Pulmonary aspergillosis
Infection with fungi of the genus Aspergillus can occur in several forms depending on the patient's immune status.
- Intracavitary mycetoma (fungus ball): Develops against the background of chronic lung diseases. A round soft-tissue formation is identified within a pre-existing cavity, most commonly in the upper lobe. Clinically, it is often accompanied by mild hemoptysis.
- Allergic bronchopulmonary aspergillosis: Manifests with asthma and cough. CT scans show non-segmental opacities and branching tubular structures — bronchi dilated due to mucus accumulation (the «glove-finger» sign).
- Angioinvasive aspergillosis: Occurs in severe neutropenia. The fungus invades blood vessels, causing infarctions and necrosis. A typical early CT sign is the halo sign, representing nodules surrounded by a zone of ground-glass opacity. Later, as immunity recovers, the air crescent sign appears.
- Semi-invasive aspergillosis: Affects patients with mild immunodeficiency (COPD, diabetes mellitus). Manifests as rounded areas of homogeneous opacity that gradually become more distinct.
Opportunistic mycoses: Pneumocystosis and Candidiasis
These infections are extremely rare in healthy individuals and serve as a marker of marked immunosuppression.
Pneumocystis pneumonia is caused by Pneumocystis jirovecii. It is a specific marker of AIDS, as well as a complication in patients receiving immunosuppressive therapy or chemotherapy. Chest radiographs show bilateral interstitial infiltrates in 90% of cases. CT reveals focal or geographic distribution of ground-glass opacities in the perihilar regions. Cysts may form in the subacute stage.
Candidiasis, caused by Candida sp., affects patients with leukemia, lymphoma, and bone marrow transplant recipients. High-resolution CT demonstrates multiple bilateral nodules combined with areas of consolidation and ground-glass opacity.
Mucormycosis and Cryptococcosis
Mucormycosis is caused by fungi of the order Mucorales (which have broad, aseptate hyphae). On CT, it is often indistinguishable from invasive aspergillosis: areas of consolidation, nodules with the halo sign, cavitation, and the air crescent sign are observed. The multifocal form carries an extremely poor prognosis and high mortality.
Cryptococcosis (Cryptococcus neoformans) is transmitted by inhaling spores from soil and bird droppings. It is often asymptomatic, but causes severe pneumonia in patients with AIDS. CT visualizes segmental or lobar opacities, as well as formations of various sizes with blurred margins and a tendency toward cavitation.
Endemic mycoses
This group of diseases is tied to specific geographical zones and environmental conditions.
- Histoplasmosis (Histoplasma capsulatum): Infection occurs in endemic river valleys (North America) through contact with soil, bird droppings, or bat guano. CT shows diffuse nodular opacities or large nodules — histoplasmomas. The chronic form can mimic tuberculosis with upper lobe volume loss.
- Coccidioidomycosis (Coccidioides immitis): Prevalent in arid regions of the USA and Mexico. Manifests as areas of homogeneous opacity resembling bacterial pneumonia. A central zone of soft-tissue density with a ground-glass halo is typical on CT.
- Blastomycosis: Pulmonary infection can present as a spherical mass, leading to frequent misdiagnosis as bronchogenic carcinoma. It may also be accompanied by skin and bone lesions.
- Paracoccidioidomycosis: Endemic to Latin America. Characterized by a combination of nodules, areas of consolidation, and halo or reverse halo signs.
Complications
Fungal infections can lead to severe structural changes in the chest. Common outcomes include calcification and the formation of permanent cavities (cavitation). In rare cases, fibrosing mediastinitis develops—a dangerous condition causing compression of the trachea, bronchi, pulmonary arteries, and superior vena cava.
Briefly: answers to topical questions
What specific CT features can reliably distinguish pulmonary mucormycosis from invasive aspergillosis?
It is not possible to reliably distinguish pulmonary mucormycosis from angioinvasive aspergillosis based on CT features alone, as the radiologic manifestations of mucormycosis are non-specific, and the air crescent sign is radiologically indistinguishable from that seen in angioinvasive aspergillosis.
Mucormycosis may present with areas of consolidation, nodules surrounded by a zone of ground-glass opacity (halo sign), and cavitation. Angioinvasive aspergillosis is characterized by nodules with the halo sign, subpleural wedge-shaped areas of consolidation, and a late air crescent sign.
Class and exam questions
What is an intracavitary mycetoma?
This is a form of aspergillosis in which a rounded conglomeration of fungal masses (fungus ball) forms within a pre-existing pulmonary cavity. Clinically, it is often manifested by hemoptysis.
Which disease is characterized by the «glove-finger» sign?
This radiologic sign is typical for allergic bronchopulmonary aspergillosis. It is caused by bronchial dilatation due to accumulated mucus and fungi.
What does the air crescent sign indicate in angioinvasive aspergillosis?
This is a late CT sign indicating the onset of immune system recovery and the sloughing of necrotic masses.
What else to study on this topic
- Parasitic lung infections: amebiasis and malaria
- Pulmonary manifestations of echinococcosis
- Toxocariasis and larva migrans syndrome
- Differential diagnosis of fungal infections and tuberculosis
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