Description
Allergic Bronchopulmonary Aspergillosis (ABPA) is a complex hypersensitivity reaction to the fungus Aspergillus fumigatus. It typically occurs in patients with asthma or cystic fibrosis (CF), causing inflammation and mucus plugging in the bronchi with potential for lung damage over time.
Pathogenesis
The pathogenesis of ABPA centres around a heightened immune response to Aspergillus fumigatus. In susceptible individuals, inhalation of spores leads to a type I hypersensitivity reaction, with IgE mediated release of inflammatory mediators. A subsequent type III hypersensitivity reaction occurs due to antigen-antibody complex deposition leading to further lung inflammation and damage of the bronchial walls (bronchiectasis, typically central).
Bronchocentric granulomatosis (necrotising granulomatous inflammation) may destroy walls of the small bronchi and bronchioles.
Epidemiology, Risk Factors & Associations
- Predominantly affects adults with asthma or cystic fibrosis, but can affect any age group.
- Atopic patients
- ABPA complicates asthma in approximately 1-2% of patients, and CF in approximately 2-15%.
- Predisposing factors include high serum total IgE, the presence of bronchiectasis, and genetic factors (e.g., HLA-DR2 and HLA-DR5 haplotypes).
Clinical Features
- Worsening of asthma symptoms: cough, wheezing, shortness of breath.
- Haemoptysis due to bronchial damage.
- Systemic symptoms: fever, malaise, weight loss.
- Recurrent chest infections productive of orange-coloured mucous plugs.
Complications
- Bronchiectasis – permanent dilation and destruction of the bronchi.
- Pulmonary fibrosis: predominantly upper lobe, leading to respiratory failure.
- Chronic pulmonary aspergillosis, aspergilloma formation.
Pathological Features
Histopathology
- Mucus plugging within bronchi, typically eosinophilic in nature.
- Orange/brown mucous plugs
- Presence of hyphae within mucus plugs.
- Evidence of chronic inflammation and fibrosis in the lung parenchyma.
- Charcot-Leyden crystals seen in sputum
Biochemistry
- Elevated serum total IgE.
- Positive aspergillus skin test: immediate cutaneous hypersensitivity to Aspergillus fumigatus.
- Presence of serum Aspergillus fumigatus specific IgE and IgG.
- Total eosinophil count > 500 cells/mL
Radiological Features
XR
- Transient pulmonary infiltrates.
- Central bronchiectasis with normal peripheral bronchi (tubular opacities finger in glove appearance).
HRCT
- Bronchiectasis and bronchial wall thickening with mucoid impaction
- Mucoid bronchial impaction appears as branching tubular opacities (finger-in-glove morphology) which demonstrates high attenuation on (>70 – 100 HU) representing fungal debris containing calcium oxalate, iron and manganese
- Air-trapping with central or proximal upper-lobe predominance
- Tree-in-bud nodularity.
- Occasionally calcification is seen.
Grading and Staging
The Rosenberg-Patterson criteria are used for diagnosis, but also provide a sort of ‘staging’ system for ABPA:
- Stage I (Acute): Clinical and immunologic evidence of ABPA with normal chest X-ray.
- Stage II (Remission): Clinical remission with normal chest X-ray.
- Stage III (Exacerbation): Clinical and/or radiographic evidence of active disease.
- Stage IV (Corticosteroid dependent asthma): Asthma requiring corticosteroids to control symptoms.
- Stage V (Fibrotic): End-stage lung disease with fibrosis and cystic changes.
Differential Diagnosis
- Congenital Bronchial Atresia: Focal atresia of segmental bronchus, typically affects left upper lobe apicoposterior segment. Recurrent infection in 20% of patients. Tubular or branching opacity surrounded by hyperlucent lung due to air-trapping and oligaemia
- Asthma or CF with bacterial superinfection.
- Other causes of bronchiectasis, e.g., non-tuberculous mycobacterial infection.
- Other fungal lung infections (e.g., cryptococcosis, histoplasmosis).
Management
Referral to respiratory medicine is typically required for management. Treatment involves corticosteroids to reduce inflammation, and antifungal agents to reduce the fungal load. Regular monitoring with pulmonary function tests and imaging is also required.