Syndromes of Orthostatic Intolerance: Pathophysiology and Diagnosis

Orthostasis means standing upright. Thus, orthostatic intolerance (OI) can be simply defined as "the development of symptoms during upright standing, that are relieved by recumbency." However, OI might be a confusing topic in clinical practice because of the recent appreciation of the condition's clinical variant, emerging understanding of its diverse mechanisms, and its nomenclature, which seems to change annually. OI is not fatal but should be differentiated from potentially lethal disorders, including seizures or cardiogenic syncope. Typical signs and symptoms include loss of consciousness, lightheadedness, and visual difficulties. However, patients also experience multiple and nonspecific symptoms that seem unrelated to orthostatic intolerance, such as headache, fatigue, nausea, abdominal pain, and exercise intolerance. This review was aimed at expanding the comprehension of this confusing and easily missed topic by providing better understanding of the normal hemodynamic response to orthostasis and the basic pathophysiological concepts of major syndromes of OI.