Primary orthostatic tremor: further observations in six cases

Deep Brain Stimulation for Orthostatic Tremor: An Observational Study

BACKGROUND

Primary orthostatic tremor (OT) can affect patients' life. Treatment of OT with deep brain stimulation (DBS) of the thalamic ventral intermediate nucleus (Vim) is described in a limited number of patients. The Vim and posterior subthalamic area (PSA) can be targeted in a single trajectory, allowing both stimulation of the Vim and/or dentatorubrothalamic tract (DRT). In essential tremor this is currently often used with positive effects.

OBJECTIVE

To evaluate the efficacy of Vim/DRT-DBS in OT-patients, based on standing time and Quality of Life (QoL), also on the long-term. Furthermore, to relate stimulation of the Vim and DRT, medial lemniscus (ML) and pyramidal tract (PT) to beneficial clinical and side-effects.

METHODS

Nine severely affected OT-patients received bilateral Vim/DRT-DBS. Primary outcome measure was standing time; secondary measures included self-reported measures, neurophysiological measures, structural analyses, surgical complications, stimulation-induced side-effects, and QoL up to 56 months. Stimulation of volume of tissue activated (VTA) were related to outcome measures.

RESULTS

Average maximum standing time increased from 41.0 s ± 51.0 s to 109.3 s ± 65.0 s after 18 months, with improvements measured in seven of nine patients. VTA (n = 7) overlapped with the DRT in six patients and with the ML and/or PT in six patients. All patients experienced side-effects and QoL worsened during the first year after surgery, which improved again during long-term follow-up, although remaining below age-related normal values. Most patients reported a positive effect of DBS.

CONCLUSION

Vim/DRT-DBS improved standing time in patients with severe OT. Observed side-effects are possibly related to stimulation of the ML and PT.

The Phenomenology of Primary Orthostatic Tremor

Background

Objectives

Methods

Results

Conclusions

The Top 50 Most-Cited Articles in Orthostatic Tremor: A Bibliometric Review

Background

Article-level citation count is a hallmark indicating scientific impact. We aimed to pinpoint and evaluate the top 50 most-cited articles in orthostatic tremor (OT).

Methods

The ISI Web of Knowledge database and 2017 Journal Citation Report Science Edition were used to retrieve the 50 top-cited OT articles published from 1984 to April 2019. Information was collected by the Analyze Tool on the Web of Science, including number of citations, publication title, journal name, publication year, and country and institution of origin. Supplementary analyses were undertaken to clarify authorship, study design, level of evidence, and category.

Results

Up to 66% of manuscripts were recovered from five journals: Movement Disorders (n = 18), Brain (n = 4), Journal of Clinical Neurophysiology (n = 4), Neurology (n = 4), and Clinical Neurophysiology (n = 3). Articles were published between 1984 and 2018, with expert opinion as the predominant design (n = 22) and review as category (n = 17). Most articles had level 5 evidence (n = 26). According to their countries of origin, 34% of articles belonged to the United States (n = 17) leading the list, followed by United Kingdom (n = 15). University College London yielded the greater number of articles (n = 12), followed by the University of Kiel (n = 9). Most popular authors were G. Deuschl (n = 10), C.D. Marsden (n = 6), J. Jankovic (n = 5), P.D. Thompson (n = 5), J.C. Rothwell (n = 5), L.J. Findley (n = 4), and P. Brown (n = 4), who together accounted for 48% of them. All papers were in English.

Discussion

Publishing high-cited OT articles could be facilitated by source journal, study design, category, publication language, and country and institution of origin.

Walking in orthostatic tremor modulates tremor features and is characterized by impaired gait stability

Primary orthostatic tremor (OT) is characterized by high-frequency lower-limb muscle contractions and a disabling sense of unsteadiness while standing. Patients consistently report a relief of symptoms when starting to ambulate. Here, we systematically examined and linked tremor and gait characteristics in patients with OT. Tremor and gait features were examined in nine OT patients and controls on a pressure-sensitive treadmill for one minute of walking framed by two one-minute periods of standing. Tremor characteristics were assessed by time-frequency analysis of surface EMG-recordings from four leg muscles. High-frequency tremor during standing (15.29 ± 0.17 Hz) persisted while walking but was consistently reset to higher frequencies (16.34 ± 0.25 Hz; p < 0.001). Tremor intensity was phase-dependently modulated, being predominantly observable during stance phases (p < 0.001). Tremor intensity scaled with the force applied during stepping (p < 0.001) and was linked to specific gait alterations, i.e., wide base walking (p = 0.019) and increased stride-to-stride fluctuations (p = 0.002). OT during walking persists but is reset to higher frequencies, indicating the involvement of supraspinal locomotor centers in the generation of OT rhythm. Tremor intensity is modulated during the gait cycle, pointing at specific pathways mediating the peripheral manifestation of OT. Finally, OT during walking is linked to gait alterations resembling a cerebellar and/or sensory ataxic gait disorder.

Proprioceptive muscle tendon stimulation reduces symptoms in primary orthostatic tremor

INTRODUCTION

Primary orthostatic tremor (OT) is characterized by high-frequency lower limb muscle contractions and a disabling sense of unsteadiness while standing. To date, therapeutic options for OT are limited. Here, we examined the effects of proprioceptive leg muscle stimulation via muscle tendon vibration (MTV) on tremor and balance control in patients with primary OT.

