The importance of the foramen ovale (FO) during fetal life is well established, but the exact mechanism that causes it to close prematurely has been of some debate over the years. During normal fetal circulation 65% of the total caval blood flows through a FO to the left side, providing oxygenated blood both to the cerebral and coronary circulations [1]. However, in cases were the FO prematurely closes or becomes restrictive, a sequence of events may follow that can result in a spectrum of cardiac anomalies, the severity of which is believed to depend both on the gestational time the restriction occurs as well as on the extent of the restriction. This excess amount of blood flow that is re-directed to the right ventricle can result in congestive right heart failure and at the same time, because of the reduced blood flow to the left, the left sided structures can become hypoplastic [2, 3]. Chronologically earlier closure of the FO seems to be related with a more profound underdevelopment e.g. hypoplastic left heart syndrome, whereas closure later in gestation can have only a minimal impact on the left heart. However, another theory that has been proposed as an explanation of PCFO, is that of abnormal hemodynamic development, such as with preexisting aortic or mitral valve stenosis. In these situations the elevated left atrial pressures may prematurely initiate the normal closure mechanism of the FO that pushes the flexible septum primum against the more rigid septum secundum [4–7].

In certain patients with PCFO, like in our case, there is a formation of an aneurysm of the atrial septum. In favor of the theory of primary PCFO, this aneurysm consistently bulges into the left atrium [8–10] whereas in the presence of left sided obstructive pathology, the aneurysm may bulge into the right atrium [4, 11]. Furthermore, the presence of a giant septal aneurysm indicates a somewhat later closure of the foramen, explaining why in these situations the left side chambers are only mildly diminished in size. In addition, coexisting lesions like a VSD can provide a variable amount of flow to the left ventricle in terms of growth.

Though in this scenario the patient did not have the benefit or prenatal screening, PCFO can be diagnosed echocardiographically in-utero. The FO is defined as restrictive if its maximal diameter is less than 3 mm [5], however in the presence of a atrial septal aneurysm this can be somewhat more difficult to assess since the fossa ovalis is patent but the FO maybe closed or restrictive as in our case. Furthermore, an atrial septal aneurysm can be confused with other congenital anomalies e.g. cor triatriatum, in which management and prognosis are quite different.

A close monitoring for signs and symptoms of decompensation is recommended for both fetuses and infants with the diagnosis of PCFO. Prenatally, and in the phase of worsening heart failure an intervention may be warranted. Balloon atrial septostomy could be performed to improve the failing right ventricle by creating a pop-off communication at the atrial level, or to improve the elevated left atrial and pulmonary venous pressures, as in the case of hypoplastic left sided structures [12]. Also, like in other causes of in utero heart failure maternal digitalization may offer a significant improvement without the need of more radical intervention. Nonetheless, if above measures fail or not available then preterm delivery may be necessary. Postnatally, the course is determined mostly by the degree of prenatal compromise. While heart failure is expected to improve, some patients may require a significant amount of inotropic support.