Essential tremor in Rochester, Minnesota: a 45-year study

The NEDICES Study: Recent Advances in the Understanding of the Epidemiology of Essential Tremor

BACKGROUND

Essential tremor (ET) is the most common tremor disorder. ET has classically been viewed as a benign monosymptomatic condition. Yet over the past 10 years, a growing body of evidence indicates that this is a progressive condition that is clinically heterogeneous, and may be associated with a variety of different features. Large epidemiological studies such as the Neurological Disorders of Central Spain (NEDICES), a longitudinal, population-based survey, have contributed significantly to the changing view of the disease. Our aim is to review some of the main results of NEDICES within the larger framework of the epidemiology of ET.

METHODS

Data for this review were gathered from all our articles published up to October 2011 regarding NEDICES study and "Essential Tremor".

RESULTS

We have published 18 articles up to October 2011. The prevalence, incidence, and mortality of ET were analyzed in this cohort. In addition, ET was found to be associated with increased frailty and low morale, as well as with a series of non-motor manifestations, including cognitive deficits, mild cognitive impairment, dementia, depressive symptoms, and hearing impairment. Finally, the link between ET and Parkinson's disease (PD) was formally quantified in the NEDICES study, which demonstrated that the risk of developing incident PD was 4.3 times higher in prevalent ET cases than in age-matched controls without ET.

CONCLUSIONS

This review highlights the contributions of NEDICES towards the advancement of current knowledge of the epidemiology and clinical features of ET, and emphasizes the importance of population-based studies towards the understanding of complex, ageing-related diseases.

Essential tremor is not a neurodegenerative disease

The pathophysiology of essential tremor (ET) remains unknown. Standard neuropathological studies have reported no consistent changes but a detailed study found neurodegeneration in all ET cases - 24% demonstrated lower brainstem Lewy body (LB) inclusions and 76% experienced a loss of cerebellar Purkinje cells (PCs) and its sequelae. We review the evidence on neurodegeneration in ET. The prevalence of LB inclusions in ET brains is similar to that in the asymptomatic general population. These incidental LB disease cases have evidence for reduced striatal tyrosine hydroxylase levels, as found in Parkinson's disease, but there is no evidence for reduced tyrosine hydroxylase levels in ET patients. Reduced mean PC counts in ET cases compared with the controls reported by some studies could not be replicated by others. Most ET cases have the same number of PCs as controls of a comparable age. Neither the lower brainstem LB inclusions nor the cerebellar PC loss represent the neurodegenerative basis of ET. Further studies are needed to determine the pathophysiology of ET.

IgM-monoclonal gammopathy neuropathy and tremor: a first epidemiologic case control study

INTRODUCTION

Small case series suggest tremor occurs frequently in IgM-monoclonal gammopathy of undetermined significance (IgM-MGUS) neuropathy. Epidemiologic study to confirm this association is lacking. Whether the neuropathy or another remote IgM-effect is causal remains unsettled.

MATERIALS AND METHODS

An IgM-MGUS neuropathy case cohort (n = 207) was compared to age, gender, and neuropathy impairment score (NIS) matched, other-cause neuropathy controls (n = 414). Tremor details were extracted from structured neurologic evaluation. All patients underwent nerve conductions.

RESULTS

Tremor occurrence was significantly higher in IgM-MGUS case cohort (29%) than in control cohort (9.2%) (p = 0.001). In IgM-MGUS cases, tremor was associated with worse NIS (p = 0.025) and demyelinating nerve conductions (p = 0.020), but 11 of 60 (18%) IgM-MGUS cases with tremor had axonal neuropathy. In other-cause neuropathy controls, tremor was associated with axonal nerve conductions (p = 0.03) but not with NIS severity (p = 0.57). Tremor occurrence associated with older age in controls, (p = 0.004) but not in IgM-MGUS cases (p = 0.272). Most IgM-MGUS tremor cases (49/60) had a postural-kinetic tremor, 8 had rest tremor, 3 had mixed rest-action. Alternative causes of tremor was identified in 42% of IgM-MGUS cases, the most common type is inherited essential tremor 6/60 (p = 0.04).

CONCLUSIONS

This first epidemiologic case-control study validates association between IgM-MGUS neuropathy and tremor. Among IgM-MGUS neuropathy cases, severity as well as type of neuropathy (demyelinating over axonal) correlated with tremor occurrence. IgM-MGUS paraproteinemia may increase tremor expression in persons recognized with common other risk factors for tremor.

Epidemiology and etiology of Parkinson's disease: a review of the evidence

The etiology of Parkinson's disease (PD) is not well understood but likely to involve both genetic and environmental factors. Incidence and prevalence estimates vary to a large extent-at least partly due to methodological differences between studies-but are consistently higher in men than in women. Several genes that cause familial as well as sporadic PD have been identified and familial aggregation studies support a genetic component. Despite a vast literature on lifestyle and environmental possible risk or protection factors, consistent findings are few. There is compelling evidence for protective effects of smoking and coffee, but the biologic mechanisms for these possibly causal relations are poorly understood. Uric acid also seems to be associated with lower PD risk. Evidence that one or several pesticides increase PD risk is suggestive but further research is needed to identify specific compounds that may play a causal role. Evidence is limited on the role of metals, other chemicals and magnetic fields. Important methodological limitations include crude classification of exposure, low frequency and intensity of exposure, inadequate sample size, potential for confounding, retrospective study designs and lack of consistent diagnostic criteria for PD. Studies that assessed possible shared etiological components between PD and other diseases show that REM sleep behavior disorder and mental illness increase PD risk and that PD patients have lower cancer risk, but methodological concerns exist. Future epidemiologic studies of PD should be large, include detailed quantifications of exposure, and collect information on environmental exposures as well as genetic polymorphisms.

