When a Tremor is Not Just a Tremor: Cognitive and Functional Decline in Essential Tremor, a More Complex Disorder Than We Thought

Locus Coeruleus Degeneration in Essential Tremor With Mild Cognitive Impairment: A Neuromelanin MRI Study

OBJECTIVE

Our aim was to research the neuromelanin-sensitive magnetic resonance imaging (NM-MRI) features of the locus coeruleus (LC) in essential tremor (ET) patients of various cognitive states and to explore the relationships between these features and cognition.

METHODS

We recruited three groups of participants, including 30 ET patients with mild cognitive impairment (ET-MCI), 57 ET patients with normal cognition (ET-NC), and 105 healthy controls (HCs). All participants underwent MRI scanning and clinical evaluation. Through NM-MRI images, we compared the contrast-to-noise ratio of LC (CNRLC) between groups and evaluated the relationships between CNRLC and cognitive scales.

RESULTS

Compared to HCs, ET-MCI patients had a substantially lower CNRLC value (p = 0.017). The CNRLC of ET-NC patients was intermediate between that of ET-MCI patients and HCs. Furthermore, a partial correlation analysis in ET-MCI patients, controlling for age, gender, and education level, showed that higher CNRLC values correlate with better performance on the Montreal cognitive assessment test and the trail making test A.

CONCLUSION

LC degeneration in ET patients may partially contribute to cognitive decline, suggesting that the LC norepinephrine system deserves further research on the mechanism of cognitive decline of ET patients as well as the development of targeted drugs.

Essential tremor-plus: a controversial new concept

In addition to redefining essential tremor (ET), the 2018 consensus statement of the Movement Disorder Society on tremor coined a new term: essential tremor-plus (ET-plus). This term is uncertainly defined as tremor with the characteristics of ET, with additional neurological signs of uncertain clinical significance. If ET-plus had been defined on the basis of a difference in underlying pathology or an appreciable difference in prognosis, it would have a valid, scientific rationale, as does the term Parkinson-plus. However, there is no such evidence, so the basis for the term is questionable. In fact, ET-plus might only represent a state condition (ie, patients with ET might develop these additional clinical features when the disease is at a more advanced stage). We caution against coining new terms that are not supported by a firm scientific basis and encourage research into the creation of essential tremor subsets that are defined with respect to differences in underlying causes or pathophysiology.

The cognitive side of essential tremor: what are the therapeutic implications?

While essential tremor (ET) has traditionally been categorized as a pure motor disease, cross-sectional and longitudinal studies of cognition in ET have demonstrated that these patients may have cognitive dysfunction. Recent epidemiological studies demonstrate an association between ET (particularly with onset after age 65) and increased risk for cognitive impairment and dementia. Although existing studies have generally conceptualized cognitive changes in ET as consistent with a 'frontosubcortical' or 'corticocerebellar' profile, results from these same studies suggest that cognitive impairment in ET may in fact be heterogeneous. Furthermore, the underlying mechanisms remain uncertain. Cognitive changes could be a byproduct of the cerebellar dysfunction of ET itself; alternately, they may be a feature of concomitant neurodegenerative diseases that have been associated in several studies with ET, including Alzheimer's disease, Parkinson's disease or progressive supranuclear palsy. While the study of cognitive dysfunction in ET has received research attention in recent years, the results of these studies have not been translated into the clinical domain and clinical practice. This review first summarizes the current literature on the potential relationships between ET and cognitive change. We then suggest areas of further clinical evaluation and treatment; these suggestions are directed at physicians caring for ET patients who may demonstrate or complain of cognitive impairment. As we discuss, clinicians should ideally screen ET patients for possible signs or symptoms of cognitive impairment in addition to assessing for psychiatric comorbidity and quality of life. These recommendations are in contrast to most current clinical practice, which does not routinely include such assessment among ET patients. To our knowledge, there have been no pharmacotherapeutic trials to date of any agent for cognitive change associated with ET. We believe that studies for this indication are now called for. Future efforts in this direction will also need to take into account the pathobiology of cognitive changes in ET, which itself is an area that is ripe for future investigations.

Cognitive features of essential tremor: a review of the clinical aspects and possible mechanistic underpinnings

The classical concept of essential tremor (ET) as a monosymptomatic tremorogenic disorder has been questioned in the last decade as new evidence has been described. Clinical, neuroimaging, and pathological studies have described a probable structural basis (mainly in cerebellum) and evidence that ET is associated with subtle clinical cerebellar deficits and several non-motor clinical manifestations, such as cognitive and mood disorders. We performed literature searches in Medline, ISI Web of Knowledge, and PsycInfo databases. The aim of this review is to describe cognitive deficits associated with ET. First, we present a brief history of ET cognitive disorders presented. Second, we describe several clinical cross-sectional series demonstrating that ET is associated with mild cognitive deficits of attention, executive functions, several types of memory (working memory, immediate, short term, delayed, and possibly others) and, mood disorders (depression). Recent neuroimaging studies favor a cerebellar basis for these cognitive deficits. Population-based surveys confirm that mild cognitive dysfunction is not limited to severe ET cases, the entire ET group, including mild and undiagnosed cases, can be affected. Cohort studies indicated that ET cognitive deficits could be progressive and that ET patients had an increased risk of dementia. The mood and cognitive deficits in ET are in agreement with cognitive affective cerebellar syndrome described in patients with cerebellar disorders. New evidence, mainly from functional (neuroimaging) and prospective clinical studies would further bolster recent descriptions of ET clinical manifestations.

Faster rate of cognitive decline in essential tremor cases than controls: a prospective study

BACKGROUND

  Mild cognitive deficits have been reported in essential tremor (ET). However, these cognitive deficits have been assessed in cross-sectional rather than longitudinal analyses.

OBJECTIVE

  To determine whether decline in cognitive test scores occurs at a faster rate in ET cases than controls.

METHODS

  In a population-based study of older people (≥ 65 years) in central Spain (Neurological Disorders in Central Spain, NEDICES), non-demented ET cases and controls were followed prospectively. Participants with baseline or incident Parkinson's disease or dementia were excluded as were participants who developed incident ET. At baseline (1994-1995) and at follow-up (1997-1998), a 37-item version of the mini-mental state examination (37-MMSE) was administered.

RESULTS

  A total of 2319 participants (72.4 ± 5.8 years) included 135 prevalent ET cases and 2184 controls. At baseline, the mean 37-MMSE in cases was 28.8 ± 5.8 vs. 30.2 ± 4.8 in controls (P = 0.02). During the 3-year follow-up period, the 37-MMSE declined by 0.70 ± 3.2 points in cases vs. 0.11 ± 3.8 points in controls (P = 0.03). In analyses that adjusted for age, education, and other potential confounders, the case-control difference remained robust.

DISCUSSION

  In this population-based, prospective study of non-demented elders, baseline cognitive test scores were lower in ET cases than controls; moreover, during the 3-year follow-up period, these scores declined at a rate that was seven-times faster in ET cases. This study provides evidence that cognitive deficits in ET are not static, and they appear to be progressing at a faster rate than in elders without this disease.