[Orthostatic hypotension: epidemiology and mechanisms].

Orthostatic hypotension (OH) is defined as a decrease in systolic blood pressure more than or equal to 20 mm Hg or diastolic more than or equal to 10 mm Hg within three minutes of standing up. Discharge records for 0.4% of all hospitalizations include OH as a diagnosis. The prevalence of OH is related to aging, reaching 5 to 30% in unselected elders aged more than or equal to 65 years, especially those treated for hypertension and/or living in nursing homes or health care facilities. OH is associated with an increased risk of syncope and subsequent cardiovascular mortality. Standing results in pooling of 25-30% of blood in the lower extremities and splanchnic-mesenteric circulation, thereby reducing venous return, ventricular filling, cardiac output and, in turn, blood pressure. Compensatory mechanisms to maintain BP involve the baroreflex and the sympathetic nervous system (SNS). Failure of SNS results in neurogenic OH, which is severe in multiple system atrophy with dysautonomia (Shy-Drager syndrome), also commonly seen in patients with Lewy body dementia and in patients with Parkinson's disease, in whom OH is less severe and occurs later. Conversely, SNS function is preserved in secondary OH associated with hypovolemia, anemia, antihypertensive or psychotropic drugs. Clinical manifestations and tolerance of OH depends on its severity and on cerebral blood flow autoregulation, which both impact on the risk of syncope, trauma and subsequent prognosis.