[Neuropsychiatric disturbances in essential tremor]

This review focuses on the neuropsychiatric manifestations (personality disturbances, depressive symptoms, cognitive impairment and dementia), which have been described in this last decade in patients with essential tremor. We compared the data derived from the Neurologic Disorders in Central Spain (NEDICES) study, a prospective population-based survey, with those derived from the literature. The traditional view of essential tremor as a mono-symptomatic condition characterized by action tremor is now changing. First, it is known that many patients also have other motor manifestations apart from tremor (e.g., ataxic gait). Second, in the last years, the presence of a variety of non-motor neuropsychiatric features has been described. Mild cognitive changes (especially executive dysfunction) have been documented in several independent studies. Further, two population-based studies have demonstrated an association between essential tremor with mild cognitive impairment and dementia. Psychiatric manifestations include specific personality traits, anxiety, social phobia, and depressive symptoms. Taking together, the neuropsychiatric dysfunction pattern and the new data on neuropathology of essential tremor suggest that this disease is a neurodegenerative cerebellar disorder and let us to question the classical concept of essential tremor as a benign mono-symptomatic disorder.

It is time to remove the 'benign' from the essential tremor label

In recent years, studies of essential tremor (ET) have demonstrated that the disease is associated with functionally-relevant cognitive abnormalities, a mood disturbance and other psychiatric co-morbidities, a functionally significant gait disorder, hearing deficits, and a variety of types of tremor. The tremor has been shown to be progressive in nature and quite disabling for a large number of sufferers. Also, recent clinical-epidemiological studies have linked prevalent ET to other neurodegenerative diseases and at least one study has demonstrated an increased risk of mortality in an ET cohort. While many of these studies are derived from clinic-based samples, population-based studies have also substantiated these findings, suggesting that even in the general population, the disease is associated with disability and with co-morbidity. Based on these available studies, it would seem inaccurate to append the word "benign" to ET. Not unlike the word "essential" itself, the word "benign" is an antiquated and outdated term that reflects an era where little was understood about ET. Historically, the general doctors who frequently encountered the disorder would educate patients that it was a benign, non-progressive condition not associated with any co-morbidity or risk of long-term worry. This notion, conceived by prior generations of physicians, is now known to be inaccurate. It is therefore our recommendation that the medical community open the dialog to consider formally discontinuing this nosology ("benign essential tremor") and to adopt the use of the term "essential tremor." Use of the word "benign" is a mischaracterization of the disease course, and could be prove misleading especially in the evolving doctor-patient relationship. In those cases with currently-mild tremor, the nosology "mild essential tremor" would be a more accurate reflection of disease.

Essential tremor

Essential tremor (ET) is among the most common neurological disorders. This chapter reviews the epidemiology, clinical features, and pathophysiology of ET. The defining feature is a kinetic tremor of the arms. Patients often have a postural tremor as well. Other body regions may also be involved (especially the head). The severity of tremor may range from mild cases in population settings to more severe cases in treatment settings. Motor features aside from tremor have been described in ET, including tandem gait difficulty. Mild cognitive changes (especially executive dysfunction) have been documented in many studies as well. Despite being regarded as one of the most common hyperkinetic movement disorders, establishing a precise prevalence has been difficult, yet the prevalence among persons aged 40 and older seems to be 4% or higher. There are numerous examples of families in which the disease appears to be inherited yet genetic studies have not progressed to the point where ET genes have been identified. There is also a growing understanding that environmental factors are likely to contribute to the etiology of ET. More recent postmortem studies have helped localize the possible source of ET to structural alterations in the cerebellum and its connecting pathways.

Tremor in hemifacial spasm patients

Tremor often accompanies dystonia and some other movement disorders. There are no reports describing tremor in patients with hemifacial spasm (HFS). The aim of the study was to assess the occurrence, type and parameters of tremor in HFS patients. The study included 47 HFS patients (mean age: 58.6 ± 10.7-years-old) and 48 healthy volunteers matched by sex and age. Tremor was assessed clinically and objectively using a triaxial accelerometer and EMG. A load test with 500 g weight was performed in order to identify physiological tremor. The Clinical Global Impression-Severity scale (CGI-S) was used to assess the severity of HFS. Neurological examination revealed tremor in 19 patients and 5 control subjects, and the objective methods postural and kinetic hand tremor was confirmed. Among the patients we identified: one case of psychogenic tremor, two patients were diagnosed with essential tremor, six patients had essential tremor in one hand and enhanced physiologic tremor in the other, seven patients had essential tremor in both hands and three patients had physiological tremor in both hands. Five control subjects revealed enhanced the physiological tremor type. The occurrence of hand tremor was associated with higher severity of HFS, as assessed by CGI score. Tremor accompanies HFS in 40% of cases and 10% of control subjects. The aetiology of hand tremor in patients with HFS is unknown. However, it may be associated with compression of the tremor oscillator located in the medulla.

Overview of essential tremor

Essential tremor (ET) is one of the most common movement disorders in the world. Despite this, only one medication (propranolol) is approved by the Food and Drug Administration (FDA) to treat it. Fortunately, recent studies have identified some additional medications as treatment of ET. Surgical procedures, such as deep brain stimulation of the ventral intermediate nucleus of the thalamus, offer treatment for refractory tremor. The epidemiology, pathogenesis, and medical and surgical treatment of ET will be discussed in this paper.

