In adults, the primary manifestation is an increase in heart rate of more than 30 beats per minute within ten minutes of standing up. The resulting heart rate is typically more than 120 beats per minute. For people aged between 12 and 19, the minimum increase for diagnosis is 40 beats per minute. This 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. Symptoms may be exacerbated with prolonged sitting, prolonged standing, alcohol, heat, exercise, or eating a large meal. In up to one third of people with<unk> , fainting occurs in response to<unk>  changes or exercise. Migraine-like headaches are common, sometimes with symptoms worsening in an upright position<unk>  headache). 40–50% of patients with<unk>  develop acrocyanosis of extremities, a reddish-purple color in the legs and/or hands when they stand (indicative of blood pooling). 48% of people with<unk>  report chronic fatigue and 32% report sleep disturbances. Others exhibit only the cardinal symptom of<unk><unk> . Additional signs and symptoms are varied, and may include excessive sweating, a lack of sweating, heat intolerance, digestive issues such as bloating, nausea, indigestion, constipation, and diarrhea, a flu-like feeling, coat-hanger pain, forgetfulness, brain fog, and presyncope. Brain fog One of the most disabling and prevalent symptoms in<unk>  is "brain fog", a term used by patients to describe the cognitive difficulties they experience. In one survey of 138<unk>  patients, brain fog was defined as “forgetful” (91%), “difficulty thinking” (89%), and “difficulty focusing” (88%). Other common description was "Difficulty processing what others say" (80%), Confusion (71%), Lost (64%), and "Thoughts moving too quickly" (40%) The same survey described the most common triggers of brain fog to be fatigue (91%), lack of sleep (90%), prolonged standing (87%) and dehydration (86%). Neuropsychological testing has shown that a<unk> -patient has reduced attention (Ruff 2&7 speed and WAIS-III digits forward), short-term memory (WAIS-III digits back), cognitive processing speed (Symbol digits modalities test) and executive function (Stroop word color and trails B). A potential cause for brain fog is a decrease in cerebral blood flow (CBF), especially in upright position. Another theory is that interoception of excessive autonomic activity causes interoceptive prediction errors (PE) to occur. A prediction error is the mismatch between a prior expectation and reality. This would cause anxiety which then overwhelms the consciousness of<unk> -patient causing cognitive-affective symptoms. This then makes them predisposed to response-focused emotion regulation (ER) instead of an antecedent-focused ER.
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