Acute Eosinophilic Pneumonia and Peripheral Eosinophilia
Acute eosinophilic pneumonia frequently presents with a normal peripheral eosinophil count at initial presentation, making bronchoalveolar lavage (BAL) essential for diagnosis when clinical suspicion is high. 1, 2
Can AEP Present Without Peripheral Eosinophilia?
Yes, peripheral blood eosinophilia is absent in approximately 50% of AEP cases at presentation. 1 In a key histopathologic study of nine AEP patients, only four of eight patients (50%) had peripheral blood eosinophilia at presentation, despite all having marked eosinophilia in BAL fluid or lung tissue. 1 This contrasts sharply with chronic eosinophilic pneumonia, where peripheral eosinophilia is typically present. 3
The absence of peripheral eosinophilia should never exclude AEP from your differential diagnosis when a patient presents with acute respiratory failure and diffuse infiltrates. 2, 4
Diagnostic Approach
Clinical Recognition Pattern
Suspect AEP in any patient presenting with:
- Acute onset (2-21 days) of dyspnea, cough, and fever 1
- Severe hypoxemia (PaO₂ <60 mmHg) requiring supplemental oxygen 2
- Bilateral diffuse infiltrates on chest imaging 1, 2
- No history of asthma or atopy (critical distinguishing feature) 2, 4
- Recent smoking initiation or environmental exposure 4
Immediate Diagnostic Steps
Bronchoalveolar lavage is the diagnostic gold standard and must be performed urgently when AEP is suspected. 5, 2 The American Thoracic Society states that an eosinophil differential count >25% in BAL fluid is virtually diagnostic of acute or chronic eosinophilic pneumonia. 5
Key BAL findings in AEP:
- Mean eosinophil percentage of 42% (range typically 25-90%) 2
- Absence of infectious organisms on culture and stains 1, 2
- Mixed inflammatory pattern with neutrophils and lymphocytes in acute phase 1
Obtain these tests immediately:
- BAL with differential cell count (priority test) 5, 2
- Complete blood count with differential (peripheral eosinophils may be normal) 1
- Chest CT showing ground-glass opacities and/or consolidation 3
- Blood and BAL cultures to exclude infection 1, 2
- Stool microscopy for ova and parasites (three specimens) if any travel history 6, 7
Critical Exclusions Before Treatment
Before initiating corticosteroids, you must exclude:
- Parasitic infections, particularly Strongyloides stercoralis (risk of fatal hyperinfection syndrome with steroids) 6, 7
- Active bacterial or fungal pneumonia 1, 2
- Drug-induced eosinophilic pneumonia (obtain detailed medication history including recent starts) 4
For any patient with travel history to endemic regions, immediately send:
- Strongyloides serology and stool culture 6, 7
- Schistosomiasis serology if freshwater exposure in Africa/tropical regions 6, 7
- Three separate concentrated stool specimens for ova and parasites 6, 7
Treatment Protocol
Immediate Management
Initiate high-dose corticosteroids immediately once infection is excluded—delay can be fatal. 1, 2, 4 Early and accurate diagnosis is imperative as delayed treatment can lead to fatal lung disease. 4
Recommended corticosteroid regimen:
- Methylprednisolone 1 gram IV daily for 3 days (pulse therapy), then 8
- Oral prednisone 40-60 mg daily, tapered over 10 days to 12 weeks 2
Expected response timeline:
- Symptomatic improvement within 24-48 hours 2
- Mean time to clinical and radiographic improvement: 4 days 1
- Complete resolution of hypoxemia typically within 1 week 2
Monitoring During Treatment
Follow these parameters closely:
- Oxygen saturation and supplemental oxygen requirements (should improve within 48 hours) 2
- Chest radiograph at 3-7 days (should show clearing) 1
- Peripheral eosinophil count (may transiently increase as lung eosinophils mobilize) 1
Prognosis and Follow-Up
AEP has excellent prognosis with appropriate treatment and does not relapse after steroid discontinuation. 2, 3 This is a critical distinguishing feature from chronic eosinophilic pneumonia, which has a high relapse rate. 3
Post-treatment evaluation at 5-6 months should include:
- Repeat BAL showing <1% eosinophils (confirms resolution) 2
- Pulmonary function tests (typically normal, though rare mild DLCO reduction may persist) 1
- Chest imaging (should be completely clear) 2
Common Pitfalls to Avoid
Do not wait for peripheral eosinophilia to develop before pursuing BAL—50% of AEP patients have normal peripheral eosinophil counts at presentation. 1 The diagnosis requires BAL confirmation. 5, 2
Do not assume normal eosinophil counts exclude parasitic infection—many helminth-infected patients have normal peripheral eosinophil counts, and only tissue-invasive parasites cause eosinophilia. 6 Always obtain travel history and stool studies. 6, 7
Do not start corticosteroids before excluding Strongyloides—this can trigger fatal hyperinfection syndrome in immunocompromised or steroid-treated patients. 6, 7 Send serology and stool studies first if any endemic exposure. 6, 7
Do not confuse AEP with chronic eosinophilic pneumonia—AEP has acute onset (<1 month), no relapse after steroid discontinuation, and frequently lacks peripheral eosinophilia, whereas CEP has subacute onset, high relapse rates, and typically has marked peripheral eosinophilia. 3, 4
Do not delay bronchoscopy for definitive imaging—when clinical presentation suggests AEP (acute respiratory failure, diffuse infiltrates, no infection), proceed directly to BAL rather than waiting for additional imaging studies. 2, 4 Time is critical. 4