The autonomic or automatic nervous system helps control blood pressure. Diseases of the autonomic nervous system may result in a drop in blood pressure on standing in many cases leading to fainting. Diseases that affect the autonomic nervous system include pure autonomic failure, multiple system atrophy and Parkinson's disease, and can present with very similar symptoms and it is sometimes difficult to determine an exact diagnosis. The purpose of the study is to find out if the blood pressure response from taking a single dose of the medication atomoxetine can help in the diagnosis of these diseases.

This is an observational, prospective three-year longitudinal study. The investigators will enroll participants with primary neurogenic orthostatic hypotension. All participants will undergo an extensive neurological and cardiovascular evaluation, including detailed autonomic testing and quality of life assessment. The investigators will then determine the magnitude of the pressor effect produced by 18 mg atomoxetine given orally, measured 1 hour after drug administration. Participants will be followed annually or more often if there is a significant change in their clinical condition. During follow up at year 3, the investigators will repeat the initial neurological, cardiovascular and autonomic evaluation. The primary endpoint would be the final diagnosis made at year 3 after the initial evaluation (at the end of the follow-up period) or if they develop significant worsening of symptoms during follow-up phone assessments, based on specific clinical criteria.

Inclusion Criteria:

• Age 18-80 years old with Neurogenic orthostatic hypotension, ≥30 mmHg drop in SBP within 5 minutes of standing
• Associated with impaired autonomic reflexes, as determined by absence of blood pressure overshoot during phase IV of the Valsalva maneuver
• Absence of other identifiable causes of autonomic neuropathy
• Able and willing to provide informed consent

Exclusion Criteria:

• Pregnancy
• Systemic illnesses known to produce autonomic neuropathy, including but not limited to diabetes mellitus, amyloidosis, monoclonal gammopathy of unknown significance, and autoimmune neuropathies
• Known intolerance to atomoxetine, Pre-existing sustained severe hypertension (BP at least 180/110 mmHg in the sitting position)
• Clinically unstable coronary artery disease, or major cardiovascular or neurological event in the past 6 months
• Any other significant systemic, hepatic, cardiac or renal illness
• Use of MAO-I within 14 days
• Known closed-angle glaucoma or Life-threatening arrhythmias

participants with neurogenic orthostatic hypotension, which is a drop in systolic blood pressure of at least 30 mmHg within 5 minutes of standing with associated impaired autonomic reflexes.