Beta-carboline alkaloids and essential tremor: exploring the environmental determinants of one of the most prevalent neurological diseases

Essential tremor (ET) is among the most prevalent neurological diseases, yet its etiology is not well understood. Susceptibility genotypes undoubtedly underlie many ET cases, although no genes have been identified thus far. Environmental factors are also likely to contribute to the etiology of ET. Harmane (1-methyl-9H-pyrido[3,4-beta]indole) is a potent, tremor-producing beta-carboline alkaloid, and emerging literature has provided initial links between this neurotoxin and ET. In this report, we review this literature. Two studies, both in New York, have demonstrated higher blood harmane levels in ET cases than controls and, in one study, especially high levels in familial ET cases. Replication studies of populations outside of New York and studies of brain harmane levels in ET have yet to be undertaken. A small number of studies have explored several of the biological correlates of exposure to harmane in ET patients. Studies of the mechanisms of this putative elevation of harmane in ET have explored the role of increased dietary consumption, finding weak evidence of increased exogenous intake in male ET cases, and other studies have found initial evidence that the elevated harmane in ET might be due to a hereditarily reduced capacity to metabolize harmane to harmine (7-methoxy-1-methyl-9H-pyrido[3,4-beta]-indole). Studies of harmane and its possible association with ET have been intriguing. Additional studies are needed to establish more definitively whether these toxic exposures are associated with ET and are of etiological importance.

Clustering of dystonia in some pedigrees with autosomal dominant essential tremor suggests the existence of a distinct subtype of essential tremor

BACKGROUND

There is an ongoing debate whether essential tremor (ET) represents a monosymptomatic disorder or other neurologic symptoms are compatible with the diagnosis of ET. Many patients with clinically definite ET develop dystonia. It remains unknown whether tremor associated with dystonia represent a subtype of ET. We hypothesized that ET with dystonia represents a distinct subtype of ET.

METHODS

We studied patients diagnosed with familial ET and dystonia. We included only those patients whose first-degree relatives met diagnostic criteria for ET or dystonia with tremor. This cohort was ascertained for the presence of focal, segmental, multifocal, hemidystonia or generalized dystonia, and ET.

RESULTS

We included 463 patients from 97 kindreds with autosomal dominant mode of inheritance (AD), defined by the vertical transmission of the disease. ET was the predominant phenotype in every ascertained family and each was phenotypically classified as AD ET. "Pure" ET was present in 365 individuals. Focal or segmental dystonia was present in 98 of the 463 patients; 87 of the 98 patients had ET associated with dystonia, one had dystonic tremor and ten had isolated dystonia. The age of onset and tremor severity did not differ between patients with "pure" ET and ET associated with dystonia. We did not observe a random distribution of dystonia in AD ET pedigrees and all patients with dystonia associated with ET were clustered in 28% of all included pedigrees (27/97, p < 0.001).

CONCLUSIONS

Our results suggest that familial ET associated with dystonia may represent a distinct subtype of ET.

Essential tremor: evolving clinicopathological concepts in an era of intensive post-mortem enquiry

Essential tremor (ET) is one of the most common neurological disorders. In recent years, as a result of systematic post-mortem examinations, our knowledge of the pathophysiology of this disease has grown substantially. Clearly identifiable structural changes (ie, Purkinje cell loss, Lewy bodies) have been observed in the brains of individuals with ET. These changes are not uniform and seem to follow several patterns, localising to the cerebellum itself or to a collection of brainstem neurons that synapse directly with Purkinje cells. Furthermore, these changes are similar to those seen in degenerative diseases. A wealth of clinical, epidemiological, and now post-mortem data indicate that this disease, or perhaps this family of diseases, is likely to be neurodegenerative. The molecular mechanisms that underlie these structural changes in ET are unknown. However, with more controlled, tissue-based studies being done, it is hoped that these mechanisms will be elucidated, thereby laying the foundation for the development of more targeted, effective pharmacotherapeutic interventions.

Olfaction and essential tremor

OBJECTIVE

To characterize the olfactory identification in 40 essential tremor (ET) patients, with the University of Pennsylvania 12 Smell Identification Test (UPSIT), to correlate UPSIT scores to clinical and epidemiological data and to compare it to 89 aged matched controls.

METHOD

Patients were assessed using ET Clinical Scale of Evaluation and UPSIT.

RESULTS

In patients with ET, the UPSIT medium score was 9.10, similar to the control group (9.11), which was also observed in all age groups. ET severity did not correlate to UPSIT scores.

CONCLUSION

This study demonstrated normality of olfactory identification on ET, qualifying UPSIT to be an important tool on tremor differential diagnosis of undetermined origin.

Essential tremor in Parkinson's disease kindreds from a population of similar genetic background

To investigate the possible association between essential tremor (ET) and Parkinson's disease (PD) we conducted a prospective clinical and epidemiological study in a population of similar genetic background. The first-degree relatives of 303 PD probands and 249 controls from Crete were evaluated for the presence of ET. In addition, the possible co-occurrence of ET and PD in the same family or in the same individual was investigated. Results showed that ET was present in the relatives of PD patients more often than in those of controls (OR:3.64, P < 0.001). The risk was even greater (OR: 4.48) when the affected proband had tremor-dominant or mixed PD. Female relatives and siblings of PD patients were more likely to have ET than male relatives and parents of PD patients (OR: 4.36 v/s 2.89 and 4.49 v/s 2.74, respectively). Twelve subjects had both ET and PD phenotypes. While this may have occurred by chance, a number of families were identified in which ET and PD were coinherited through the same parental line. Hence, in certain families ET and PD are genetically related probably sharing common hereditary predisposition.

