Theophylline in patients with syncope without prodrome, normal heart, and normal electrocardiogram: a propensity-score matched study verified by implantable cardiac monitor

The Brain-Heart Network of Syncope

Observed and recorded in various forms since ancient times, 'syncope' is often popularly called 'fainting', such that the two terms are used synonymously. Syncope/fainting can be caused by a variety of conditions, including but not limited to head injuries, vertigo, and oxygen deficiency. Here, we draw on a large body of literature on syncope, including the role of a recently discovered set of specialized mammalian neurons. Although the etiology of syncope still remains a mystery, we have attempted to provide a comprehensive account of what is known and what still needs to be performed. Much of our understanding of syncope is owing to studies in the laboratory mouse, whereas evidence from human patients remains scarce. Interestingly, the cardioinhibitory Bezold-Jarisch reflex, recognized in the early 1900s, has an intriguing similarity to-and forms the basis of-syncope. In this review, we have integrated this minimal model into the modern view of the brain-neuron-heart signaling loop of syncope, to which several signaling events contribute. Molecular signaling is our major focus here, presented in terms of a normal heart, and thus, syncope due to abnormal or weak heart activity is not discussed in detail. In addition, we have offered possible directions for clinical intervention based on this model. Overall, this article is expected to generate interest in chronic vertigo and syncope/fainting, an enigmatic condition that affects most humans at some point in life; it is also hoped that this may lead to a mechanism-based clinical intervention in the future.

Mechanism-based therapy of non-cardiac syncope: a practical guide

The term non-cardiac syncope includes all forms of syncope, in which primary intrinsic cardiac mechanism and non-syncopal transient loss of consciousness can be ruled out. Reflex syncope and orthostatic hypotension are the most frequent aetiologies of non-cardiac syncope. As no specific therapy is effective for all types of non-cardiac syncope, identifying the underlying haemodynamic mechanism is the essential prerequisite for an effective personalized therapy and prevention of syncope recurrences. Indeed, choice of appropriate therapy and its efficacy are largely determined by the syncope mechanism rather than its aetiology and clinical presentation. The two main haemodynamic phenomena leading to non-cardiac syncope include either profound hypotension or extrinsic asystole/pronounced bradycardia, corresponding to two different haemodynamic syncope phenotypes, the hypotensive and bradycardic phenotypes. The choice of therapy-aimed at counteracting hypotension or bradycardia-depends on the given phenotype. Discontinuation of blood pressure-lowering drugs, elastic garments, and blood pressure-elevating agents such as fludrocortisone and midodrine are the most effective therapies in patients with hypotensive phenotype. Cardiac pacing, cardioneuroablation, and drugs preventing bradycardia such as theophylline are the most effective therapies in patients with bradycardic phenotype of extrinsic cause.

Syncope without prodromes is associated with excessive plasma release of adenosine at the time of syncope during head-up tilt table test

BACKGROUND

In syncopal patients without underlying structural disease, we sought to investigate the association of Adenosine Plasma Levels (ADP) with the clinical presentation of neurally mediated syncope (NMS) and the outcomes of Head-Up Tilt Table Test (HUTT) and Adenosine test (ADT).

METHODS

We studied 124 patients with different clinical types of NMS, i.e., Vasovagal (VVS, n=58), non-prodromes (NPS, n=18), or situational syncope (SS, n=48), using a standard protocol including HUTT and ADT. During HUTT, ADP was measured in the supine position, at table tilting and in syncope.

RESULTS

Baseline ADP did not differ among groups. ADP at syncope were higher in NPS (n=5) compared to VVS (n=20): 0.23 vs. 0.12 μΜ, p=0.03, and SS (n=22): 0.04 μΜ, p=0.02. In NPS, ADP increased from supine to syncope (n=5): 0.15 vs. 0.23 μΜ, p=0.04. In VVS, ADP increased only from supine to tilt position: 0.11 vs. 0.14 μΜ, p=0.02. In SS, ADP did not change during HUTT. In positive vasodepressor HUTT, ADP increased from supine to tilt position (p=0.002) and at syncope (p=0.01). In SS, 20.0% exhibited cardioinhibitory HUTT vs. 6.8% in other forms of syncope (p=0.04). In SS, 22.9% manifested positive ADT vs 6.6% in other types of syncope (p=0.012).

CONCLUSION

The subset of NPS patients with positive HUTT, show excessive ADP release at the time of syncope. This may explain the lack of prodromes in this form of syncope. Such observations contribute to the understanding of distinct profiles of clinical forms of syncope and may differentiate the management approach accordingly.

Adenosine, Adenosine Receptors and Neurohumoral Syncope: From Molecular Basis to Personalized Treatment

Adenosine is a ubiquitous nucleoside that is implicated in the occurrence of clinical manifestations of neuro-humoral syncope (NHS). NHS is characterized by a drop in blood pressure due to vasodepression together with cardio inhibition. These manifestations are often preceded by prodromes such as headaches, abdominal pain, feeling of discomfort or sweating. There is evidence that adenosine is implicated in NHS. Adenosine acts via four subtypes of receptors, named A1 (A1R), A2A (A2AR), A2B (A2BR) and A3 (A3R) receptors, with all subtypes belonging to G protein membrane receptors. The main effects of adenosine on the cardiovascular system occurs via the modulation of potassium ion channels (IK Ado, K ATP), voltage-gate calcium channels and via cAMP production inhibition (A1R and A3R) or, conversely, through the increased production of cAMP (A2A/BR) in target cells. However, it turns out that adenosine, via the activation of A1R, leads to bradycardia, sinus arrest or atrioventricular block, while the activation of A2AR leads to vasodilation; these same manifestations are found during episodes of syncope. The use of adenosine receptor antagonists, such as theophylline or caffeine, should be useful in the treatment of some forms of NHS. The aim of this review was to summarize the main data regarding the link between the adenosinergic system and NHS and the possible consequences on NHS treatment by means of adenosine receptor antagonists.

Adenosine and neurohumoral syncope

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 lowadenosine 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 two controlled studies.