From Wikipedia, the free encyclopedia - View original article
|Classification and external resources|
|Classification and external resources|
Orthostatic intolerance (OI) is the development of symptoms when standing upright which are relieved when sitting back down again. There are many types of orthostatic intolerance. OI can be a subcategory of dysautonomia, a disorder of the autonomic nervous system occurring when an individual stands up.
It affects more women than men (female-to-male ratio is at least 4:1), usually under the age of 35. Up to 97% of those who have chronic fatigue syndrome (CFIDS) have been said been shown in studies to have some form of OI. 
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. Changes in heart rate, blood pressure, and cerebral blood flow that produce OI may be caused by abnormalities in the interactions between blood volume control, the cardiovascular system, the nervous system and circulation control systems.
Symptoms of OI are triggered by the following:
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."
A classic manifestation of acute OI is a soldier who faints after standing rigidly at attention for an extended period of time.
Patients with chronic orthostatic intolerance have symptoms on most or all days. Their symptoms may include most of the symptoms of acute OI, plus:
OI is "notoriously difficult to diagnose." As a result, many patients have gone undiagnosed or misdiagnosed and either untreated or treated for other disorders. Current tests for OI (Tilt table test, autonomic assessment, and vascular integrity) can also specify and simplify treatment. (See Dr. Julian Stewart's article, "Orthostatic Intolerance: An Overview" for a more detailed description of OI tests.)
Most patients experience an improvement of their symptoms, but for some, OI can be gravely disabling and can be progressive in nature, particularly if it is caused by an underlying condition which is deteriorating. The ways in which symptoms present themselves vary greatly from patient to patient; as a result, individualized treatment plans are necessary.
OI is treated both pharmacologically and non-pharmacologically. Treatment does not cure OI; rather, it controls symptoms.
Physicians who specialize in treating OI agree that the single most important treatment is drinking more than two liters (eight cups) of fluids each day. A steady, large supply of water or other fluids reduces most, and for some patients all, of the major symptoms of this condition. Typically, patients fare best when they drink a glass of water no less frequently than every two hours during the day, instead of drinking a large quantity of water at a single point in the day.
For most severe cases and some milder cases, a combination of medications are used. Individual responses to different medications vary widely, and a drug which dramatically improves one patient's symptoms may make another patient's symptoms much worse. Medications focus on three main issues:
Medications that increase blood volume:
Medications that inhibit acetylcholinesterase:
Medications that improve vasoconstriction:
Behavioral changes that patients with OI can make are:
A notable sufferer of orthostatic intolerance is Greg Page, from The Wiggles. It is due to this illness that Greg chose to leave the group in 2006. He went on to create his own fund for OI, to help fund research into this little known disorder.  Greg recovered sufficiently to announce his return to The Wiggles in 2012.