Hypersensitivity pneumonitis

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HP Pathophysiology - The Allergic Lung

  • Definition: A diffuse, granulomatous interstitial lung disease (ILD) from repeated inhalation of an organic antigen in a sensitized individual.
  • Immunologic Mechanism: A combination of hypersensitivity reactions, distinct from asthma.
    • Type III (Immune Complex): Antigen-antibody complexes deposit in alveoli, activating complement and causing acute inflammation.
    • Type IV (Delayed, Cell-Mediated): T-cell activation leads to the formation of characteristic granulomas upon chronic exposure.

⭐ The presence of poorly formed non-caseating granulomas on biopsy is a classic finding, but not pathognomonic; it's also a key feature of sarcoidosis.

Histopathology of Hypersensitivity Pneumonitis

Etiologies & Antigens - Feather & Spore Culprits

Inhaled organic dusts containing antigens are the primary triggers. Diagnosis hinges on identifying the specific exposure. Key syndromes include:

SyndromeAntigen Source & ExposureAntigen(s)
Farmer's LungMoldy hay, grain, compostSaccharopolyspora rectivirgula
Bird Fancier's LungBird droppings & feathersAvian proteins
Hot Tub LungContaminated hot tubs, humidifiersMycobacterium avium complex
BagassosisMoldy sugarcane fiber (bagasse)Thermophilic actinomycetes

⭐ Farmer's Lung is the most common type of hypersensitivity pneumonitis.

Clinical Picture & Diagnosis - The Telltale Crackles

  • Clinical Presentations Vary:
    • Acute: Abrupt onset (4-8 hrs post-exposure) of flu-like symptoms (fever, myalgia), cough, & dyspnea. Resolves with antigen avoidance.
    • Subacute: Insidious onset of productive cough & dyspnea over weeks to months.
    • Chronic: Progressive dyspnea, cough, and fatigue, leading to irreversible pulmonary fibrosis.

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⭐ On high-resolution CT (HRCT), the combination of ground-glass opacities, centrilobular nodules, and air trapping (mosaic attenuation) is highly suggestive of HP.

Management & Prognosis - Dodge, Suppress, Survive

  • Antigen Avoidance: The absolute cornerstone of management. Complete removal of the offending agent is paramount for halting disease progression.
  • Corticosteroids: Mainstay for symptomatic acute or subacute disease (e.g., prednisone) to suppress inflammation.
  • Chronic/Fibrotic HP: May require long-term steroids or other immunosuppressants (e.g., mycophenolate, azathioprine). Lung transplantation is the final option for severe, end-stage fibrotic disease.
  • Prognosis: Excellent for acute HP if the antigen is removed; guarded to poor for chronic fibrotic HP.

⭐ The single most important factor determining prognosis in hypersensitivity pneumonitis is the identification and removal of the causative antigen.

  • A Type III/IV hypersensitivity reaction to inhaled organic antigens (e.g., moldy hay in Farmer's Lung, avian proteins in Bird Fancier's Lung).
  • Acute form presents with sudden fever, cough, and dyspnea 4-8 hours post-exposure.
  • Chronic exposure leads to insidious progressive dyspnea and irreversible pulmonary fibrosis.
  • Imaging shows diffuse ground-glass opacities (acute) or reticular/nodular patterns with honeycombing (chronic).
  • BAL reveals marked lymphocytosis.
  • Antigen avoidance is the most critical step in management.
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Practice Questions: Hypersensitivity pneumonitis

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A 23-year-old man presents to the emergency department with shortness of breath. The patient was at a lunch hosted by his employer. He started to feel his symptoms begin when he started playing football outside with a few of the other employees. The patient has a past medical history of atopic dermatitis and asthma. His temperature is 98.3°F (36.8°C), blood pressure is 87/58 mmHg, pulse is 150/min, respirations are 22/min, and oxygen saturation is 85% on room air. Which of the following is the best next step in management?

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Flashcards: Hypersensitivity pneumonitis

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What is the treatment for idiopathic pulmonary fibrosis? _____

TAP TO REVEAL ANSWER

What is the treatment for idiopathic pulmonary fibrosis? _____

Lung transplant

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