Syncope (Greek, synkope, meaning cessation, pause) is a transient loss of consciousness and postural tone with spontaneous recovery and no neurologic sequelae. Syncope is caused by a global reversible reduction of blood flow to the reticular activating system, the neuronal network in the brain stem responsible for supporting consciousness. Syncope most frequently occurs while standing because in the vertical position blood pressure and blood flow to the brain are exquisitely dependent on the normal functioning of the cardiovascular system. Thus, abnormalities in cardiac output or in autonomic reflexes controlling blood pressure can cause syncope while standing. The most common cause of syncope is orthostatic (Greek, orthos, straight; statikos, causing to stand) hypotension caused by an acute and transient dysfunction of autonomic reflexes (i.e., neurally mediated syncope). It is important to keep in mind, however, that syncope may also occur without systemic hypotension when hypocapnia-induced diffuse cerebral vasoconstriction results in a global reduction in cerebral blood flow. Finally, because the brain is enclosed in the nondistensible cranium, maneuvers that raise intracranial pressure may reduce cerebral blood flow drastically inducing symptoms of cerebral ischemia including syncope.

Syncope occurs often, at least once in 3% of men and in 3.5% of women.<sup>76 Savage D., Corwin L., McGee D. , et al. Epidemiologic features of isolated syncope: The Framingham Study Stroke 1985 ;  16 : 62

Cliquez ici pour aller à la section Références</sup> The causes of syncope are numerous. Further complicating the differential diagnosis of syncope are other causes of loss of consciousness, such as seizures and metabolic and psychiatric disorders, which may simulate syncope. This article reviews the autonomic and neuroendocrine mechanisms involved in maintaining blood pressure and blood flow to the brain in the standing position, the causes of syncope, its differential diagnosis, and its treatment.