Horseshoe lung: clinical, pathologic, and radiologic features and a new plain film finding.

Horseshoe lung is a rare congenital malformation in which an isthmus of pulmonary parenchyma extends from the right lung base across the midline behind the pericardium and fuses with the base of the left lung. Six cases are presented and eight previously published case reports are reviewed. Eleven of these 14 cases occur in conjunction with scimitar syndrome, the complex of anomalies including hypoplasia of the right lung, anomalous right pulmonary venous return, and anomalous arterial supply to the right lung. Differentiation of scimitar syndrome from horseshoe lung can be made on the plain chest film, if, in addition to the typical radiographic findings of scimitar syndrome, there is evidence of a fine linear density in the medial aspect of the left base representing the lateral extent of the pulmonary isthmus. Recognition of horseshoe lung is important because children present in the first few years of life with significant respiratory symptoms, whereas the scimitar syndrome alone is usually discovered fortuitously after the first decade of life.