Acute Eosinophilic Pneumonia (AEP) — 15-Mark Exam Answer

Definition

Epidemiology

• Typically affects males aged 20–40 years
• No prior history of asthma required (unlike chronic eosinophilic pneumonia)
• No definite seasonal variation
• Up to 70% of patients have a smoking history
• Can occur at any age and in either sex, but male predominance is well established

Etiology & Pathogenesis

1. Inhaled agent → epithelial injury → release of DAMPs (IL-33, uric acid, ATP)
2. IL-33 activates ILC2 cells → rapid innate immune activation
3. CD4+ Th2 lymphocyte adaptive response (requires days to weeks)
4. Release of type 2 cytokines: IL-4, IL-5, IL-13
5. IL-5 → bone marrow eosinophil production and maturation; IL-13 → eosinophil chemotactic factors
6. Eosinophil chemokines (eotaxins CCL11, CCL24, CCL26; LTB4) → massive eosinophil migration into lung
7. Eosinophil granule proteins → tissue damage and diffuse alveolar injury

Clinical Features

• Acute onset fever (high-grade), chills, malaise, myalgias
• Dyspnea and dry/nonproductive cough
• Pleuritic chest pain
• Night sweats
• Rapid progression from mild dyspnea → overt respiratory failure requiring mechanical ventilation

• Tachypnea, tachycardia
• High fever
• Bilateral basilar crackles (inspiratory); wheezing may be present
• Signs of hypoxemic respiratory failure (PaO₂ < 60 mmHg on room air)
• No signs of multiorgan dysfunction (this distinguishes AEP from sepsis/ARDS)

• No preceding infection or systemic illness
• BAL eosinophilia >25%
• Dramatic response to corticosteroids
• No multisystem organ failure

Laboratory Findings

Critical point: Blood eosinophilia is typically ABSENT at presentation — its absence should not exclude AEP. BAL eosinophilia is the cornerstone of diagnosis.

Imaging

• Early: subtle patchy infiltrates, Kerley B lines
• Later: bilateral alveolar and interstitial infiltrates (diffuse)
• Bilateral pleural effusions in 50–70% of patients

Diffuse bilateral ground-glass opacity and consolidation in AEP — Fishman's Pulmonary Diseases and Disorders

• Bilateral ground-glass opacification and consolidation in a random (non-segmental) distribution
• Interlobular septal thickening
• Thickening of bronchovascular bundles
• Small to moderate bilateral pleural effusions
• Mediastinal lymphadenopathy (common)
• CT appearance can mimic pulmonary edema or DAD

Pathology

Eosinophilic pneumonia: dense eosinophilic infiltrates in fibrin-rich alveolar exudate (H&E ×200). Inset: eosinophils at high magnification (×400). — Murray & Nadel's Textbook of Respiratory Medicine

• Prominent eosinophil infiltration in interstitium, alveolar spaces, and bronchial walls
• Diffuse alveolar damage (DAD) pattern with hyaline membranes — distinctive for AEP
• Type 2 pneumocyte hyperplasia
• Intra-alveolar fibrinous exudate
• Lymphocytic interstitial infiltration
• Granulomas, alveolar hemorrhage, and non-necrotic perivascular inflammation reported
• Basal lamina damage is unusual

Biopsy is generally NOT required if BAL eosinophilia >25% is present.

Diagnostic Criteria (Modified Philit Criteria)

• Hypoxemic respiratory failure
• Quick clinical response to corticosteroids
• Failure to relapse after discontinuation of corticosteroids

Differential Diagnosis

Treatment

• Mechanism: Induce apoptosis of eosinophils → rapid resolution
• Oral prednisone: 40–60 mg/day for mild-moderate disease
• IV methylprednisolone: 60–125 mg every 6–8 hours for severe/intubated patients
• Duration: typically 2–4 weeks, then taper over 1–2 months
• Response: Fever and respiratory symptoms abate within 12–48 hours; radiographic resolution within 1–2 weeks; complete resolution within several weeks

• ICU admission for severe hypoxemia
• Non-invasive ventilation (CPAP/BiPAP) for moderate respiratory failure
• Invasive mechanical ventilation for respiratory failure unresponsive to NPPV
• Supplemental oxygen

• Do NOT delay steroids while waiting for investigation results if AEP is strongly suspected
• Antibiotics are NOT indicated (unless concurrent infection excluded)
• No role for antifungal or antiparasitic therapy unless infection confirmed

Prognosis

• Excellent prognosis — characteristic feature of AEP
• Complete clinical and radiographic recovery in almost all patients within several weeks
• No recurrence in truly idiopathic AEP after corticosteroid discontinuation
• No residual pulmonary sequelae (unlike ARDS)
• Relapse possible if smokers resume smoking — disease may recur upon re-exposure

AEP vs Chronic Eosinophilic Pneumonia — Key Comparison

Summary Flowchart

Young male + acute fever + dyspnea + bilateral infiltrates + hypoxemia
        ↓
Suspect AEP (especially if recent smoking onset or new inhaled exposure)
        ↓
BAL → >25% eosinophils? Yes → Exclude infection/drugs → AEP CONFIRMED
        ↓
Remove exposure + IV methylprednisolone → rapid response within 24–48h
        ↓
Complete recovery, no recurrence