The Relationship Between Dysarthria, Primary Motor Profile, and Cognition in Idiopathic Parkinson’s Disease

Abstract

While previously considered purely a motor disorder, Parkinson’s disease (PD) is now recognized as a complex constellation of motor and non-motor symptoms. Current research to better understand the heterogeneity of symptoms has uncovered potential links between motor and non-motor features. Specifically, converging evidence suggests an association between the presence of dysarthria and cognitive decline, and that this relationship may be mediated by primary motor phenotype (i.e., tremor-dominant and non-tremor dominant). The aim of this study is to examine this purported relationship through two distinct, yet related, experiments. Experiment 1 utilized perceptual ratings of naturalness, articulatory precision, and intelligibility to examine the relationship between dysarthria, primary motor presentation (tremor versus non-tremor), and cognition in 9 individuals with PD. 10 experienced SLPs used a visual analog scale to rate naturalness and articulatory precision. Orthographic transcription was used to calculate intelligibility. Cognition was assessed with the Parkinson’s Disease Cognitive Rating Scale (PD-CRS) and primary motor presentation was rated with the Unified Parkinson’s Disease Rating Scale (UPDRS). Results aligned with the hypothesis and indicated that participants with increased non-tremor symptom severity had more impaired ratings of speech naturalness and articulatory precision. Furthermore, participants with mild cognitive impairment (MCI) had more impaired speech ratings than individuals without cognitive impairment, which is consistent with a small, but growing, literature. In Experiment 2, 192 individuals with PD were examined from the Parkinson’s Progression Marker Initiative (PPMI) – a longitudinal, controlled database of individuals with de novo PD. Speech ratings, and tremor and non-tremor symptom severity, as measured by the UPDRS, in early PD (T1) were examined as predictive factors of cognitive status 4-5 years later (T2). Results are consistent with the hypothesis and suggest that speech functioning around the time of diagnosis is associated with a non-tremor dominant motor profile, and is predictive of dementia 5-7 years post diagnosis. Additionally, the tremor dominant group performed worse or equal to the non-tremor dominant group across cognitive domains, which is contrary to the literature that suggests worse cognitive functioning in non-tremor dominant PD. Findings from both experiments align, in part, with current literature implicating breakdowns in cholinergic-mediated channels, thought to be more severely depleted in individuals with pervasive cognitive decline and a non-tremor presentation, as a possible contributor to dysarthria from PD. More research is needed to better understand these relationships.