Double Discordance: "Congenitally Corrected Transposition of the Great Arteries"

Classically, identifying a chamber with a right ventricular (RV) morphology on two-dimensional echocardiography relies on: (1) a coarse and irregular endocardial surface, (2) chordae tendineae attaching directly to the ventricular septum, (3) the presence of an infundibulum, (4) a triangular-shaped ventricular cavity, (5) the presence of a prominent moderator band, and (6) recognition of the atrioventricular (A-V) valve as tricuspid. 

The features associated with left ventricular (LV) morphology include: (1) a smooth and uniform endocardial surface, (2) two distinct groups of papillary muscles, (3) an ellipsoid-shaped ventricular cavity, and (4) identification of the A-V valve as mitral.

Double Discordance is characterized by a defect in laterality and an abnormal cardiac (L-)loop, leading to both atrioventricular and ventriculo-arterial discordance. In this condition, the morphologic right ventricle is situated on the left side, often described as a "left-handed" RV morphology. In France, it is recommended to use the term "double discordance" over "congenitally corrected TGA" (cCTGA) - often used in North America.

• The primary reason for this preference is that the essence of double discordance lies in the atrioventricular discordance.

• The orientation of the outflow tract and septal band, supports this perspective since it is similar (though mirrored) to a normal heart, implying a degree of outflow tract rotation that is not present in d-TGA (d-transposition of the great arteries).

• Therefore, the ventriculo-arterial discordance (transposition) is considered a secondary phenomenon to the primary atrioventricular discordance.

This distinction highlights that the defining feature of the condition is the atrioventricular discordance, making "double discordance" a more accurate and preferred descriptor.

The RV morphology in double discordance is always abnormal, a consensus shared by experts like Van Praagh and Anderson:

• It does not simply present as a mirror image of a normal RV; the septal band can appear abnormal.

• Anomalies typically affect the inlet, the tricuspid valve, and often the RV apex, which is frequently hypoplastic.

• There can be a constriction at the junction of the outflow tract and the admission chamber, often due to abnormal tricuspid attachments to the septum or septal band, making recognition of the septal band difficult.

• The septal band is frequently abnormal: specifically, the postero-inferior branch of the Y-shaped septal band is often virtually absent, leaving predominantly an anterior branch.

• Despite the ventriculo-arterial discordance, studies have shown that the orientation of the outflow tract and septal band in double discordance is similar (though mirrored) to a normal heart, in contrast to TGA where it is rectilinear. This suggests a degree of outflow tract rotation may occur in double discordance, unlike TGA. This finding underscores why the term "double discordance" is preferred over "congenitally corrected TGA," as the defining feature is the atrioventricular discordance.

Tricuspid Valve Anomalies in Double Discordance: The tricuspid valve is always anatomically abnormal in double discordance:

• It can be dysplastic, with leaflets appearing fused or atypical (e.g., the anterior leaflet appearing as two fused leaflets), leading to potential regurgitation.

• Epstein-like anomalies are common, though often they involve a displacement or verticalization of the tricuspid annulus rather than a true absence of leaflet delamination. While true Epstein's (with delamination absence) can occur, they are not the majority of "Epstein-like" valves seen in this condition.

• The posterior wall does not undergo atrialization; it remains muscular despite being thinner.

Conduction System Anomalies: The conduction pathways are also abnormal in double discordance:.

• An anterior atrioventricular node is typical.

• The bundle of His courses along the anterior border of an associated VSD (if present), or within the septal band, often in proximity to the pulmonary valve in the morphologic left ventricle.

Reference: Particularités du VD dans les TGV, doubles discordances etc by Dr Lucile Houyel - M3C Academy - YouTube Link.

In Van Praagh's classification, congenitally corrected transposition of the great arteries (CCTGA) is referred to as ventricular inversion or L-transposition and is described as follows:

Key Features in Van Praagh's Classification:

1. Atrioventricular (AV) Discordance

   • The morphologic right atrium (RA) is connected to the morphologic left ventricle (LV), and the morphologic left atrium (LA) is connected to the morphologic right ventricle (RV).

   • This is termed AV discordance because the atria and ventricles are mismatched in morphology.

2. Ventriculoarterial (VA) Discordance

   • The morphologic LV ejects blood into the pulmonary artery (PA), while the morphologic RV ejects blood into the aorta - (depends on if there is valvular patency)

   • This is termed VA discordance because the ventricles are mismatched with the great arteries they supply.

3. Physiological Correction

   • Despite the discordant connections, the circulation is "physiologically corrected" because deoxygenated blood flows to the lungs, and oxygenated blood flows to the systemic circulation. However, the RV, not the LV, functions as the systemic ventricle, which poses long-term challenges.

4. Anatomical Description

   • Situs solitus: The normal positioning of the atria.

   • L-looping of the ventricles: The morphologic LV is positioned on the right, and the morphologic RV is on the left (hence the "L" in L-transposition).

   • The full description would be S,L,L (situs solitus, L-looped ventricles, L-transposition).

5. Associated Lesions

   • CCTGA is often accompanied by other congenital abnormalities, such as:

     • Ventricular septal defect (VSD)

     • Pulmonary stenosis (PS) or atresia.

     • Tricuspid valve anomalies (e.g., Ebstein-like malformation or regurgitation)

     • Conduction system abnormalities, including heart block.

Echocardiography example

Case of cc-TGA (L-TGA) with pulmonary atresia. Here there is no heart block during the echocardiography. 

Case of atrial situs solitus; laevocardia (heart predominantly in the left hemithorax) and congenitally corrected transposition of great arteries (discordant atrioventricular & ventriculo-arterial connections, ventricular septal defect and pulmonary atresia, Ebstein's malformation of tricuspid valve. Tortuous patent arterial duct (pulmonary atresia type - aortic arch to pulmonary artery) shunting left to right. Large nonrestrictive perimembranous central ventricular septal defect (VSD) and Left aortic arch.

It is important to outline the coronary branching in cases of ccTGA. 

Presentation by Dr Sariya Sahussarungsi and Dr Abdullah Alghamdi - January 2025

Important Articles

Anatomy and Classification of Congenital Heart Disease (external link)

Nomenclature and Anatomic Evaluation in Congenital Heart Disease (external link)

"Describing Congenital Heart Disease by Using Three-Part Segmental Notation"