Hypersensitivity Pneumonitis, Chronic

Jud W. Gurney, MD, FACR

Key Facts

Terminology

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

Imaging Findings

Ground-glass opacities + centrilobular nodules + lobular hyperinflation + signs of fibrosis (traction bronchiectasis, irregular reticular lines, honeycombing)
Mid lung more common, especially in bird breeders and those with continuous exposure
Upper lung zone more common in farmers (intermittent exposure)
Emphysema (20%) more common in farmer’s lung as compared to other agents
Patterns of fibrosis may be either NSIP or IPF
Mild degrees of irregular linear opacities and traction bronchiectasis may be partially or fully reversible

Top Differential Diagnoses

Idiopathic Pulmonary Fibrosis (IPF)
Nonspecific Interstitial Pneumonia (NSIP)
Sarcoidosis

Pathology

Chronic HP thought to result from low-level or recurrent exposure

Clinical Issues

Diagnosis often delayed for years, mean 3 years
Disease may progress despite elimination of antigen exposure
Prognosis depends on degree of fibrosis

Frontal radiograph shows reticular opacities

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

in the right upper lobe. The background pattern is mosaic attenuation

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