The atrial septal pouch—a new source of thrombus?

January 29, 2010

Orange,CA- A pouchlike structure, commonly found in the left atrium of the heart, has the potential to be a potent generator of thrombus and might be the cause of many ischemic events, particularly strokes in younger individuals, a new paper reports [1].

The paper, published in the January 2010 issue of the Journal of the American College of Cardiology: Cardiovascular Interventions, was written by Drs Subramaniam Krishnan and Miguel Salazar (University of California at Irvine Medical Center, Orange).

"Paradigm shift" in understanding?

A septal pouch on autopsy (Photo courtesy of Dr Krishnan)

To heart wire , Krishnan said: "We are describing this novel cardiac anatomy for the first time. Increased recognition of this left atrial septal pouch has the potential to cause a paradigm shift in the understanding of thromboembolic conditions. The pouchlike structure can result in stasis of blood with consequent thromboembolic complications."

He believes this has the potential to fill a "knowledge gap" in patients with ischemic strokes. He explained that, "in 25% to 30% of patients with an ischemic stroke, the cause or source of the stroke may not be clear. Termed cryptogenic strokes, these represent a definite knowledge gap in the field and can be a source of intense frustration to the treating physician. We believe that, similar to the left atrial appendage, the septal pouch represents an additional structure that is capable of causing thromboembolic strokes. We are currently examining the prevalence of the pouch in patients who have already had strokes, especially where the cause is not clear.

"If we can prove our theory that these pouches are indeed thrombogenic and account for the large number of strokes without known causes, it could result in more effective treatment for these patients, with an improvement in their prognosis," Krishnan suggested.

Incomplete closure of the foramen ovale

Krishnan believes that the left atrial septal pouch, which is found in about 30% of the population, represents a stage in the natural history of closure of the foramen ovale. In this paper, he and Salazar explain that a patent foramen ovale is an opening in the interatrial septum that is normally present in fetal life. In most adults, the foramen ovale is closed, but in around 25% of adults, it remains open, which is considered a risk factor for several serious clinical conditions, including embolic stroke, MI, and complications of pulmonary embolism. They note that a patent foramen ovale tends to decrease in prevalence with advancing age, from 34% during the first three decades of life to 20% during the ninth decade.

Normal structure and left atrial septal pouch

Krishnan told heart wire that "it is believed that the closure of foramen ovale happens [when adhesions occur]. But if adhesions happen only in part of the structure, then closure is only partial, and a pouch is formed. In the majority of people with these pouches, the openings are in the left atrium. We believe the process of having a patent foramen ovale, then having a pouch, then having full closure is a continuum that moves forward during a person's life. Therefore, like patent foramen ovales, pouches should also be more common in younger people."

In the current study, Krishnan and Salazar examined the interatrial septum in 94 randomly selected autopsied hearts. They found that 26 had a patent foramen ovale, 27 had complete closure, and 41 had incomplete fusion. Of those with incomplete fusion, 37 had a septal pouch that opened into the left atrial cavity, and four had a pouch accessible from the right atrium. Hearts with left-sided pouches tended to be younger (average age 50 years) than hearts in which there was complete fusion (average age 63 years).

Krishnan told heart wire that, in addition to this autopsy series, they have collected a registry of around 600 live cases in which they have visualized the pouch with either transesophageal echo or CT scans.

Confirmation in case reports

He added: "According to conventional teaching, except for the left atrial appendage, the body of the left atrium is smooth and continuous and hence is unlikely to provide a location for the formation of thrombi. We show that in a third of individuals, this is not true, and the septal pouch is another structure in the left atrium that is also not smooth or continuous." He noted that there have been some case reports of patients with clots in the left atrial pouch with embolic complications, such as MIs and transient ischemic attacks.

One such case report, by Breithardt et al [2], describes a 25-year-old woman who had an embolic event. On scanning, it was shown that she had a thrombus in this pouch. Krishnan commented: "I was shocked—but happily so—to see this report, as it really confirmed our theories."

He noted that the formation of clots in this pouch is even more likely in low-flow states, such as rheumatic valve disease and atrial fibrillation. "In patients with rheumatic mitral-valve disease, when a thrombus occurs in the left atrium, it is present in the left atrial appendage only 50% of the time, so it could well be in the pouch in the remaining cases." Some of the pouches are deeper and bigger than others, and he believes these would be the most problematic.

Krishnan is the owner of intellectual property related to closure of patent foramen ovales.

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