Limbic Alzheimer's co-pathology in multiple system atrophy is associated with cognitive impairment and diagnostic inaccuracy.

The clinical heterogeneity of multiple system atrophy (MSA) may, along with the primary aggregation of alpha-synuclein (α-syn), partly be shaped by co-pathologies, such as amyloid-beta (Aβ), phosphorylated (p)-tau, and pTDP-43, though their relevance remains unclear. Here, we aimed to characterize the prevalence, morphology, regional patterns, and clinical relevance of co-pathologies in a well-characterized MSA autopsy cohort. Regional load (%area) and morphological characterization of α-syn (KM51), Aβ (6F/3D), p-tau (AT8), and pTDP-43 (pSer409) pathology were assessed in limbic regions of MSA (n = 70) donors from the Netherlands Brain Bank. APOE-ε4 genotyping and clinical parameters of the cohort were collected. Associations with clinical features were analyzed using ANCOVA and mixed linear models, adjusted for age and sex. Aβ, p-tau, and pTDP-43 pathology were detected in 31%, 91%, and 11% of all MSA cases, respectively, with the highest burdens in the entorhinal cortex and amygdala. Mixed MSA + AD cases had a higher age at death (75 ± 7 vs. 64 ± 7, p < 0.001), and more frequently APOE-ε4 alleles (p < 0.001) than pure MSA. Cognitive impairment (CDR scores) was associated with diffuse and compact Aβ plaques across all regions (r ≥ 0.24, p ≤ 0.015), p-tau pathology in CA1 and CA3 + 4 (r ≥ 0.32, p ≤ 0.020), and neuronal α-syn inclusions in the amygdala (r = 0.54, p < 0.001). No robust correlations were found between total α-syn burden and Aβ or p-tau. Misdiagnoses increased with co-pathology burden (Aβ: p = 0.040; p-tau: p = 0.020) and age at onset (80% in those with onset > 75 years). Our results demonstrate that limbic Aβ, p-tau, and neuronal α-syn pathologies occur in a substantial proportion of MSA donors and are independently associated with cognitive decline and diagnostic inaccuracy, particularly among those with older age at onset. By providing a systematic quantitative and morphological assessment of co-pathologies in limbic regions of MSA, our study advances beyond prior prevalence reports and highlights their direct clinical relevance. These findings highlight the need for refined diagnostic criteria and co-pathology-informed biomarker strategies for MSA.