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Expanding clinical dimensions of essential tremor
  1. L J Findley
  1. Correspondence to:
 Professor L J Findley
 Essex Neurosciences Unit, Oldchurch Hospital, Romford, Essex RM7 0BE, UK; ljfindleyuk-consultants.co.uk

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The non-motor manifestations of essential tremor may be important

The paper in this issue by Chatterjee et al (page 958)1 is the first large cross sectional study of personality in people with essential tremor compared with a control group. This careful study showed higher scores in the essential tremor group on the transdimensional personality questionnaire (TPQ) in the domain of harm avoidance—implying a personality with increased levels of pessimism, fearfulness, shyness, and anxiety, and easy fatigability.

Essential tremor is the commonest movement disorder seen in clinical practice and has hitherto been considered a pure motor disorder without evidence of neuronal degeneration or widespread changes in the central nervous system. The age specific prevalence is reported to be between 1% and 3% of the general population. It is often given the prefix “benign,” which is unfortunate as many affected individuals have physical, social, and psychological handicaps, and some are totally disabled.2

As with essential tremor, the early descriptions of other less common movement disorders, such as Parkinson’s disease, did not mention or emphasise the non-motor manifestations, though these are now recognised to be an integral part of Parkinson’s disease. However, in one of the earliest large studies of essential tremor, Minor described higher intelligence, fecundity, and longevity in the essential tremor group.3 Considering the overlap and commonality in phenomenology between essential tremor and Parkinson’s disease,4 it is perhaps not surprising that in recent years non-motor manifestations have been increasingly recognised as an integral part of essential tremor.

The limited studies thus far have described abnormalities of cognition, affect, and personality in essential tremor. The cognitive impairments include deficits in verbal fluency, naming, recent memory, working memory, and mental set shifting.5 Higher levels of depression, anxiety, and obsessive-compulsive disorder are described in comparison with control groups. Interestingly, above average performances in the essential tremor compared with controls have been reported in the areas of general verbal and intellectual abilities,5 and this would be in line with the early observation of Minor.3 The severity of the cognitive deficits ranges from unnoticeable to severe. The largest impairments have been described in verbal fluency and mental set shifting. In some studies cognitive impairment and depression were of sufficient severity to interfere with activities of daily living. In some individuals the personality changes were significant enough to cause disturbance of psychosocial functioning or to provoke comment from family members. In general, patients with Parkinson’s disease have more widespread and severe impairments.

When considering changes in the psychology, mood, and results of tests of cognition in essential tremor, consideration has to be given to the direct, or indirect, effects of the tremor itself. In none of the studies so far has any significant correlation been found between tremor severity and any measure of psychological or cognitive change. However, I am not convinced that sufficient numbers of patients with very severe essential tremor have yet been examined. Such cases may represent a separable subgroup—for example, some show clear evidence of cerebellar deficits.6 The concept that severe essential tremor represents a separable category was expounded eloquently by the late David Marsden.

Further longitudinal prospective comparative studies will be required to unravel the link between the tremor of essential tremor and the underlying mechanisms producing cognitive, personality, and psychological change. From the complexity of the non-motor manifestations and knowledge on the generation of essential tremor, it would seem unlikely that the phenomena can be linked to a change in a single neurotransmitter system. Non-motor manifestations of essential tremor—including changes in mood and personality and the disparate cognitive abnormalities—could be subserved by abnormalities in frontal/subcortical pathways.5 However, the constellation of cognitive and affective changes resembles those described in the “cerebellar cognitive affective syndrome,” which is found in cerebellar syndromes.7 Although the pathogenesis of essential tremor is still not understood, there is overwhelming evidence of involvement of the cerebellum, and current concepts and studies have shown that the cerebellum is functionally connected to the frontal cerebral cortex through feed forward and feed backward pathways.7 The cerebellar cognitive affective syndrome is more pronounced in patients with acute cerebellar lesions and therefore the slow onset of essential tremor may account for the generally milder symptoms described in the studies of this condition.

Non-motor manifestations of essential tremor will have to be considered in the assessment of patients under consideration for invasive treatments, such as stereotactic surgery or insertion of a deep brain stimulator. Limited evidence thus far available would suggest such procedures do not produce any deleterious effects on cognition and may result in a significant reduction in anxiety and an improvement in the quality of life.

The non-motor manifestations of essential tremor may be important

REFERENCES

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  • Competing interests: none declared

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