Orthostatic hypotension
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Orthostatic hypotension (also known as postural hypotension and, colloquially, as head rush) is a sudden fall in blood pressure that occurs when a person assumes a standing position. Symptoms, which generally occur after sudden standing, include dizziness, lightheadedness, blurred vision, and syncope (temporary loss of consciousness).
It may be caused by hypovolemia (a decreased amount of blood in the body), resulting from the excessive use of diuretics, vasodilators, or other types of drugs, dehydration, or prolonged bed rest. The disorder may be associated with Addison's disease, atherosclerosis (build-up of fatty deposits in the arteries), diabetes, and certain neurological disorders including Shy-Drager syndrome and other dysautonomias.
When orthostatic hypotension is caused by hypovolemia due to medications, the disorder may be reversed by adjusting the dosage or by discontinuing the medication. When the condition is caused by prolonged bed rest, improvement may occur by sitting up with increasing frequency each day. In some cases, physical counterpressure such as elastic hose or whole-body inflatable suits may be required. Dehydration is treated with salt and fluids.
The prognosis for individuals with orthostatic hypotension depends on the underlying cause of the condition.
References
Adapted from public-domain resource at http://www.ninds.nih.gov/health_and_medical/disorders/orthosta_doc.htm