Atrial Septal Defect

Defect or opening of the atrial septum resulting in communication between right and left atria.
 

The incidence of atrial septal defects (ASDs) is approximately 1 per 1500 live births, with isolated secundum defects accounting for 10-15% of congenital heart disease. (1,2) They can occur in isolation or be associated with additional congenital heart disease.

The majority of ASDs are isolated defects and not associated with hereditary or genetic anomalies.  In some cases, ASDs may occur in families with inheritable genetic anomalies such as Holt-Oram syndrome, which is characterized by skeletal abnormalities of the upper extremities and cardiac defects (most commonly secundum ASD and cardiac conduction disorder). A TBX5 gene mutation is identified in 74% of affected individuals with Holt-Oram syndrome. (3) Other genetic mutations reported for inheritable ASDs include NKX2-5, GATA4, MYH6, and chromosome 14q12. (5,6)

There are four subtypes of ASDs classified according to their anatomical location in the atrial septum: 

•Secundum defects result from a deficiency in the foramen ovale flap and are the most common, accounting for up to 80% of ASDs.
•Primum defects result from a deficiency within the embryological septum primum and are associated with the atrioventricular septal defect (AVSD) disease spectrum. Primum ASDs account for approximately 15% of ASDs.
•Sinus venosus defects involve the malalignment between the posterior wall of the superior vena cava (SVC) or the inferior vena cava (IVC) with the atrial septum.  Sinus venosus ASDs account for approximately 5% of ASD.  Sinus venosus ASDs are often associated with partial anomalous pulmonary venous return.
•Coronary sinus ASD, the rarest form, results from a defect in the wall between the coronary sinus and the posterior aspect of the left atrium.  The true atrial septum is typically intact and the atrial shunt occurs through the unroofed coronary sinus.  A persistent left superior vena cava to the coronary sinus is often associated with this type of ASD.

ASDs may be isolated or part of more complex cardiac congenital heart disease. Isolated ASDs generally have no impact on fetal physiology due to the normal fetal circulation and are difficult to diagnose prenatally. The detection of ASDs postnatally is high, accounting for up to 15% of infants born with congenital heart defects. (7) 

Prenatally detected ASDs are commonly associated with additional congenital heart disease, including atrioventricular septal defects (AVSD) and partial anomalous pulmonary venous return (PAPVR). Primum ASDs fall on the spectrum of atrioventricular septal defects and is often associated with trisomy 21. (4)

ASDs are challenging to diagnose prenatally. Secundum defects occur in the region of the foramen ovale flap, and its complete closure can only be assessed after birth. Primum defects are identified by a deficiency in the atrial septum primum with linear insertion of the atrioventricular valves. The use of colour Doppler imaging can confirm the right-to-left shunting of blood through the septal defect.

Once an ASD is suspected, a detailed evaluation of the fetal heart should be warranted to rule out associated cardiac anomalies, such as an atrioventricular septal defect (AVSD).

Isolated ASDs have no impact on fetal physiology due to the normal fetal circulation. 
After birth, small ASDs may be followed conservatively to monitor for spontaneous closure.  However, moderate to large ASDs typically require closure by cardiac catheterization or surgical repair early in life to avoid volume overload in the right atrium. Sinus venosus and coronary sinus ASDs do not typically close spontaneously and require surgical closure.  

1. Samanek M: Children with congenital heart disease: probability of natural survival. Pediatr Cardiol.  13:152-158 1992

2. Porter BJ, Edwards WD: Moss and Adams’ Heart Disease in Infants, Children, and Adolescents: Including the Fetus and Young Adult. 7th ed 2008 Wolters Kluwer Health/Lippincott Williams and Wilkens Philadelphia 

3. Li QY, Newbury-Ecob RA, Terrett JA, et al.: Holt-Oram syndrome is caused by mutations in TBX5, a member of the Brachyury (T) gene family. Nat Genet. 15:21-29 1997

4. Paladini D, Volpe P, Sglavo G, et al. Partial atrioventricular defect in the fetus: diagnostic features and associations in a multicenter series of 30 cases. Ultrasound Obstet Gynecol. 2009; 34:268-273.

5. Srivastava D, Olsen EN: A genetic blueprint for cardiac development. Nature.  407:221-226 2000

6. Ching YH, Ghosh TK, Cross SJ, et al.: Mutation in myosin heavy chain 6 causes atrial septal defect.  Nat Genet. 37:423-428 2005

7. Hoffman JI, Christianson R. Congenital heart disease in a cohort of 19, 502 births with long term follow-up. Am J Cardiol. 1978; 42:641-647
 

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