SudoScan as a Biomarker of Parkinson's Disease

Inclusion Criteria: 1. PD patients: All will meet criteria for probable PD, according to the new MDS Clinical Diagnostic criteria 2. Non-PD parkinsonism patients: They will have with progressive supranuclear palsy, multiple system atrophy, 'vascular parkinsonism' or corticobasal syndrome. All patients will have parkinsonism according to UK brain bank criteria, with a diagnosis of one of the above conditions made according to gold-standard expert evaluation. No patient will meet MDS Criteria for probable PD. 3. iRBD patients: All patients will have polysomnogram-confirmed RBD according to American Academy of Sleep Medicine Criteria. Patients will be free of parkinsonism and dementia according to neurological examination and will have no untreated sleep apnea, epilepsy, or other abnormalities that could cause dream enactment behavior. 4. Controls: These will be age matched (within 5 years) and sex-matched (with \>90% concordance). All controls will have an examination confirming the absence of parkinsonism, and will have no symptoms of REM sleep behavior disorder, as assessed with the RBD1Q and expert interview. Exclusion Criteria: 1. Diabetes Mellitus - In addition to causing autonomic neuropathy, hyperglycemia itself is known to interfere with results of the sudomotor scan 2. Any preceding diagnosis of autonomic neuropathy (of a cause other than PD) 3. Dementia of severity sufficient to preclude informed consent, MoCA \<23. 4. Prescription of medications that directly alter peripheral autonomic function, including beta-blockers, sympatholytics (i.e. clonidine) and non-specific alpha-blockers.