Cognitive functioning and intra-individual variability in essential tremor and Parkinson's disease: A comparative study.

Objective: Cognitive decline may be more prevalent in essential tremor (ET) than previously recognized. Dispersion-based intraindividual variability (IIV-D) is proposed as a marker of neurological integrity. While elevated IIV-D has been noted in Parkinson's disease (PD), it has not been explored in ET. This study investigated cognitive functioning and IIV-D in ET compared to PD and cognitively intact controls (IC). Method: Neuropsychological data from 188 PD, 53 ET, and 135 IC patients were reviewed. Group comparisons on neuropsychological tests were examined. IIV-D was calculated via the intraindividual standard deviation (iSD) and the coefficient of variance (CoV) across 13 neurocognitive scores. Secondary analyses examined IIV-D among PD, ET, and IC with "No Cognitive Disorder" (NCD). Results: PD and ET performed worse than IC on oral processing speed, mental tracking, set-shifting, and semantic fluency tasks. Greater IIV-D was observed in PD and ET using CoV (PD vs. IC: p < .001, d = 0.680; ET vs. IC: p < .001, d = 0.894), but only ET using iSD (p = .001, d = 0.614), after controlling for mean performance and covariates. In the NCD subset, ET showed greater IIV-D than IC (p = .001, d = 0.867) when using CoV. Conclusions: This study is the first to examine IIV-D in ET, revealing individuals with ET exhibit greater IIV-D than IC, even in the absence of a neurocognitive diagnosis. Lack of assessment for clinical confounds (e.g. medications, motor severity) may limit findings. Nevertheless, the research enhances our understanding of cognitive characteristics in ET.