Epidemiology and management of essential tremor in children

Essential tremor (ET) is a common, often familial, movement disorder characterized by tremor of the limbs, head, and voice. Epidemiological surveys indicate that up to 5% of the adult population has ET, and 5-30% of adults with ET report symptom onset during childhood. There is, however, little published regarding ET in the pediatric population, and no prospective studies targeted specifically to children. Retrospective studies from subspecialty movement disorder clinics indicate that childhood-onset ET is usually hereditary, begins at a mean age of 6 years, and affects boys three times as often as girls. While ET occasionally results in disability during childhood, only one-quarter of children seeing a neurologist for ET require pharmacotherapy. Small case series suggest that propranolol is effective in approximately 50% of children with ET, but controlled treatment trials are lacking.

NonParkinsonism Movement Disorders in the Elderly

OBJECTIVE

The primary objective of this article is to review the clinical presentation and pharmacologic management of essential tremor and tardive dyskinesia.

DATA SOURCES

The MEDLINE (1966-August 2005), Cochrane Database of Systematic Reviews, and International Pharmaceutical Abstracts (1970-August 2005) databases were searched for original research and review articles published in English. The search terms were essential tremor and tardive dyskinesia. Reference lists from articles were also consulted.

DATA SYNTHESIS

Essential tremor is most commonly manifested as a postural or action tremor of the upper extremities. Midline regions such as the head and voice are also commonly affected. Based on review of the literature, propranolol and primidone are the current pharmacologic mainstays for treating essential tremor of the upper extremities. The choice of agent depends on patient-specific factors such as underlying medical conditions. Other agents with demonstrated efficacy include gabapentin and topiramate. Benzodiazepines are effective adjunctive agents, but should be utilized judiciously in the elderly. Botulinum toxin is effective for essential tremor of the voice and head. Surgery is very effective, but may not be appropriate in the frail elderly and should be avoided in the presence of cognitive impairment. Tardive dyskinesia is characterized by involuntary choreoathetoid movements of the orofacial region. For the management of tardive dyskinesia, emphasis is placed on primary prevention and early recognition of signs and symptoms. In some cases, discontinuation of the offending agent can result in reversal of symptoms. If a conventional neuroleptic is the causative agent, switching to an atypical antipsychotic may be helpful. Overall, few treatments have proven to be consistently useful. Other agents that may be helpful include acetylcholinesterase inhibitors, amantadine, baclofen, benzodiazepines, branched chain amino acids, gabapentin, levetiracetam, pyridoxine, verapamil, and vitamin E.

CONCLUSION

Current pharmacologic agents for essential tremor can be expected to provide partial benefit. However, agents for the symptomatic management of tardive dyskinesia are limited, and additional research is warranted in this area.

Essential tremor might be less frequent than Parkinson's disease in North Israel Arab villages

Essential tremor (ET) is much more prevalent than Parkinson's disease (PD) in Western countries. We estimated ET and PD prevalence in Wadi Ara Arabic villages in Northern Israel. In this door-to-door survey, all consenting residents aged >or=65 years were systematically examined by an Arabic speaking team. No prescreening questionnaires were used. A random sample of 900 subjects [437 males, mean age (SD) = 72.6 years (6.6)] of the 2,163 eligible residents were evaluated. Sixteen subjects had an action, intentional tremor. Tremor prevalence was estimated as 1.78% (95% CI 1.1-2.87). Nine of these had another likely cause of tremor. Only 7 patients were diagnosed as ET [prevalence 0.78% (95% CI 0.38-1.6)]. PD was diagnosed in 13 subjects. PD prevalence was 1.44% (95% CI 0.84-2.45). ET is unusually uncommon in this population and possibly even less frequent than PD. The PD prevalence in Wadi Ara is similar to that reported in Western countries.

Tremor in childhood

Tremor in childhood is not commonly described in the literature; but it is also likely underappreciated. The etiology of childhood tremor encompasses a wide variety of pathologic processes. Tremor may occur in isolation, or in association with other neurologic findings or systemic disorders. This article aims to provide an overview of tremorogenic mechanisms with respect to neuroanatomy and neurophysiology, particularly as they relate to children. Classification of tremors, diagnostic entities in childhood, and treatment will also be discussed. With improved recognition and characterization of childhood tremors, we may gain a better understanding of the pathophysiology of the disease and determine more age-appropriate treatment strategies.

Prevalence of essential tremor in the city of Kolkata, India: a house-to-house survey

BACKGROUND AND PURPOSE

Prevalence of essential tremor (ET), one of the most common movement disorders, has not been properly evaluated amongst heterogeneous population in India.

METHODS

We conducted a cross-sectional epidemiological study on the prevalence of ET in a randomly stratified population in the metropolitan city of Kolkata (erstwhile Calcutta), India, by a field team headed by a specialist doctor. A two-stage house-to-house survey was carried out with a validated screening instrument.

RESULTS

A population of 52377 was screened and a total of 184 cases of ET were identified as per pre-defined criteria and the prevalence rate adjusted to World Standard Population was 3.95 per 1000 (95% CI: 3.40-4.56). Age-specific prevalence showed increasing prevalence with aging. Sex-specific prevalence did not show significant difference between men and women. Socio-economic factors appeared to influence the prevalence and risk of ET was higher amongst slum dwellers versus non-slum population (odd ratio-2.29). Family history was positive in about one-fifth of the cases.

CONCLUSION

This study has documented that the prevalence of ET above 40 years in India is similar to that of many countries globally and common amongst slum dwellers and there are fewer familial cases.

