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Orthostatic intolerance (OI) is a subcategory of dysautonomia, a disorder of the autonomic nervous system[1] occurring when an individual stands up.[2]
OI can also be defined as “the development of symptoms during upright standing relieved by recumbency,” or by sitting back down again.[3] It affects more women than men (female-to-male ratio is at least 4:1), usually under the age of 35.[4]
Orthostatic intolerance occurs in humans because standing upright is a fundamental stressor and requires rapid and effective circulatory and neurologic compensations to maintain blood pressure, cerebral blood flow, and consciousness. When a human stands, approximately 750 mL of thoracic blood is abruptly translocated downward. People who suffer from OI lack the basic mechanisms to compensate for this deficit.[3] Changes in heart rate, blood pressure, and cerebral blood flow that produce OI “may be related to abnormalities in the interplay between blood volume control, the cardiovascular system, the autonomic nervous system and local circulatory mechanisms that regulate these basic physiological functions.”[5]
Symptoms of OI are triggered by the following:
Orthostatic intolerance is divided, roughly based on patient history, in two variants: acute and chronic.
Patients who suffer from acute OI usually manifest the disorder by a temporary loss of consciousness and posture, with rapid recovery (simple faints, or syncope), as well as remaining conscious during their loss of posture. This is different than a syncope caused by cardiac problems because there are known triggers for the fainting spell (standing, heat, emotion) and identifiable prodromal symptoms (nausea, blurred vision, headache). As Dr. Julian M. Stewart, an expert in OI from New York Medical College states, “Many syncopal patients have no intercurrent illness; between faints, they are well.”[3]
Symptoms:[5]
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