Semantic fluency and processing speed are reduced in non-cognitively impaired participants with Parkinson’s disease

Introduction: Parkinson’s disease (PD) is associated with a range of cognitive deficits. Few studies have carefully examined the subtle impacts of PD on cognition among patients who do not meet formal criteria for MCI or dementia. The aim of the current study was thus to describe the impact of PD on cognition in those without cognitive impairment in a well-characterized cohort. Methods: Non-cognitively impaired participants (122 with PD, 122 age- and sex-matched healthy volunteers) underwent extensive cognitive testing. Linear regression analyses compared diagnostic group performance across cognitive measures. For cognitive tasks that were significantly different between groups, additional analyses examined group differences restricting the group inclusion to PD participants with mild motor symptoms or disease duration less than 10 years. Results: Processing speed and semantic verbal fluency were significantly lower in the PD group (B = −3.77, 95% CIs [−5.76 to −1.77], p < .001, and B = −2.02, 95% CIs [−3.12, −0.92], p < .001, respectively), even after excluding those with moderate to severe motor symptoms (B = −2.73, 95% CIs [−4.94 to −0.53], p = .015 and B = −2.11, 95% CIs [−3.32 to −0.91], p < .001, respectively) or longer disease duration (B = −3.89, 95% CIs [−6.14 to −1.63], p < .001 and B = −1.58, 95% CIs [−2.78 to −0.37], p = .010, respectively). Semantic verbal fluency remained significantly negatively associated with PD diagnosis after controlling for processing speed (B = −1.66, 95% CIs [−2.79 to −0.53], p = .004). Conclusions: Subtle decline in specific cognitive domains may be present among people diagnosed with PD but without evidence to support a formal cognitive diagnosis. These results suggest the importance of early awareness of the potential for diminishing aspects of cognition in PD even among those without mild cognitive impairment or dementia.