Movement disorders

Abnormal involuntary movements are major features of a large group of neurologic disorders, some of which are neurodegenerative and pose a significant diagnostic and treatment challenge to treating physicians. This article presents a concise review of clinical features, pathogenesis, epidemiology, and management of seven of the most common movement disorders encountered in a primary care clinic routinely. The disorders discussed are Parkinson disease, essential tremor, restless legs syndrome, Huntington disease, drug-induced movement disorder, Wilson disease, and Tourette syndrome.

Prevalence of essential tremor in a multiethnic, community-based study in northern Manhattan, New York, N.Y

BACKGROUND

Our aims were to: (1) estimate the prevalence of essential tremor (ET) in a community-based study in northern Manhattan, New York, N.Y., USA; (2) compare prevalence across ethnic groups, and (3) provide prevalence estimates for the oldest old.

METHODS

This study did not rely on a screening questionnaire. Rather, as part of an in-person neurological evaluation, each participant produced several handwriting samples, from which ET diagnoses were assigned.

RESULTS

There were 1,965 participants (76.7 +/- 6.9 years, range = 66-102 years); 108 had ET [5.5%, 95% confidence interval (CI) = 4.5-6.5%]. Odds of ET were robustly associated with Hispanic ethnicity versus white ethnicity [odds ratio (OR) = 2.19, 95% CI = 1.03-4.64, p = 0.04] and age (OR = 1.14, 95% CI = 1.03-1.26, p = 0.01), i.e. with every 1 year advance in age, the odds of ET increased by 14%. Prevalence reached 21.7% among the oldest old (age > or = 95 years).

CONCLUSIONS

This study reports a significant ethnic difference in the prevalence of ET. The prevalence of ET was high overall (5.5%) and rose markedly with age so that in the oldest old, more than 1 in 5 individuals had this disease.

Fertility in essential tremor. Data from population-based and clinical sources

BACKGROUND

The view of essential tremor (ET) as a degenerative disorder, which is now gaining support from postmortem studies, is at odds with traditional views of ET as a 'super-healthy' condition characterized by increased longevity and fertility. Longevity has recently been re-examined in ET, yet fertility, measured by number of offspring, has never been critically assessed in this disease. The objective was to determine whether ET cases and controls differ in terms of number of children.

METHODS

Family history data were collected on ET cases and controls from 2 distinct sources, a population and a clinical (referred) sample.

RESULTS

In the population, the number of children was similar in 59 cases vs. 72 controls [mean +/- SD (median) = 2.3 +/- 2.9 (1) vs. 2.2 +/- 1.8 (2); p = 0.26]. In the referred sample, number of children was similar in 184 cases vs. 241 controls [2.0 +/- 1.5 (2) vs. 1.9 +/- 1.7 (2); p = 0.33]. In adjusted analyses, results were similar.

CONCLUSIONS

Fertility, measured by number of children, was similar in cases and controls. With a newer understanding of the biological substrate of ET emerging from postmortem studies, it is important to critically reassess this and other fundamental biological questions about the disease.

Environmental epidemiology of essential tremor

BACKGROUND

Essential tremor (ET) is one of the most common neurological disorders. Despite this, the disease mechanisms and etiology are not well understood. While susceptibility genotypes undoubtedly underlie many ET cases, no ET genes have been identified thus far. As with many other progressive, degenerative neurological disorders, it is likely that environmental factors contribute to the etiology of ET. Environmental epidemiology is the study in specific populations or communities of the effect on human health of physical, biologic and chemical factors in the external environment. The purpose of this article is to review current knowledge with regards to the environmental epidemiology of ET.

RESULTS

As will be discussed, a series of preliminary case-control studies in recent years has begun to explore several candidate toxins/exposures, including harmane (1-methyl-9H-pyrido[3,4-b]indole), lead and agricultural exposures/pesticides.

CONCLUSIONS

While several initial results are promising, as will be discussed, additional studies are needed to more definitively establish whether these exposures are associated with ET and if they are of etiological importance.

Irregularity distinguishes limb tremor in cervical dystonia from essential tremor

INTRODUCTION

Patients with cervical dystonia (CD) often have limb tremor that is clinically indistinguishable from essential tremor (ET). Whether a common central mechanism underlies the tremor in these conditions is unknown. We addressed this issue by quantifying limb tremor in 19 patients with CD and 35 patients with ET.

METHOD

Postural, resting and kinetic tremors were quantified (amplitude, mean frequency and regularity) using a three-axis accelerometer.

RESULTS

The amplitude of limb tremor in ET was significantly higher than in CD, but the mean frequency was not significantly different between the groups. The cycle-to-cycle variability of the frequency (ie the tremor irregularity), however, was significantly greater (approximately 50%) in CD. Analysis of covariance excluded the possibility that the increased irregularity was related to the smaller amplitude of tremor in CD (ANCOVA: p = 0.007, F = 5.31).

DISCUSSION

We propose that tremor in CD arises from oscillators with different dynamic characteristics, producing a more irregular output, whereas the tremor in ET arises from oscillators with similar dynamic characteristics, producing a more regular output. We suggest that variability of tremor is an important parameter for distinguishing tremor mechanisms. It is possible that changes in membrane kinetics based on the pattern of ion channel expression underlie the differences in tremor in some diseases.

The emerging neuropathology of essential tremor

Essential tremor (ET) is one of the most prevalent neurological disorders. At the same time, it is among the most poorly-understood of these disorders. The underlying anatomical pathology of ET has been elusive until recently. Postmortem studies have begun to display some of the underlying brain changes in patients with this disease. These types of investigations are likely to lead the way to additional insights into the pathophysiology of ET and guide the development of therapies for this common movement disorder.

