Hypersensitivity Pneumonitis

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Summary1)First Aid: USMLE Step 1 by Tao Le

Hypersensitivity Pneumonitis is an immunologically mediated response to an extrinsic antigen that involves type III and type IV hypersensitivity reactions affecting the alveoli.

Pathophysiology 2) USMLE Step 1 Secrets by Thomas Brown

High resolution CT – ground glass (Cleveland Clinic)

Hypersensitivity pneumonitis is an acute inflammatory disease of the alveoli; it results from an allergic reaction to a wide variety of allergens, and causes acute shortness of breath. It subsumes a wide variety of diseases with the same underlying etiology and pathogenesis, but can be due to exposure to different allergens (e.g., farmer’s lung, pigeon handler’s lung, humidifier lung)

Causes 3)USMLE Step 1 Secrets by Thomas Brown

  • Exposure to different allergens (e.g., farmer’s lung, pigeon handler’s lung, humidifier lung)

Giant cells and granulomas (Cleveland Clinic)

Symptoms 4)Robbins Basic Pathology by Vinay Kumar

  • Acute reaction – fever, cough, dyspnea, and constitutional signs and symptoms arising 4 to 8 hours after exposure 
  • Chronic disease – characterized by insidious onset of cough, dyspnea, malaise, and weight loss

Diagnosis 5)USMLE Step 1 Secrets by Thomas Brown 6) Robbins Basic Pathology by Vinay Kumar

  • Chest X-ray – diffuse infiltrates 
  • High Res CT –  Ground glass opacity (due to chronic interstitial inflammation)
  • Pulmonary function tests (PFTs) – restrictive patterns from eventual fibrosis 
  • Pathology – loosely formed interstitial granulomas and chronic inflammation

Treatment 7)USMLE Step 1 Secrets by Thomas Brown

  • Most often reversible if diagnosed early and offending agent removed 
  • Steroids

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