Hypersensitivity Pneumonitis, Acute-Subcute

Jud W. Gurney, MD, FACR

Key Facts

Terminology

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

Imaging Findings

Ground-glass centrilobular nodules & mosaic perfusion
Geographic ground-glass attenuation + normal lung + mosaic perfusion + air-trapping = head cheese sign
Air-trapping expiratory scan (95%)
Tree-in-bud pattern rare
Lung cysts (10%), always seen in conjunction with diffuse ground-glass opacities

Top Differential Diagnoses

Nonspecific Interstitial Pneumonia (NSIP)
Langerhans Cell Histiocytosis
Vasculitis, includes Churg-Strauss, microscopic polyangiitis, lupus erythematosus
Sarcoidosis

Clinical Issues

Diagnosis often delayed more than 1 year
Depends largely on avoiding antigen and removal from offending environment
Nonspecific symptoms in acute disease often mistaken as pneumonia
- Since disease self-limited, antibiotics may appear to improve patient’s condition

Diagnostic Checklist

Normal chest radiograph and strikingly diffuse abnormal CT commonly seen with HP

Axial CECT shows diffuse ground-glass opacities

and faint centrilobular nodules

. Chest radiograph was normal.

Axial CECT shows diffuse ground-glass opacities. Clustered centrilobular nodules

and lobular air-trapping

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