Fossa ovalis

Fossa ovalis is a depression of varying shapes in the right atrium of the heart, located in the inferior aspect of the right interatrial septum, the wall between right and left atrium ¹. The fossa ovalis is the remnant of an interatrial opening, the foramen ovale, which has a significant role in fetal circulation (allowing blood to flow from the right atrium to the left atrium during fetal development) ². The fossa ovalis is formed by the fusion of the septum primum and septum secundum ³. Although the fossa ovalis appears two-dimensional, it is, in fact, a three-dimensional structure – consisting of the septum primum, septum secundum and the annulus or limbus fossa ovalis which raises around the perimeter of the fossa ovalis ⁴. Important neighboring structures are as follows: below and rightward of the fossa ovalis lies the inferior vena cava opening; the location of the coronary sinus is anterior to the fossa ovalis; along the same horizontal plane of the fossa ovalis is the His bundle ³.

The fossa ovalis forms part of the atrial septum which appears expansive from the right atrial view. However, the true and false atrial septa are clinically important features that need to be distinguished ⁵. The true interatrial septum contains the fossa ovalis and comprises only 20% of the entire septum. It is the only area through which the interatrial septum may be traversed without the risk of cardiac perforation ⁵.

The fossa ovalis may assume various shapes and dimensions ³. The fossa ovalis most commonly assumes an oval shape but may be circular or even elliptical. The dimensions of the fossa ovalis vary among populations as well as by the measuring tools employed but may correlate with the patient’s weight of the heart, age, and body weight ⁵. Significant variations of the fossa ovalis exist, with recesses, slits, aneurysms and fibrous strands found in certain specimens ³.

There is presence of probe patency of foramen ovale in about 15–35% of general population ⁶. But sometimes foramen ovale persists to a varying degree (patent foramen ovale [PFO]), causing mixing of oxygenated and deoxygenated blood, which leads to a number of clinical and subclinical conditions like migraine, cryptogenic stroke, decompression sickness in divers, and platypnea orthodeoxia ⁷. It may also lead to cerebral embolism. Most patients with cryptogenic stroke have a patent foramen ovale which serves as a channel for paradoxical emboli leading to a greater incidence of cerebrovascular accidents ⁸. Proper screening for patent foramen ovale requires transthoracic agitated saline contrast echocardiography during a cough or in conjunction with Valsalva maneuver ⁹. Interatrial shunting in confirmed by the presence of more than five bubbles in the left atrial chamber within three cardiac cycles ⁹. Transcatheter closure of patent foramen ovale is indicated in these settings ¹⁰ and prior to device selection, echocardiography is done to look for the dimensions of foramen ovalev and foramen ovale, its position along the limbus, redundancy, etc.

The interatrial septum receives vascular supply by anastomoses between the left and right branches of the anterior and posterior atrial branches of the right and left coronary arteries; termed Kugel’s artery or arteria anastomotica aucularis magna ¹¹. The fossa ovalis together with the middle part of the interatrial septum receives the least vascular supply, additionally, the number and density of these vessels’ network decrease with age, whereas the anteroinferior portion of the septum receives the densest vascular network ¹².

Figure 1. Fossa ovalis

Abbreviations: FOv = fossa ovalis; CS = coronary sinus ostium.

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Figure 3. Shapes of fossa ovalis

Footnote: (A) Oval and (B) circular. Double-headed red arrow indicates the long axis.

Abbreviation: FOv = fossa ovalis.

[Source ⁶ ]

Figure 4. Annulus fossa ovalis or limbus fossa ovalis was found to be flat or raised at the margin. Annulus was raised in 46 cases (92%) and flat in 4 cases (8%) 

Footnote: (A) The annulus is raised along the whole circumference (black arrows). The fossa ovalis show redundancy. (B) The annulus is flat except in its upper part which shows a prominent recess (red arrows). Fossa ovalis is aneurysmal bulging towards right atrium.

Abbreviation: FOv = fossa ovalis.

[Source ⁶ ]

Fossa ovalis function

In the normal heart, the fossa ovalis serves to prevent blood flow, i.e., shunting of blood, across the interatrial septum ¹³. During fetal development, the foramen ovale allows blood to pass from the right atrium to the left atrium, bypassing the nonfunctional fetal lungs while the fetus obtains its oxygen from the placenta ¹⁴. A flap of tissue called the septum primum and septum secundum form a one-way flap valve over the foramen ovale during that time, that allows shunting of blood from right atrium to the left atrium ⁴. The septum primum which is on the left atrial side forms the floor of the fossa ovalis, whereas the septum secundum derived from an infolding of the roof of the atrium forms the limbus of the fossa ovalis (annulus ovalis) which is the prominent oval margin of the fossa ovalis in the right atrium ³. After birth, the introduction of air into the lungs causes the pressure in the pulmonary circulatory system to drop. Physiologic increase in the left heart pressure at the time of birth allows the septum primum to create a tight seal with the septum secundum, creating a depression called the fossa ovalis ¹⁵, This fusion of the septum primum with the septum secundum and subsequent formation of the fossa ovalis occurs in 75% of cases, failure of which leads to a patent foramen ovale. Depending on the circumstances, a patent foramen ovale may be completely asymptomatic, or may require surgery ¹⁶. Also, the fossa ovalis undergoes thinning in utero ¹⁷.

