Air crescent (lung)

Last revised by Liz Silverstone on 11 Aug 2024

Air crescent describes the crescent of gas between an intra-cavitary mass and the cavity wall. The intra-cavitary mass may be due to necrotic tissue or a fungus ball 6.

Terminology

The descriptor Monod sign 2 is commonly used to describe the combination of a gas crescent and a mobile fungus ball. The 2024 Fleischner glossary no longer includes this term and uses ‘air crescent’ to refer to gas in cavities that contain mobile fungal balls as well as those that contain fixed infarcted tissue 8.

Pathology

Aetiology

Common causes include angio-invasive aspergillosis with retraction of infarcted lung, fungal colonisation of a preexisting cavity or blood clot 8. Fungal balls can be mobile.

Causes include 5:

In angioinvasive fungal infection, the nodules are composed of infected haemorrhagic and infarcted lung tissue. As the neutrophil count recovers and the patient mounts an immune response, peripheral reabsorption of necrotic tissue causes the retraction of the infarcted centre and air fills the space in between 7. This creates an air crescent within the nodules and is a good prognostic finding because it marks the recovery phase of the infection. This sign is seen in ~50% of patients 2.

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