A Study of Pyridostigmine in Postural Tachycardia Syndrome

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