Causes of Syncope

Neurocardiogenic syncope or vasovagal syncope (35-50% of cases) arises from an initial increase in sympathetic outflow followed by a rebound reduction in sympathetic activity leaving unopposed parasympathetic activity causing vasodilatation, bradycardia and hypotension (Fig 1, opposite).

Cardiac (10-30% of cases): arrhythmias, cardiac failure, ventricular dysfunction (e.g. hypertrophic obstructive cardiomyopathy HOCM) and valvular heart disease can all cause syncope.

Orthostatic syncope (10% of cases) is defined as an orthostatic drop of >20 mm systolic blood pressure or >10 mm diastolic blood pressure. This may be due to absolute volume depletion from dehydration or haemorrhage or to venodilatation caused by medications or autonomic insufficiency (eg as occurs in Parkinson’s disease).

Neurological/psychiatric syncope (5% of cases) include basilar artery migraine, vestibular dysfunction and vertebrobasilar ischaemia. Psychiatric syncope is a recognised syndrome consisting of syncopal episodes found in anxiety, depression and conversion disorder that resolve with treatment of the psychiatric disorder.

No cause found (35-50%of cases)


There are a wide variety of important causes for a patient presenting with syncope and up to 50% may not reach a diagnosis in the Emergency Department.

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Good module for revision

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