METHODS

Tremor in nine patients with primary OT was examined during four conditions: standing (1), standing with MTV on the bilateral soleus muscles (2), lying (3), and lying with MTV (4). Tremor characteristics were assessed by frequency domain analysis of surface EMG recordings from four leg muscles. Body sway was analyzed using posturographic recordings.

RESULTS

During standing, all patients showed a coherent high-frequency tremor in leg muscles and body sway that was absent during lying (p < 0.001). MTV during standing did not reset tremor frequency, but resulted in a decreased tremor intensity (p < 0.001; mean reduction: 32.5 ± 7.1%) and body sway (p = 0.032; mean reduction: 37.2 ± 6.8%). MTV did not affect muscle activity during lying. Four patients further reported a noticeable relief from unsteadiness during stimulation.

CONCLUSION

Proprioceptive stimulation did not reset tremor frequency consistent with the presumed central origin of OT. However, continuous MTV influenced the emergence of OT symptoms resulting in reduced tremor intensity, improved posture, and a relief from unsteadiness in half of the examined patients. These findings indicate that MTV either directly interferes with the peripheral manifestation of the central oscillatory pattern or prevents proprioceptive afferent feedback from becoming extensively synchronized at the tremor frequency.

Tremor

Tremor is a phenomenon observed in a broad spectrum of diseases with different pathophysiologies. While patients with tremor may not complain in the clinic of symptoms of imbalance, gait difficulties, or falls, laboratory research studies using quantitative analysis of gait and posture and neurophysiologic techniques have demonstrated impaired gait and balance across a variety of tremor etiologies. These findings have been supported by careful epidemiologic studies assessing symptoms of imbalance. Imaging and neurophysiologic studies have identified cerebellar networks as important mediators of tremor, and therefore a likely common site of dysfunction to explain the phenomenologic overlap between impaired postural and gait control with tremor. Further understanding of these mechanisms and networks is of crucial importance in the development of new treatments, particularly surgical or minimally invasive lesional therapies.

Pathogenesis of Primary Orthostatic Tremor: Current Concepts and Controversies

Background

Orthostatic tremor (OT), a rare and complex movement disorder, is characterized by rapid tremor of both legs and the trunk while standing. These disappear while the patient is either lying down or walking. OT may be idiopathic/primary or it may coexist with several neurological conditions (secondary OT/OT plus). Primary OT remains an enigmatic movement disorder and its pathogenesis and neural correlates are not fully understood.

Methods

A PubMed search was conducted in July 2017 to identify articles for this review.

Results

Structural and functional neuroimaging studies of OT suggest possible alterations in the cerebello-thalamo-cortical network. As with essential tremor, the presence of a central oscillator has been postulated for OT; however, the location of the oscillator within the tremor network remains elusive. Studies have speculated a possible dopaminergic deficit in the pathogenesis of primary OT; however, the evidence in favor of this concept is not particularly robust. There is also limited evidence favoring the concept that primary OT is a neurodegenerative disorder, as a magnetic resonance spectroscopic imaging study revealed significant reduction in cerebral and cerebellar N-acetyl aspartate (NAA) levels, a marker of neuronal compromise or loss.

Discussion

Based on the above, it is clear that the pathogenesis of primary OT still remains unclear. However, the available evidence most strongly favors the existence of a central oscillatory network, and involvement of the cerebellum and its connections.

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

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.

Orthostatic Tremor: An Update on a Rare Entity

Background

Orthostatic tremor (OT) remains among the most intriguing and poorly understood of movement disorders. Compared to Parkinson’s disease or even essential tremor, there are very few articles addressing more basic science issues. In this review, we will discuss the findings of main case series on OT, including data on etiology, pathophysiology, diagnostic approach, treatment strategies, and outcome.

Methods

Data for this review were identified by searching PUBMED (January 1966 to August 2016) for the terms “orthostatic tremor” or “shaky leg syndrome,” which yielded 219 entries. We did not exclude papers on the basis of language, country, or publication date. The electronic database searches were supplemented by articles in the authors’ files that pertained to this topic.

Results

Owing to its rarity, the current understanding of OT is limited and is mostly based on small case series or case reports. Despite this, a growing body of evidence indicates that OT might be a progressive condition that is clinically heterogeneous (primary vs. secondary cases) with a broader spectrum of clinical features, mainly cerebellar signs, and possible cognitive impairment and personality disturbances. Along with this, advanced neuroimaging techniques are now demonstrating distinct anatomical and functional changes, some of which are consistent with neuronal loss.

Discussion

OT might be a family of diseases, unified by the presence of leg tremor, but further characterized by etiological and clinical heterogeneity. More work is needed to understand the pathogenesis of this condition.

