Vasovagal syncope is a temporary loss of consciousness caused by a sudden drop in blood pressure and, often, a slowing of the heart rate. It is the most common cause of blackout and fainting, accounting for the majority of syncopal episodes seen in otherwise healthy people. The mechanism involves an abnormal reflex response of the vagus nerve that causes blood vessels to dilate and the heart rate to slow, reducing blood flow to the brain.
Common triggers include prolonged standing, heat, dehydration, pain, emotional distress, or the sight of blood. A typical episode involves warning symptoms (lightheadedness, nausea, sweating, tunnel vision, a feeling of impending faint) before loss of consciousness. Recovery is usually rapid once the person is horizontal. Brief convulsive movements can occur and do not necessarily indicate epilepsy.
Vasovagal syncope is important in the context of [cardiac arrest](/glossary/cardiac-arrest/) because the two can be confused, and because some people with a history of unexplained blackout are later found to have an underlying [arrhythmia](/glossary/cardiac-arrhythmia/). The distinction matters: vasovagal syncope is benign and self-limiting, while blackout caused by [ventricular tachycardia](/glossary/ventricular-tachycardia/) or [ventricular fibrillation](/glossary/ventricular-fibrillation/) is life-threatening and requires urgent investigation and treatment. A [tilt table test](/glossary/tilt-table-test/) can help diagnose vasovagal syncope by reproducing the reflex under controlled conditions.
Management of vasovagal syncope focuses on identifying and avoiding triggers, increasing fluid and salt intake, physical counterpressure manoeuvres (tensing the legs and abdomen when warning symptoms begin), and advice on posture changes. [Pacemaker](/glossary/pacemaker/) implantation is occasionally considered in severe cases with a predominantly cardioinhibitory (heart-rate-dropping) component.
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