To date, numerous causes of syncope have been recognized (Table 1). This presents a diagnostic challenge to clinicians. In the initial evaluation of syncope patients, the diagnostic rate is estimated to be only 20% to 50% [1]. Even after extensive diagnostic work-up, no cause can be identified in 15% to 30% of patients [1]. There is no diagnostic 'gold standard' for syncope. A careful history, physical
examination and electrocardiogram (ECG) are usually the most efficacious means used to establish a diagnosis or determine the need for further diagnostic testing [11]. Unlike history and physical examination, ECG actually has a low diagnostic yield [1]. It should be included in the initial syncope evaluation, however, because it is noninvasive, relatively inexpensive and can detect potentially life-threatening conditions. Extensive laboratory and imaging studies rarely provide useful diagnostic information unless specifically indicated. Unfortunately, the evaluation
of patients presenting with syncope is frequently unable to reveal a clear etiology and many cases remain unexplained.