Revisiting Parkinson's disease definition and classification: insights from two emerging biological frameworks.

Parkinson's disease (PD) is increasingly recognized as a heterogeneous neurodegenerative entity with diverse clinical presentations, genetic contributors, and neuropathological features. Central to its pathogenesis is misfolded and aggregated α-synuclein, which collectively form Lewy pathology. Recent advances in biomarker and genetic research have enabled biologically grounded models of PD classification, diagnosis and staging. This review summarizes key principles, differences, and ongoing challenges of two emerging research frameworks: the SynNeurGe criteria and the Neuronal α-Synuclein Disease Integrated Staging System (NSD-ISS)-the former proposed a biologically based classification, while the latter proposed a more restrictive biological definition and staging schema. SynNeurGe incorporates synucleinopathy (S), neurodegeneration (N), genetic risk (G) and clinical status (C) to classify etiologic subtypes across the disease spectrum, emphasizing clinical heterogeneity and multifaceted underlying biological processes. In contrast, the NSD-ISS defines "neuronal α-synuclein disease" (NSD) based on specific molecular (S) and dopaminergic dysfunction (D) markers and a single genetic anchor (SNCA) (G), and maps disease progression across seven clinical stages. While both aim to improve early detection and to advance PD research, they differ in scope, operational definitions, implementation principles, and intended applications. Prevailing challenges include current limitations in mechanistic insights, biomarker standardization and accessibility, underrepresentation of genetic diversity, and ethical considerations around disease labeling and risk disclosure, particularly in asymptomatic cases. These frameworks represent a pivotal shift toward biologically based concepts of PD and related disorders, with future success contingent on continued refinement, validation, and equitable implementation.