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.

Even among the 355 patients thought to have had a different cause for syncope, an embolism was detected in 45 (12.7%, 95% CI: 9.2 to 16.1).

“Not surprisingly, patients with dyspnea, tachycardia, hypotension, or clinical signs or symptoms of deep-vein thrombosis were more likely to have pulmonary embolism, as were those with active cancer. However, the proportion of patients who did not have these features yet had an objective confirmation of pulmonary embolism was not negligible,” the Prandoni team found.

Ultimately, no definitive cause of the syncope was found in 205 of the 560 patients.

Dr. Prandoni told Reuters Health in an email that, based on the results, “All existing guidelines dealing with the diagnostic work-up of patients hospitalized for syncope should be rapidly reconsidered, and the execution of a diagnostic work-up for pulmonary embolism should be strongly encouraged in all such patients, including those in whom there is an apparent explanation for the episode of syncope.”

Because pulmonary embolism “is a serious and potentially lethal complication that can be treated effectively and safely with the available antithrombotic drugs, its prompt detection is crucial,” he said. “In addition, the cost of the additional diagnostic workup for pulmonary embolism is acceptably low.”