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Hypersensitivity Pneumonitis, Chronic | Radiology Key

Hypersensitivity Pneumonitis, Chronic



Hypersensitivity Pneumonitis, Chronic


Jud W. Gurney, MD, FACR










Frontal radiograph shows reticular opacities image in the mid and upper lung in this farmer with chronic dyspnea. Upper lobe fibrosis is more common in farmers.






Axial HRCT shows irregular linear opacities and traction bronchiectasis image in the right upper lobe. The background pattern is mosaic attenuation image.


TERMINOLOGY


Abbreviations and Synonyms



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


Definitions



  • Chronic granulomatous lung disease caused by inhalation of variety of organic and chemical antigens



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


IMAGING FINDINGS


General Features



  • Best diagnostic clue: Ground-glass opacities + centrilobular nodules + lobular hyperinflation + signs of fibrosis (traction bronchiectasis, irregular reticular lines, honeycombing)


  • Patient position/location



    • Mid lung more common, especially in bird breeders and those with continuous exposure


    • Upper lung zone more common in farmers (intermittent exposure)


CT Findings



  • Background of subacute findings: Ground-glass opacities and small ill-defined centrilobular nodules



    • Ground-glass opacities, often in geographic distribution (100%)


    • Centrilobular nodules (60%), usually ground-glass density < 5 mm in diameter


    • Mosaic perfusion pattern (60%)


    • Lung cysts (30%), nearly always seen in conjunction with ground-glass opacities



      • Thin-walled 3-25 mm in diameter, usually few in number (mean 4) range from 1-15


    • Individual signs alone nonspecific, combinations more helpful



      • Geographic ground-glass attenuation + normal lung + mosaic perfusion + centrilobular nodules


      • Ground-glass opacities + centrilobular nodules + lobular air-trapping


  • Chronic HP depends on signs of fibrosis



    • Irregular linear opacities (40%)


    • Traction bronchiectasis (20%)



    • Honeycombing (50%)


    • Emphysema (20%)



      • Patients with HP tend not to be smokers


      • Emphysema more common in farmer’s lung as compared to other etiologies


  • Patterns of fibrosis may be either NSIP or IPF



    • In IPF, most inferior lung (posterior costophrenic angles) is usually severely involved (relatively spared in HP)


    • In NSIP, peribronchovascular ground-glass opacities and basilar distribution similar (centrilobular nodules and lobular hyperinflation uncommon in NSIP)


  • Distribution of disease



    • Bird breeders: Mid lung zone predominant


    • Farmer’s lung: Upper lung zone predominant


    • Distinction may reflect intermittent exposure (farmers) vs. continuous exposure (bird breeders) to offending antigen


    • Relative sparing costophrenic angles



      • Posterior costophrenic angles less involved than other areas in lung, in contrast to IPF in which this lung usually is most severely involved


  • Associated findings



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


    • Pleural effusions rare


  • Resolution



    • Mild degrees of irregular linear opacities and traction bronchiectasis may be partially or fully reversible


  • Accuracy of diagnosis



    • If highly confident of diagnosis (up to 60% of cases of chronic HP) you will be correct 90% of the time



      • Most common mimics: IPF and NSIP


Radiographic Findings



  • Radiography



    • Findings of fibrosis



      • Architectural distortion, volume loss, variable distribution: Upper, mid, or lower zone predominant


Imaging Recommendations



  • Best imaging tool: HRCT much more specific than chest radiography for fibrosis


DIFFERENTIAL DIAGNOSIS


Idiopathic Pulmonary Fibrosis (IPF)



  • Striking subpleural distribution, not as common with HP


  • Honeycombing prominent, ground-glass opacities less common


  • Inferior costophrenic angles usually most severely abnormal lung (basilar peripheral lung)



    • May not be spared but not most severely involved lung in HP


  • Air-trapping not a feature


Nonspecific Interstitial Pneumonia (NSIP)



  • Ground-glass opacities > reticulation



    • Traction bronchiectasis usually out of proportion to degree of reticulation


  • Peripheral &/or peribronchovascular distribution


  • Centrilobular nodules not a feature


  • Air-trapping not a feature


Sarcoidosis



  • Peribronchovascular distribution, subpleural nodules, adenopathy



    • Subpleural lymphatic deposits rare in HP


  • Predominantly upper lung zones


Silicosis



  • Occupational history of dust exposure


  • Predominantly upper lung zones



    • May develop progressive massive fibrosis, not seen with chronic HP


  • Subpleural lymphatic deposits rare in HP


  • Air-trapping not a feature


Ankylosing Spondylitis



  • Upper lung zone distribution



  • Severe architectural distortion with scarring and bullous lung disease


  • Spine nearly always ankylosed and longstanding history of spinal disease

Sep 20, 2016 | Posted by in RESPIRATORY IMAGING | Comments Off on Hypersensitivity Pneumonitis, Chronic
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