Inclusion Criteria:

• Diagnosis of postural tachycardia syndrome using the following criteria:
  • Orthostatic heart rate increment greater than or equal to 30 beats per minute (bpm) within 5 minutes of head-up tilt.
  • Symptoms of orthostatic intolerance. These include weakness, lightheadedness, blurred vision, nausea, palpitations, and difficulty with concentration and thinking.
• Both criteria must be fulfilled.

Exclusion Criteria:

• Pregnant or lactating women
• Presence of failure of other organ systems or systemic illness that could affect autonomic function or the patient's ability to cooperate with the study
• Hypothyroidism or hyperthyroidism
• Clinically significant coronary artery disease
• Medications that could interfere with autonomic testing
• Previous treatment with pyridostigmine for POTS. Patients must not have taken pyridostigmine in the past month