Syncope is defined as transient loss of consciousness due to global cerebral hypoperfusion of abrupt onset and short duration, and spontaneous complete recovery.

The term neurally mediated syncope refers to the vasovagal syndrome, carotid sinus syndrome and less common syndromes such as micturition syncope, in which triggering of an autonomic nervous system reflex results in syncope due to inappropriate bradycardia, and/or hypotension caused by vasodilatation.

Neurally mediated syncope should be considered in patients with unexplained syncope when there is no electrocardiographic evidence of the sick sinus syndrome or atrioventricular (AV) block.

Malignant vasovagal syndrome

The malignant vasovagal syndrome, also termed neurocardiogenic syncope, is characterised by recurrent, abrupt syncope when standing or sitting (including car driving), and a positive tilt-table test. Tests for sick sinus syndrome and atrioventricular block are negative. The term ‘malignant’ is used to indicate that episodes occur without obvious prodromal symptoms or an apparent triggering stimulus such as occurs in situational faints.

Syncope is thought to result from pooling of blood in the lower extremities during standing or sitting. Reduced venous return leads to hypotension which is detected by baroreceptors in the aortic arch and carotid arteries and leads to reflex-enhanced sympathetic nervous system activity and thus increased force of myocardial contraction. Because of reduced venous return, the left ventricle in diastole is relatively empty. Systole results in excessive stimulation of ventricular mechanoreceptors, which in patients with the vasovagal syndrome trigger inappropriate reflex vasodilatation and bradycardia. Reflex control of venous tone has also been shown to be abnormal. In some patients ‘cardioinhibition’ (i.e. bradycardia, either sinus arrest or AV block) predominates, while in others it is the ‘vasodepressor’ element (i.e. vasodilatation) that is the main problem.

Even though a period of asystole may occur, the syndrome is not a cause of sudden death. Frequency of recurrence of attacks is variable and unpredictable; episodes may occur in clusters. It occurs in both young and elderly people. In contrast to syncope caused by the sick sinus syndrome or AV block, loss of consciousness may be prolonged due to persistent hypotension, and fitting and incontinence can sometimes occur.

Tilt-table test

The patient is gently secured on a tilt table and rapidly tilted from the supine position to a 60-degree angle, and then stays in this position, standing on a footplate, for up to 45 minutes. The ECG and blood pressure are continuously monitored. The test is positive if the patient’s typical spontaneous symptoms result from profound bradycardia (often asystole) and/or hypotension (Figures 17.1–17.3). Blood pressure and heart rate are rapidly restored on returning to a horizontal position (Figure 17.4).

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