Objectives: This study evaluated morphologic features of fossa ovalis membrane aneurysm and associated cardiac abnormalities that may predispose to systemic embolism.

Background: An increasing number of fossa ovalis membrane aneurysms are diagnosed by echocardiography. Higher frequencies of such aneurysms have been reported in patients with embolic stroke.

Methods: The hearts of 20 adults with fossa ovalis membrane aneurysms (mean [ +/- SD] age 62 +/- 19 years, range 24 to 87; 12 women [60%], 8 men [40%]) were examined. The areas of excised atrial septum, fossa ovalis membrane and fossa ovalis were measured. An aneurysm was defined as the ratio of the fossa ovalis membrane to fossa ovalis area > or = 1.5.

Results: Compared with a control group of 20 adults (mean age 58 +/- 21 years, range 18 to 86; 12 women [60%], 8 men [40%]), the 20 patients with fossa ovalis membrane aneurysm had larger (711 +/- 240 vs. 203 +/- 105 mm2, p < 0.0001) and thinner fossa ovalis membranes (0.6 +/- 0.1 vs. 1.9 +/- 0.9 mm2, p < 0.0001). The mean ratio of the fossa ovalis membrane to fossa ovalis area was 2.1 +/- 0.4 in patients with and 1.2 +/- 0.1 in those without fossa ovalis membrane aneurysm. Patent foramen ovale was seen in 14 (70%) of 20 patients with fossa ovalis membrane aneurysm and in 4 (20%) of 20 control subjects (p = 0.0005).

Conclusions: Fossa ovalis membrane aneurysm is characterized by thinning and marked redundancy without adherent thrombi or fibrin tags. Mitral valve prolapse, dilated atria, intracardiac thrombi and patent foramen ovale are frequently seen in association with fossa ovalis membrane aneurysm and may explain the increased frequency of embolic stroke in patients with such aneurysms.