α-Synuclein seed amplification assay positivity beyond synucleinopathies.

Neurodegenerative diseases are increasingly recognized as complex disorders involving multiple protein pathologies, with α-synuclein frequently observed beyond classical synucleinopathies such as Parkinson's disease and multiple system atrophy. Recent advances in seed amplification assays (SAAs) have enabled the highly sensitive and specific detection of misfolded α-synuclein in vivo, particularly in cerebrospinal fluid (CSF). This review focuses on CSF-based α-synuclein SAAs and their application in detecting co-pathology across non-synucleinopathies, including Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, idiopathic normal pressure hydrocephalus, and traumatic brain injury. Evidence indicates a role for α-synuclein in clinical heterogeneity and disease progression. Emerging diagnostic frameworks increasingly support integrating co-pathologies into classification and therapeutic strategies. Addressing key knowledge gaps, such as α-synuclein interactions with other protein pathologies, and current limitations of α-syn SAA, such as the lack of quantification of misfolded α-synuclein seeds, will refine precision medicine and improve outcomes for patients with neurodegenerative diseases.