Parkinson’s disease (PD) is typically characterized by unilateral onset of motor symptoms (i.e., tremors, rigidity) which is caused by dopaminergic degeneration of the substantia nigra that influences basal ganglia-prefrontal circuitry. Over time, motor symptoms become more bilateral, though continue to remain asymmetric. Many neuropsychological studies suggest that laterality of motor onset may be linked to hemispheric specific cognitive or mood changes. Namely, worse verbal/language performance may be present in individuals with right body (left hemisphere) onset and conversely for visuospatial performance, with depression symptoms relating more so to individuals with right body (left hemisphere) onset. To date, findings are often inconsistent, with some studies showing evidence for laterality effects and others not. The basis for this inconsistency is unclear, though one possibility relates to small sample sizes and varying methodologies. Thus, the goal of this study was to examine potential cognitive and mood laterality effects in a large clinical sample of individuals with PD.

Participants included a convenience sample of 600 nondemented individuals with idiopathic PD from the University of Florida Fixel Institute Movement Disorders Center. As a group, participants were around 60 years of age (Mean Age=63.9+9.4), well educated (Mean years=14.9+2.7), predominantly male (70%), and white non-Hispanic (93%). Side of initial motor symptom onset was based on self-report: Right (N=337) and Left (N=263). Approximately 79% were tremor predominant. All received mood and neurocognitive measures as part of standard clinical care, including indices of executive function (Stroop Color-Word, Trails B, Letter Fluency), recent verbal memory (delayed recall: Hopkin’s Verbal Learning Test, WMS-III Logical Memory), language (Boston Naming Test, Animal fluency), visuospatial skills (Judgment of Line Orientation, Facial Recognition Test). Evaluation of emotion symptoms included: depression (Beck Depression Inventory-II), apathy (Apathy Scale), and anxiety (State-Trait Anxiety Inventory). Analyses used raw scores from these measures. Due to non-normality of most measures’ distributions, laterality effects were examined using bootstrapped multivariate methods (multivariate analysis of variance [MANOVA]). Separate MANOVA’s were run for each cognitive domain (i.e., EF, language, etc.) and mood measures.

The right and left sided onset groups did not significantly differ in demographic (age, education, sex) or disease characteristics (duration, PD subtype). Results of the MANOVA’s with cognitive variables were all nonsignificant broadly (all with F’s ranging from .33 to .94) and at the single test level. Similarly, the left and right onset groups did not significantly differ (a=0.05) across standard scales of depression (F=0.031), anxiety (Trait F=0.463; State F=3.29), and apathy (F=0.74).

We found no evidence that laterality of initial motor symptoms influenced cognitive or mood symptoms in a large cohort of 600 individuals with PD. These findings raise questions about importance of motor onset laterality for cognitive and emotion related changes in PD. Future studies should move beyond self-report and behavioral motor scales for determining hemispheric contributions. In PD, use of refined metrics for determining the extent of asymmetric dopaminergic degeneration (e.g., DAT scan) at the hemispheric level coupled with sensitive neuropsychological measures may provide clearer understanding of potential neural circuitry relationships.