Cognition in non-demented Parkinson's disease vs essential tremor: A population-based study

Objectives: Patients with Parkinson's disease (PD) and essential tremor (ET) have a higher risk of cognitive impairment than age-matched controls. Only a few small studies (11-18 subjects per group) have directly compared the cognitive profile of these conditions. Our aim was to compare the cognitive profile of patients with these two conditions to each other and to healthy individuals in a population-based study of non-demented participants. Materials and methods: This investigation was part of the NEDICES study, a survey of the elderly in which 2438 dementia-free participants underwent a short neuropsychological battery. We used nonparametric techniques to evaluate whether there are differences and/or a gradient of impairment across the groups (PD, ET, and controls). Also, we performed a head-to-head comparison of ET and PD, adjusting for age and education. Results: Patients with PD (N=46) and ET (N=180) had poorer cognition than controls (N=2212). An impaired gradient of performance was evident. PD scored lower than ET, and then each of these lower than controls, in memory (P<.05) and verbal fluency (P<.001) tasks. When we compared PD and ET, the former had lower scores in verbal fluency (P<.05), whereas the later had a poorer cognitive processing speed (P<.05). Conclusions: This large population-based study demonstrates that both conditions influence cognitive performance, that a continuum exists from normal controls to ET to PD (most severe), and that although deficits are in many of the same cognitive domains, the affected cognitive domains do not overlap completely.