Acute interstitial pneumonia (AIP), classically known as Hamman-Rich syndrome (described in 1944), is a rare and fulminant form of idiopathic interstitial pneumonia (IIP) defined by ATS/ERS classification. It manifests as rapidly progressive acute respiratory failure resembling acute respiratory distress syndrome (ARDS) but without identifiable precipitating cause. Histologically, it is characterized by diffuse alveolar damage (DAD), the same pattern seen in ARDS but without etiologic trigger.
Clinically, AIP affects previously healthy individuals (mean age 50, slight female predominance) with no preexisting lung disease. Onset is acute with prodromal viral-like illness (fever, malaise, myalgia) followed by rapidly progressive dyspnea, dry cough, and hypoxemic respiratory failure within days to 1-3 weeks. Auscultation reveals bilateral crackles. ABG shows severe hypoxemia (PaO2/FiO2 less than 200, often less than 100). Diagnosis requires exclusion of identifiable ARDS causes including infection (viral, bacterial, fungal — comprehensive bronchoalveolar lavage with PCR), drug-induced lung injury, transfusion-related acute lung injury (TRALI), autoimmune disease flare, vasculitis (especially ANCA-associated), and acute exacerbation of preexisting interstitial lung disease.
Imaging shows bilateral diffuse ground-glass opacities with consolidation, often dependent or peripheral predominant, with traction bronchiectasis indicating underlying fibrosis in later stages. Surgical lung biopsy shows three histologic phases of DAD: exudative (hyaline membranes, edema), proliferative (fibroblastic foci, type II pneumocyte hyperplasia), and fibrotic phases. Treatment is supportive in the ICU: low-tidal-volume lung-protective mechanical ventilation (6 mL/kg ideal body weight), prone positioning for severe hypoxemia, neuromuscular blockade if needed, conservative fluid strategy, and ECMO for refractory cases. Pulse methylprednisolone (1g IV daily for 3 days followed by oral taper) is widely used despite limited evidence. Empiric broad-spectrum antibiotics and antivirals are administered until infection is excluded. Mortality exceeds 50% (some series 70-80%); survivors may develop residual fibrotic interstitial lung disease requiring long-term pulmonary function follow-up and consideration of antifibrotic therapy.