Treatment of Orthostatic Intolerance in Patients With Parkinson's Disease Using Midodrine

This will be a cross-over study where participants with OI will be randomized to initially receive midodrine or placebo then crossed over to the opposite treatment after three weeks (2 weeks on midodrine or placebo plus one week wash out period). The control group will consist of participants with OH and PD being treated with midodrine. Basic demographic data including will be collected from the medical record of each participant after consent. At each study visit, each participant will undergo traditional measurement of blood pressure and heart rate as well as measurement of beat-to-beat blood pressure and heart rate using CNAP™ during valsalva maneuver and in response to standing for 5 minutes after sitting. Symptoms of orthostatic intolerance will be measured during the study visit using Domain 1 of the Non-Motor Symptoms Scale for Parkinson's Disease (NMSS). At the initial study visit, participants will also be administered first dose of midodrine or placebo. Supine sitting, and standing systolic and diastolic blood pressure and pulse rates will be measured immediately before and 1 hour after the administration of drug or placebo. This blood pressure monitoring process will take place at each of the four study visits.