[Orthostatic intolerance and syncope]

To describe the current state of knowledge and clinical features of patients with othostatic intolerance.

Patients with orthostatic intolerance can be classified in two types. In one of them, sympathetic activity is diminished and they have hypotension when they stand up. This becomes chronic in patients with central or peripheral neurodegenerative diseases and is acute in those with vasovagal syncope. In the second type of orthostatic intolerance patients display an increase in sympathetic activity, without arterial hypotension and a notable tachycardia, this is called postural tachycardia syndrome. The mechanism responsible for this type of orthostatic intolerance and the exact cause of the characteristic symptoms are unknown. Diverse theories include a neuropathic lesion that affects the nerves in the lower part of the body in a selective fashion or a congenital or acquired defect in the neuronal norepinephrine transporter.

Orthostatic intolerance is the difficulty experienced by patients to remain standing due to a dysfunction of the autonomic nervous system. Orthostatic intolerance occurs when efferent sympathetic activity is notably diminished or, paradoxically, in patients in whom efferent sympathetic activity increases in an exaggerated manner when they stand up.