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USMLE – Syncope

The common causes of syncope, drop attacks or sudden loss of consciousness are arrhythmia, inappropriate vasodilation, neurogenic, and metabolic (hypoglycemia). These conditions may also be responsible for recurrent episodes of dizziness, lightheadedness or presyncope.

Cardiac syncope is caused by a sudden drop in cardiac output and cerebral perfusion due to an arrhythmia or a mechanical problem. Inappropriate vasodilatation also causes symptoms by reducing cerebral perfusion.

Vasovagal syncope
This is mediated by the Bezold-Jarisch reflex and is usually triggered by a reduction in venous return due to prolonged standing, excessive heat or a large meal. Concomitant sympathetic activation then leads to vigorous contraction of the relatively under filled ventricles and engages the reflex by stimulating ventricular mechanoreceptors. This produces parasympathetic (vagal) activation and sympathetic withdrawal causing bradycardia, vasodilatation or both. Head-up tilt testing can be used to confirm the diagnosis by inducing a typical attack. Treatment is often unnecessary but in severe cases, beta-adrenoceptor antagonists, disopyramide and a dual-chamber pacemaker are all partially effective remedies.

Differential diagnosis
Whenever possible, an accurate description of the attack should be obtained from the patient and a witness. Particular attention should be paid to the recovery phase and possible precipitants or triggers such as medication, exercise and alcohol. In cardiac syncope, recovery is usually rapid, whereas patients with vasovagal syncope often feel nauseated and unwell for several minutes, and patients with neurogenic syncope usually take more than 5 minutes to recover.

A careful history and clinical examination will often reveal the cause of recurrent syncope without recourse to complex and expensive investigations. In the remaining cases the pattern and description of the patient’s symptoms should indicate the probable mechanism of syncope and will therefore determine subsequent investigations. The discovery of common pathology (e.g, paroxysmal atrial fibrillation or cervical spondylosis in elderly subjects) does not necessarily explain the patient’s complaint and in many cases a definitive diagnosis can only be made if it is possible to provoke typical symptoms or demonstrate a close temporal relationship between the patient’s symptoms and an arrhythmia.

Lillian Thompson By Lillian Thompson

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