Acute respiratory distress syndrome. Etiology, pathogenesis, pulmonary pathomorphology
Acute respiratory distress syndrome (ARDS) is caused by diffuse alveolar capillary and epithelial damage, causing rapid onset respiratory insufficiency, cyanosis and hypoxemia that can result in multiorgan failure and death. Histologically, we see diffuse alveolar damage that can occur as a result of direct or indirect insult to the lungs. There is additionally a newborn respiratory distress syndrome, but this is separate, caused by deficiency of surfactant.
Etiological causes are:
- Direct
- Pneumonia and aspiration of gastric contents most commonly
- Pulmonary contusion, fat embolism, near drowning, inhalational injury and reperfusion injury after lung transplant more rarely
- Indirect
- Sepsis and severe trauma with shock commonly
- Cardiopulmonary bypass, acute pancreatitis, drug overdose, transfusion of blood products and uremia more rarely
The Pathogenesis is summed up below:
- Alveolar-capillary membrane is formed of microvascular endothelium and alveolar epithelium
- The integrity of this barrier in ARDS is then broken causing increased vascular permeability and alveolar flooding leading to loss of diffusion capacity, widespread surfactant abnormalities caused by damage to type II pneumocytes.
- The damage to the cells and subsequent widespread release of inflammatory mediators leads to endothelial activation and activation of neutrophils in pulmonary capillaries
- The large number of neutrophils is visible on the slide
Pulmonary pathomorphology
- In the acute phase, lungs are dark red, firm, airless and heavy, microscopic examination shows capillary congestion, necrosis of alveolar epithelial cells, and intra-alveolar edema and hemorrhage.
- Most characteristically, observe hyaline membranes lining the distended alveolar ducts
- In the organisation stage, we see proliferation of type II pneumocytes
- Resolution is unusual, commonly there is organisation of the fibrin exudates causing intra-alveolar fibrosis and thickening of the alveolar septa
H&E, acute (exudative) phase of diffuse alveolar damage: wavy, bright pink hyaline membranes line the alveolar walls; the septa are oedematous and inflamed.
Image: “Diffuse alveolar damage (DAD) (4564039878)” by Yale Rosen from USA, CC BY-SA 2.0, via Wikimedia Commons.
Remember: hyaline membranes are made of fibrin-rich oedema fluid mixed with the remains of necrotic alveolar epithelial cells. Histologically, ARDS is diffuse alveolar damage in three phases: exudative (oedema, hyaline membranes) → proliferative (type II pneumocyte hyperplasia, organisation) → fibrotic.