NEW YORK (Reuters Health) – A study of 560 Italians hospitalized for syncope has found that nearly one in six had a pulmonary embolism, suggesting that such clots may be a major under-recognized source of fainting.

When there was no obvious explanation for syncope, 25% of patients turned out to have an embolism. When a cause was suspected, the embolism rate was still nearly 13%.

“Current guidelines pay little attention to a diagnostic workup for pulmonary embolism in these patients,” said the research team, led by Dr. Paolo Prandoni at the University of Padua in Italy. “Hence, when a patient is admitted to a hospital for an episode of syncope, pulmonary embolism – a potentially fatal disease that can be effectively treated – is rarely considered as a possible cause.”

The study, published online October 19 in The New England Journal of Medicine, was an attempt to determine the prevalence of emboli as a source of syncope.

Patients in the study, known as PESIT, were enrolled from 11 hospitals. It was their first episode of syncope. All were unconscious for less than a minute and all regained consciousness spontaneously. There was no obvious cause for the fainting.

The Prandoni team used two measures to try to rule out emboli. The first was a D-dimer assay which detects fresh thrombi in the blood simply and cheaply, and with high sensitivity. The second was a scoring system known as a Wells Score, which tries to predict an embolism using measures such as heart rate, cancer, hemoptysis and evidence of deep-vein thrombosis.

Negative results from both were used to rule on an embolism in 58.9% of the 560 cases. Of the 230 patients remaining, 135 had a positive D-dimer assay only and three had a high clinical probability of an embolism on the Wells test but a negative D-dimer result.

Among the 180 who underwent CT scanning, 72 (or 40.0%) turned out to have a pulmonary embolism, usually in the main pulmonary artery (41.7% of the cases) or in a lobar artery (25.0%). An embolism was found in 24 (49%) of the 49 patients who underwent ventilation-perfusion scanning.

“Hence, pulmonary embolism was confirmed in 97 of the patients who had a positive D-dimer assay, a high pretest clinical probability, or both (42.2%; 95% confidence interval, 35.8 to 48.6). In the entire cohort, the prevalence of pulmonary embolism was 17.3% (95% CI, 14.2 to 20.5),” the researchers wrote.