Patent foramen ovale

A patent foramen ovale (PFO) is an anatomical variant of the atrial septum in which there is incomplete fusion of the interface between the embryologic septum primum and secundum; this may result in intracardiac shunting. 

Terminology

Despite anatomical proximity and potential hemodynamic similarities, a PFO is considered distinct from pathologic congenital cardiac defects (e.g. atrial septal defects) ²¹;

• its presence is normal during embryologic life
• persistence into adult life is common with an incidence of 20–35% of the population
• shunting is functional rather than anatomic, and typically requires reversal of the physiologic gradient of interatrial pressure, commonly absent without provocation 

It is therefore most commonly referred to as an anatomical variant of the interatrial septum as opposed to a pathologic defect.

Epidemiology

Associations

The presence of a patent foramen ovale has been variably associated with a higher prevalence of the following ²²:

• acute ischemic stroke
  • defect size and degree of shunting may correlate with risk, closure may be selectively considered to mitigate recurrence 
  • common association in younger populations without conventional historical risk factors
• chronic migraine headaches
• platypnea-orthodeoxia ²⁶
• dysbarism
  • associated with severe decompression sickness in divers ²³
  • integumentary and neurologic manifestations common
• carcinoid heart disease

Pathology

Etiology

Formation of the ostium secundum from confluent perforations in septum primum during cardiac development facilitates shunting of oxygenated blood from the inferior vena cava preferentially to the left atrium, and thereafter the systemic circulation. While the septum secundum grows to largely, but incompletely, overlap the septum primum and overlie this communication, the orifice remains open due to a physiologic gradient of pressure right-to-left facilitated by the high resistance pulmonary circulation and the jet of pressure generated by the orientation of the Eustachian valve.

Reversal in the interatrial pressure gradient at birth results in apposition of the septum primum and secundum; this may result in subsequent fusion (typically complete within the first year of life) and closure, or a highly variable persistence of this interatrial communication ranging from a subtle flap-like orifice to a widely patent, long and serpentine tunnel ¹¹.

Classification

One method of classifying PFOs uses binary division into simple or complex, with the latter characterized by one or more of the following characteristics ¹⁰:

• defect anatomy
  • length exceeding 8 mm ("long tunnel")
  • wide atrial orifice
• degree of shunting
  • shunting present at rest
  • complete left atrial opacification with agitated saline administration
• associated features
  • atrial septal aneurysm
  • increased thickness of the septum secundum
  • prominent Eustachian ridge

Radiographic features

Ultrasound

Transthoracic echocardiography may suggest the presence of a patent foramen ovale by demonstrating right-to-left shunting not explained by a structural anomaly (e.g. atrial septal defect). As shunting may be intermittent and the interatrial pressure gradient insufficiently large to result in reliable detection with color flow Doppler, provocative maneuvers (e.g. Valsalva) and contrast enhancement are utilized to optimize detection ¹². Transesophageal echocardiography is superior in its ability to diagnose the presence of a PFO and fully delineate atrial septal anatomy and the presence of associated defects . Other modalities which have been variably utilized include:

• 3 dimensional echocardiography ²¹
• intracardiac echocardiography ²⁷
• transcranial Doppler 

Transthoracic echocardiography

Subcostal¹⁶ or apical windows are commonly utilized, with the apical 4 chamber view typically considered optimal. The atrial septum is examined using 2D and color flow Doppler, and the presence of shunting assessed;

• following the opacification of the right atrium with a positive contrast medium (commonly an intravenous admixture of saline and air, referred to as "agitated saline") a right-to-left shunt is diagnosed when any of the echogenic contrast bubbles appear in the left heart ¹⁵
  • prompt appearance (e.g. within fewer than three to six cardiac cycles) may favor intracardiac over intrapulmonary shunting ^23,24
  • amount of contrast present sometimes used to semi-quantify size ¹⁹
• shunting is dependent upon the interatrial pressure gradient, which may be augmented by maneuvers such as:
  • alteration of intrathoracic pressure by performing a Valsalva maneuver or alteration of mechanical ventilator settings ¹⁸
  • manual abdominal compression, patient repositioning 
  • alteration of this gradient is dynamically confirmed by a shift of the convexity of the atrial septum toward the lower pressure chamber 20
• bubbles crossing the mitral valve may result in high intensity transient signals deforming the velocity envelope visualized with pulsed wave Doppler (PWD) ¹⁷

Transesophageal echocardiography

• commonly used imaging planes/views and salient features include:
  • mid esophageal aortic valve short axis
    • allows visualization of shunting at the superior extent of the atrial septum ¹³
  • mid esophageal bicaval view
    • visualization of the flaplike interface between the limbus of the septum secundum and the septum primum 
      • measurements (e.g. tunnel/septal length) from this view important for planning closure
    • anatomical variants of structures associated with a higher prevalence of shunting which should be assessed include ¹⁴
      • eustachian valve
      • chiari network
      • atrial septal aneurysm

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

On ECG gated CT four types inter-atrial septum have been described ⁶

• type 1: an inter-atrial septum (IAS) with no visible channel: no visible septal flap
• type 2: a closed channel
• type 3: an open channel with no visible jet flow of contrast material between the two atria
• type 4: an open channel with a visible jet flow of contrast material between the atria

Types 3 and 4 comprised of PFO's

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.