Are Standardized Tests Sensitive to Early Cognitive Change in Parkinson's Disease?

Cognitive deficits within the first years of Parkinson's disease (PD) diagnosis are commonly reported, and progression to dementia greatly impacts independence. Identifying measures sensitive to early changes is critical for trials of symptomatic therapies and neuroprotection.

A sample of 253 newly diagnosed PD patients and 134 Health Controls (HC) completed a brief cognitive battery annually over a 5-year period through the Parkinson's Progression Markers Initiative (PPMI). The battery included standardized measures of memory, visuospatial functions, processing speed, working memory, and verbal fluency. Inclusion criterion for HCs was performance above a cutoff for possible Mild Cognitive Impairment (pMCI) on cognitive screening (MoCA ⩾ 27) The PD sample was therefore divided to match HCs on baseline cognitive testing (PD-normal n = 169; PD-pMCI n = 84). The multivariate approach to repeated measures examined rates of change between groups on cognitive measures.

An interaction indicating slightly greater decline over time in PD-normal relative to HCs was observed on a measure of working memory: letter-number sequencing. Differential rates of change were not observed on any other measures. Motor symptoms on the dominant right upper extremity accounted for performance differences on a test with writing demands (Symbol-Digit Modality Test). PD-pMCI performed worse than PD-normal on all cognitive measures at baseline, but did not decline faster.

Working memory appears to decline slightly faster in early PD compared to HCs, while other domains remain similar. Within PD, faster decline was not associated with lower baseline cognition. These findings have implications for clinical trial outcome selection and study design.