Definition, epidemiology, classification, and pathophysiology
Syncope can closely resemble other disorders with which it shares an apparent short-lived transient loss of consciousness. Together, these disorders are labelled as ‘transient loss of consciousness (T-LOC)’. Syncope is the form of T-LOC caused by cerebral hypoperfusion; the other main T-LOC forms are several types of epileptic seizures and the psychogenic conditions that resemble either syncope or epileptic seizures. The main forms of syncope are reflex syncope, syncope due to orthostatic hypotension, and cardiac syncope, also comprising cardiopulmonary causes and disorders of the great vessels. All forms of syncope share cerebral hypoperfusion and arterial hypotension as a final common pathway. They differ in the mechanism of hypotension: cardiac syncope is largely due to a low cardiac output, but both low peripheral resistance and low cardiac output contribute to syncope due to orthostatic hypotension and reflex syncope. The clinical expression of the main forms is tightly linked to their pathophysiology, which is therefore important for differential diagnosis.