Patent foramen ovale (PFO) is a form of atrial septal defect (ASD). It is a congenital heart formation abnormality, a vestige of the foetal development period.

 

The human heart   

The heart is divided into 4 distinct chambers, divided by permeable walls called septums and valves that open and shut, letting blood flow in the right direction.

One can compare the heart with a house where there are several doors, each one being accessible through a door of its own.

 

Embryonic development of the heart

The human heart develops in the first few weeks after conception. When the heart begins to form, it looks like a tube with dilatations and constrictions.

At the beginning, the heart has only one atrium, the primitive atrium, and one primitive ventricle.

 

Development of the interatrial septum

A partition or wall, the interatrial septum, divides the primitive atrium into 2 parts, the right and left atriums.

Formation of the interatrial septum occurs in several stages. It is made up of 2 layers, as one might say.

 

 

 

 

 

Two layers

The first layer has a central hole while the second one has an oval-shaped opening   located at its base. Hence, it is rightly called " foramen ovale ".

 

 

 

 

 

 

The 2 layers do not overlap in the fetus, therefore the 2 atriums can communicate.

 

Normal fetal interatrial communication

At this stage, interatrial communication is normal. It is a way of bypassing the lungs, which are not yet in use. During fetal life, the lungs require a minimum amount of blood only.

The placenta acts as fetal lungs during intrauterine life. It is responsible for providing oxygen and nutrients that are essential to the developing fetus.

Most of the blood that flows into the right atrium travels to the left atrium by way of the ostium secundum; the blood glides between the 2 layers of the septum and exits the left atrium through the patent foramen ovale. Then, the left ventricle propels the blood, which goes on to feed the fetus and ensure its development.

 

READ: Atrial development and venous return

 

Atrial septal defect (ASD) normally closes at birth

The lungs are ready to breathe the moment the baby is born. Pressure in the right pulmonary cavities decreases due to lung expansion and easier blood circulation therein.

Lower right atrium pressure makes left atrium pressure higher than that of its neighbor's.

The pressure gap crushes the tunnel between the 2 layers of the fetal ASD septum.

According to some U.S. statistics, 75% of fetal ASDs close permanently after a few months. Persistence of the communication, the connection between the 2 upper heart chambers, is called patent foramen ovale (PFO).

 

What does it imply exactly?

At birth, blood flows through the blood vessels, heart and lungs, according to a pre-established pathway that cannot be reversed.   

On the other hand, when a patent foramen ovale exists, it is possible for the blood to flow from one atrium to the other.

Left atrial pressure being higher than in the right atrium, it is easier for some of the blood coming from the lungs to go from there and into the right atrium through the PFO.

The extra volume of blood in the right atrium is minimal and cannot have any impact on the heart

 

 

Although passage from the left atrium to the right one is favoured by the difference in their pressure degrees; passing from the right to the left atrium is possible as well.

 

 

 

Medical implication

PFO medical implication relates to the brain.

Tiny blood clots of a few millimeters in size develop inside blood vessels, more often in the lower legs area.

A natural destruction process disaggregates these blood clots, which normally balances the coagulation system called "thrombolysis". 

When the thrombolysis system does not succeed in dissolving all the blood clots, the inferior vena cava carries them to the right ventricle from where they are propelled toward the lungs. Once there, the clots are entrapped and filtered without any adverse consequences. As a result, they cannot get into the bloodstream.

The PFO makes it possible for a small blood clot to cross over from the right to the left atrium.

This passage is paradoxical, given that the blood to the PFO normally flows from the left to the right atrium, the pressure being lower on that side.

 

Paradoxical embolism

Once the small blood clot gets to the left side of the heart, the ventricle propels it into the bloodstream. 

A trajectory of cerebral blood flow can be fraught with consequences and cause a  transient ischaemic attack (TIA) or, worse still, a stroke. 

This type of embolism is called paradoxical embolism (PDE) because the embolus results from its unusual right-to-left ventricular transit. Paradoxical embolism is a low-risk type of stroke that rarely happens, but it does occur.

 

What are the possible symptoms?

It is estimated that PFO is present in 25% of the population.

PFO hardly ever shows any symptoms.

In fact, PFO symptoms frequently lack evidence and are generally wrongly attributed; symptoms are often found to be of a neurological nature.     

Studies have shown that there is a possible connection between chronic migraine and PFO, however, the evidence is unclear and is still being debated.

Its most significant involvement is in the event of a transient ischaemic attack (TIA) or a stroke.

When such an incident occurs, several tests are done to help determine what caused the TIA or stroke. If no obvious cause is found, and a foramen ovale is present, the latter is deemed responsible for the disorder.  

It is interesting to point out that for half of the patients with PFO no cause was found to explain their TIA or stroke.  The causal connection is not clear as the other half of the patients, who did not have a PFO, suffered a TIA as well. Therefore, it is a presumptive diagnosis, and particularly so in the case of patients under 50 years of age.

 

How is this anomaly diagnosed?

Unlike congenital atrial septal defect (ASD), patent foramen ovale (PFO) does not cause a heart murmur.

It is often discovered totally by chance during a transthoracic echocardiogram requested for an entirely different purpose.

A specific test is needed as a medical examination cannot detect a PFO.

 

Sonographers have their own secret method!

Shake a bottle of water and take a look, the water is blurred, full of small bubbles or micro-bubbles.

Sonographers strongly agitate a water/air mixture, creating minute bubbles that are ultrasound reflective; they inject the bubbly solution into a vein and watch as it reaches the right atrium. Seeing micro-bubbles in the left atrium is an indication that there is a possible passage through the septum. A diagnosis can now be established accordingly.

Lungs filter the micro-bubbles and none should be able to get inside the left atrium.

 

 A transesophageal echocardiography (TEE) is more precise

For a transthoracic echocardiogram (TTE), a device or transducer is placed firmly on the patient's chest. This test is often insufficient in detecting a congenital atrial septal defect vs. a patent foramen ovale.

For more detailed images of the patient's heart, another test, a transesophageal echocardiography (TEE) may be recommended and performed.

A flexible tube containing a tranducer is guided down the patient's throat, somewhat like a gastroscopy procedure. A transesophageal echocardiography gives clearer images of the patient's heart and provides a precise identification of the existing type of congenital atrial septal defect or patent foramen ovale. 

 

What can be done in the case of PFO?

There is no obvious answer to this question. When the problem is not due to a   neurological disorder, no therapy is necessary.

The case is not quite as simple when PFO is diagnosed in a transient ischemic attack (TIA) or stroke situation.  

Treatment is usually made up of an agent acting on blood coagulation. Antiplatelet or a stronger anticoagulant therapy is recommended, namely, Coumadin or Novel Oral Anticoagulants (NOAC), now called Direct Oral Anticoagulants (DOAC).

Possibilities for closing the PFO can be discussed between the cardiologist and the neurologist who specialize in this area, taking into account the patient's medical history and medical condition at present.

The patient is met before the intervention in order for him to be properly informed about the procedure as well as the benefits and potential risks involved.

 

Enjoying a normal life

People with PFO can lead a normal life, including pregnancy, if appllicable.

 

No preventive antibiotic therapy            

It is unmecessary for people with PFO to take antibiotics before having dental work done or any other type of surgery. Good oral hygiene is recommended.

 

When intervention is unavoidable

For a very small number of patients, an intervention to close the PFO is inevitable. It is a non-surgical procedure called "Percutaneous closure of PFO".