Patent foramen ovale

A patent foramen ovale (PFO) is a type of atrial septal defect in which there is channel-like communication between the atria through an unfused fossa ovale.

Gross anatomy

The foramen ovale in the interatrial septum normally develops into the fossa ovalis when the flaps of the atrial septa primum and secundum normally fuse during development. The foramen ovale remains patent if there is incompletely fusion. This variant occurs in 25-33% of adults ^1,2. The prevalence may decrease with advancing age.

Patent foramen ovales/foramina ovalia are subdivided into:

• probe-patent foramen ovale
• patent foramen ovale

A "probe patent" foramen ovale is defined as a defect in the fossa ovalis that would be revealed with instrument probing.

Pathology

A patent foramen ovale has been associated with paradoxical embolism and ischemic stroke because of the potential for a venous thromboembolism to pass from the right atrium to the systemic circulation, bypassing the lungs.

PFOs are small enough to be haemodynamically insignificant.

Radiographic features

Transesophageal echocardiography

• may be detected on a "bubble study" (IV injection of agitated saline); the contrast material appears in the left atrium before the normal time
  • the significance of the shunt through a PFO often estimated as a function of the number of bubbles directly visualized passing from the right to left atrium
  • severe shunt present when more than 30 bubbles (25 is the TTE cutoff) are seen crossing the IAS
• best assessed from a bicaval view (mid esophageal level, 90 degree view)
  • interatrial septum appears in near field, with the confluence of the superior and inferior vena cavae into the right atrium in the far field
  • color flow doppler applied across interatrial septum to assess for PFO or secundum ASD
  • if possible, a valsalva maneuver is performed (also possible with transthoracic echocardiography as a supplement), which is crucial to elicit right to left shunting 
• if visualized, the maximum discontinuity between the primum and secundum should be measured (valsalva phase IV)
  • defects > 4 mm considered severe, <2 mm considered small ⁵

Cardiac CT

• abnormal communication of contrast material between the atria through a channel-like tunnel in the interatrial septum
• a channel-like tunnel alone is a normal variant of the fossa ovalis, and is not diagnostic

Contrast-enhanced cardiac MRI

• not a first-line study, but may be diagnosed by visual assessment or computation of signal–time curves in the pulmonary vein and the left atrium ³

A patent foramen ovale can be differentiated from an atrial septal defect because a PFO takes a tunneled intraseptal course, or with the presence of a flap valve on the left atrial side of the foramen ². 

Treatment and prognosis

Closure devices, both surgically open and percutaneous, have been developed and are currently implemented in some centers for PFO.