Mind the gaps: What we don't know about cognitive impairment in essential tremor

Abstract

Introduction: Although the hallmark feature of essential tremor (ET) is tremor, there is growing appreciation that cognitive impairment also occurs, including increased prevalence of mild cognitive impairment (MCI) and increased prevalence and incidence of dementia. With emerging knowledge of ET-cognitive impairment, come fundamental questions regarding its course, bases, predictors and clinical outcomes. Studies in the general population and in Parkinson's disease (PD), a related movement disorder, offer a starting point from which to begin filling these clinically important knowledge gaps. Methods: A PubMed search (June 2018) identified articles for this review. Results: Much of our knowledge of cognitive impairment in ET is of the static condition (e.g., prevalence of cognitive impairment in ET), with nearly no information on its bases, predictors and dynamics (i.e., course, and clinical outcomes). In PD, where such data have been published, rates of cognitive decline and conversion to MCI/dementia are higher than in the general population. Predictors of cognitive change in PD and the general population have also been identified, yet they only partially overlap one another.

Essential tremor: the most common form of cerebellar degeneration?

“…Aside from motor features, there is a growing literature that demonstrates the presence of non-motor features in patients with ET [88]. Among these is cognitive impairment, which may range from mild to marked [89]. While some of this impairment likely has its basis in degenerative pathology in the cerebral cortex [90,91], some is also likely based in the cerebellum, as a cerebellar cognitive affective syndrome is well-known to occur and includes hallmark deficits in executive function [92].…”

“…While some of this impairment likely has its basis in degenerative pathology in the cerebral cortex [90,91], some is also likely based in the cerebellum, as a cerebellar cognitive affective syndrome is well-known to occur and includes hallmark deficits in executive function [92]. That executive dysfunction is a common feature of the cognitive impairment of both ET and cerebellar-related cognitive dysfunction further suggests links between ET and the cerebellum [89].…”

Distinct non‐motor features of essential tremor with head tremor patients

“…In our study, the ETh group showed higher scores and frequency of ecmnesia than those in the ETol group. We obtained little information on the cognitive level in the patients with ETh; however, neuroimaging studies have demonstrated that the poorer cognitive performance in patients with ET might be related to gray matter atrophy or cerebellar‐cortex dysfunction, 10,26 which suggests that worse deterioration occurs in patients with ETh.…”

“…In addition, both patient subgroups showed poor performance in the attention/memory domain; nearly half of the patients claimed to frequently forget things, which might be considered as a cognitive decline manifestation. Previous studies report 10 of increased prevalence of mild cognitive impairment and dementia in patients with ET compared with age‐matched controls 10,25 . Further, some studies have reported that besides executive dysfunction, which is the most common domain deficit in patients with ET, memory deficiency is also common.…”

“…Some studies have demonstrated that non‐motor symptoms (NMS) could be long‐term consequences of tremor and are related to tremor severity 9 . Tremor severity is correlated with poor cognitive performance and might accelerate cognitive impairment progression 10 . It remains unknown whether symptoms such as head tremor might aggravate NMS in patients with ET.…”