Interactions of motor and non-motor symptoms in Parkinson's disease
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Parkinson’s disease (PD) is characterized by motor dysfunction and multiple non-motor symptoms. Though motor/non-motor interactions are common, the lines of research focusing on motor and non-motor symptoms mainly remain separate. The present studies assessed interactions between several motor aspects of PD (impaired gait, side of motor-symptom onset, tremor, motor-symptom severity) and non-motor symptoms (cognition, anxiety, self-perceived stigma) in non-demented individuals with idiopathic PD. Study 1 examined cognitive and motor performance during dual tasking, specifically executive function while walking. The impact of dual tasking on walking (speed, stride frequency) was greater for PD (N=19) than NC participants (N=13). The PD group had fewer set-shifts than NC on dual tasking, and demonstrated greater cognitive variability on dual tasking. Study 2 considered mechanisms of visuospatial dysfunction in PD (N=79) by assessing how side of motor-symptom onset (left versus right) and cognition (attention, executive function) affect spatial judgment on a dynamic line bisection task. In contrast to a rightward-biased parietal-neglect pattern, the PD group showed a leftward bias that occurred when attention was directed to the left side of space, regardless of side of onset. The extent and variability of bias correlated with frontally-mediated neuropsychological performance for PD but not NC (N=67). Both results suggested frontal-attentional rather than parietal-neglect mechanisms of spatial bias. Study 3 assessed how motor symptoms contribute to self-reported anxiety on the Beck Anxiety Inventory (BAI). Factor analysis identified a five-item PD motor factor, which correlated with motor-symptom severity and mediated the difference on BAI total scores between PD (N=100) and NC (N=74). Removal of the motor-factor items (e.g., “hands trembling”) significantly reduced BAI scores for PD relative to NC and reduced the size of the correlation between the BAI and motor-symptom severity. Study 4 examined the contributions of motor and non-motor symptoms to self-perceived stigma in PD (N=362). Contrary to expectations, perceived stigma was not predicted by motor symptoms but rather by depression and, for men only, by younger age. These studies provide insight into interactions that occur between motor and non-motor symptoms in PD in multiple aspects of daily function, highlighting potential avenues for future research and intervention.