Causes of syncope (syncope = fainting)
The causes of syncope can be grouped into cardiac and non-cardiac causes (box 1). Cardiac syncope is associated with increased mortality and may lead to sudden death. Syncope affects 30% of adults at some point in their lives and is a common problem in people presenting at emergency departments. 1 One reason postulated for syncope being so common reflects our incomplete evolutionary adaptation to the upright body position as standing upright requires blood pressure regulation to drive blood against gravity up to the head. 3
Box 1: Causes of syncope
Non-cardiac causes of syncope
- Vasovagal syncope
- Situational syncope
- Urinating (micturition syncope)
- Defecating (defecation syncope)
- Swallowing (deglutition syncope)
- Coughing (cough syncope)
- Post-exercise/Post-prandial syncope
- Postural (orthostatic) hypotension
- Dehydration/volume depletion causing a decrease in blood volume
- Blood pressure medications leading to low blood pressure.
- Autonomic failure, eg, some patients with Parkinson's disease, diabetic neuropathy
- Carotid sinus syncope
- Drug induced syncope
- Hypoglycaemic
Cardiac causes of syncope
- Heart conditions that can cause syncope or fainting due to temporary loss of consciousness include:
- Abnormal heart rhythms
- Abnormalities of the heart valves (eg, aortic stenosis)
- Pulmonary artery hypertension
- Aortic dissection
- Cardiomyopathy
Vasovagal syncope
In young people, neurocardiogenic (vasovagal) syncope is the most common form. The vagus nerve transmits afferent signals from the aortic arch baroreceptors, regulating arterial pressure. Increases in arterial flow stimulate the vagus nerve, inhibiting sympathetic drive and decreasing blood pressure. When someone stands in an upright position, blood pools in the lower limbs and activates the autonomic cycle. The resulting increase in sympathetic nervous activity causes vasoconstriction (narrowing of vessels), increased heart rate, and maintains the blood pressure.
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