Horseshoe lung: report on a new variant--"inverted" horseshoe lung--with embryological reflections on the formal pathogenesis of horseshoe lungs.

The term "horseshoe lung" is used to describe a rare congenital anomaly of the lungs that is characterized by the presence of a midline isthmus of pulmonary parenchyma connecting the posterobasal regions of the right and left lungs. Since the introduction of the term horseshoe lung in the 1960's, almost 40 cases have been reported in the literature. In all these cases, the right and left lungs were joined in their posterobasal regions, the situation resembling that found in horseshoe kidneys. Here we present a case of connection between the right and left lungs found during necropsy of a human fetus. In this case, a midline isthmus of pulmonary parenchyma covered by visceral pleura joined the apical regions of the right and left lungs behind the trachea and esophagus. This connection resulted in a "horseshoe"-shaped lung in which the "horseshoe" was turned by 180 degrees compared to classical cases of horseshoe lung. To the best of our knowledge, this is the first report on an "inverted" horseshoe lung. Embryological reflections on the formal pathogenesis of inverted and classical horseshoe lungs are presented.