The aim of this review is to provide an update of the current knowledge of the physiological mechanisms underlying reflex syncope. Carotid sinus syncope will be used as the classical example of an autonomic reflex with relatively well-established afferent, central and efferent pathways. These pathways, as well as the pathophysiology of carotid sinus hypersensitivity (CSH) and the haemodynamic effects of cardiac standstill and vasodilatation will be discussed. We will demonstrate that continuous recordings of arterial pressure provide a better understanding of the cardiovascular mechanisms mediating arterial hypotension and cerebral hypoperfusion in patients with reflex syncope. Finally we will demonstrate that the current criteria to diagnose CSH are too lenient and that the conventional classification of carotid sinus syncope as cardioinhibitory, mixed and vasodepressor subtypes should be revised because isolated cardioinhibitory CSH (asystole without a fall in arterial pressure) does not occur. Instead, we suggest that all patients with CSH should be thought of as being 'mixed', between cardioinhibition and vasodepression. The proposed stricter set of criteria for CSH should be evaluated in future studies.
Either central or peripheral baroreceptor reflex abnormalities and/or alterations in neurohumoral mechanisms play a pivotal role in the genesis of neurally mediated syncope. Thus, improving our knowledge of the biochemical mechanisms underlying specific forms of neurally mediated syncope (more properly termed ‘neurohumoral syncope’) might allow the development of new therapies that are effective in this specific subgroup. A low-adenosine phenotype of neurohumoral syncope has recently been identified. Patients who suffer syncope without prodromes and have a normal heart display a purinergic profile which is the opposite of that observed in vasovagal syncope patients and is characterized by very low-adenosine plasma level values, low expression of A2A receptors and the predominance of the TC variant in the single nucleotide c.1364 C>T polymorphism of the A2A receptor gene. The typical mechanism of syncope is an idiopathic paroxysmal atrioventricular block or sinus bradycardia, most often followed by sinus arrest. Since patients with low plasma adenosine levels are highly susceptible to endogenous adenosine, chronic treatment of these patients with theophylline, a non-selective adenosine receptor antagonist, is expected to prevent syncopal recurrences. This hypothesis is supported by results from series of cases and from observational controlled studies.
NCT01509534; EudraCT2013-004364-63; Results.
Total word count: 2583 (excluding front page)Statement. We declare no relationship with industry. The study was not funded. AbstractBackground.Adenosine, an ATP derivative, may be implicated in some kinds of unexplained syncope. In patients with normal heart, normal ECG and recurrent sudden-onset syncope without prodromes have been shown to present with lowplasmatic adenosine levels and a high susceptibility to exogenous adenosine.The term "low-adenosine syncope" has been launched to describe this distinct clinical entity. Objectives. We decided to investigate whether chronic treatment of these patients with theophylline, a non-selective adenosine receptor antagonist, results in clinical benefit. Methods. We report on the prolonged clinical observation of 6 "low-adenosine" syncope patients (mean age 50±20 years, 4 females) treated with oral theophylline within the therapeutic range of 12-18 μg/ml. We were able to make an intrapatient comparison between a period with and a period without theophylline therapy. Results. In five patients, symptoms disappeared and the number of prolonged asystolic pauses detected by implantable loop recorder (ILR) fell impressively from a median of 1.11 per month (interquartile range 0.4 -1.8)during 13 months of no-treatment (range 2-36) to 0 per month(0-0.7) during 20 months of theophylline treatment (range 6-120). The 6 th patient, was unresponsive to theophylline therapy, and a different mechanism of syncope was hypothesized. Conclusion. In this small series of highlyselected patients affected by syncope with low circulating adenosine levels, theophylline proved to be an effective therapy in most patients. The logical inference is that the adenosine pathway has a causal role in the mechanism of syncope in such patients.
Carotid sinus massage by means of the 'method of symptoms' indentifies a clinical syndrome with definite features and outcome. A treatment strategy involving lifestyle measures, reduction of antihypertensive drugs and cardiac pacing when appropriate is effective in reducing the syncopal recurrence rate.
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