<unk>  is most commonly diagnosed by a cardiologist (41%), cardiac electrophysiologist (15%), or Neurologist (19%). The average number of physicians seen before receiving diagnosis is seven, and the average delay before diagnosis is 4.7 years. Diagnostic criteria A<unk>  diagnosis requires the following characteristics: * For patients age 20 or older, increase in heart rate ≥30 bpm within ten minutes of upright posture (tilt test or standing) from a supine position * For patients age 12–19, heart rate increase must be >40 bpm * Associated with related symptoms that are worse with upright posture and that improve with recumbence * Chronic symptoms that have lasted for longer than six months * In the absence of other disorders, medications, or functional states that are known to predispose to<unk><unk>  Autoantibodies against G-protein coupled receptor Measurement of G protein–coupled receptor activity may be used as a diagnostic tool in the near future.<unk>  intolerance An increase in heart rate upon moving to an upright posture is known as<unk>  (upright)<unk>  (fast heart rate). It occurs without any coinciding drop in blood pressure, as that would indicate<unk>  hypotension. Certain medications to treat<unk>  may cause<unk>  hypotension. It is accompanied by other features of<unk>  intolerance—symptoms that develop in an upright position and are relieved by reclining. These<unk>  symptoms include palpitations, light-headedness, chest discomfort, shortness of breath, nausea, weakness or "heaviness" in the lower legs, blurred vision, and cognitive difficulties. Differential diagnoses A variety of autonomic tests are employed to exclude autonomic disorders that could underlie symptoms, while endocrine testing is used to exclude hyperthyroidism and rarer endocrine conditions. Electrocardiography is normally performed on all patients to exclude other possible causes of<unk> . In cases where a particular associated condition or complicating factor are suspected, other non-autonomic tests may be used: echocardiography to exclude mitral valve prolapse, and thermal threshold tests for small-fiber neuropathy. Testing the cardiovascular response to prolonged head-up tilting, exercise, eating, and heat stress may help determine the best strategy for managing symptoms.<unk>  has also been divided into several types (see § Causes), which may benefit from distinct treatments. People with neuropathic<unk>  show a loss of sweating in the feet during sweat tests, as well as impaired norepinephrine release in the leg, but not arm. This is believed to reflect peripheral sympathetic denervation in the lower limbs. People with hyperadrenergic<unk>  show a marked increase of blood pressure and norepinephrine levels when standing, and are more likely to suffer from prominent palpitations, anxiety, and<unk> . People with<unk>  can be misdiagnosed with inappropriate sinus<unk>  as they present similarly. One distinguishing feature is those with<unk>  rarely exhibit >100 bpm while in a supine position, while patients with IST often have a resting heart rate >100 bpm. Additionally patients with<unk>  display a more pronounced change in heart rate in response to<unk>  change.
Postural orthostatic tachycardia syndrome
Da Costa's syndrome  Da Costa's syndrome  1
Heat syncope
Laughter-induced syncope
Vasoplegic syndrome
Reflex syncope
Permissive hypotension
Postanesthetic shivering