The cardiac shadow is seen in the midline, with almost equal spread to either side, suggesting mesocardia. In levocardia the heart is predominantly in the left hemithorax and the apex points to the left. In dextrocardia, the heart is predominantly in the right hemithorax and the apex points to the right.
Normally the aorta is dextro-posed and the ascending aorta is seen along the right cardiac border. In levo-posed (L-posed) aorta, the ascending aorta is on the left border, as in this case. In dextro-transposition of great arteries (D-TGA), aorta is dextro-posed and in levo transposition of great arteries, aorta is levo-posed.
D-transposition is the usual transposition of great arteries, which a cyanotic congenital heart disease with increased pulmonary blood flow (unless there is associated severe pulmonary stenosis – left ventricular outflow tract obstruction). L-transposition is usually a corrected transposition of great arteries with atrioventricular and ventriculoarterial discordance so that right atrial blood reaches the pulmonary artery and left atrial blood reaches the aorta. Hence it is a non cyanotic condition and may be missed unless there are other associated anomalies like ventricular septal defect, pulmonary stenosis or congenital complete heart block, which are the common associations of L-TGA.
Important differential diagnosis for a shadow along the left upper cardiac border are:
L – posed aorta
Dilated main pulmonary artery
Aneurysm of arch or upper descending thoracic aorta
Vertical vein in total anomalous or hemi-anomalous pulmonary venous connection
Aneurysm of the ductus arteriosus
Partial absence of left pericardium causing bulge of cardiac structures to the left
Left atrial appendage
Submitral aneurysm (a little lower on the silhouette)