Effects of Midodrine and Droxidopa on Splanchnic Capacitance in Autonomic Failure

Inclusion Criteria: * Male and female subjects, age 40-80 years with possible or probable Multiple System Atrophy, Pure Autonomic Failure, or Parkinson disease, as defined by Consensus Criteria. * Neurogenic orthostatic hypotension defined as a ≥30-mmHg decrease in systolic blood pressure within 3 minutes of standing associated with impaired autonomic reflexes determined by autonomic testing in the absence of other identifiable causes. * Subjects able and willing to provide informed consent. Exclusion Criteria: * Supine hypertension, defined as systolic blood pressure of ≥ 160 mmHg measured on two separate occasions. * Pregnancy. * Systemic illnesses known to produce autonomic neuropathy, including but not limited to diabetes mellitus, amyloidosis, monoclonal gammopathies, and autoimmune neuropathies. * History of known aortic aneurisms, thoracic, abdominal or pelvic surgery in the past 6 months. * Symptomatic abdominal or inguinal hernias. * Severe gastroesophageal reflux. * Recent fractures or fissures of ribs, thoracic or lumbar spine. * Medical devices implanted on the abdominal wall or abdomen that would interfere with the abdominal compression. * Intolerance to any increase in intraabdominal pressure. * Clinically unstable coronary artery disease or major cardiovascular or neurological event in the past 6 months, and other factors which in the investigator's opinion would prevent the subject from completing the protocol including clinically significant abnormalities in clinical, mental or laboratory testing.