Mediastinal pseudomass caused by compression of the thymus in neonates with anterior pneumothorax.

We studied the radiographs of 115 neonates with anterior pneumothoraces to determine how often the pneumothorax created the impression of a mediastinal pseudomass. The pseudomass results from compression of the thymus gland by air under pressure. To the unwary, the resulting configuration can lead to an erroneous diagnosis of a mediastinal mass. In experienced hands, however, it can serve as a strong clue to the presence of an underlying anterior pneumothorax. A pseudomass was present in 27 (33%) of 82 neonates with unilateral pneumothoraces and in 29 (88%) of the 33 patients with bilateral anterior pneumothoraces. A free lung edge was visualized in 26% of the patients with a unilateral pseudomass and in 55% of the neonates with a bilateral pseudomass. The pseudomass was large enough to potentially lead to a misdiagnosis in one third of the cases. In the other neonates the mass was smaller and not particularly problematic. The majority of the neonates with pseudomasses were large, not intubated, and not on positive-pressure assisted ventilation. Over half had no underlying pulmonary disease. The appearance of a mediastinal pseudomass on radiographs of neonates can obscure a pneumothorax, and lead to an erroneous diagnosis. Recognition of the pseudomass as a manifestation of pneumothoraces is important.