Further evidence of genetic heterogeneity in familial essential tremor

Familial essential tremor (FET) is a common hereditary movement disorder with phenotypic variability and genetic heterogeneity. To date, linkage analyses revealed three loci associated to essential tremor (ET) (ETM1 on 3q13, ETM2 on 2p22-25, and a locus on 6p23). We performed a genetic analysis of these candidate chromosomal regions in a fifth-generation Italian kindred with autosomal-dominant ET. Of the 22 clinically evaluated family members, nine were affected by ET. The genetic study indicates that the ET in this family is not associated to any of the known ET loci. These findings support evidence of further genetic heterogeneity for such disease.

A population-based study of mortality in essential tremor

BACKGROUND

Although data are sparse, people with essential tremor (ET) are usually assumed to have mortality rates similar to those in the general population. Because ET is common, particularly among older adults, an influence of ET on the life span would have important public health implications. The authors compared the risks of mortality in patients with ET and control subjects without ET.

METHODS

A prospective, population-based design was used to compare the risk of mortality in participants with ET vs controls in three communities in central Spain. Participants were evaluated at baseline (1994 to 1995) and at follow-up 3 years later (1997 to 1998). The relative risk (RR) of mortality (ET vs controls) was estimated using Cox proportional hazards models that excluded participants with Parkinson disease or dementia.

RESULTS

Mean baseline age was 73.5 +/- 6.4 years. There were 33 (16.4%) deaths among 201 ET cases and 465 (13.9%) among 3,337 controls. In an unadjusted Cox model, risk of mortality was increased in ET (RR = 1.59, 95% CI = 1.11 to 2.27, p = 0.01). In a Cox model that adjusted for baseline age, gender, educational category, current ethanol drinking, use of antidepressant medication, and community, RR = 1.45, 95% CI = 1.01 to 2.08, p = 0.04. In an adjusted Cox model restricted to persons with longer (>3 years) follow-up, RR = 4.69 (95% CI = 2.18 to 10.07, p = 0.001).

CONCLUSIONS

In this longitudinal, prospective study, the risk of mortality was increased in essential tremor. Additional studies of incident cases are needed to confirm these results.

Does age of onset in essential tremor have a bimodal distribution? Data from a tertiary referral setting and a population-based study

BACKGROUND/AIMS

The distribution of age of onset of essential tremor (ET) is unclear, with discrepancies in the literature. Some data suggest a bimodal distribution and other data 1 late-life peak. We studied age of ET onset in 2 distinct settings: a population-based study and a tertiary referral center.

METHODS

Age of onset data were collected.

RESULTS

In the population, there was only a small peak at the age of <or=30 years (14.1% of cases) but a clear peak in later life (85.9% of cases). In the tertiary referral center, a bimodal distribution was apparent with 1 large peak (42.2% of cases) at the age of <or=40 years and the second large peak (57.8% of cases) in later life. Familial cases accounted for only 52.6% of young-onset cases from the population, yet 82.7% from the tertiary center.

DISCUSSION

In the population-based study, a peak in later life was clearly present but a young-onset peak was barely discernable, comprising few cases. By contrast, in a tertiary referral center, age of onset was clearly bimodal. While age of ET onset is often said to be bimodal, this may be due to the preferential referral to tertiary centers of patients with young-onset, familial ET.

Essential tremor: Phenotypic expression in a clinical cohort

OBJECTIVES

Examine the characteristics of an essential tremor (ET) clinical cohort including base-rate variability of several commonly accepted diagnostic criteria.

METHODS

A clinical series of 487 consecutive individuals diagnosed with ET were included for study.

RESULTS

The sample was 53% male, had a mean age of onset of 52, and a mean age of 71. Half of the sample had a family history of ET. Half presented with asymmetrical disease and tremor affected the arms (97%), voice (62%), and head or neck (48%). There was considerable variability in the base rate of individuals fulfilling various commonly used diagnostic criteria of ET.

CONCLUSION

The sample was deemed representative of ET clinical cohorts. Asymmetric disease was common, and there was considerable base-rate variation across traditional ET diagnostic criteria.

Update on pathogenesis and treatment of essential tremor

PURPOSE OF REVIEW

This review focuses on recent findings on the aetiological, clinical, pathological and genetic heterogeneity of essential tremor and new therapeutic approaches.

RECENT FINDINGS

Although essential tremor is one of the most common movement disorders, understanding of the causes and mechanisms of the disease is still very limited. Studies on the clinical presentation of essential tremor have expanded the clinical dimension, now including nontremor manifestations such as cerebellar signs, neuropsychological characteristics, distinct personality traits and behavioural symptoms. Results of neuropathologic and imaging studies are conflicting, with hints of neurodegeneration or a nondegenerative disturbance of functional circuits or receptors. Genetic heterogeneity of essential tremor has been demonstrated by linkage to three different chromosomal loci so far, and several negative genetic studies. New animal models are reinforcing previous hypotheses about gamma-aminobutyrate (GABA)-ergic mechanisms in essential tremor. New therapeutic agents for essential tremor have been tested and demonstrated to be partly effective.

SUMMARY

The traditional view of essential tremor as a single disease entity has been replaced with the concept that this disorder is a complex and heterogeneous disease. Heterogeneity of the condition, and lack of diagnostic criteria and objective diagnostic tests add to this problem. Many conflicting results may be due to differences in patient selection.

Parkinsonism and tremor disorders. A clinical approach

Differentiation of idiopathic Parkinson's disease from other causes of Parkinsonism, such as Multiple System Atrophy, Progressive Supranuclar Palsy and Vascular Parkinsonism can be difficult. Clinicopathological studies suggest that the clinical diagnosis of idiopathic Parkinson's disease is 76% reliable. Also, clinical differentiation of tremor prominent Parkinsonism from Essential Tremor or Drug induced Parkinsonism may be problematic, especially in the early stages of the disease. Since these disorders are obviously different in clinical progress, it is important for the clinician to address the patient's and family's concerns about prognosis from a firm diagnostic footing. In this article the clinical features of the common and important causes of Parkinsonism and tremor disorders are reviewed and a practical approach is suggested.