Long-term follow-up of chronic spinal cord stimulation for medically intractable orthostatic tremor

Orthostatic tremor (OT) is a rare form of tremor occurring in the legs when standing upright. Medical treatment frequently is unsatisfactory, thus in selected cases, surgical treatment, such as spinal cord stimulation (SCS) or thalamic deep brain stimulation has been proposed. We report the long-term results (follow-up (FU) 34-133 months) of SCS in four patients with medically intractable OT. Outcome was assessed by recording the time tolerated to stand still pre- and post-operatively and by a patient self-rating (PSR) scale (0 = poor to 6 = excellent). Furthermore, surface electromyography (EMG) recordings of different leg muscles were performed to estimate tremor activity with and without SCS post-operatively. With chronic SCS, all four patients showed an improvement of unsteadiness occurring in the presence of stimulation-induced paraesthesia of the legs. The mean standing time improved from 51 s (SD 47 s, range 4-120 s) pre-operatively to 220 s (SD 184 s, range 10-480 s) with SCS at last available FU. Tremor activity in the EMG of the anterior tibial muscle was reduced by 30-60 % with SCS compared with off SCS. PSR score was 4 or 5 in three patients and 3 in the other. In conclusion, SCS is an effective long-term treatment option in patients with otherwise intractable OT.

Orthostatic tremor: current challenges and future prospects

This review provides an outlook of orthostatic tremor (OT), a rare adult-onset tremor characterized by subjective unsteadiness during standing that is relieved by sitting or walking. Recent case series with a long-time follow-up have shown that the disease is slowly progressive, spatially spreads to the upper limbs, and other neurological disorders may develop in about one-third of the patients. The diagnosis of OT hinges on the typical history of unsteadiness during standing, which is confirmed by electromyographic findings of a 13–18 Hz tremor that is typically absent during tonic activation while the patient is sitting and lying. Although the tremor is generated by a central oscillator, cerebellar and/or basal ganglia dysfunction are needed for its manifestation (double lesion hypothesis). However, functional neuroimaging findings have not consistently implicated the dopaminergic system in its pathogenesis. Drug treatments have been largely disappointing with no sustained benefits, although thalamic deep brain stimulation has helped some patients. Large-scale follow-up studies, more drug trials, and novel therapies are urgently needed.

The long-term outcome of orthostatic tremor

OBJECTIVES

Orthostatic tremor is a rare condition characterised by high-frequency tremor that appears on standing. Although the essential clinical features of orthostatic tremor are well established, little is known about the natural progression of the disorder. We report the long-term outcome based on the largest multicentre cohort of patients with orthostatic tremor.

METHODS

Clinical information of 68 patients with clinical and electrophysiological diagnosis of orthostatic tremor and a minimum follow-up of 5 years is presented.

RESULTS

There was a clear female preponderance (76.5%) with a mean age of onset at 54 years. Median follow-up was 6 years (range 5-25). On diagnosis, 86.8% of patients presented with isolated orthostatic tremor and 13.2% had additional neurological features. At follow-up, seven patients who initially had isolated orthostatic tremor later developed further neurological signs. A total 79.4% of patients reported worsening of orthostatic tremor symptoms. These patients had significantly longer symptom duration than those without reported worsening (median 15.5 vs 10.5 years, respectively; p=0.005). There was no change in orthostatic tremor frequency over time. Structural imaging was largely unremarkable and dopaminergic neuroimaging (DaTSCAN) was normal in 18/19 cases. Pharmacological treatments were disappointing. Two patients were treated surgically and showed improvement.

CONCLUSIONS

Orthostatic tremor is a progressive disorder with increased disability although tremor frequency is unchanged over time. In most cases, orthostatic tremor represents an isolated syndrome. Drug treatments are unsatisfactory but surgery may hold promise.

Approach to a tremor patient

Tremors are commonly encountered in clinical practice and are the most common movement disorders seen. It is defined as a rhythmic, involuntary oscillatory movement of a body part around one or more joints. In the majority of the population, tremor tends to be mild. They have varying etiology; hence, classifying them appropriately helps in identifying the underlying cause. Clinically, tremor is classified as occurring at rest or action. They can also be classified based on their frequency, amplitude, and body part involved. Parkinsonian tremor is the most common cause of rest tremor. Essential tremor (ET) and enhanced physiological tremor are the most common causes of action tremor. Isolated head tremor is more likely to be dystonic rather than ET. Isolated voice tremor could be considered to be a spectrum of ET. Psychogenic tremor is not a diagnosis of exclusion; rather, demonstration of various clinical signs is needed to establish the diagnosis. Severity of tremor and response to treatment can be assessed using clinical rating scales as well as using electrophysiological measurements. The treatment of tremor is symptomatic. Medications are effective in half the cases of essential hand tremor and in refractory patients; deep brain stimulation is an alternative therapy. Midline tremors benefit from botulinum toxin injections. It is also the treatment of choice in dystonic tremor and primary writing tremor.

Orthostatic Tremor: A Spectrum of Fast and Slow Frequencies or Distinct Entities?

BACKGROUND

Orthostatic tremor (OT) is defined by the presence of a high-frequency (13-18 Hz) tremor of the legs upon standing associated with a feeling of unsteadiness. However, some patients have discharge frequencies of <13 Hz, so-called "slow OT". The aim of this study was to characterize patients with unsteadiness upon standing found to have <13 Hz tremor discharges on neurophysiologic testing.

METHODS

A retrospective review was performed on all subjects with a diagnosis of OT who were referred to the Mayo Clinic, Scottsdale, AZ, between 1999 and 2013 for confirmation using neurophysiology.

