The extrapleural air sign is one of the many signs of pneumomediastinum, and was first described by Lillard and Allen in 1965. It is defined as the presence of gas between the parietal pleura and the diaphragm. On a lateral projection the gas forms a radiolucent pocket of gas posterior to the dome of the hemidiaphragm. The reader should be aware that this term is not synonymous with the term extrapleural sign by itself.
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