Diagnostic evaluation of syncope and transient loss of consciousness

Various guidelines have been published for evaluating syncope.8,9,22,23 Our preferred diagnostic algorithm is shown in Box 4. The recent consensus statement of the American Heart Association and the American College of Cardiology Foundation23 is primarily useful for providing a diagnostic path for excluding cardiac causes of syncope associated with sudden death. It does not address diagnosis and management of postural syncope and is neglectful of the clinical importance of disorders of circulatory control.24

Taking a detailed history is the most powerful tool for diagnosing syncope. Measuring supine and standing blood pressures is crucial: typical orthostatic hypotension is characterised by a fall in blood pressure of > 20 mmHg (systolic) and/or 10 mmHg (diastolic) after 3 minutes of standing. It is also important to test blood pressure after prolonged standing (10 minutes), as delayed hypotension is common. 25 The absence of reflex tachycardia on standing suggests autonomic failure.

Routine blood investigations should be done only if clinically indicated. In general, the yield in syncope patients is low.

Cardiovascular investigations

For cardiovascular investigations, a 12-lead ECG is essential (Box 5). It may identify arrhythmias or preconditions for arrhythmia (eg, a long QT interval) or indicate underlying ischaemic heart disease. An abnormal ECG has prognostic value. Yet despite its obvious clinical importance, only 59% of patients with syncope presenting to US emergency departments in 2004 were given an ECG at presentation.26

Twenty-four-hour blood pressure monitoring, coupled with diary recording by the patient, is extremely useful — for example, it may demonstrate persistently low blood pressure. A relatively fixed heart rate is an important clue to autonomic failure.

Cardiac rhythm monitoring is probably overused in patients with postural syncope, which is infrequently caused by cardiac arrhythmias.

Twenty-four-hour Holter monitoring has a sensitivity of only 10% in unexplained syncope, and should only be used in patients who experience daily events.

External loop recorders, which may be worn for 1 month, require patient activation when symptoms occur. They increase diagnostic yields to 25%,27 but problems with dermal electrode tolerance and forgetting to trigger recording at the onset of symptoms limit their effectiveness.28

Implantable loop recorders are inserted under the skin of the chest wall, requiring only local anaesthetic. A recent study of patients with at least three severe episodes of neurally mediated syncope (orthostatic hypotension excluded) demonstrated the value of early loop recorder implantation and subsequent guided therapy.29

Prolonged cardiac monitoring should only be employed when “symptom–rhythm” correlation is of clinical value (particularly when arrhythmia is thought to be a likely culprit).

Echocardiography, although rarely diagnostic, is important for the exclusion of structural heart disease if clinically indicated — for example, in cases of impaired left ventricular function, abnormal right ventricular function suggestive of arrhythmogenic right ventricular dysplasia, and hypertrophic cardiomyopathy, a common cause of sudden death in the young.

Electrophysiological testing in cases of unexplained syncope remains controversial and is limited to select patients, usually those with structural heart disease.8

Tilt-table testing is a widely used method for evaluating postural syncope. It is predominantly used to diagnose vasovagal syncope. It may also be used to train patients to recognise the vasovagal prodrome — some patients can then abort a vasovagal event using counter-manoeuvres such as leg-crossing.30 The diagnosis should only be made if the event during tilt-table testing reproduces the patient’s real-world event. The use of tilt-table testing remains controversial:31 publications report a wide range of sensitivity and specificity, with lack of specificity being a particular problem. The use of provocation testing with isoprenaline is of questionable merit32 — it is possible to induce vasovagal syncope in almost anyone with an adequately aggressive tilt test. In a patient with a normal ECG, structurally normal heart, no neurological features, normal supine blood pressure and a negative tilt test, vasovagal syncope is still the likely diagnosis.

Neurological investigations should only be performed when clinically indicated and do not form part of the routine investigation of syncope.