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Hypersensitivity Pneumonitis, Acute-Subcute | Radiology Key

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.


TERMINOLOGY


Abbreviations and Synonyms



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


Definitions



  • 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


IMAGING FINDINGS


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


DIFFERENTIAL DIAGNOSIS


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


Vasculitis



  • 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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