orthostatic intolerance, an abnormal response to standing upright that results from decreased blood pressure and inadequate blood flow to the brain, characterized by a variety of symptoms, including light-headedness, palpitations, tremulousness, visual disturbances, and syncope. It occurs in both acute and chronic forms and is frequently seen as a transient condition in space travelers returning from microgravity. The cause is unknown but may be related to abnormalities in the autonomic regulation of cardiovascular function.
