Differential Diagnosis for Hypersensitivity Pneumonitis vs. CTD-related ILD

When differentiating Hypersensitivity Pneumonitis (HP) from Connective Tissue Disease (CTD)-related Interstitial Lung Disease (ILD), it's crucial to consider various factors including clinical presentation, exposure history, serological tests, and imaging findings. Here's a structured approach to the differential diagnosis:

• Single Most Likely Diagnosis

  • Hypersensitivity Pneumonitis: This is a likely diagnosis if there's a clear history of exposure to antigens known to cause HP (e.g., bird fancier's lung, farmer's lung) and symptoms improve upon removal from the antigen.
  • CTD-related ILD: If there are systemic symptoms suggestive of a CTD (like arthritis, skin rash, or esophageal dysmotility) along with ILD on imaging, this becomes a more likely diagnosis.

• Other Likely Diagnoses

  • Idiopathic Pulmonary Fibrosis (IPF): Presents with progressive dyspnea and fibrosis on imaging, more common in older adults with a history of smoking.
  • Sarcoidosis: Can mimic both HP and CTD-related ILD, characterized by non-caseating granulomas in lung tissue and often involves lymph nodes.
  • Occupational or Environmental Lung Diseases: Depending on the exposure history, other occupational lung diseases could be considered, such as silicosis or asbestosis.

• Do Not Miss Diagnoses

  • Lymphangitic Carcinomatosis: Metastatic cancer to the lungs can mimic ILD and is critical to diagnose early due to its poor prognosis.
  • Pulmonary Vasculitis (e.g., Wegener's Granulomatosis): Can present with lung infiltrates and is a medical emergency due to potential for rapid progression and organ failure.
  • Invasive Fungal Infections: In immunocompromised patients, fungal infections can cause lung infiltrates that resemble ILD and require prompt antifungal therapy.

• Rare Diagnoses

  • Eosinophilic Pneumonia: Characterized by eosinophilia and pulmonary infiltrates, can be acute or chronic.
  • Lymphocytic Interstitial Pneumonia (LIP): Often associated with CTD or immunodeficiency, presents with diffuse lung infiltrates.
  • Pulmonary Alveolar Proteinosis: Rare condition characterized by accumulation of surfactant-like protein in alveoli, can mimic ILD on imaging.

Each of these diagnoses requires careful consideration of clinical, radiological, and pathological findings to accurately differentiate HP from CTD-related ILD and to not miss critical diagnoses that could significantly impact patient outcomes.