1. classic findings of Scimitar syndrome (hypoplasia of right upper lung lobe, diffuse hypoplasia of right pulmonary artery, cardiac dextroposition, anomalous drainage of a single pulmonary venous trunk into IVC/right atrial junction, systemic collateral supply from Descending aorta to the right lower lung lobe (sequestered lobe) and dilatation of right sided cardiac structures due to volume overload).
2. Characteristic hypoechoic vessel/bronchus seen bridging between basal segments of both lower lung lobes (arising from right lower lobar artery/bronchus) with a bridging isthmus of pulmonary parenchymal tissue consistent with a horseshoe lung malformation rather than just a crossover lung segment.

Images 3-9, videos 2-11: sequential 2D and color Doppler images and videos representing the hypoplastic RPA with normal LPA and dilated ductus arteriosus due to volume load, cardiac dextroposition, hypoplastic right upper lung lobe, anomalous small branch from LPA connecting the anomalous mediastinal vessel to the left of main pulmonary artery, having an arterial signal and ends into mildly dilated coronary sinus.

Images 10-17, videos 12-17: Images and videos to illustrate to anomalous drainage of right lower lobar pulmonary venous trunk into IVC/RA junction with usual low velocity, phasic Doppler waveform pattern to exclude obstructed venous drainage and the systemic collateral supply to the right lower lung lobe.

Images 18-22 videos 18: shows anomalous pulmonary venous trunk draining into IVC/RA junction and the horseshoe lung malformation with the characteristic bridging vessel/bronchus sign.

Images 23 and videos 19 and 20: Glass body and color Doppler STIC volume acquisition showing the anomalous vein and the mediastinal vascular structures.

Comment:

Horseshoe lung with scimitar syndrome is a well-known postnatal congenital broncho-pulmonary malformation with a very rare incidence¹. Previous prenatal reports of horseshoe lung focused upon its distinct appearance on fetal MRI as a retro cardiac confluent isthmus of normal lung parenchyma in front of the descending aorta and esophagus^{2, 3}. However, the presence of a bridging vessel/bronchus, as in our case, can be the most definite sonographic sign to diagnose horseshoe lung malformation rather than just a crossover lung segment if an isthmus of lung tissue is seen in front of descending thoracic aorta. Imaging findings of classic scimitar syndrome was much more helpful in diagnosing such imaging findings as 80% of horseshoe lung malformations are associated with scimitar syndrome. As well, recent advances in ultrasound technology were helpful in visualizing the systemic collateral supply of the right lower lung lobe that was difficult to accomplish in the previous study of Bhide and his colleagues⁴, completing the full picture of scimitar syndrome.

References:

1-Bhardwaj, H. and Bhardwaj, B., 2014. A rare case of scimitar syndrome with horseshoe lung. European Respiratory Review, 23(131), pp.153-154.

2-Goldberg, S., Ringertz, H. and Barth, R.A., 2006. Prenatal diagnosis of horseshoe lung and esophageal atresia. Pediatric radiology, 36(9), pp.983-986.

3-Tilea, B., Garel, C., Delezoide, A.L., Vuillard, E., Azancot, A., Oury, J.F. and Sebag, G., 2005. Prenatal diagnosis of horseshoe lung: contribution of MRI. Pediatric radiology, 35(10), pp.1010-1013.

4-Bhide, A., Murphy, D., Thilaganathan, B. and Carvalho, J.S., 2010. Prenatal findings and differential diagnosis of scimitar syndrome and pulmonary sequestration. Ultrasound in Obstetrics & Gynecology, 35(4), pp.398-404.