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US Findings:
There was a persistent, somewhat mobile, rounded anechoic
structure within the Left Atrium that appeared to be in
contact with the atrial septum. No fetal cardiac
arrhythmias were present during the exam. The remainder of
the fetal "Well-Being" exam was normal.
Diagnosis: Foramen of Ovale Aneurysm Also referred to as "Redundant Foramen of Ovale" or "Redundant Septum Primum".
Embryology:
Remember: On a coronal view of the fetal heart, the SEPTUM
PRIMUM forms first, from superiorly down towards the dorsal
endocardial cushion. Perforations in the Septum Primum
form the Ostium Primum. The SEPTUM SECUNDUM forms just to
the right of the Septum Primum. The Ostium Secundum - with a
portion of the Ostium Primum - forms the Foramen of Ovale.
The superior portion of the Septum Secundum and inferior
flap of the Septum Primum close the Foramen Ovale post-
natally.
Many fetuses have a recognizable flap (Septum Primum flap) that projects into the left atrium. However, this flap shows little mobility during the cardiac cycle, and has limited extension: it is a flap, NOT an aneurysm. An atrial septal ANEURYSM (redundant atrial septal tissue) is very mobile, and may extend across to the far left atrial wall, or even herniate through the mitral valve!
Associations:
Foramen of Ovale Aneurysms are often Isolated. However,
they are associated with fetal atrial arrhythmias in up to
67% of some series. The majority of these arrhythmias are
solitary PAC's; there may rarely be short runs of - or even
sustained - SVT. FOA's have been seen with Hypoplastic RV,
ASD, Mitral and Tricuspid Valve Prolapse, Thromboembolism,
AV Valve Obstruction, and Pulmonary Venous Obstruction.
Etiology: Chicken Vs. Egg
Many theories have been proposed, such as abnormal atrial
pressures, weakened interatrial septum, abnormal flow
patterns, or a combination of these. One could see how
floppy atrial tissue could cause arrhythmias; but perhaps an
arrhythmia could cause abnormal flow patterns that result in
atrial septal redundancy/aneurysm. In two cases cited in
the literature, atrial septal aneurysms resolved upon
control of atrial arrhythmias; in two other cases, resection
of atrial aneurysms led to resolution of atrial arrhythmias.
Prognosis:
The high frequency of FOA in biased study cohorts compared
to the low frequency in children & adults also raises the
possibility that FOA is a variant of normal, and
spontaneously regresses in the vast majority of cases.
Those associated with other cardiac anomalies tend to be
overshadowed by those anomalies.
References:
1. "Fetal Atrial Septal Aneurysm: A Cause of Fetal Atrial
Arrhythmias", by Mary Jo Rice, MD, et al; JACC Vol.12,
No.5; Nov.1988:1292-7.
2. "Fetal Atrial Arrhythmias Associated with
Redundancy/Aneurysm of the Foramen Ovale", by Patricia
A. Stewart, et al; J. Clin Ultrasound 16:643-650, Nov/Dec 1988.
3. "In Utero Sonographic Appearance of the Atrial Septum
Primum and Septum Secundum", by Parul Kachalia, et al; J
Ultrasound Med 10:423-426, 1991.
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