Syncope without prodromes in patients with normal heart and normal electrocardiogram: a distinct entity.

OBJECTIVES This study sought to investigate the clinical and laboratory findings of patients affected by sudden-onset syncope without prodromes who had a normal heart and normal electrocardiogram. BACKGROUND The pathophysiology of syncope in these patients is uncertain. METHODS We compared the clinical and laboratory findings of 15 patients with sudden-onset syncope without prodromes who had a normal heart and normal electrocardiogram (the study group) with those of 31 patients with established vasovagal syncope (VVS). RESULTS The patients in the study group were older than those with VVS (age 61 ± 12 years vs. 46 ± 17 years) and had a history of fewer episodes of syncope (median of 2 [interquartile range [IQR]: 1 to 2.5] vs. 9 [IQR: 4 to 15] years) that were of more recent onset (median of 1 [IQR: 0 to 1] vs. 10.5 [IQR: 3.3 to 27] years). The study group had lower median baseline adenosine plasmatic levels than the VVS group (0.25 μmol/l [95% confidence interval: 0.10 to 1.51] vs. 0.85 μmol/l [95% confidence interval: 0.32 to 2.80]). On receiver-operating characteristic curve analysis, the adenosine plasmatic level of ≤0.36 best discriminated between groups, displaying 73% sensitivity and 93% specificity. Tilt table testing was more frequently positive in patients with VVS than in the study group (74% vs. 33%). A similarly high positivity rate of adenosine/adenosine triphosphate testing was found in both groups. CONCLUSIONS Common clinical features and a low adenosine plasmatic level define a distinct form of syncope, distinguish it from VVS, and suggest a causal role of the adenosine pathway.