Definition
• Rapidly progressive form of idiopathic interstitial pneumonia with diffuse alveolar damage (DAD) and respiratory failure
• Synonymous with Hamman-Rich syndrome, occurring in patients without preexisting lung disease
Pathogenesis
• Etiology and mechanism of injury unknown
Clinical features
Epidemiology
• Mean age 50 years with a wide range
• No gender predilection
Presentation
• Generally occurs in previously healthy individuals without a history of lung disease who present with symptoms suggestive of upper respiratory tract viral infection and acute respiratory failure (<3 weeks’ duration)
• Pulmonary function tests show a restrictive pattern
• Similar in presentation to acute respiratory distress syndrome (ARDS); probably corresponds to a subset of idiopathic ARDS
• High-resolution CT shows ground-glass attenuation, bronchial dilatation, and architectural distortion
Prognosis and treatment
• Management is largely supportive and consists of oxygen supplementation; ventilation with positive end-expiratory pressure eventually needed
• Greater than 70% mortality in 3 months, despite mechanical ventilation
• In the surviving population, recurrences and chronic interstitial disease are common
Pathology
Histology
• Characterized by the histological pattern of DAD. AIP cannot be distinguished from DAD on histology alone (see DAD section)
Immunopathology/special stains
• Not contributory
Main differential diagnoses
• ARDS: known etiology
• Idiopathic pulmonary fibrosis/usual interstitial pneumonia: patchy fibrosis varying in intensity and time (fibroblastic foci)
• Organizing pneumonia: intraalveolar Masson bodies
Fig 1 Acute interstitial pneumonia. A 72-year-old patient seen with increasing shortness of breath. Wedge lung biopsy shows patchy hyaline membranes. Note that the word diffuse in diffuse alveolar damage implies that the whole alveolus is involved, not the whole lung.
Fig 2 Acute interstitial pneumonia. High power shows bright pink hyaline membranes. There is interstitial edema and early fibrosis.