Syncope without prodromes is associated with excessive plasma release of adenosine at the time of syncope during...

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).

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.

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).

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.