Hamman-Rich syndrome

**Definition/Nomenclature:** Described by Louis Hamman and Arnold Rich in 1935 (and 1944) as "acute diffuse interstitial fibrosis of the lung." It is now classified as acute interstitial pneumonia (AIP), an idiopathic interstitial pneumonia. In essence, it is idiopathic ARDS. Do not confuse it with Hamman sign or Hamman syndrome, which is spontaneous pneumomediastinum.

**Pathophysiology:** The histologic pattern is diffuse alveolar damage (DAD), the same pattern seen in ARDS. - Exudative phase (first week): hyaline membranes, alveolar and interstitial edema, and type I pneumocyte necrosis. - Organizing/proliferative phase: type II pneumocyte hyperplasia, fibroblast proliferation, and interstitial widening. - Late fibrotic changes can occur, but the cause is unknown (idiopathic). Lung injury is thought to follow epithelial and endothelial damage, with increased permeability and impaired gas exchange.

**Clinical presentation:** - Usually previously healthy adults, average age about 50-60, with no sex predominance. - A viral-like prodrome (fever, cough, myalgias) for days to 2 weeks, then rapidly progressive dyspnea and hypoxemic respiratory failure, often within days. - Most patients need mechanical ventilation.

**Diagnosis:** - Chest x-ray shows bilateral diffuse airspace opacities. - HRCT shows bilateral ground-glass opacities and airspace consolidation, often with traction bronchiectasis later. - Hypoxemia is severe, with a low PaO2/FiO2 ratio. - It is a diagnosis of exclusion. Rule out infection (including viral and atypical pathogens), cardiogenic pulmonary edema, drug toxicity, connective tissue disease, vasculitis/diffuse alveolar hemorrhage, and other known ARDS causes. - Bronchoalveolar lavage shows nonspecific inflammation, and biopsy (if done) shows DAD.

**Management:** - Supportive care with lung-protective ventilation (low tidal volumes, 6 mL/kg ideal body weight, as in ARDS). - High-dose corticosteroids are commonly used empirically, but evidence is limited. - Treat any coexisting infection while excluding other causes.

**Prognosis:** - In-hospital mortality is high, around 50% or more, and most deaths occur within 1-2 months. - Survivors may recover substantially but can have residual restrictive impairment, and recurrence or chronic fibrosis is possible.

**High-yield comparisons:** - AIP (Hamman-Rich) is acute and idiopathic with DAD histology. - IPF has a UIP pattern, with a subacute/chronic course over years, subpleural and basal honeycombing, and "Velcro" crackles. It is seen in older patients and treated with nintedanib or pirfenidone. - Cryptogenic organizing pneumonia has a subacute course, patchy consolidations, and Masson bodies, and responds to steroids. - Acute exacerbation of IPF can mimic AIP.

**Board pearls:** A previously healthy adult with a flu-like illness who develops rapidly progressive respiratory failure and bilateral infiltrates, with no cause found, points to AIP (Hamman-Rich syndrome). Histology shows diffuse alveolar damage with hyaline membranes.