Proposed biological definitions of Parkinson's disease confuse understanding without delivering meaningful advances.

This article reflects on two proposals to classify and stage Parkinson's disease (PD) and related conditions based on the presence of aggregated alpha-synuclein (ASN) detected by alpha-synuclein aggregation assay (SAA). The authors highlight the significant shortcomings of the proposals: detection of ASN by SAA is not specific of PD or other well-established clinical conditions; they do not allow prediction of clinical course and prognosis; and they are not suitable as an endpoint for clinical trials. To move forward with proposals reflecting so many uncertainties and unknowns will just sow confusion and misunderstanding within the PD community.