Monod sign (lungs)

Last revised by Benjamin Layton on 4 Aug 2022

The Monod sign simply describes gas that surrounds a mycetoma (most commonly an aspergilloma) in a pre-existing pulmonary cavity 1-3.

It should not be confused with the air crescent sign which is seen in recovering angioinvasive aspergillosis 4. The air crescent sign heralds improvement in the condition.

The sign implies a freely mobile mass, which moves on mobilizing the patient. This is best demonstrated by acquiring images in a prone position, causing the mass to fall, to a gravity-dependent location and favoring a Monod sign; as compared to an air crescent sign. 

In practice, it is likely that the term Monod sign will receive blank stares. The gas around the mycetoma is often crescent-shaped and hence, the term air crescent sign is used interchangeably by many to refer to both pathological processes.

History and etymology

The sign was first described by the French physicians G D Pesle and Olivier Monod in 1954 6.

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Cases and figures

  • Figure 1: gross pathology
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  • Case 1
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  • Case 2
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  • Case 3
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  • Case 4
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