Cognitive and motor profiles in biologically defined Parkinson's and Alzheimer's disease.

BackgroundParkinson'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.ObjectiveTo 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.MethodsWe 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.ResultsAmong 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.ConclusionsBiomarker-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.