TGA
The heart is enlarged with a narrow \"pedicle\" giving the so called \"egg on a string\" appearance. The superior mediastinum appears narrow due to the antero-posterior relationship of the transposed great vessels and \"radiologic-absence of the thymus\".The stress of hypoxia in the newborn period is believed responsible for thymic regression. Right posterior oblique view demonstrates widening of structures in the superior mediastinum due to the anteroposterior relationship of the aorta and pulmonary artery.
D-TGA accounts for some 5% of all congenital heart defects. D-TGA describes patients in whom there is atrioventricular concordance with ventriculoarterial discordance i.e. the pulmonary artery arises from the left ventricle and the aorta arises from the right ventricle.
Embryology: Abnormal looping of the great vessels is believed to occur. In the normal heart a bilateral conus develops beneath both semilunar valves with progressive development of the subpulmonary conus and regression of the subaortic conus, resulting in the great vessels arising from the appropriate ventricle. In d-TGA the reverse occurs with progressive growth of the subaortic conus and regression of the supulmonary conus resulting in transposition of the great arteries.
Associated lesions: These include VSDs (typically outlet or muscular septum defects) often with malalignment of the outlet septum which is deviated rightward, resulting in overriding of the pulmonary valve. If there is significant override this may represent Taussig-Bing anomaly (double outlet right ventricle with subpulmonary VSD). Other associated lesions include pulmonary stenosis (30%), patent ductus arteriosus, coarctation, coronary artery anomalies.