Monod sign | Radiology Signs

Answer:

The Monod sign is a crescent of air surrounding a mobile intracavitary mass within a pre-existing pulmonary cavity, the appearance of an aspergilloma or mycetoma colonising an old cavity. The classic setting is a healed tuberculous cavity, but sarcoidosis, bullous emphysema, previous abscess, cystic bronchiectasis and treated cancer cavities all provide the same substrate, and the upper lobes are the usual site. Two features confirm the diagnosis: the mass is mobile, changing position between supine and prone imaging, and the adjacent pleura is thickened, which is often the earliest sign of colonisation. The distinction from the air crescent sign of angioinvasive aspergillosis is fundamental and frequently confused: in that condition the cavity is created by the infection itself in a neutropenic patient, the crescent appears during neutrophil recovery as necrotic lung retracts, and the illness is invasive and life-threatening, whereas an aspergilloma is a saprophytic ball of hyphae, fibrin and cellular debris in an immunocompetent or mildly immunosuppressed host. Management differs accordingly: aspergilloma is often observed, but massive haemoptysis from bronchial artery hypertrophy is the main risk and is treated with bronchial artery embolisation and, in fit patients with recurrent bleeding, surgical resection, since systemic antifungals penetrate the fungus ball poorly.

Why is it called so?

The sign carries the name of Monod, who described the air crescent around a fungus ball in a pre-existing cavity.

Pathophysiology

Aspergillus species are ubiquitous in the environment and their spores are inhaled continuously. In a host with normal immunity they are cleared, but a chronic pulmonary cavity provides a space that is poorly cleared by mucociliary transport and in which the organism can grow saprophytically without invading tissue. The mass that forms consists of matted hyphae, fibrin, mucus and inflammatory debris, and it lies free within the cavity, which is why it is gravity-dependent and mobile. Chronic inflammation of the cavity wall induces hypertrophy and tortuosity of the bronchial arteries supplying it, and these fragile vessels are the source of the haemoptysis that dominates the clinical course.

Alternative names: Fungus ball sign; mycetoma sign

Other associated named signs: Air crescent sign of angioinvasive aspergillosis and the halo sign that precedes it

References

1. Soewondo W, Kusumaningrum CS, Hanafi M, Adiputri A, Hayuningrat PK. Co-existing active pulmonary tuberculosis with aspergilloma in a diabetic patient: A rare case report. Radiol Case Rep. 2022;17(4):1136-1142. PMID: 35169416.