Primary: 1. Pure autonomic failure 2. Autonomic failure with multiple system atrophy 3. Familial autonomic dysfunction 4. Dopamine beta hydroxylase deficiency Secondary: 1. Chronic alcoholism 2. Parkinson’s disease 3. Diabetes 4. Porphyria 5. Amyloidosis 6. Malignancy 7. Vitamin B1 or B12 deficiency Other: 1. Hypovolemia (e.g. bleeding, burns, hemodialysis) and dehydration 2. Poor hemodynamic regulation (e.g. old age, pregnancy, fever, or prolonged bed rest) 3. Medications (e.g. antihypertensives, insulin, tricyclic antidepressants)