Orthostatic tremor

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.

The pathophysiology of tremor

Tremor is defined as rhythmic oscillatory activity of body parts. Four physiological basic mechanisms for such oscillatory activity have been described: mechanical oscillations; oscillations based on reflexes; oscillations due to central neuronal pacemakers; and oscillations because of disturbed feedforward or feedback loops. New methodological approaches with animal models, positron emission tomography, and mathematical analysis of electromyographic and electroencephalographic signals have provided new insights into the mechanisms underlying specific forms of tremor. Physiological tremor is due to mechanical and central components. Psychogenic tremor is considered to depend on a clonus mechanism and is thus believed to be mediated by reflex mechanisms. Symptomatic palatal tremor is most likely due to rhythmic activity of the inferior olive, and there is much evidence that essential tremor is also generated within the olivocerebellar circuits. Orthostatic tremor is likely to originate in hitherto unidentified brainstem nuclei. Rest tremor of Parkinson's disease is probably generated in the basal ganglia loop, and dystonic tremor may also originate within the basal ganglia. Cerebellar tremor is at least in part caused by a disturbance of the cerebellar feedforward control of voluntary movements, and Holmes' tremor is due to the combination of the mechanisms producing parkinsonian and cerebellar tremor. Neuropathic tremor is believed to be caused by abnormally functioning reflex pathways and a wide variety of causes underlies toxic and drug-induced tremors. The understanding of the pathophysiology of tremor has made significant progress but many hypotheses are not yet based on sufficient data. Modern neurology needs to develop and test such hypotheses, because this is the only way to develop rational medical and surgical therapies.

Involvement of cranial muscles and high intermuscular coherence in orthostatic tremor

Electromyographic recordings were conducted from limb, trunk, and cranial muscles in 6 patients with orthostatic tremor. Spectral analysis revealed a high-frequency tremor not only in the muscles of the limbs and trunk, but also in cranial muscles. The cross spectra were analyzed between various pairs of muscles that displayed a high-frequency tremor pattern. The resulting peak correlations were uniformly very high (near one) suggesting a high level of coherence. The involvement of cranial muscles suggests that supraspinal mechanisms are involved in the generation of orthostatic tremor. The high intermuscular coherence between all muscles indicates the existence of either a unique oscillator that generates tremor in all involved muscles on both sides of the body or a linking mechanism probably at a supraspinal level. The high-frequency tremor was only found when the muscles were contracted isometrically, irrespective of body posture. Thus, tremor generation might be more closely linked to mechanisms responsible for isometric force control than to those involved in stance regulation.