may be more frequent than currently recognized. The shared weight-bearing induction of OT and OM may indicate a common pathophysiology. Further research, including use of animal models, is necessary to better define the prevalence and pathophysiology of OT and OM, in order to improve their treatment, and provide additional insights into basic balance and gait mechanisms.

Keywords: Electrophysiology; imaging; shaky legs.

Figure 1. Classical Orthostatic Tremor. (A) A 14 Hz, highly synchronized tremor is present in the patient’s legs as the patient stands. (B) As the patient leans onto a chair, the tremor transfers to the left triceps. It remains in the legs, but its amplitude is reduced. ADM, Abductor Digiti Minimi; MG, Medial Gastrocnemius; Quad, Quadriceps; TA, Tibial Anterior; Tri, Triceps; WE, Wrist Extensors.

Figure 2. Typical orthostatic myoclonus. (A) While the patient stands, frequent bursts of motor activity lasting <50 ms are present in the TAs and MGs. These are often synchronous between homologous muscles and occasionally have an alternating pattern between ipsilateral muscle antagonists. (B) Leaning onto a chair, the activity in the legs becomes virtually quiescent and high amplitude, tonic motor activity is present in the arm. ADM, Abductor Digiti Minimi; MG, Medial Gastrocnemius; Quad, Quadriceps; TA, Tibial Anterior; Tri, Triceps; WE, Wrist Extensors.

Figure 3. Asymmetric Orthostatic Myoclonus in a Patient Who Suffered a Remote Right MCA (Middle Cerebral Artery) Stroke. (A) As he stands, frequent, high amplitude bursts lasting <50 ms are present in his left TA, while only an isolated, lower amplitude burst <50 ms is present in the right TA. (B) Leaning onto a chair, the activity in both legs is tonic and of very low amplitude, predominantly in the quadriceps. It is tonic and high in amplitude in the triceps. ADM, Abductor Digiti Minimi; MG, Medial Gastrocnemius; Quad, Quadriceps; TA, Tibial Anterior; Tri, Triceps.

Figure 4. Mixed Orthostatic Myoclonus and Slow Orthostatic Tremor. (A) While the patient is standing in his preferred manner, intermittent bursts of motor activity lasting <50 ms are present, particularly in the TAs. He describes his legs as “trembling.” (B) Standing with his knees slightly flexed, a synchronous 5 Hz tremor is present in the TAs, and he describes his legs as “shaky.” ADM, Abductor Digiti Minimi; MG, Medial Gastrocnemius; Quad, Quadriceps; TA, Tibial Anterior; Tri, Triceps.