RESULTS

Fourteen of 28 subjects (50%) had OT discharges of <13 Hz, of whom eight had frequencies of <10 Hz and six had frequencies of 10-13 Hz. Lower frequency discharges tended to have a broader spectral peak, greater variability in discharge duration, and lower inter-muscular coherence. Subjects with <13 Hz OT had shorter mean disease duration at time of neurophysiology testing (2.00 years in <10 Hz group, 7.96 years 10-13 Hz group, and 11.43 years >13 Hz; p = 0.002). The proportion of subjects who experienced gait unsteadiness (85.7% vs. 66.6% vs. 21.4%; p = 0.016), falls (37.5% vs. 50% vs. 0%; p = 0.010), and had abnormal gait on examination (71.4% vs. 66.0% vs. 14.3%; p = 0.017) was greater in those with low and intermediate frequencies.

DISCUSSION

Slow tremor electromyography frequencies (<13 Hz) may characterize a substantial proportion of patients labeled as OT. These subjects may have greater gait involvement and higher likelihood of falls leading to earlier presentation to subspecialty care.

Shaking on Standing: A Critical Review

Orthostatic tremor is a rare condition, though its exact prevalence is unknown, which is clinically characterized by a feeling of unsteadiness or being about to fall on standing and which disappears on walking, sitting, or lying down. It is generally accepted that classic orthostatic tremor manifests with a high-frequency tremor (>13 Hz) of the legs when standing. However, a number of patients initially reported as orthostatic tremor did not actually have such electrophysiological features. It is our experience that there is a clinical spectrum of different conditions presenting as shaking on standing, and this highlights the importance of the electrophysiology to aid the differential diagnosis of these disorders. Here, we provide a critical review of the clinical spectrum of shaking on standing, along with demonstrative electrophysiological recordings of some of these conditions.

The central oscillatory network of orthostatic tremor

Orthostatic tremor (OT) is a movement disorder of the legs and trunk that is present in the standing position but typically absent when sitting. The pathological central network involved in orthostatic tremor is still unknown. In this study we analyzed 15 patients with simultaneous high-resolution electroencephalography and electromyography recording to assess corticomuscular coherence. In 1 patient we were able to simultaneously record the local field potential in the ventrolateral thalamus and electroencephalography. Dynamic imaging of coherent source analysis was used to find the sources in the brain that are coherent with the peripheral tremor signal. When standing, the network for the tremor frequency consisted of unilateral activation in the primary motor leg area, supplementary motor area, primary sensory cortex, two prefrontal/premotor sources, thalamus, and cerebellum for the whole 30-second segment recorded. The source coherence dynamics for the primary leg area and the thalamic source signals with the tibialis anterior muscle showed that they were highly coherent for the whole 30 seconds for the contralateral side but markedly decreased after 15 seconds for the ipsilateral side. The source signal and the recorded thalamus signal followed the same time frequency dynamics of coherence in 1 patient. The corticomuscular interaction in OT follows a consistent pattern with an initially bilateral pattern and then a segregated unilateral pattern after 15 seconds. This may add to the feeling of unsteadiness. It also makes the thalamus unlikely as the main source of orthostatic tremor.

Levetiracetam in primary orthostatic tremor: a double-blind placebo-controlled crossover study

BACKGROUND

In a double-blind crossover study we evaluated the antitremor effect of a 4-week treatment with either escalating dosages of levetiracetam or placebo in orthostatic tremor.

METHODS

Twelve patients with orthostatic tremor participated in the study. Primary end point was improvement in stance duration. Secondary end points were total track length of the sway path and tremor total power. The patients' impression of impairment was assessed by a visual analog scale and quality of life by the SF-36.

RESULTS

We found no significant effect of dosage or treatment on stance duration (P = .175), total track length (P = .690), total power (P = .280), or visual analog scale (P =.735). Neither was SF-36 differentially changed by levetiracetam or placebo (SF-36, Physical Component Summary: P = .079; SF-36, Mental Component Summary: P = .073). Side effects like dizziness, fatigue, or nausea were only mild to moderate.

CONCLUSIONS

Levetiracetam is ineffective in the treatment of orthostatic tremor.

Diagnosis and treatment of common forms of tremor

Tremor is the most common movement disorder presenting to an outpatient neurology practice and is defined as a rhythmical, involuntary oscillatory movement of a body part. The authors review the clinical examination, classification, and diagnosis of tremor. The pathophysiology of the more common forms of tremor is outlined, and treatment options are discussed. Essential tremor is characterized primarily by postural and action tremors, may be a neurodegenerative disorder with pathologic changes in the cerebellum, and can be treated with a wide range of pharmacologic and nonpharmacologic methods. Tremor at rest is typical for Parkinson's disease, but may arise independently of a dopaminergic deficit. Enhanced physiologic tremor, intention tremor, and dystonic tremor are discussed. Further differential diagnoses described in this review include drug- or toxin-induced tremor, neuropathic tremor, psychogenic tremor, orthostatic tremor, palatal tremor, tremor in Wilson's disease, and tremor secondary to cerebral lesions, such as Holmes' tremor (midbrain tremor). An individualized approach to treatment of tremor patients is important, taking into account the degree of disability, including social embarrassment, which the tremor causes in the patient's life.