The limbus of the fossa ovalis (annulus ovalis) represents the most direct anatomical landmark for atrial septal puncture for various procedures and may be located using the His bundle electrogram locating method ¹⁸. The following are significant procedures that require location of the fossa ovalis for transseptal puncture ⁵:

• Patent foramen ovale and atrial septal defect repair
• Right heart catheterization
• Percutaneous balloon valvuloplasty
• Radiofrequency catheter ablation
• Pulmonary vein isolation
• Left atrial appendage closure
• Catheter-based mitral valve repair
• Hemodynamic assessment of the mitral valve
• Paravalvular leak closure
• Alternative access to the left ventricle in the presence of a prosthetic aortic valve

Intracardiac echocardiography and fluoroscopy are useful for identifying the fossa ovalis, a critical landmark for electrophysiological procedures ¹⁹.

Fossa ovalis clinical significance

The fossa ovalis may be a frequent site of cardiac pathology. Known pathologies involving the fossa ovalis include:

• Premature closure of the foramen ovale and patent foramen ovale (PFO)
• Atrial septal defect (ASD)
• Rheumatic heart disease
• Amyloidosis
• Tumors: Prichard’s structures, myxomas, lipomatous hypertrophy of cardiac interatrial septum)
• Fossa ovalis membrane aneurysm
• Cardiac arrhythmias

Patent foramen ovale and atrial septal defect represent functionally similar pathologies; however, they differ in their etiology as well as clinical severity. Atrial septal defects are the result of the failure to form either of the interatrial septa (septum primum, septum secundum) or sinus venosus, whereas patent foramen ovales result when, in approximately 25% of the population, there is a failure of closure of the foramen ovale created by the septa ¹⁵. Both diseases are characterized clinically by the presence of migraines, paradoxical emboli and strokes, and decompression sickness ¹. The size of a patent foramen ovale largely correlates with its clinical severity; larger lesions correlate with more severe illness and a higher risk of paradoxical emboli ²⁰.

Atrial septal defects may occur in isolation or as part of a syndrome. Several genes have been implicated in the development of atrial septal defects. NKX2.5 gene is known to produce secundum type defects, TBX5 gene mutations result in Holt-Oram syndrome which features atrial septal defect ³. Atrial septal defects may undergo spontaneous closure depending on its initial size at the time of diagnosis, found to be the best predictor of the progression of such defects. Holes in the fossa ovalis measuring less than 8 mm and/or containing aneurysms may diminish in size or resolve spontaneously ²¹. However, failure of closure usually leads to enlargement of the defect and the need for surgical closure ²¹.

The structure and dimensions of the fossa ovalis are altered in the course of rheumatic heart disease ²². Rheumatic heart disease is characterized by extensive scarring of valves and other structures within the heart ²³. The fossa ovalis may assume a more horizontal orientation in addition to a larger surface area; hemodynamic alterations are also prominent in this setting ²².

Amyloidosis, characterized by diffuse thickening of heart valves, does not spare the interatrial septum. Thickening of the interatrial septum is 100% specific for amyloidosis ²⁴.

The fossa ovalis is a frequent site of atrial myxomas. Atrial myxomas are true neoplasms, appearing as pedunculated friable masses attached to the interatrial septum ²⁵. These masses present with a wide range of symptoms ranging from dyspnea to hemiplegia ²⁶.

Prichard’s structures are also benign structures derived from mature endothelial cells, they are more frequently found in individuals beyond 60 years of age and are unrelated to cardiac myxomas ²⁷. These structures may be subendothelial or located within the atrial cavity, commonly on the left side of the fossa ovalis and may be the result of hemodynamic alterations in blood flow within the heart ²⁸.

Lipomatous hypertrophy is fat accumulation greater than 2cm located on the interatrial septum. The mass characteristically spares the fossa ovalis. Arrhythmias and sudden cardiac death may be accompanying complications ²⁹. Obstruction of the vena cava ostium may be present in larger lesions ³⁰. The occurrence of lipomatous hypertrophy of the septum has correlations with obesity, older age, female gender, steroid use ³⁰.

The fossa ovalis is found to be aneurysmal in some cases but is present with increased incidence in patients with stroke ³. Fossa ovalis membrane aneurysms present as focal bulging of the interatrial septum, with displacement towards the left or right atrium ³¹. Fossa ovalis membrane aneurysm carries a significant risk of fibrosis and thrombus formation within its wall with related complications, as well as the formation of an atrial septal defect (ASD) from a preexisting patent foramen ovale (PFO) leading to increased intracardiac shunting. It is often the result of elevated intracardiac pressures arising from secondary cardiac pathologies, most commonly ischemic heart disease, aortic and mitral valvular diseases ³¹. Transesophageal echocardiography represents the best method of visualizing fossa ovalis membrane aneurysms in patients suspected of cardiogenic emboli, although the emboli may arise from a different source ³².

Electrophysiological studies may reveal increased automaticity in the region of the limbus fossa ovalis. Such automaticity may manifest as reversible focal atrial tachycardia and chaotic atrial rhythm and is responsive to focal radiofrequency ablation of the limbus with the restoration of cardiac function ³³.

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