What is a Patent Foramen Ovale (PFO)?

Patent Foramen Ovale (PFO): Definition and Clinical Significance

A patent foramen ovale (PFO) is a remnant of the fetal circulation that persists after birth, creating an interatrial communication that allows potential right-to-left shunting of blood between the atria. 1

Anatomical Definition and Prevalence

• PFO is a normal fetal communication between the right and left atria that fails to close after birth, creating a potential opening between the cardiac chambers 2
• It is present in approximately 25% of the general adult population, making it the most common congenital heart abnormality of fetal origin 1, 2
• The prevalence of PFO appears to decrease with age, from about 35% in younger individuals to approximately 20% in older adults 3
• PFO is defined specifically as a small interatrial communication confined to the region of the fossa ovalis without deficiency of the septum primum or septum secundum 1

Diagnostic Criteria

• The diagnosis of PFO is established by demonstration of an interatrial communication with right-to-left transit of contrast microbubbles within 3 to 4 cardiac cycles of right atrial opacification 1
• PFO can be detected using several imaging modalities:
  • Transesophageal echocardiography (TEE)
  • Transthoracic echocardiography (TTE) with contrast
  • Transcranial Doppler 2
• Contrast TEE with provocative maneuvers is considered the diagnostic method of choice as it allows direct visualization of the shunt 4

Clinical Significance and Associated Conditions

• PFO provides an anatomic substrate for paradoxical embolization, where venous thrombi can shunt directly from the right to left atrium, bypassing pulmonary circulation 1
• PFO has been associated with several clinical conditions:
  • Cryptogenic stroke (particularly in patients younger than 55 years)
  • Migraine with aura
  • Decompression illness in divers
  • High-altitude pulmonary edema
  • Systemic arterial embolism
  • Obstructive sleep apnea syndrome 5, 3
• PFO is found in approximately 50% of patients 60 years or younger with embolic stroke of undetermined source (cryptogenic stroke), compared to 25% in the general population 6

Anatomical Variations and Risk Factors

• Some PFOs may be associated with an atrial septal aneurysm, defined as a redundant and hypermobile portion of the interatrial septum that demonstrates more than 10-mm excursion from the centerline during the cardiac cycle 1
• An atrial septal aneurysm is present in approximately 2.5% of the general population, most often in association with PFO 1
• The size of the PFO, magnitude of the right-to-left shunt, and presence of an atrial septal aneurysm may influence the risk of clinical events 1, 6

Mechanisms of Clinical Events

• Paradoxical embolism through a PFO accounts for about 5% of all strokes and 10% of strokes in younger patients 6
• Other potential mechanisms of stroke among patients with PFO include:
  • Paroxysmal atrial fibrillation (which may be unrelated to the PFO itself)
  • Formation and release of thrombus from the rim of the defect
  • Thrombus formation on the left atrial aspect of an associated atrial septal aneurysm
  • Passage of vasoactive substances that escape pulmonary degradation 1

Clinical Evaluation Tools

• The Risk of Paradoxical Embolism (RoPE) score incorporates clinical characteristics to predict the likelihood that a cryptogenic stroke was caused by a PFO 6
• The PFO-Associated Stroke Causal Likelihood (PASCAL) classification system combines the RoPE score with anatomical criteria from echocardiography to classify PFO as the "probable," "possible," or "unlikely" cause of cryptogenic stroke 6

In most individuals, a PFO poses no health threat, but in certain circumstances, it can contribute to significant clinical events, particularly cryptogenic stroke in younger patients 2, 5.