Limitations of studies on vasovagal syncope

The report written by Ermis et al on the catecholamine response during tilt–table-induced vasovagal syncope illustrates 2 kinds of difficulties of the vasovagal syncope studies. The first is the lack of a true control group. The second is the tilt-test limitations (the only one diagnostic tool available) that include the timing of the blood withdrawals during the test. The control group of the study of Ermis et al is a group of patients with an iterative syncope history but a negative outcome to the tilt test. Medical history is the deciding factor of the diagnosis of vasovagal syncope. A typical history makes the diagnosis, and no further evaluations are needed excepted a clinical examination, an electrocardiogram, and supine and standing blood pressure measurements. Study of vasovagal syncope should include patients with no doubt about the diagnosis. A positive outcome to a tilt test coming with a typical history allows the clinician to confirm the diagnosis and permits inclusion of the patient in a study. The group with a negative outcome to a tilt test, but a typical history of syncope, is certainly not a control group. It is hard to conceive a true control group for comparison with patients with vasovagal symptoms. A clinical examination, and even further investigations performed between episodes, cannot make the difference between a group of normal subjects and the patients. The incidence of unexplained syncope is so large (one in 6) that anybody may experience vasovagal symptoms at least once in a whole life. It is why regular designs for clinical investigations (controlled studies) are probably not appropriate to study vasovagal syncope. However, it is difficult to draw a clear conclusion from uncontrolled studies.