Progression of tremor and ataxia in male carriers of the FMR1 premutation

Premutation alleles of the fragile X mental retardation 1 (FMR1) gene give rise to a late-onset movement disorder, fragile X-associated tremor/ataxia syndrome (FXTAS), characterized by progressive intention tremor and gait ataxia, with associated dementia and global brain atrophy. The natural history of FXTAS is largely unknown. To address this issue, a family-based, retrospective, longitudinal study was conducted with a cohort of 55 male premutation carriers. Analysis of the progression of the major motor signs of FXTAS, tremor and ataxia, shows that tremor usually occurs first, with median onset at approximately 60 years of age. From the onset of the initial motor sign, median delay of onset of ataxia was 2 years; onset of falls, 6 years; dependence on a walking aid, 15 years; and death, 21 years. Preliminary data on life expectancy are variable, with a range from 5 to 25 years.

Exploring the relationship between essential tremor and Parkinson's disease

Although essential tremor (ET) and Parkinson's disease (PD) are considered distinct disorders, there is overlap in some clinical features. In some PD patients, a long-standing postural tremor in the hands may precede the onset of parkinsonian features by several years or decades. Furthermore, large families with both ET and PD phenotypes have been described and autopsy studies have demonstrated Lewy body pathology in brains of ET patients. Functional neuroimaging suggests that some ET patients have dopaminergic deficit. We examine here the evidence for and against an association between ET and PD, and critically review data supporting the notion that a subset of ET patients is predisposed to developing PD.

Study of possible factors associated with age of onset in essential tremor

Factors associated with the age of onset of essential tremor (ET) have not been studied in detail. Identification of modifiable factors could lead to strategies to delay disease onset and identification of nonmodifiable factors would be useful while counseling at risk individuals. The objective of this study was to identify factors associated with age of onset of tremor in ET. One hundred ninety-five ET cases were enrolled in an environmental epidemiological study. Clinical questionnaires included questions on age of onset, demographics (age, sex, race, education), early-life exposures (birth order, childhood household size), exposures prior to tremor onset (head trauma, well water, rural living, estrogen replacement therapy), and family history. In unadjusted analyses, age of onset was associated with family history of tremor (40.9 +/- 22.0 years for cases with a family history of tremor vs. 57.3 +/- 18.4 years for cases without a history; P < 0.001), history of head trauma, younger current age, greater tremor severity, and white race. Ninety-one percent of cases with onset before age 20 years had a family history of tremor. Age ofonset was not associated with other variables of interest (e.g., sex, well water, rural living). In an adjusted linear regression model, age of tremor onset was strongly associated with family history of tremor (P < 0.001). The familial form of ET is characterized by an earlier age of onset than the sporadic form. This study did not detect any other exposures that modified the age of onset of ET. Follow-up studies are needed to examine additional factors of potential interest.

Familial essential tremor with apparent autosomal dominant inheritance: should we also consider other inheritance modes?

A positive family history is present in many patients with essential tremor (ET), but twin studies and segregation analysis have suggested that ET is not entirely a genetic disorder. Two genetic loci have been identified in autosomal dominant (AD) ET and polymorphisms in the DRD3 and HS1-BP3 genes have been proposed as the possible susceptibility factors for ET. There is also evidence for further genetic heterogeneity. We evaluated 4 unrelated large kindreds with ET with an apparent AD mode of transmission. Each kindred spanned at least 3 generations and contained at least 13 living affected subjects who met criteria for definitive ET. None of the pedigrees had evidence for inheritance of ET from both parents. Known genetic ET loci were excluded in these families. We detected a preferential transmission of ET in every kindred and the proportion of affected offspring varied from 75% to 90% (P < 0.05) in the generations with complete ascertainment. Our data indicate that non-Mendelian preferential transmission of an affected allele is a feature in many ET kindreds with multiple affected members and an apparent AD mode of inheritance. ET may have a complex etiology. Additional genetic models need to be considered, including an interaction of susceptibility genes and environmental risk factors.

Increased risk of essential tremor in first-degree relatives of patients with Parkinson's disease

We conducted a historical cohort study of 981 first-degree relatives of 162 patients with Parkinson's disease (PD) and of 838 first-degree relatives of 147 controls representative of the population of Olmsted County, Minnesota. In addition, we studied 2,684 first-degree relatives of 411 patients with PD referred to the Mayo Clinic. Relatives were interviewed and screened for tremor either directly or through a proxy, and those who screened positive were examined or copies of their medical records were obtained to confirm the diagnosis of essential tremor (ET). We also obtained ET information from a medical records-linkage system (family study method). In the population-based sample, the risk of ET was significantly increased for relatives of patients with onset of PD<or=66 years (first tertile; hazard ratio [HR]=2.24; 95% confidence interval [95% CI]=1.26-3.98; P=0.006). In the referral-based sample, the risk of ET among relatives increased with younger onset of PD in patients (linear trend; P=0.001), and was higher in relatives of PD patients with the tremor-predominant or mixed form when compared with relatives of patients with the akinetic-rigid form, and in men compared with women. These findings suggest that PD and ET may share familial susceptibility factors.

Essential tremor

Essential tremor (ET) is among the most common neurologic disorders. The traditional view of this disorder as a benign, monosymptomatic, familial condition is being replaced by a more complex view of ET as perhaps a family of diseases with etiologic, clinical, and pathologic heterogeneity. This article discusses the major clinical features of ET and approaches to its diagnosis and treatment.