Orthostatic tremor - a review

Orthostatic tremor (OT) is a rare syndrome characterized by unsteadiness on standing due to a high-frequency tremor involving the legs. Symptoms usually start in the sixth decade. Typically, the symptoms rapidly improve on sitting or walking, and the need to sit down or to move can be so strong that patients avoid situations where they have to stand still. A polygraphic recording of a fast and synchronous tremor of the legs, between 13 and 18Hz, is mandatory to confirm the diagnosis of OT. Many patients also suffer from tremor, often involving lower frequencies, of the face, hands, or trunk. Recent studies suggest that this is perhaps due to subharmonics of the high-frequency tremor spreading through the body. Most cases of OT seem to be idiopathic, though symptomatic forms have been occasionally described. Several cases of OT have been reported in Parkinson's disease (PD), either preceding the onset of OT or developing in long-standing PD, suggesting a dopaminergic control of the central oscillator, possibly in the posterior fossa. The response to treatment is often disappointing. Clonazepam is widely used as a first-line agent, but gabapentin and dopaminergic drugs may be helpful in some patients.

The clinical approach to movement disorders

Movement disorders are commonly encountered in the clinic. In this Review, aimed at trainees and general neurologists, we provide a practical step-by-step approach to help clinicians in their 'pattern recognition' of movement disorders, as part of a process that ultimately leads to the diagnosis. The key to success is establishing the phenomenology of the clinical syndrome, which is determined from the specific combination of the dominant movement disorder, other abnormal movements in patients presenting with a mixed movement disorder, and a set of associated neurological and non-neurological abnormalities. Definition of the clinical syndrome in this manner should, in turn, result in a differential diagnosis. Sometimes, simple pattern recognition will suffice and lead directly to the diagnosis, but often ancillary investigations, guided by the dominant movement disorder, are required. We illustrate this diagnostic process for the most common types of movement disorder, namely, akinetic-rigid syndromes and the various types of hyperkinetic disorders (myoclonus, chorea, tics, dystonia and tremor).

Chronic spinal cord stimulation in medically intractable orthostatic tremor

BACKGROUND

Orthostatic tremor with its sense of unsteadiness when standing may have a devastating effect on affected persons. Currently, there are no other treatment options in those who do not respond or who do not tolerate medical treatment.

OBJECTIVES

To report on a pilot study on spinal cord stimulation in medically intractable orthostatic tremor.

METHODS

Chronic spinal cord stimulation (SCS) was performed in two patients with medically-intractable orthostatic tremor via quadripolar plate electrodes implanted at the lower thoracic spine. The electrodes were connected to implantable pulse generators.

RESULTS

Subjective and objective improvement of unsteadiness was achieved within a frequency range of 50 to 150 Hz, and occurred in the presence of stimulation-induced paraesthesia. With optimized stimulation settings polygraphic electromyelogram (EMG) recordings continued to show the typical 14-16 Hz EMG activity. The beneficial effect of SCS was maintained at long-term follow-up.

CONCLUSIONS

The results of this pilot study indicate that SCS may be an option in patients with otherwise intractable orthostatic tremor.

Sway patterns in orthostatic tremor: impairment of postural control mechanisms

Sway parameters in orthostatic tremor (OT) patients were compared with age-matched controls. The effects of vision (eyes open or closed), stance width (feet apart or together), and external support (with or without) on sway and 14-18 Hz energy were measured. Sway in OT patients decreased in the presence of each of the stabilizing factors but the extent of benefit obtained by OT patients was significantly less than controls for support for the sagittal plane (22% vs. 42% decrease; P < 0.01) and feet apart in the mediolateral plane (38% vs. 65% decrease; P < 0.01). Three patients with proprioceptive loss also had a reduced response to these factors. Energy in the 14-18 Hz range did not always change in parallel with sway levels. Vision suppresses activity at the OT frequency in patients. Postural control mechanisms in OT remain responsive to postural conditions, but patients differ significantly from normals in the degree of their responsiveness. OT appears to have disruption of the normal generation or processing of proprioceptive signals as one of its important components.

Clinical and neurophysiologic spectrum of orthostatic tremor: Case series of 26 subjects

Orthostatic tremor (OT) is a condition described as high-frequency tremors predominantly in the legs and trunk, which are present not only in the standing position but also during isometric contraction of the limb muscles. This report is one of the largest OT series describing clinical and neurophysiologic findings in 26 subjects with OT. The main findings included 13.0 to 18.6 Hz leg tremors while standing with varied patterns of phase relationships between the antagonists of the ipsilateral leg and between the homologous muscles of the contralateral leg, short latency tremor onset upon standing with abrupt cessation after sitting, coexistence of tremors in the cranial structures and the arms, and sense of unsteadiness without actual falls. Although the oscillator of OT is most likely located in the brainstem, cerebral cortex, basal ganglia, and cerebellum may also be involved in its pathogenesis.

