Is There a Relationship Between Movement and Sleep Disturbances in Essential Tremor?

Background: Essential tremor (ET) is a clinically heterogeneous disorder involving motor and non-motor features. Voluntary movement abnormalities, including movement slowness (bradykinesia), have been frequently described in ET. Among non-motor symptoms, insomnia is also frequently reported, raising the possibility of shared underlying mechanisms with bradykinesia (e.g., brainstem noradrenergic dysfunction involving the locus coeruleus due to Lewy body pathology). We investigated the relationship between movement abnormalities, as objectively quantified using finger-tapping kinematic analysis, and sleep disturbances in ET. Methods: A subsample of 29 ET patients included in a previous study underwent kinematic analysis of finger-tapping. Insomnia was evaluated using the Insomnia Severity Index (ISI). Patients were stratified according to the presence of bradykinesia (e.g, movement slowness during finger-tapping) and insomnia (ISI ≥ 8). Group comparisons and correlational analyses were performed to assess the association between kinematic measures of bradykinesia, insomnia severity and other clinical features. Results: Fourteen subjects (48.3%) exhibited bradykinesia on kinematic analysis, and eleven patients (37.9%) met the criteria for insomnia. The prevalence of insomnia was similar between patients with and without bradykinesia. Likewise, no significant differences in finger-tapping kinematics were observed between ET patients with and without insomnia. Kinematic measures of bradykinesia did not correlate with ISI scores (all p > 0.05), whereas ISI scores were significantly positively correlated with clinical tremor severity. Conclusions: The current kinematic analysis suggests no relationship between movement abnormalities and sleep disturbances in ET. While bradykinesia in ET possibly relies on the dysfunction of cerebellar-basal ganglia circuits, insomnia in ET may reflect prominent brainstem dysfunction. Larger studies integrating kinematic assessments, neuroimaging, and longitudinal designs are needed to clarify the relationship between movement and sleep disturbances in ET.