Factor analysis of motor and nonmotor signs in essential tremor: are these signs all part of the same underlying pathogenic process?

Essential tremor plus rest tremor: current concepts and controversies

Since the initial description of Essential Tremor (ET), the entity of ET with rest tremor has proven to be a controversial concept. Some authors argued it could be a late manifestation of ET, others suggested it could be a variant of ET, yet others suggested it could represent a transitional state between ET and Parkinson's disease. The novel tremor classification has proposed the construct of ET-plus to differentiate patients with rest tremor from pure ET. However, there is no clarity of what ET-plus rest tremor represents. With the aim of shedding light on this controversial entity, we have, therefore, systematically reviewed all clinical, electrophysiological, imaging and anatomopathological studies indexed in the Medline database published both before and after the new tremor classification and involving patients with ET-plus rest tremor. Forty-four studies involving 4028 patients were included in this review and analyzed in detail by means of descriptive statistics. The results of the current review suggest that ET-plus rest tremor is a heterogenous group of conditions: thus, rest tremor might represent a late feature of ET, might reflect a different disorder with higher age at onset and lower dependance on genetic susceptibility than ET, might suggest the development of Parkinson's disease or might indicate a misdiagnosis of ET. The reviewed lines of evidence refuse recent claims arguing against the construct of ET-plus, which should be viewed as a syndrome with different possible underpinnings, and highlights methodological issues to be solved in future research.

Essential tremor: from bedside to bench and back to bedside

PURPOSE OF REVIEW

The last several years have witnessed a remarkable increase in research on essential tremor, with consequent advances in our understanding of this entity. An attempt to both summarize and frame this work has not been undertaken.

RECENT FINDINGS

Here, I show that observations on essential tremor arising from clinical practice/clinical studies have guided scientific studies of this disorder. In turn, the results of scientific studies are beginning to be translated back to the bedside to improve treatment. Recent essential tremor research has given rise to several novel and intriguing ideas about the disease. These include the following: essential tremor may represent a family of diseases rather than a single disease; essential tremor seems to be a disease of the cerebellum or cerebellar system; essential tremor may be neurodegenerative; low gamma aminobutyric acid tone seems to be a central feature of essential tremor. As with many emerging ideas, there is significant discussion and debate over these emerging ideas, and this fuels additional scientific studies.

SUMMARY

The flow of ideas from clinical observations about essential tremor, to their translation into scientific studies, and their translation back to the bedside, is expected to eventually lead to improvements at the patient interface.

Medication non-adherence in essential tremor

BACKGROUND

There are numerous studies of medication adherence in a variety of chronic diseases including Parkinson's disease; however, there are no such studies in patients with essential tremor (ET). This study aimed to (1) present self-report data on medication adherence in ET cases, (2) examine the demographic and clinical factors that might be associated with lower medication adherence.

METHODS

151 ET cases were enrolled in a clinical-epidemiological study at Columbia University. An 11-item medication adherence questionnaire, modeled after the Morisky medication adherence questionnaire, was administered.

RESULTS

Seventy-three (48.3%) of 151 cases were taking daily medication for ET. One-third (24/73; 32.9%) of cases reported that they sometimes forgot to take their medication, and 1 in 5 (15/73; 20.5%) reported missed doses within the past week. Most striking was that nearly 1 in 4 (17/73; 23.3%) reported that there were whole days in the past two weeks in which they had not taken their medication. A factor analysis revealed four factors that captured different aspects of non-adherence. Higher non-adherence was associated with more depressive symptoms, younger age, and less severe tremor but was not associated with type or number of ET medications.

CONCLUSIONS

Approximately one in four ET patients reported whole days in the past two weeks in which they had not taken their medication. It is possible that this relatively high rate of non-adherence could be a function of the poor therapeutic efficacy of the medications currently available to treat ET.

'Essential tremor' or 'the essential tremors': is this one disease or a family of diseases?

