Can pulmonary aspergillosis cause pulmonary fibrosis, what are the characteristic high‑resolution CT findings and potential mimics, and what is meant by hypersensitivity in this context?

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Can Aspergillosis Cause Pulmonary Fibrosis?

Yes, aspergillosis can definitively cause pulmonary fibrosis, specifically through chronic fibrosing pulmonary aspergillosis (CFPA), which represents the terminal fibrosing evolution of untreated chronic cavitary pulmonary aspergillosis (CCPA). 1

Mechanisms of Fibrosis in Aspergillosis

There are two distinct pathways by which Aspergillus leads to pulmonary fibrosis:

1. Chronic Fibrosing Pulmonary Aspergillosis (CFPA)

  • CFPA develops when CCPA remains untreated over years, resulting in severe fibrotic destruction of at least two lobes of lung with major loss of lung function 1
  • The fibrosis typically appears solid on imaging but may contain large or small cavities with surrounding fibrosis
  • This represents direct fungal-mediated chronic inflammation leading to progressive fibrosis
  • Aspergillomas may be present within the fibrotic areas 1

2. Allergic Bronchopulmonary Aspergillosis with Chronic Pleuropulmonary Fibrosis (ABPA-CPF)

  • ABPA-CPF is a hypersensitivity-mediated form where immune overreaction to Aspergillus fumigatus causes fibrosis 2
  • Defined by two or more of: pulmonary fibrosis, fibro-cavitary lesions, fungal ball, and pleural thickening 2
  • This is the most severe form of ABPA and must be distinguished from chronic pulmonary aspergillosis complicating ABPA 2

High-Resolution CT Findings

CFPA (Chronic Fibrosing Pulmonary Aspergillosis)

Characteristic features:

  • Extensive fibrotic consolidation affecting at least two lobes, often appearing solid 1
  • Multiple thick-walled cavities (median wall thickness 6mm, higher in CFPA) with or without aspergillomas 3
  • Pericavitary fibrosis present in 100% of CFPA cases 3
  • Pleural thickening (median 8mm) with hyperattenuated extrapleural fat 1
  • Mediastinal shift toward the affected side due to volume loss 3
  • Upper lobe predominance (particularly left upper lobe in 68% of cases) 3
  • Cavities may contain fungal balls, irregular intraluminal material, or strands of Aspergillus 1

ABPA-CPF (Allergic Bronchopulmonary Aspergillosis with Fibrosis)

Characteristic features:

  • Central bronchiectasis (cylindrical or cystic) extending to peripheral zones in 33-43% 2, 4
  • High-attenuation mucus (HAM) - pathognomonic finding appearing hyperdense on mediastinal windows 2, 5, 6
  • Mucoid impaction in bronchiectatic airways 2, 4
  • Centrilobular nodules with tree-in-bud appearance 2
  • Consolidation (though often represents mucus-filled bronchiectatic cavities rather than true consolidation) 4
  • Mosaic attenuation and atelectasis 2
  • Fibrosis, fibro-cavitary lesions, and pleural thickening in advanced cases 2

CT Mimics and Differential Diagnosis

CFPA Mimics:

  • Active tuberculosis - most common mimic, look for tree-in-bud nodules, lymphadenopathy 1
  • Non-tuberculous mycobacterial infection - similar cavitary disease pattern 1
  • Lung carcinoma - particularly necrotic tumors; may coexist with Aspergillus infection 1
  • Histoplasmosis, actinomycosis, coccidioidomycosis - similar cavitary patterns 1
  • Aspergillus nodules can mimic tuberculoma, carcinoma, metastases, cryptococcal nodules 1

ABPA Mimics:

  • Primary lung cancer - ABPA can present as large pulmonary mass in heavy smokers 7
  • Idiopathic pulmonary fibrosis (IPF) - particularly when ABPA presents insidiously 8
  • Chronic hypersensitivity pneumonitis - can be complicated by pulmonary aspergilloma 8

Key Distinguishing Features:

To differentiate CFPA from tuberculosis:

  • CFPA shows thicker cavity walls, more pleural thickening, and hyperattenuated extrapleural fat 1
  • Post-tubercular changes are present in 97.1% of CPA cases as underlying substrate 3

To differentiate ABPA from IPF:

  • ABPA shows central bronchiectasis, high-attenuation mucus (pathognomonic), and centrilobular nodules 2, 5, 6
  • IPF shows honeycombing, traction bronchiectasis, subpleural predominance without HAM 9

What is "Hypersensitivity" in This Context?

Hypersensitivity refers to allergic bronchopulmonary aspergillosis (ABPA), which is a type I and type III hypersensitivity reaction to Aspergillus fumigatus 5, 7, 6

Key Characteristics:

  • Immune-mediated disease rather than direct fungal invasion
  • Occurs primarily in patients with asthma or cystic fibrosis 5
  • Represents an exaggerated type 2 immune response to Aspergillus antigens 2
  • Treatment targets immune response with glucocorticoids or biologicals targeting type 2 immunity, not just antifungal therapy 2

Clinical Distinction:

  • ABPA (hypersensitivity): Allergic reaction in immunocompetent hosts with asthma/CF
  • CPA/CFPA (chronic infection): Direct fungal colonization in structurally abnormal lungs (post-TB, COPD) in non-immunocompromised patients 1
  • Invasive aspergillosis: Tissue invasion in severely immunocompromised patients (not discussed here)

Critical Pitfalls to Avoid

  1. Do not assume consolidation on chest X-ray represents true consolidation - in ABPA, this often represents mucus-filled bronchiectatic cavities on HRCT 4

  2. Do not miss high-attenuation mucus - must view mediastinal windows, not just lung windows; this is pathognomonic for ABPA 2, 5, 6

  3. Do not diagnose "central bronchiectasis" as specific for ABPA - peripheral bronchiectasis occurs in 33-43% of ABPA cases 2, 4, 6

  4. Do not confuse dilated esophagus with cavity on axial CT images 1

  5. In ABPA-CPF, always exclude chronic pulmonary aspergillosis complicating ABPA - these are distinct entities requiring different management 2

  6. Normal HRCT does not exclude ABPA - 22-33% of ABPA patients have normal HRCT (serologic ABPA) 4, 6

  7. Quantitative assessment improves accuracy - measure cavity wall thickness, pleural thickness, and cavity number to differentiate CPA subtypes 3

References

6
Research

Pictorial essay: Allergic bronchopulmonary aspergillosis.

The Indian journal of radiology & imaging, 2011

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