Syncope accounts for about 2% of emergency department visits. About 40% of people experience a syncopal episode at least once in their lifetime. There are multiple different possible causes of syncope. Most cases of syncope are due to benign etiologies, several of which are described here. However, syncope can sometimes be a symptom of a life-threatening condition such as a cardiac arrhythmia (cardiac syncope). About 5% of patients who present for syncope will suffer a serious adverse event in the 30 days following the syncopal episode. A mnemonic for causes of transient loss of consciousness is head, heart, vessels.

Vasovagal Syncope
This is the most common cause of syncope. Vasovagal, or neurocardiogenic, syncope is a form of neurally mediated syncope that leads to a transient loss of consciousness. It most frequently occurs in otherwise young and healthy individuals but may occur in older individuals. Vasovagal syncope is thought to be due to a reflexive activation of the parasympathetic nervous system, often in response to a trigger. This can lead to bradycardia and decreased cardiac contractility, termed a cardioinhibitory response. Alternatively, there may be vasodilation with a corresponding decrease in blood pressure in the absence of a significant change in heart rate, termed a vasodepressor response. These hemodynamic changes result in systemic hypotension and cerebral hypoperfusion.

Often, it is triggered by an emotional or orthostatic stress such as painful stimuli, venipuncture, fear of bodily injury, prolonged standing, exertion, or heat exposure. Some medications can also predispose to syncopal episodes due to provoking orthostatic hypotension. Patients typically describe a prodrome of nausea, pallor, and diaphoresis followed by a brief episode of syncope from either the sitting or standing position. Consciousness is restored with the supine position, but patients may feel fatigued for some time after the event, and persistent low blood pressure and bradycardia can cause syncope to recur upon standing.

Drug-Induced Syncope
Both syncope due to cardiac arrhythmias and syncope due to orthostatic hypotension can be the result of a medication, alcohol, or illicit drug. See drug-induced syncope for more information, including drug reaction data.  

Carotid Sinus Syncope
Hypersensitivity of the carotid sinus such that manipulation causes a vagal response with bradycardia and vasodilation (asystole and drop in blood pressure) causes syncope. Abnormal response to carotid sinus massage is a strong indication of the condition. Other findings may include hypotension, dizziness, presyncope, sinus bradycardia, headache, nausea, pallor, and diaphoresis. It can be triggered by fast neck movements, shaving, constrictive collars, neck tumors, lymphadenopathy, or neck surgery. This condition is more common in elderly patients. Treatment is case dependent and may include permanent pacemaker placement for more severe cases.

Situational Syncope
These are neurally mediated conditions in which specific actions can cause syncope. Examples include cough syncope, syncope during sneezing or when playing a wind instrument, defecation syncope (syncope triggered by the act of defecation), micturition syncope (syncope during urination), deglutition syncope (syncope triggered by oral intake), coital syncope (syncope during coitus or orgasm), and laughter-induced syncope. Cough syncope and micturition syncope are the most common. Laughter-induced syncope in children may be caused by tickling. Situational syncope is characterized by vasodilatation and/or bradycardia resulting in transient hypotension and cerebral hypoperfusion. Treatment is case dependent and may include permanent pacemaker placement.