Hypersensitivity Pneumonitis CT Patterns and Treatment
The typical CT pattern of hypersensitivity pneumonitis (HP) includes diffusely distributed ground-glass opacities, mosaic attenuation, and small airway disease features such as ill-defined centrilobular nodules and air trapping, with different presentations depending on whether the disease is nonfibrotic or fibrotic. 1
Nonfibrotic HP CT Patterns
Typical Nonfibrotic HP Pattern
- Diffusely distributed ground-glass opacities (GGO) and mosaic attenuation as features of lung infiltration 1
- Ill-defined, small (<5 mm) centrilobular nodules on inspiratory images 1
- Air trapping on expiratory CT images 1, 2
- Craniocaudal distribution is typically diffuse, sometimes with relative basal sparing 1
- Axial distribution is typically diffuse without central or peripheral predominance 1
Compatible with Nonfibrotic HP Pattern
- Uniform and subtle ground-glass opacities 1
- Airspace consolidation 1
- Lung cysts 1
- Diffuse distribution (variant: peribronchovascular) 1
Fibrotic HP CT Patterns
Typical Fibrotic HP Pattern
Coexisting lung fibrosis and signs of bronchiolar obstruction 1
Irregular fine or coarse reticulation with architectural distortion 1
Septal thickening with or without traction bronchiectasis in areas of GGO 1
"Three-density pattern" (also called "headcheese sign") - a highly specific pattern showing three different lung densities: 1
- Ground-glass opacities
- Lobules of decreased attenuation and vascularity
- Normal-appearing lung
Fibrosis typically most severe in mid or mid and lower lung zones 1
Relative basal sparing, which helps differentiate from idiopathic pulmonary fibrosis 1, 3
No central or peripheral predominance of fibrosis on axial images 1
Indeterminate for HP Pattern
- UIP pattern alone (honeycombing with subpleural and basal predominance) 1
- Fibrotic nonspecific interstitial pneumonia (NSIP) 1
- Organizing pneumonia 1
CT Acquisition Parameters for HP Diagnosis
- Noncontrast examination 1
- Volumetric acquisition with submillimetric collimation 1
- Thin-section CT images (<1.5 mm) 1
- Both inspiratory and expiratory acquisitions (expiratory to evaluate air trapping) 1
- Recommended radiation dose: 1-3 mSv ("reduced" dose) 1
Diagnostic Considerations
- CT findings alone are not sufficient to make a definitive diagnosis of HP, especially in fibrotic HP 1
- Integration with exposure history and clinical information is essential 1
- Multidisciplinary discussion (MDD) is recommended for definitive diagnosis 1
- In chronic HP, CT may show findings that overlap with other interstitial lung diseases 2, 3
- Severity of traction bronchiectasis on CT is a strong predictor of mortality in chronic HP 4
Common Pitfalls in CT Interpretation
- Chronic HP may have findings identical to usual interstitial pneumonia, making differentiation difficult 3
- Desquamative interstitial pneumonia cannot reliably be distinguished from acute/subacute HP on CT 3
- Mosaic attenuation pattern in chronic HP may represent patchy interstitial lung disease rather than small airways disease 5
- Quantitative CT analysis shows that low attenuation areas can be overestimated by visual assessment 5
Treatment Considerations
While the question asks about treatment, the provided evidence focuses primarily on diagnostic imaging patterns rather than treatment approaches. Based on general medical knowledge, treatment typically includes:
- Avoidance of the causative antigen when identified 1
- Corticosteroids for symptomatic disease, particularly in acute and subacute forms 1
- Immunosuppressive agents may be considered in chronic, progressive disease 1
- Lung transplantation may be considered in end-stage disease 1
The prognosis is significantly influenced by the extent of fibrosis, with the presence of traction bronchiectasis and honeycombing on HRCT being strong predictors of mortality 4.