Background
The foramen ovale is a normal cardiac structure found in all newborns and can be best described as a "door" between the right and left atria.
The foramen ovale is essential for proper fetal circulation, directing oxygenated, nutrient-rich blood from the placenta, preferentially to the developing fetal brain. During fetal life, the "door" is open, and blood passes from the right to left atrium. However, with separation from the placenta and with the first few breaths, the left atrium fills with blood returning from the lungs and closes the "door."
During the first years of life, the foramen ovale seals shut and becomes a true wall that separates the right and left atria. However, in a significant proportion of people, the foramen ovale does not seal shut and remains a potential trapdoor between the two atria. A patent foramen ovale (PFO) is defined as a foramen ovale that does not seal. See the images below.
This 2-dimensional echocardiogram in an infant (subcostal long-axis view) shows a patent foramen ovale. Right atrium (RA) and left atrium (LA).
Color Doppler of the patent foramen ovale (PFO) seen in the previous image. A small amount of left-to-right flow is present. This left-to-right flow pattern is typical for PFO seen in newborn infants.
All newborns are expected to have a patent foramen ovale. The time frame over which most seal shut varies. However, adult autopsy studies have shown that 20-34% of adults from the third to ninth decades of life have at least a small patent foramen ovale. [1]
Although it is a normal structure, a foramen ovale has several special circumstances under which it may be implicated in disease.
Pathophysiology
The foramen ovale is an interatrial communication that permits blood from the inferior vena cava to freely enter the left atrium in utero. Anatomically, a thick muscular ridge, the limbus of the fossa ovalis, borders the foramen ovale. A thin tissue flap on the left atrial side of the septum, which represents an embryological remnant of the septum primum, forms the valve of the fossa ovalis. At birth, the left atrial pressure exceeds the right atrial pressure and forces the valve against the limbus, thus achieving physiological closure. During the first weeks of life, Doppler echocardiographic studies in healthy newborns can often demonstrate incompetence of the valve that allows some degree of left-to-right shunting. Shunting generally resolves by age 1 year as the foramen ovale seals shut. [2, 3, 4, 5]
Persistent left atrial enlargement associated with specific cardiac lesions, such as mitral valve stenosis, mitral valve regurgitation, patent ductus arteriosus, or ventricular septal defect, can render the foramen ovale "incompetent." Atrial left-to-right shunting can continue as a result.
Right-to-left shunting can occur through a patent foramen ovale, especially in conditions associated with elevated right atrial pressure such as tricuspid atresia, tricuspid valve stenosis or right ventricular hypoplasia with decreased right ventricle compliance. Patients with persistent or transient elevation of right atrial pressure can experience a paradoxical embolus through a patent foramen ovale. Some congenital heart lesions depend on the foramen ovale for obligatory left-to-right (mitral atresia) or right-to-left (tricuspid atresia, total anomalous pulmonary venous return) shunting to maintain adequate cardiac output.
Epidemiology
International data
Several echocardiography and postmortem studies indicate that the foramen remains competent in 30% of patients with otherwise normal cardiac anatomy.
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This 2-dimensional echocardiogram in an infant (subcostal long-axis view) shows a patent foramen ovale. Right atrium (RA) and left atrium (LA).
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Color Doppler of the patent foramen ovale (PFO) seen in the previous image. A small amount of left-to-right flow is present. This left-to-right flow pattern is typical for PFO seen in newborn infants.
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Transesophageal echocardiogram showing the atrial septum. The "flap" of the septum primum is seen. The diagnosis of patent foramen ovale (PFO) cannot be made until right-to-left bubble contrast is demonstrated. LA = Left atrium; RA = Right atrium.
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Bubble-contrast injection during Valsalva maneuver. The "flap" of the foramen ovale is opened and bubbles are seen crossing from the right atrium to the left atrium (arrow).
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Transesophageal echocardiogram revealing a 25-mm Amplatzer patent foramen ovale (PFO) occluder in place across the PFO shown in the previous 2 images.
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Transesophageal echocardiogram of a patent foramen ovale (PFO) closed with 25-mm Amplatzer PFO occluder. Bubble-contrast study with Valsalva post-device placement shows no residual right-to-left bubble passage.
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Transcranial Doppler (TCD) study with bubble-contrast study. A Doppler probe is used to interrogate the right middle cerebral artery. Frame 1 shows normal findings. Note the absence of bubble artifact of Doppler signal in the middle cerebral artery. Frame 2 shows strongly positive (5/5) bubble transit seen in a patient with a patent foramen ovale (PFO) during Valsalva maneuver. TCD is a useful screening tool for PFO because of its ease-of-use and ability to easily quantify the amount of potential right-to-left shunt. One of the pitfalls is the inability to differentiate between other sources of right-to-left shunt, such as pulmonary arteriovenous malformation and a PFO.
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Transesophageal echocardiogram of bubble contrast study showing right-to-left passage of bubble-contrast with a Valsalva maneuver. RA = Right atrium. LA = Left atrium. Arrow shows bubble passage to LA.
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Transesophageal echocardiogram showing a 10-mm Amplatzer Septal Occluder in place across a patent foramen ovale.
