Neurophysiology of Atypical Parkinsonian Syndromes: A Study Group Position Paper.

Atypical parkinsonian syndromes (APs) are characterized by parkinsonian features combined with additional motor and non-motor signs and symptoms. Neurophysiological studies have contributed to clarifying differences and similarities between APs and idiopathic Parkinson's disease (PD) and to unravel specific pathophysiological features of APs. A comprehensive and updated evaluation of the potential clinical utility of the available neurophysiological tools in APs is, however, currently needed. The Neurophysiology Study Group of the International Parkinson and Movement Disorder Society reviewed previously published neurophysiological studies including those based on electromyography, electroencephalography, and evoked potentials, transcranial magnetic stimulation and kinematics, in most relevant APs, including progressive supranuclear palsy, multiple system atrophy, corticobasal syndrome, Lewy body dementia, fronto-temporal dementia, vascular parkinsonism, normal pressure hydrocephalus, and drug-induced parkinsonism. Following a critical narrative review of all the available information for each AP, the study group examined the most relevant pathophysiological advances achieved in the field owing to the application of specific neurophysiological tools. Furthermore, the review includes statements regarding the potential role in a research context (ie, pathophysiological investigation) as well as in the clinical setting (ie, clinical utility) of each neurophysiological technique, through an estimation of the corresponding levels of evidence, based on the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) system. Finally, an example of a possible stepwise approach based on the sequential application of specific neurophysiological techniques for better supporting the clinical differential diagnosis of PD and APs is proposed. © 2025 International Parkinson and Movement Disorder Society.