The term orthostasis means to stand up. Orthostatic intolerance is the presentation of symptoms which interfere with or prevent standing up. Orthostatic intolerance (OI) affects heart rate, blood pressure and blood distribution in the brain. This can present as a number of symptoms with multiple root causes.
The autonomic nervous system is responsible for making quick changes to the cardiovascular system based upon changes in the environment. It adjusts the circulatory system by changing heart rate, constricting blood vessels and inducing secretion from the adrenal gland to sustain a normal blood pressure.
The sympathetic nervous system is one part of the autonomic nervous system and its function is to activate the fight-or-flight response. When it malfunctions as in OI, it causes pallor, headache, high blood pressure, palpitations, sweating, tremor and anxiety. When the autonomic nervous system is unbalanced with the parasympathetic system being more active, low blood pressure, slow heart rate, cold hands and feet and constriction of the pupil may occur. This is called vagotonia. Another principle symptom of orthostatic intolerance is intolerance to exercise.
Other parts of the nervous system are involved here as well. Misbehavior in the central nervous system can cause loss of consciousness, dizziness and cognitive issues. Malfunction of the vagus nerve can cause tachycardia, abdominal pain and nausea/vomiting.
There are three common forms of orthostatic intolerance.
Orthostatic hypotension is a consistent reduction of systolic blood pressure of more than 20 mm Hg or diastolic blood pressure of more than 10 mm Hg within three minutes of standing or a head up tilt of at least 60°. Orthostatic hypotension can occur for many reasons, including dehydration, blood loss or conditions that cause acute or chronic hypovolemia. Neurogenic OH is caused by insufficient norepinephrine released from cells of the sympathetic nervous system, causing inadequate vasoconstriction. Neurogenic OH typically occurs secondary to a systemic disease.
POTS patients suffer daily OI symptoms in conjunction with excessive tachycardia when standing, but not with low blood pressure. In adults, excessive tachycardia is defined as an increase of 30 bpm when standing or over 120 bpm. In children, excessive tachycardia is an increase of 40 bpm. Tachycardia is not sufficient for diagnosis; patients must also have OI symptoms. There are multiple subcategories of POTS, which I have previously covered and will cover in more detail elsewhere.
Postural syncope can be caused by acute orthostatic intolerance, simple fainting or vagovagal syncope (VVS). Syncope, also called fainting, is the loss of consciousness due to temporarily insufficient blood supply to the brain, followed by complete recovery. In short, this means fainting upon standing up. About 40% of people will faint at some point in their lives. About half of these people have their initial episode during adolescence, most around the age of 15. Syncope can be cardiovascular, from arrhythmia or structural abnormalities, or reflex/neurologic.
Mast cell disease (both mastocytosis and MCAS) has a known propensity for causing orthostatic intolerance.
References:
Stewart, Julian M. Update on the theory and management of orthostatic intolerance and related syndromes in adolescents and children. Expert Rev Cardiovasc Ther 2012 November; 10(11): 1387-1399.
Figueroa, Juan J., et al. Preventing and treating orthostatic hypotension: As easy as A, B, C. Cleve Clin J Med 2010 May; 77(5): 298-306.
Medow MS, Stewart JM, Sanyal S, Mumtaz A, Sica D, Frishman WH. Pathophysiology, diagnosis, and treatment of orthostatic hypotension and vasovagal syncope. Cardiol. Rev. 2008; 16(1):4–20.
Bayles R, Kn H, Lambert E, et al. Epigenetic modification of the norepinephrine transporter gene in postural tachycardia syndrome. Arterioscler. Thromb. Vasc. Biol. 2012; 32(8):1910–1916.