Cognitive and motor profiles in biologically defined Parkinson's and Alzheimer's disease
Background Parkinson's disease (PD) patients may harbor coexisting Alzheimer's disease (AD) pathology that accelerates cognitive and motor decline. Defining biomarker-defined AD in PD is important for prognosis, patient counseling, and trial stratification. Objective To determine the prevalence of AD biomarker positivity in α-synuclein seed amplification assay (αSyn-SAA) positive PD and assess its impact on cognitive and motor progression. Methods We analyzed data from the Parkinson's Progression Markers Initiative, a multinational prospective cohort of de novo PD patients. Baseline CSF biomarkers included αSyn-SAA, amyloid-β1-42 (Aβ1-42), and phosphorylated tau181 (p-tau181). AD biomarker positivity was defined by a low Aβ1-42/high p-tau181 profile using the CSF Aβ1-42/p-tau181 ratio (<39.2). αSyn-SAA-positive PD participants with and without AD biomarker positivity were compared. Outcomes included cognition, Montreal Cognitive Assessment (MoCA), neuropsychological testing, and MDS-UPDRS motor scores over follow-up. Results Among 449 αSyn-SAA-positive PD patients, 42 (9.3%) met AD biomarker criteria (PD-AD). Baseline cognition and motor scores did not differ between PD-AD and PD without AD biomarkers (PD-nonAD). Over time, PD-AD patients showed greater cognitive decline, with lower MoCA scores and higher MCI prevalence at 5 years. At 8 years, PD-AD patients also demonstrated worse motor outcomes. MoCA <24 predicted AD biomarker positivity (PPV 15%; OR 6.1). APOE ε4 status was not associated with cognition. Conclusions Biomarker-defined AD identifies a distinct PD subgroup with accelerated cognitive and motor decline, independent of APOE ε4. A ratio-based CSF Aβ1-42/p-tau181 framework offers a practical approach to detect AD copathology in PD and refine prognostic stratification.