There is accumulating evidence that the entity referred to as 'essential tremor' (ET) is not a single disease. It may be a family of diseases better referred to as 'the ETs'. This review will summarize the following evidence: (1) the presence of etiological heterogeneity; (2) the heterogeneity of findings in postmortem studies, thus suggesting several diseases; (3) the recent discussion that age of onset may be an important marker of disease heterogeneity; (4) the clinical expansion of the concept of ET in recent years to include a broader range of tremor phenomenology, other motor features (gait ataxia), other involuntary movements (dystonia), and nonmotor features (cognitive problems, psychiatric problems), some of which could be primary; (5) the heterogeneity of pharmacological response profiles and clinical progression, and (6) the association of ET with Parkinson's disease, Alzheimer's disease, and possibly progressive supranuclear palsy, with the possibility that some ET patients are more predisposed to develop one of these. © 2013 S. Karger AG, Basel.

What is essential tremor?

Classic essential tremor is a clinical syndrome of action tremor in the upper limbs (at least 95 % of patients) and less commonly the head, face/jaw, voice, tongue, trunk, and lower limbs, in the absence of other neurologic signs. However, the longstanding notion that essential tremor is a monosymptomatic tremor disorder is being challenged by a growing literature describing associated disturbances of tandem walking, personality, mood, hearing, and cognition. There is also epidemiologic, pathologic, and genetic evidence that essential tremor is pathophysiologically heterogeneous. Misdiagnosis of essential tremor is common because clinicians frequently overlook other neurologic signs and because action tremor in the hands is caused by many conditions, including dystonia, Parkinson disease, and drug-induced tremor. Thus, essential tremor is nothing more than a syndrome of idiopathic tremulousness, and the challenge for researchers and clinicians is to find specific etiologies of this syndrome.

The balance and gait disorder of essential tremor: what does this mean for patients?

Essential tremor (ET), once considered a monosymptomatic disease, is now viewed as a syndrome characterized by action tremor as well as a variety of other neurological features. These features include deficits of both balance (i.e. the ability to maintain the body within its base of support) and gait. Balance and gait abnormalities in ET have been studied most extensively in the laboratory, with the disability and burden on life, as experienced by patients with ET, being a subject of relatively less scrutiny. In this paper, we review the data on balance and gait impairment in ET. These include data both from objective assessments (bedside examination or laboratory gait analysis) and subjective assessments (patients' reports of imbalance). We discuss the clinical implications of the current data, and suggest a tiered approach to begin to address the still unanswered questions in this field. This tiered approach starts with an attempt to isolate a subpopulation of patients who are at greatest risk of difficulty with balance and gait, and then later focuses on this subgroup to assess the burden of any balance and gait impairments as well as the impact of potential therapeutic interventions.

Cognitive motor interference during dual-task gait in essential tremor

BACKGROUND

Functional ambulation requires concurrent performance of motor and cognitive tasks, which may create interference (degraded performance) in either or both tasks. People with essential tremor (ET) demonstrate impairments in gait and cognitive function. In this study we examined the extent of interference between gait and cognition in people with ET and controls during dual-task gait.

METHODS

We tested 62 controls and 151 ET participants (age range: 72-102). ET participants were divided into two groups based on median score on the modified Mini Mental State Examination. Participants walked at their preferred speed, and performed a verbal fluency task while walking. We analyzed gait velocity, cadence, stride length, double support time, stride time, step width, step time difference, coefficient of variation (CV) of stride time and stride length.

RESULTS

Verbal fluency performance during gait was similar across groups (p=0.68). Velocity, cadence and stride length were lowest whereas step time difference (p=0.003), double support time (p=0.009), stride time (p=0.002) and stride time CV (p=0.007) were highest for ET participants with lower cognitive scores (ETp-LCS), compared with ET participants with higher cognitive scores (ETp-HCS) and controls. ETp-LCS demonstrated greatest interference for double support time (p=0.005), step time difference (p=0.013) and stride time coefficient of variation (p=0.03).

CONCLUSIONS

ETp-LCS demonstrated high levels of cognitive motor interference. Gait impairments during complex tasks may increase risk for falls for this subgroup and underscore the importance of clinical assessment of gait under simple and dual-task conditions.

Non-motor features in essential tremor

INTRODUCTION

Essential tremor (ET) is increasingly recognized to have several non-motor manifestations. The aim of this study was to determine the prevalence of non-motor manifestations in ET and its impact on the quality of life (QOL).