Natural history and syndromic associations of orthostatic tremor: a review of 41 patients

Orthostatic tremor (OT) is a rare condition characterized by unsteadiness when standing still that is relieved when sitting or walking and is thought to arise from a central generator in the cerebellum or brainstem. OT is considered to be a distinct, discrete condition, and little is known about its demographic characteristics, natural history, associated features, and treatment response. We have reviewed these aspects in 41 OT patients fulfilling current diagnostic criteria, seen at our institution between 1986 and 2001. We classified 31 (75%) as having idiopathic "primary OT" either with (n = 24) or without an associated postural arm tremor. We found that 10 of 41 (25%) cases had additional neurological features, and we defined this group as having "OT plus" syndrome. Of these 10, 6 had parkinsonism; 4 of these had typical Parkinson's disease (PD), 1 had vascular and 1 had drug-induced parkinsonism. Among the remaining 4 patients, 2 had restless legs syndrome (RLS), 1 had tardive dyskinesia, and 1 orofacial dyskinesias of uncertain etiology. One patient with PD and the patient with vascular parkinsonism also had RLS. Age at onset was significantly earlier in the "primary OT" (mean +/- SD, 50.4 +/- 15.1) than in the "OT plus" (61.8 +/- 6.4; z = 2.7; P =.006) group. In 7 of the 10 "OT plus" patients, OT leg symptoms preceded the onset of additional neurological features. OT appeared to be underdiagnosed, and on average, it took 5.7 years from the initial complaints until a diagnosis was made. In general, treatment response to a variety of drugs such as clonazepam, primidone, and levodopa was poor. In most cases, OT symptoms remain relatively unchanged over the years, but in 6 of 41 cases (15%), the condition gradually worsened over the years, and in some of these cases, symptoms spread proximally to involve the trunk and arms. OT may not be a discrete disorder as commonly believed and associated features like parkinsonism present in nearly 25% of cases. Dopaminergic dysfunction may have a role in the pathophysiology of this disorder.

Impact of different stimulation types on orthostatic tremor

OBJECTIVE

Primary orthostatic tremor (OT) is thought to be generated by a unique supraspinal tremor generator. Here we studied the effect of ipsi- and contralateral stimulation of the central and peripheral nervous system on OT.

METHODS

In 7 patients with primary OT, surface EMG was recorded from both tibialis anterior muscles. We performed transcranial magnetic stimulation (TMS) over the vertex, and lumbar magnetic stimulation (LMS) over the lumbar spine. Supramaximal electrical nerve stimuli were applied to the tibial or peroneal nerve at the knee. Proprioceptive input was evoked by rhythmical submaximal stimulation of the tibial, peroneal or sural nerve at the ankle.

RESULTS

TMS reset OT significantly in the contralateral as well as the ipsilateral tibialis anterior muscle. The resetting in both muscles was identical. In contrast, peripheral input by means of LMS, supra- or submaximal nerve stimulation had no impact on OT.

CONCLUSIONS

Transcranial magnetic stimulation of one cortical leg area resets OT in both legs whereas OT is not modified by any peripheral stimuli applied in this study.

SIGNIFICANCE

Our results support the hypothesis of n unique supraspinal OT generator. This generator receives a modulating input from the motor cortex.

Sway patterns in a case of orthostatic tremor responsive to alcohol

We describe changes in the extent of sway in a man with orthostatic tremor (OT) who reported increased stability after alcohol. He was tested at baseline and again after 40 g (0.5 g/kg) of alcohol. These results were compared to those of 3 age-matched controls (no alcohol). The patient's baseline sway was greater than controls, larger in the lateral than the anteroposterior plane, but retained normal responsiveness to the use of external support, increasing stance width, and vision. Tremor frequency significantly decreased after alcohol, as did low- and high-frequency tremor amplitude and the extent of body sway. Despite these findings, sway remained greater than controls. OT thus may show functionally important alcohol responsiveness.

A case of combined orthostatic tremor and primary gait ignition failure

We report a case of a woman with both orthostatic tremor and primary gait ignition failure, a novel combination. We review the literature on both of these conditions, and discuss possible neuroanatomic substrates.

Acoustic startle evokes bilaterally synchronous oscillatory EMG activity in the healthy human

Despite animal evidence that the reticulospinal system is of major importance to movement, this motor pathway has remained relatively inaccessible to experimentation in the human. Consequently, little is known about its function in health and disease. Here, we use the acoustic startle response to demonstrate that one type of reticulospinal activity in the human is associated with a characteristic pattern of bilateral synchronization between motor units. Surface electromyography (EMG) was recorded from upper limb muscles in 15 healthy subjects during the reflex startle to unexpected acoustic stimulation, voluntary movements mimicking the startle and during sustained voluntary tonic contraction. Frequency analysis demonstrated autospectral peaks at approximately 14 Hz in deltoid and biceps muscles only during the startle reflex. Similarly, coherence spectra of the EMG recorded between homologous proximal upper limb muscles demonstrated a peak centered approximately 12-16 Hz during reflex startles. Coherence in the 10- to 20-Hz band was significantly greater in the startle reflex than during voluntary sham startles or voluntary tonic contraction for deltoid, but not first dorsal interosseous, muscles. The coherence at 10-20 Hz between EMGs from homologous muscles represents a potential surrogate measure of reticulospinal activity that may be useful in determining the contribution of the reticulospinal system to different types of movement in health and disease.

