Pediatr Radiol (1990) 20:580-584 Pediatric Radiology 9 Springer-Verlag 1990 Horseshoe lung: differential diagnosis N. D. Hawass 1, M. G. B adawi 2, A. M. A1-Muzrakchi 1, A. I. A1-Sammarai 3, A. J. Jawad 3, M. A. Abdullah 4 and H. Bahakim 4 Departments of 1Radiology, 2Anatomy, 3 Surgery, and 4 Pediatrics, College of Medicine, King Saud University Riyadh, Saudi Arabia Received: 20 September 1989; accepted: 8 April 1990 Abstract. A detailed radiologic and anatomic study of one horseshoe lung syndrome has already been published from this centre. A further case of horseshoe lung, which was also diagnosed prospectively by radiology, is de- scribed. Alongside, this is a set of a group of three condi- tions: we describe a right accessory lung with a peculiar feeding arterial branch originating from the left basal pul- monary artery The bronchogram and oesophagogram of this case were normal. The other is a case of bridging bron- chus where the left lower lobe bronchus originates from the right main stem bronchus. This was discovered at post mortem bronchogram performed on a 19 weeks foetus. Finally, in the dog, we found that the pulmonary anglo- gram and bronchogram display close similarity to human horseshoe lung though the dog has two separate lungs. The common denominator of all above cases is the presence of a pulmonary arterial branch or bronchus crossing the midline from the ipsi to the contralateral side. Therefore, it may be concluded "in contradiction to the common belief" that pulmonary angiography or bron- chography alone is not sufficient for the diagnosis of horseshoe lung. When only one of these investigations is savailable, computerized tomography is necessary to show the isthmic lung tissue before the diagnosis of horse: shoe lung is confirmed. The congenital malformation in which a pulmonary arte- rial branch and/or bronchus crosses the midline from one side to the other is rare. This has been described in horse- shoe lung, 'pulmonary sling' and bridging bronchus. In this report, we describe a new case of horseshoe lung and a case of accessory lung and one example of a bridging bronchus and accessory lung in a living dog. All of these conditions share one or more features of horse- shoe lung. The aim of this paper is to discuss the radiologi- cal differential diagnosis of these features with brief de- scription of their embryological basis. Case material 1. A case of horseshoe lung syndrome with hypoplastic left lung has already been reported from this centre [11]. A second case of horse- shoe lung in a two-year old boy with asymmetry of the chest wall as the main complaint was diagnosed initially on plain films and compu- terized tomography (Figs. 1-2). The syndrome was confirmed by pulmonary angiography and bronchography (Figs. 3-4). The anom- aly was associated with cardiac dextro position, atrial septal defect, hypoplastic right lung and hemi-anomalous pulmonary venous drainage of the right lung into the superior vena cava. 2. A similar angiographic finding to the first case of"horseshoe lung" has also been encountered during a post-mortem foetal pulmonary angiogram in which a branch from the left basal pulmonary artery curved across the midline to the right side (Fig. 5). The bronchogram and oesophagogram of this case were normal (Fig. 6). Detailed au- topsy showed an unusual right accessory lung. It was situated just above the right hemidiaphragm and sheathed with its own visceral pleura (Fig. 7). The cross-over left pulmonary arterial branch proved to be the feeding artery to the accessory lung. The draining vein was followed and it terminated in the right inferior pulmonary vein. There was no connection to the bronchi or oesophagus. 3. A case of bridging bronchus has been encountered which showed radiological findings which were the reverse of the previous case, i. e. a normal pulmonary angiogram but with an abnormal bronchogram. The bronchogram showed an aberrant bronchus originating from the main stem of the right main bronchus crossing the midline to sup- ply the left lower lobe without a uniting isthmus (Fig. 8). Permission for autopsy of this case was not obtained. 4. Finally, it is interesting to know that a pulmonary angiogram and a bronchogram in dog showed the typical appearance of a human horseshoe lung (Figs. 9, 10), though the dog has two separate lungs. The common denominator in all these examples is the presence of a bronchus or pulmonary artery crossing the midline from the ipsi to the contralateral lung. This forms the subject of discussion and dif- ferential diagnosis presented in this paper. Discussion The definition of horseshoe lung is well described. How- ever, the hall/mark of the syndrome is not very well em- phasized. Congenital horseshoe lung is characterized by the presence of unilateral pulmonary hypoplasia and a midline isthmus, fusing the two lungs at their caudal end. Therefore, it must be emphasized that the cardinal feature of this syndrome is the presence of fusion occurring through a pleural parietal defect which allows the right and left lung parenchymal components to adhere to each other either directly (i. e. without a fissure) or indirectly through intervening visceral pleura, thus forming an isth- mic fissure. Therefore, it should be understood simply as being a one lung in a horseshoe fashion sheathed with one common parietal pleura. Accordingly and on theoretical