METHODS

This was a cross-sectional case-control questionnaire-based study. The subjects were 50 patients with ET and 50 matched healthy controls. All subjects were assessed by Pittsburgh Sleep Quality Index, Epworth Sleepiness Scale, Parkinson Fatigue Scale, Brief Pain Inventory, Hamilton Anxiety Rating Scale, and Hamilton Depression Rating Scale. In addition, QOL in Essential Tremor questionnaire was administered to patients with ET.

RESULTS

Patients with ET, when compared with controls, had significantly higher prevalence and higher mean scores of sleep disturbances (46% vs 8%, P < 0.001; 5.9 ± 4.6 vs 2.6 ± 2.3, P < 0.001), fatigue (30% vs 8%, P = 0.009; 5.8 ± 0.8 vs 2.5 ± 0.4, P < 0.001), anxiety (66% vs 18%, P = 0.009; 7.4 ± 9.0 vs 0.7 ± 2.6, P < 0.001), depression (44% vs 8%, P = 0.009; 7.8 ± 7.9 vs 1.7 ± 3.3, P < 0.001) as well as higher mean score of pain severity (1.9 ± 2.3 vs 0.6 ± 1.2, P = 0.001) and interference owing to pain (2.0 ± 2.9 vs 0.5 ± 1.2, P = 0.001). Following hierarchical regression analysis, depression was the only non-motor feature that affected the QOL.

CONCLUSION

There was a significantly higher prevalence and greater severity of sleep disturbances, fatigue, pain, anxiety, and depression in patients with ET and depression significantly affected the QOL.

Diffusion tensor imaging: tract based spatial statistics study in essential tremor

INTRODUCTION

Essential tremor (ET) is a common movement disorder with motor and non-motor symptoms. We aimed to investigate the neurodegenerative changes in the brain white matter of patients with ET using Diffusion Tensor Imaging (DTI).

METHODS

Clinical and MRI data from 20 patients (5 women and 15 men; age-38.2 ± 16.5 yrs) with ET and 17 controls (3 women and 14 men; age-40.7 ± 16.5 yrs) were collected prospectively. The DTI data were analyzed using tract based spatial statistics (TBSS) software for tract wise analysis. Further region of interest (ROI) analysis was carried out in the genu of corpus callosum, anterior limb of internal capsule (ALIC), corticospinal tract (CS), and cerebellar peduncles. Effect of tremor severity, disease duration and age of onset on DTI metrics was also studied.

RESULTS

Patients with ET in comparison to controls showed significant (P(corrected) < 0.05) increase of mean diffusivity and radial diffusivity in right frontoparietal white matter. Axial diffusivity increase was seen in bilateral cerebral hemispheres, thalamus, brainstem and cerebellar hemisphere white matter. No significant change in fractional anisotropy of the white matter was seen. ROI analysis also revealed abnormalities in the ALIC and cerebellar peduncles. There was no correlation between the severity of white matter changes and clinical tremor severity score as well as disease duration.

CONCLUSIONS

This study provides in vivo evidence for axonal disintegration of the cerebral and cerebellar white matter fibres in patients with ET.

Association between essential tremor and other neurodegenerative diseases: what is the epidemiological evidence?

BACKGROUND

The possible links between essential tremor (ET) and Parkinson's disease (PD) and, more recently, between ET and Alzheimer's disease (AD) have been of great interest, particularly with the emergence of postmortem data suggesting that ET itself may be a neurodegenerative disease. Given the very high population prevalence of ET, its possible role as a risk factor for PD and AD is of special significance. At the center of this debate is the burgeoning evidence from epidemiological studies, examining whether there is an increased co-occurrence of these conditions within the same individuals or within families.

METHODS

We conducted a PubMed search from 1966 to March 2011 and reviewed the epidemiological evidence, restricting our analyses to studies that generated quantifiable measures of association (odds ratios or relative risks), thereby being able to assess the role of chance.

RESULTS

The most robust evidence, from case-control, prospective and familial aggregation studies, indicates that ET is associated with increased odds and increased risks of both PD and AD.

CONCLUSIONS

There is reasonable epidemiological evidence to support a link between ET and these neurodegenerative diseases. Further studies are needed to corroborate the current results, provide additional estimates of effect and begin to explore the mechanistic underpinnings for these intriguing associations.