Parkinsonian signs and cognitive trajectories in older adults: a population-based longitudinal study

Cognitive impairment is common in parkinsonian disorders, but its extent among older adults with subtle motor abnormalities without a diagnosed movement disorder remains unclear. We examined cognitive decline, domain-specific cognitive function and incident dementia in older adults with subthreshold parkinsonism and parkinsonism (primary analysis). Sensitivity analyses were repeated using an additional definition of Mild Parkinsonian Signs for comparison.

In the longitudinal general-population study Good Ageing in Skåne, 398 older adults aged 80-101 years (mean 86) were examined using the Unified Parkinson's Disease Rating Scale part III and a comprehensive cognitive test battery. Participants were followed for a mean of 2.67 (0-9) years. Linear mixed models adjusted for age, sex and education were used to examine cognitive trajectories.

At baseline, 54.4% of participants met the criteria for subthreshold parkinsonism and 23.6% for parkinsonism. Subthreshold parkinsonism was associated with reduced performance in perceptual speed (z-score estimate: -0.51), language (-0.36), executive function (-0.36) and global cognition (-0.33). Parkinsonism was associated with reduced performance in memory (-0.30), perceptual speed (-0.40), language (-0.33), executive function (-0.32) and global cognition (-0.34). Memory decline was most pronounced in both groups. During follow-up, 18% of participants developed dementia; of these, 46% had subthreshold parkinsonism and 26% parkinsonism at baseline.

Subthreshold parkinsonism and parkinsonism are associated with cognitive impairment, particularly perceptual speed, language, executive function and global cognition that progress over time. These findings support parkinsonian signs as a prodromal state and highlight the importance of early detection for dementia prevention.