Hypersensitivity Pneumonitis, Acute-Subcute

Hypersensitivity Pneumonitis, Acute-Subcute

Jud W. Gurney, MD, FACR

Axial CECT shows diffuse ground-glass opacities image and faint centrilobular nodules image. Chest radiograph was normal.

Axial CECT shows diffuse ground-glass opacities. Clustered centrilobular nodules image and lobular air-trapping image was visible in this patient with subacute hypersensitivity pneumonitis.


Abbreviations and Synonyms

  • Extrinsic allergic alveolitis, hypersensitivity pneumonitis (HP), farmer’s lung


  • Diffuse granulomatous interstitial lung disease caused by inhalation of various antigenic particles (microbes, animal proteins, and low-molecular weight chemicals)

    • Farmer’s lung and bird fancier’s lung are most common forms

    • “Hot tub” lung latest source


General Features

  • Best diagnostic clue: Ground-glass centrilobular nodules & mosaic perfusion (or lobular air-trapping)

  • Patient position/location: Diffuse mid lung most common, typically spares costophrenic angles

  • Morphology: Predominant ground-glass opacities forming small ill-defined centrilobular nodules

CT Findings

  • More sensitive than chest radiography but may be normal

    • Sensitivity in 1 population-based study that used 1990’s technology, only 50% (sensitivity of chest radiographs even worse at 10%)

  • CT signs

    • Ground-glass opacities (100%)

      • Geographic distribution in central and peripheral portions of lung, nonspecific

    • Centrilobular nodules (70%)

      • Ground-glass density with ill-defined edges usually < 5 mm in diameter

      • Pleural surfaces usually spared

    • Mosaic perfusion (80%) (usually from air-trapping)

    • Air-trapping expiratory scan (95%)

    • Individual signs nonspecific, combined signs more specific

      • Geographic ground-glass attenuation + normal lung + mosaic perfusion + air-trapping = head cheese sign

      • Ground-glass centrilobular nodules + mosaic perfusion (or lobular hyperinflation) = hypersensitivity pneumonitis until proven wrong

  • Distribution of disease

    • Most prominent mid to lower lungs, commonly spares (or less severe) costophrenic angles

  • Acute stage

    • Diffuse ground-glass opacities

    • Small ill-defined centrilobular nodules, nearly always in conjunction with ground-glass opacities

      • Centrilobular nodules more likely to be found in less severely involved lung

    • Air-trapping common, usually at lobular level

    • Tree-in-bud pattern rare

  • Subacute stage

    • Ground-glass opacities (patchy distribution) to mosaic perfusion

    • Ill-defined centrilobular nodules (< 5 mm diameter) more common than in acute stage

    • Lung cysts (10%), nearly always seen in conjunction with diffuse ground-glass opacities

      • Thin-walled 3-25 mm diameter

      • Mean number 4 cysts per patient (range 1-15)

  • Associated findings

    • Mediastinal adenopathy (50%), nodes < 20 mm short axis diameter

    • Pleural effusion rare

  • Resolution: Lung may return to normal with avoidance of antigen or steroid therapy

Radiographic Findings

  • Radiography

    • Acute stage

      • Chest radiography abnormal in only about 10%

      • Nonspecific fine nodular or reticulonodular pattern, consolidation rare (usually signifies community acquired pneumonia)

    • Subacute stage

      • Chest radiograph more often abnormal (90%) (but may be subtle)

      • Poorly defined small nodules (miliary pattern) or areas of ground-glass opacities

Imaging Recommendations

  • Best imaging tool: 1 clue to diagnosis of HP is marked disparity between normal chest radiograph and striking diffuse abnormal CT

  • Protocol advice: Expiratory scanning may be useful to show air-trapping


Nonspecific Interstitial Pneumonia (NSIP)

  • Ground-glass opacities > reticulation

    • Traction bronchiectasis usually out of proportion to degree of reticulation

  • Peripheral &/or peribronchovascular distribution

  • Air-trapping not a feature as it is in HP

  • Centrilobular nodules uncommon

Metastatic Pulmonary Calcification

  • Ill-defined centrilobular nodules similar to HP

  • Nodules may have calcific attenuation, not seen with HP

  • Usually upper lung zone in distribution

  • Seen in patients with disorders of calcium metabolism, most commonly renal failure


  • Includes Church-Strauss syndrome and

    • Systemic lupus erythematosus

      • Anemia with hemorrhage, not seen with HP

      • Air-trapping uncommon

      • Often have renal disease

Lymphocytic Interstitial Pneumonia

  • Similar CT findings: Ground-glass opacities, centrilobular nodules and cysts

  • Air-trapping not a feature

  • Often have dysproteinemias or Sjögren syndrome

Langerhans Cell Histiocytosis

Sep 20, 2016 | Posted by in RESPIRATORY IMAGING | Comments Off on Hypersensitivity Pneumonitis, Acute-Subcute

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