Slow Orthostatic Tremor: Review of the Current Evidence.

Background: Orthostatic tremor (OT) is defined as tremor in the legs and trunk evoked during standing. While the classical description is tremor of ≥13 Hz, slower frequencies are recognized. There is disagreement as to whether the latter represents a slow variant of classical OT, or different tremor disorder(s) given frequent coexistent neurological disease. Methods: A systematic literature search of PubMed was performed in February 2019 for "slow orthostatic tremor" and related terms which generated 573 abstracts, of which 61 were included. Results: Between 1970 and 2019, there were 70 cases of electrophysiologically confirmed slow OT. Two-thirds were female, of mean age 60 years (range 26-86), and mean disease duration 6 years (range 0-32). One-third of cases were isolated, and two-thirds had a coexistent disorder(s), including parkinsonism (30%), ataxia (12%), and dystonia (10%). Postural arm tremor was present in 34%. Median tremor frequency was 6-7 Hz (range 3-12). Tremor bursts ranged from 50 to 150 ms duration, and were alternating or synchronous in antagonistic and/or analogous muscles. Low and high coherences were reported. Five cases (7%) had coexistent classical OT. Clonazepam was the most effective medication across all frequencies, and levodopa was effective for 4-7 Hz OT with coexistent parkinsonism. Two cases resolved with the treatment of Graves' disease. Electrophysiology and imaging predominantly support a central tremor generator. Discussion: While multiple lines of evidence separate slow OT from classical OT, clinical and electrophysiological overlap may occur. Primary and secondary causes are identified, similar to classical OT. Further exploration to clarify these slow OT subtypes, clinically and neurophysiologically, is proposed.