[123I]-FP-CIT-SPECT demonstrates dopaminergic deficit in orthostatic tremor

There is increasing evidence of a potential role of the dopaminergic system in orthostatic tremor (OT): Association with parkinsonism and treatment effects of L-dopa and dopamine agonists have been reported. Eleven patients with isolated OT had single-photon emission computed tomography (SPECT) using (123)I-FP-CIT ([(123)I]-2 beta-carbomethoxy-3beta-(-4-iodophenyl)-N-(3-fluoropropyl)-nortropane) as dopamine transporter tracer. Results were compared with 12 age-matched normal controls and 12 patients with Parkinson's disease (PD). A marked reduction in striatal tracer binding was found in OT compared to normal controls (p < 0.001). Tracer uptake was significantly higher and more symmetrical than in PD, and caudate and putamen were equally affected. L-dopa challenges, performed in seven patients, showed a small but non-significant improvement on EMG and a small but significant improvement in clinical parameters on blinded video rating. Two-month open-label L-dopa treatment (600 mg/day) led to a small improvement in two of five patients but no significant overall change. Olfactory function on University of Pennsylvania Smell Identification Test was normal. Our finding of a marked tracer uptake reduction on dopamine transporter SPECT supports a role of the dopaminergic system in OT. Lack of evidence of a clinically relevant therapeutic response to L-dopa suggests that other mechanisms must also be involved in the pathogenesis.

Primary orthostatic tremor associated with a persistent cerebrospinal fluid monoclonal IgG band

Primary orthostatic tremor is of unknown aetiology and is believed to be a distinct entity rather than a subtype of essential tremor. We describe the first patient with a typical phenotype of primary orthostatic tremor who has a persistent isolated monoclonal immunoglobulin G band in the cerebrospinal fluid.

Primary orthostatic tremor is an exaggeration of a physiological response to instability

Primary orthostatic tremor (POT) is a rare disorder characterised by an intense sense of unsteadiness upon standing and a 16-Hz tremor in which the timing between tremor bursts in different muscles (unilateral and bilateral) remains constant. Hitherto, similar EMG activity has not been described in healthy subjects and it has been postulated that the oscillations seen in POT are primarily pathological. In this study, EMG was recorded from tibialis anterior in healthy subjects who were made unsteady through vestibular galvanic stimulation or leaning backwards. Under these conditions, a peak at approximately 16 Hz was seen in the coherence between the left and right tibialis anterior. This bilateral coherence was absent when the subjects activated the same muscles when not unsteady. These data indicate the existence of a physiological system involved in organising postural responses under circumstances of imbalance and characterised by a highly synchronised output at approximately 16 Hz. In addition, the results suggest that the core abnormality in POT may be an exaggerated sense of unsteadiness when standing still, which then elicits activity from a 16-Hz oscillator normally engaged in postural responses.

Bilaterally coherent tremor resembling enhanced physiological tremor: report of three cases

The contribution of the central nervous system to tremor pathogenesis is unclear. Poor side-to-side coherence in physiological, essential, and parkinsonian tremors suggests distinct bilateral generators. By contrast, significant bilateral coherence demonstrated in orthostatic tremor and in enhanced physiological tremor (EPT) in patients with persistent mirror movements favours single or closely linked bilateral oscillators. We describe three patients (aged 21-37 years) who developed unusual bilateral postural and kinetic tremors at 6-13 Hz resembling EPT. The tremor involved all limbs, and in two cases the face or jaw, in the absence of other significant neurological features. Significant side-to-side coherence was demonstrated in each case using cross-correlation of electromyographic recordings from homologous muscle pairs. We postulate that these unusual tremors originate from a single brainstem source or from bilateral oscillators closely linked at or below this level.

Phenomenology and treatment of tremor disorders

Tremor is classified according to anatomic distribution among body parts, and by frequency and amplitude during rest, postural maintenance, movement, intention, and the performance of specific tasks. Key historical features include age at onset, progression over time, family history, exacerbating and remitting factors and behaviors, response to alcohol and medications, and additional neurological signs and symptoms. Accurate diagnosis is a critical factor in predicting the natural history and response to treatment.

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.

Orthostatic tremor arises from an oscillator in the posterior fossa

We tested the hypotheses that orthostatic tremor is generated by a central oscillator and that the tremor is expressed through spinal Ib interneurons. Six patients with orthostatic tremor were examined. The tremor was reset by electrical stimulation over the posterior fossa at intensities that were below the threshold for a motor evoked potential (MEP) but was not reset by transcranial magnetic stimulation over the motor cortex that did produce an MEP. It is argued that the oscillator involves the cerebellum or brainstem. The inhibition of voluntary EMG produced by stimulation over tendons, which has been attributed to effects from Golgi tendon organs (GTO), was not modulated in synchrony with the tremor. We were unable to demonstrate, therefore, that the tremor is expressed through GTO interneurons with this method.

