Insidious Hypersensitivity Pneumonitis

Axial CT image shows bilateral numerous centrilobular groundglass opacities (arrows) and areas of air trapping (heterogeneous low attenuation in lung parenchyma) in this 57-year-old nonsmoker.

Facts: Hypersensitivity Pneumonitis

• Numerous etiology: exposure to microbes, animal protein, plant protein, low-molecular-weight chemicals
• Clinical features depend on the amount of allergen, duration of exposure, nature of antigen and host factors
• Clinical syndromes divided into three types: 1. acute (or episodic) with improvement between attacks, 2. insidious with superimposed acute attack, 3. insidious without acute attacks
• Histology shows cellular bronchiolitis, diffuse chronic interstitial infiltrates, nonnecrotizing granulomas or giant cells in alveoli/interstitium
• Much more common in nonsmokers

Imaging Appearance

• Usually not seen on chest radiograph. Bilateral groundglass opacities and fine reticulations can be seen (similar appearance to pulmonary edema). If fibrosis occurs, reticular opacities and honeycombing is visualized
• On CT: centrilobular ground glass opacities (less than 5 mm ill-defined nodules in centrilobular distribution), air trapping and diffuse groundglass opacities
• Usually symmetric in the mid/lower lungs

Diagnosis

• Clinical history + lab test (Antibody testing) + bronchoalveolar lavage finding + imaging
• In BAL, there is increased lymphocytes with CD4:CD8 ratio of less than 1

Our case: insidious hypersensitivity pneumonitis due to unknown etiology, the diagnosis was confirmed with BAL and lung biopsy. The main differential diagnosis based on imaging in this case was NSIP.

Reference:

Hirschmann JV, Pipavath SNJ, Godwin JD. Hypersensitivity pneumonitis: a historical, clinical and radiologic review. RadioGraphics 2009;29:1921-1938.