Key Facts

Pulmonary eosinophilia encompasses a spectrum of disorders characterised by eosinophilic infiltration of the lung parenchyma and/or airways, typically with peripheral blood eosinophilia. Diagnosis requires integration of clinical features, imaging, BAL findings, and exclusion of secondary causes.

Epidemiology

Pulmonary eosinophilia is uncommon overall. AEP has an incidence of approximately 1 per 100,000 per year. CEP is slightly more common in women (2:1) and non-smokers, with mean age of onset 45 years. Drug-induced pulmonary eosinophilia depends on prescribing patterns. Parasitic causes are more common in returning travellers and immigrants from endemic areas.

Aetiology

• Parasitic: Ascaris lumbricoides (Löffler syndrome), Strongyloides stercoralis, hookworm, Toxocara, Schistosoma
• Drug-induced: NSAIDs, nitrofurantoin, daptomycin, sulphonamides, phenytoin, carbamazepine, methotrexate
• Fungal: ABPA (Aspergillus fumigatus), coccidioidomycosis
• Idiopathic: AEP, CEP
• Systemic: EGPA (Churg-Strauss), hypereosinophilic syndrome

Pathophysiology

Eosinophil recruitment to the lungs is driven by Th2 cytokines (IL-5, IL-13, IL-4) and eotaxin. Eosinophils release toxic granule proteins (major basic protein, eosinophil peroxidase, eosinophil cationic protein) causing tissue damage. In parasitic infection, eosinophilia represents a response to larval migration through the lungs (e.g. Ascaris transpulmonary passage). In AEP and CEP, the trigger is unclear but likely involves dysregulated eosinophilic inflammation.

Löffler Syndrome

• Mild cough, low-grade fever, wheeze
• Transient, migratory pulmonary infiltrates
• Resolves spontaneously within 2-4 weeks

Acute Eosinophilic Pneumonia

• Acute onset (<7 days): fever, cough, dyspnoea, myalgia
• May progress to respiratory failure requiring ventilation
• Often in young adults, frequently new smokers or recent dust/smoke exposure

Chronic Eosinophilic Pneumonia

• Subacute onset (weeks-months): cough, dyspnoea, fever, weight loss, night sweats
• Over 50% have co-existing asthma
• Peripheral blood eosinophilia prominent

Red Flags

• Rapidly progressive respiratory failure (AEP can be fulminant)
• Systemic vasculitis features (EGPA): neuropathy, purpura, cardiac involvement
• Recent travel to tropical areas (parasitic cause)
• New drug exposure within preceding weeks

Bedside

• Detailed history: drug exposure, travel, smoking, occupational exposure, pet exposure
• SpO₂: may be significantly low in AEP
• Stool samples: ova, cysts, and parasites (×3)

Bloods

• FBC: peripheral eosinophilia (>0.5 × 10⁹/L; often >1.5 in CEP and EGPA)
• Total IgE: often raised
• Parasite serology: Strongyloides, Toxocara, Schistosoma
• ANCA: pANCA/anti-MPO (EGPA)
• Tryptase, vitamin B12, FIP1L1-PDGFRA: hypereosinophilic syndrome screen
• CRP/ESR: raised

Imaging

• CXR: migratory infiltrates (Löffler), peripheral consolidation (CEP — "photographic negative of pulmonary oedema"), diffuse infiltrates (AEP)
• HRCT: ground-glass opacities, consolidation; distribution pattern helps differentiate subtypes

Special Tests

• BAL: eosinophils >25% (AEP), >40% (CEP); essential for diagnosis
• Spirometry: obstructive (if co-existing asthma), restrictive, or mixed
• Lung biopsy: rarely needed but diagnostic if atypical
• Echocardiography: if EGPA suspected (cardiac involvement in 60%)

Non-pharmacological

• Identify and remove cause: stop offending drug, treat parasitic infection, allergen avoidance
• ALWAYS treat parasitic infection before steroids — steroids can cause Strongyloides hyperinfection

Pharmacological

• AEP: methylprednisolone 125-250 mg IV QDS for 48-72 hours, then prednisolone 40-60 mg OD tapering over 2-4 weeks; complete resolution usual; relapse uncommon
• CEP: prednisolone 0.5-1 mg/kg/day (max 60 mg) for 2-4 weeks, then taper over 6-12 months; relapse rate ~50% — some require long-term low-dose maintenance (5-10 mg/day)
• Löffler syndrome: usually self-limiting; treat underlying parasitic infection (e.g. albendazole 400 mg stat for Ascaris, ivermectin 200 mcg/kg OD for 2 days for Strongyloides)
• Drug-induced: withdraw offending drug; short course of steroids if severe
• EGPA: see separate entry — high-dose steroids + cyclophosphamide/rituximab for severe disease

Surgical/Interventional

• Bronchoscopy + BAL: diagnostic; therapeutic mucus plug removal if needed

Referral Criteria

• Respiratory specialist referral for all cases
• Tropical medicine/infectious disease if parasitic cause suspected
• Haematology referral if hypereosinophilic syndrome suspected
• Rheumatology if EGPA features

AEP has an excellent prognosis — near 100% recovery with steroids; relapse is rare. CEP responds well to steroids (>90%) but has a high relapse rate (~50%) requiring prolonged treatment. Löffler syndrome is typically self-limiting. Drug-induced pulmonary eosinophilia resolves on drug withdrawal. EGPA has a more guarded prognosis, particularly with cardiac involvement (cardiac disease accounts for ~50% of EGPA mortality).

Classification of Pulmonary Eosinophilia