Essential tremor: emerging views of a common disorder

Essential tremor (ET) is the most prevalent tremor disorder. ET has traditionally been viewed as a monosymptomatic disorder characterized by a kinetic arm tremor, but this definition is gradually being replaced. The clinical spectrum has come to include several motor features, including tremor and ataxia, and several non-motor features, including possible cognitive impairment and personality disturbances. Postmortem studies are revealing several different patterns of pathology. The emerging view is that ET might be a family of diseases, unified by the presence of kinetic tremor, but further characterized by etiological, clinical and pathological heterogeneity. Effective pharmacological treatments for the disorder remain limited, although new insights into disease mechanisms might result in more-effective therapies. In addition, recent investigations of environmental toxicants that might be linked to ET open the way towards primary disease prevention through a reduction in exposure to these factors.

Clinical features of childhood onset essential tremor

Childhood onset essential tremor (ET) is uncommon. It is not clear as to whether ethnicity-specific differences may influence the phenotypic features. To determine the frequency and clinical characteristics of childhood ET in a tertiary referral center. In a prospective evaluation of 120 consecutive ET patients in a movement disorders clinic, we found a 15.5% (19) frequency of childhood onset ET patients. The mean age of onset and mean age was 10.8 +/- 4.1 (6-16) years and 25.7 +/- 15.0 (16-73) years consisting of 73.6% (14/19) men and 26.4% (5/19) women. A positive family history of ET was present in 11 of 19 (52.6%). Presence of a head tremor was observed in 2/19 (10.5%). We highlighted a relatively high frequency (15,5%) of childhood ET in our Asian cohort. In addition, we drew attention to the male preponderance and the low frequency of head tremor in childhood ET corroborating study findings in white ET patients. These observations appear to transcend ethnic and cultural differences and lend further support that gender difference may play a role in the pathogenesis and expression of ET.

Pharmacological treatment of disabling tremor

Tremor is often a disabling primary condition or secondary to another disorder. No universally effective pharmacological agent exists for the treatment of essential tremor, and patients differ greatly in their response to therapies, thus requiring individualised regimens. Deep brain stimulation is the best option for patients with disabling, drug-resistant essential tremor. Resting tremor in Parkinson's disease is usually not the primary disabling feature, and in most cases, levodopa/carbidopa is satisfactory for many years. Young Parkinson's patients with dominant, disabling tremor benefit from anticholinergics in addition to dopaminergic therapies. However, older Parkinson's patients with more disabling tremor may suffer from dose-dependent side effects, and deep brain stimulation should be considered. This article outlines the available pharmacological agents and treatment considerations for various disabling tremors, including essential tremor and Parkinson's disease.

Benefits and risks of pharmacological and surgical treatments for essential tremor: disease mechanisms and current management

Essential tremor (ET) is a neurological disease (and possibly a family of diseases) whose most recognisable feature is an action tremor of the hands and occasionally of the voice and head. Current data support the view that CNS gamma-amino-butyric acid (GABA)-ergic mechanisms may underlie ET and that the tremor may be further modulated by peripheral (muscle) adrenoreceptors. Potential pharmacotherapeutic options, targeted to influence the activity of the neurotransmitter GABA within the CNS and the peripheral adrenergic receptors, are part of the current armamentarium to treat ET. As such, primidone and propranolol remain the mainstays of the therapy for ET. Intramuscular injections of botulinum toxin A may play a role in the treatment of voice and head tremor. Surgical options, which are reserved for patients with severe, medically-refractory tremor, provide adequate tremor control in the majority of patients. As with other progressive neurological disorders of late life, the ability to use neuroprotective medications to intervene in the developing disease to either slow or halt the progression of the pathological process, would involve an understanding of underlying disease mechanisms. The understanding of these mechanisms in ET is limited and further study of these mechanisms is critical for the development of such therapies.

Report from a U.S. conference on essential tremor

Seventy researchers met in Washington, DC, on 20-21 October 2005 to identify and discuss the most pressing research issues in essential tremor (ET). The conference attendees concluded that the following six objectives are of immediate and overriding importance: (1) a collaborative network of research centers; (2) an international committee for developing a standard protocol for the diagnosis and quantification of ET; (3) the identification of one or more genes for ET; (4) a centralized repository of DNA and, ideally, immortalized cell lines from well-characterized ET families and healthy controls; (5) a reliable and efficient repository of optimally prepared and categorized brain samples for hypothesis-driven neuropathological examinations in well-characterized ET patients; and (6) animal models of ET for screening promising drugs. The conference attendees hope that this statement from the United States will engender international collaboration in finding a cure for ET.

Case-Control Study of Nutritional Antioxidant Intake in Essential Tremor

The theory that oxidative stress is involved in the pathogenesis of neurodegenerative diseases has received considerable attention and studies have linked these diseases to the diminished use of antioxidant vitamins (vitamins E and C) and other dietary antioxidants. Essential tremor (ET) is a chronic, progressive disease. One possible disease mechanism is neurodegenerative. Whether nutritional antioxidant use differs between ET cases and controls is not known. Using a case-control design, we conducted detailed dietary assessments and tested the hypothesis that diminished use of nutritional antioxidants is associated with ET. Data on diet were collected on 156 ET cases and 220 controls using a semi-quantitative food-frequency questionnaire. There was no evidence that current nutritional antioxidant exposure differs in ET cases and controls. This does not exclude the possibility that nutritional antioxidant exposure was lower in ET cases prior to their disease onset.