A dissociation between subjective and objective unsteadiness in primary orthostatic tremor

Patients with primary orthostatic tremor (OT) experience a disabling sense of unsteadiness but rarely fall. In order to study the relationship between the development of subjective unsteadiness, objective unsteadiness and tremor, we recorded standing under four conditions (eyes open or closed, feet together or apart) in six patients with OT. Subjective unsteadiness was indicated by the patients on a four-point scale using a hand-held slider. Objective unsteadiness was assessed by measuring the path lengths of the centre of foot pressure and body motion at the level of the cervical spine. Tremor was measured by surface electromyography from leg and paraspinal muscles. OT patients were objectively more unsteady than controls. Objective unsteadiness also increased disproportionately in patients when standing with eyes closed. These findings suggest that balance control in OT is abnormal and shows increased visual dependence. Subjective unsteadiness increased from mild to severe over seconds to minutes. The increase was faster when standing with eyes closed or feet together. However, although escalating subjective unsteadiness was paralleled by an increase in leg tremor, there were no comparable changes in either paraspinal tremor or objective unsteadiness during the course of a stand. We conclude that there is a dissociation between subjective and objective unsteadiness. This implies that subjective unsteadiness does not arise simply from an awareness of increased body sway. We postulate that the sensation of unsteadiness arises from a tremulous disruption of proprioceptive afferent activity from the legs. This disturbance gives rise to increased co-contracting drive to the leg muscles in order to stiffen the joints and increase stability. Since muscle activity remains tremor-locked, the tremulous proprioceptive feedback is increased, which then further increases the sensation of unsteadiness, and so on in a vicious circle of escalating activity.

[Neurophysiologic study of tremor]

Tremor is frequently encountered in neurologic practice. Clinical examination supplies information regarding its approximative frequency, regularity, amplitude, topography and activation conditions. The neurophysiological study helps in confirming the tremor, in differentiating it from other movement disorders like myoclonus, and may provide distinctive features which are important for the aetiological diagnosis. The neurophysiological investigation includes accelerometry, which analyses the mechanics of the movement. Spectral analysis with FFT allows the determination of frequency and amplitude. Accelerometry is always associated with surface EMG of at least two antagonistic muscles. It may show rhythmic bursts, their frequency, duration and activation pattern (alternating or synchronous). This neurophysiological approach to tremor has multiple interests. It may help in differentiating a true rest tremor from a postural tremor seemingly persisting at rest. Brief interruptions or rhythm breaks during distraction manoeuvers are seen in psychogenic tremors. Surface EMG may also demonstrate positive myoclonic bursts, or brief silent periods corresponding to negative myoclonus, sometimes pseudorhythmic, thus appearing clinically like a tremor, but investigations, aetiologies and treatment are different. Several features, especially slow frequency, may suggest a midbrain tremor, thus requiring brain imagery centered around the posterior fossa. Finally, the neurophysiological examination is the only way to demonstrate a primary writing tremor, or a primary orthostatic tremor, the frequency of which is pathognomonic while clinical symptoms are rather misleading.

The timing of primary orthostatic tremor bursts has a task-specific plasticity

Primary orthostatic tremor is characterized by unsteadiness and shakiness of the legs while standing. It is due to a remarkably strong and regular EMG modulation at approximately 16 Hz that is thought to be of CNS origin. Previous studies have shown that the tremor frequency is the same in all involved muscles and that the time relation between bursts of activity in different muscles may be fixed (e.g. always co-contracting or always contracting in an alternating pattern). Here we have used frequency domain analysis of postural muscle EMG signals in five primary orthostatic tremor patients and in two normal controls to explore the nature of such fixed timing patterns. The timing is found not to relate simply to the relative conduction times for passage of rhythmic bursts from a central oscillation to different muscles. Indeed, although the timing pattern (expressed as phase) of the 16-Hz EMG bursts in different postural muscles remains constant while the subject adopts a certain steady posture, it is different for different subjects and also changes when the same subject adopts a different posture. It seems unlikely that such complex task-dependent timing relations of rhythmic postural muscle activity are due to the primary pathology of primary orthostatic tremor. Instead, we suggest that the abnormally strong peripheral manifestation of a 16-Hz CNS oscillation merely unmasks normal central processes so that the timing patterns may provide a clue to the nature of postural motor control.

Side-to-side correlation of muscle activity in physiological and pathological human tremors

OBJECTIVE

Many tremors occur always or often bilaterally. The question arises whether this could be explained by a common source or commonly transmitting pathways or by bilaterally represented, independent structures with the same oscillatory properties. A similar tremor frequency does not provide sufficient information to clarify this question.

METHODS

We analyze coherencies between surface electromyographies (EMG) to investigate if bilateral physiologic (PT), essential (ET), Parkinsonian (PD) and orthostatic (OT) tremors originate from a common source for both sides of the body. We show that commonly used techniques to test whether coherencies are significant could lead to false positive results for tremor EMGs. A new estimation procedure is proposed to test EMG tremor time series on their linear independence. We apply this test to bilateral tremors.

RESULTS

All measured EMG-pairs in OT (n = 7) were highly coherent between both sides with reproducible coherency values of up to 0.99. All other investigated tremors, i.e. PT and enhanced physiological tremors (EPT, n = 117), ET (n = 76) and PD resting and postural tremors (n = 70) do not show a significant side-to-side correlation.

CONCLUSIONS

This finding shows that the pathophysiologies of OT and other pathological tremors are definitely different. Either they have different origins or different kinds of transmitting pathways. The proposed method might also be used to investigate other electrophysiological data and is a helpful, easy to use investigation for a daily clinical routine.