Parkinson's disease (PD) is primarily considered to be a progressive degenerative motor disease associated with the degeneration of striatal dopamine neurons. However, increasing evidence has suggested progressive cognitive and psychiatric changes as well. Forty-six patients with PD, ranging in severity from Hoehn and Yahr (H-Y) score of 1:4, were recruited from a clinic specializing in PD. Various cognitive and neuropsychological measures were used to discover if there were indeed differences due to the progression of PD. As H-Y stage significantly increased, so did age and levodopa equivalency dose of medications, both independent of one another. Years of education had a significant negative relationship with H-Y score. Measures of general cognition divulged a significant decrease as H-Y score increased. Finally, as H-Y score increased, magical ideation decreased, and religious group social support increased. Mechanistically, the significant cognitive decline occurring with H-Y staging may be linked to a reduced dopaminergic function. Significant cognitive and neuropsychological changes are associated with the progression of PD and its possible relationship to Reward Deficiency Syndrome (RDS).

BackgroundParkinson's disease (PD) usually emerges with a unilateral side‐of‐onset (left‐onset: LOPD; right‐onset: ROPD; Marinus & van Hilten, 2015) due to an asymmetrical degeneration of striatal dopaminergic neurons (Donnemiller et al., Brain, 135, 2012, 3348). This has led to a body of research exploring the cognitive, neuropsychological, and clinical differences between LOPD and ROPD (e.g., Verreyt et al., Neuropsychology Review, 21, 2011, 405).MethodsThirty ROPD and 14 LOPD cases were drawn from a Boston clinic specializing in PD. Various cognitive and neuropsychological measures were used in an attempt to discover if there were indeed any differences between LOPD and ROPD in this cohort.ResultsFor LOPD, duration of illness was found to be significantly greater than that of ROPD. However, further testing was able to confirm that despite this difference, it was not the cause of the other significant differences found. Furthermore, this increased duration was consistent with a previous study (Munhoz et al., Parkinsonism and Related Disorders, 19, 2013, 77). Performance on the Digit Span Backward (DSB) was found to be significantly poorer in LOPD than ROPD, suggesting compromised executive function in LOPD. Additionally, LOPD had significantly greater anxiety on the DASS Anxiety scales than ROPD. However, unlike Foster et al (Cognitive and Behavioral Neurology, 23, 2010, 4), this increased anxiety could not account for the poorer performance on the DSB for LOPD. Finally, ROPD had significantly greater magical ideation than LOPD, which can be explained by the theory put forth by Brugger and Graves (European Archives of Psychiatry, 247, 1997, 55).ConclusionClear and significant differences between LOPD and ROPD were found within our cohort. LOPD showed greater impairment of working memory, greater anxiety, and greater duration of illness—all independent of one another; whereas, those with ROPD had greater magical ideation, also independent of any other variables.