Pacing in Reflex (Neurally-Mediated) Syncopes

The Clinical Spectrum of Reflex Syncopes

Reflex syncope (synonym: neurally-mediated [reflex] syncope ) refers to a reflex response that, when triggered, gives rise to vaso-/venodilation and/or bradycardia; however, the contribution of each of these two factors to systemic hypotension and cerebral hypoperfusion may differ considerably among affected individuals and may even differ in the same patient at different times. As all forms of reflex syncope share essentially the same basic pathophysiology (i.e., they basically represent the same disorder as best as we currently understand), the apparent clinical differences only reflect primarily which specific trigger elicits the reflex.

There is an overabundance of names for reflex syncope, and this has proved confusing. Vasovagal syncope (VVS) is best reserved for a specific form of reflex syncope (see later), but several authors prefer to consider it as a synonym for the broad range of all reflex faints. Other diagnostic synonyms include neurogenic, neurocardiogenic, neuromediated, vasodepressor, etc. The terminology in children is particularly confusing, in that pallid breath-holding spells and reflex anoxic seizures concern reflex syncope in infants with pronounced cardioinhibition often leading to asystole. In the end, the Task Force on Syncope of the European Society of Cardiology recommended that the broad condition be termed reflex or neurally-mediated syncope. Specific subsets can then be specifically identified (e.g., vasovagal, carotid sinus syndrome, etc.).

The presence of a trigger of a recognizable type is important for the diagnosis of reflex syncope (in which case, the general term situational syncope is often employed) ( Box 15-1 ). Most variants are in fact named for their triggers, such as cough syncope, micturition syncope, swallow syncope, and so on.

Often, reflex syncope has an atypical presentation. The term atypical form is used to describe those situations in which reflex syncope occurs with uncertain or even apparently absent triggers. The diagnosis then rests less on history taking alone, and more on the exclusion of other causes of syncope (absence of structural heart disease) and on reproducing similar symptoms with carotid sinus massage, tilt-table testing, or other tests.

In extreme cases, there may be the situation of patients with unexplained syncope without prodrome and absence of any identifiable trigger. Some of these patients have a very low baseline plasma adenosine value and show a positive response to adenosine triphosphate test or adenosine infusion, which suggests a role of the purinergic pathway in the genesis of syncope.

The clinical spectrum of reflex syncope demonstrates much overlap among the clinical forms. Frequently, patients with recurrent syncope have their episodes triggered by different stimuli. In this regard, age is an important determinant of the clinical presentation of reflex syncope ( Fig. 15-1 ). Situational syncope and tilt-induced syncope are observed at all ages. By contrast, typical vasovagal syncope is observed in youth but only rarely in old age. Because the elderly are not immune to emotional stimuli (fear, severe pain, strong emotion), the difference between young and older patients suggests that in old age the responsiveness to afferent neural signals located in cortical sites is decreased or altered. By contrast, positive responses to carotid sinus massage (CSM) increase with age; indeed, patients with carotid sinus syncope (CSS) and those with so-called “atypical” presentation are elderly. Because autonomic responses tend to decrease with advancing age, a positive response to CSM probably implies a reduction in compensatory mechanisms in a part of the reflex arc. Finally, low-adenosine syncope seems to be present at any age.

An abnormal reflex plays a role in causing syncope in different clinical settings where more than one pathophysiologic factor may contribute to the symptoms. For instance, in the setting of valvular aortic stenosis or left ventricular outflow tract obstruction, syncope is not solely the result of restricted cardiac output, but may be in part due to inappropriate neurally-mediated reflex vaso/venodilation and/or primary cardiac arrhythmias. Similarly, a neural reflex component (preventing or delaying vasoconstrictor compensation) appears to play an important role when syncope occurs in association with certain bradyarrhythmias and tachyarrhythmias. In rare cases, reflex syncope is associated with epilepsy (the so-called “ictal asystole”) (see Case Study 15-1 ).

Vasovagal Syncope (Typical and Atypical Forms)

Epidemiology

About 40% of people faint at least once in their life, and at least 20% of adults faint more than once. Fainters usually present first in their teenage years and twenties and may faint sporadically for decades. This long, usually benign, and sporadic history can make for difficult decisions about therapy. Syncope is responsible for 1% to 6% of emergency room visits and 1% to 3% of hospital admissions. Tilt tests are often used as a diagnostic tool, although they are limited by difficulties with sensitivity, specificity, and reproducibility, and with little evidence-based agreement on methodologic details and outcome criteria. Positive tilt tests are characterized by presyncope, syncope, bradycardia, and hypotension, as well as a reproduction of the patient's perisyncopal symptoms ( Fig. 15-2 ). Although many patients of all ages simply have vasovagal syncope, clinicians need to remain vigilant and look for other causes, including valvular and structural heart disease, sick sinus syndrome, carotid sinus syndrome (CSS), and orthostatic hypotension.