What HRCT (High-Resolution Computed Tomography) patterns are characteristic of hypersensitivity pneumonitis (HP) in a patient with a history of emphysema, pulmonary fibrosis, and autoimmune features?

HRCT Patterns for Diagnosis of Hypersensitivity Pneumonitis

The diagnosis of hypersensitivity pneumonitis on HRCT requires identifying both fibrosis patterns AND small airway disease features together—the presence of fibrosis alone without small airway signs is indeterminate and cannot establish the diagnosis. 1

Fibrotic HP HRCT Patterns

The 2020 ATS/JRS/ALAT guidelines categorize fibrotic HP into three diagnostic confidence levels based on specific HRCT features 1:

Typical HP Pattern (Highest Diagnostic Confidence)

This pattern requires BOTH components to be present simultaneously: 1

Fibrosis features:

• Irregular linear opacities/coarse reticulation with lung distortion 1
• Traction bronchiectasis and honeycombing may be present but do NOT predominate 1
• Distribution patterns that strongly suggest HP:
  • Random distribution both axially and craniocaudally 1
  • Mid lung zone predominant 1, 2
  • Relative sparing of lower lung zones 1, 2

Small airway disease features (MUST be present):

• Ill-defined centrilobular nodules and/or ground-glass opacities 1, 2
• Mosaic attenuation, three-density pattern (formerly "headcheese sign"), and/or air trapping in lobular distribution 1, 2

Compatible with HP Pattern (Intermediate Confidence)

Variant fibrosis patterns that still suggest HP when accompanied by small airway disease signs: 1

• UIP pattern: basal and subpleural honeycombing with/without traction bronchiectasis 1
• Extensive ground-glass opacities with superimposed subtle fibrosis features 1
• These variant patterns MUST still show signs of small airway disease to be considered compatible with HP 1

Indeterminate for HP Pattern (Cannot Diagnose HP)

Lone patterns without accompanying small airway disease features: 1

• UIP pattern alone 1
• Probable UIP pattern alone 1
• Fibrotic NSIP pattern alone 1
• Organizing pneumonia-like pattern alone 1
• Any truly indeterminate HRCT pattern 1

Nonfibrotic HP HRCT Patterns

The CHEST guidelines provide distinct criteria for nonfibrotic HP 1:

Typical Nonfibrotic HP

• Profuse poorly defined centrilobular nodules of ground-glass opacity affecting all lung zones 1, 3
• Inspiratory mosaic attenuation with three-density sign 1
• Inspiratory mosaic attenuation and air-trapping associated with centrilobular nodules 1

Compatible with Nonfibrotic HP

• Centrilobular nodules of ground-glass attenuation that are not profuse or diffuse, without mosaic attenuation or lobular air-trapping 1
• Patchy or diffuse ground-glass opacity 1
• Mosaic attenuation and lobular air-trapping without centrilobular nodules or ground-glass abnormality 1

Critical Diagnostic Features

The Three-Density Pattern (Pathognomonic When Present)

This highly specific sign shows three distinct lung densities sharply demarcated from each other: 1, 2

• High attenuation (ground-glass opacity) 1
• Lucent lung (regions of decreased attenuation and decreased vascular markings) 1
• Normal lung 1

Essential Imaging Protocol

To properly evaluate for HP, HRCT must include: 2

• Both inspiratory AND expiratory acquisitions (expiratory images are essential to evaluate air trapping) 2
• Thin-section CT images (<1.5 mm) 2
• Volumetric acquisition with submillimetric collimation 2
• Noncontrast examination 2

Special Considerations for Your Patient Context

In patients with emphysema and pulmonary fibrosis (combined pulmonary fibrosis and emphysema), fibrotic HP can still occur and should be actively considered: 1

• Fibrotic HP may present as a component of combined pulmonary fibrosis and emphysema 1
• A pure emphysematous form of HP exists 1
• The key distinguishing feature remains the presence of small airway disease signs (centrilobular nodules, mosaic attenuation, air trapping) alongside the fibrosis 1

In patients with autoimmune features, distinguishing fibrotic HP from connective tissue disease-related ILD is critical: 4

• High titers of ANA, rheumatoid factor, or anti-CCP antibodies suggest CTD-related ILD rather than HP 4
• However, both can coexist, requiring multidisciplinary discussion 4

Critical Pitfalls to Avoid

Never diagnose HP based on fibrosis patterns alone—small airway disease features MUST be present. 1 The absence of centrilobular nodules, mosaic attenuation, or air trapping makes the HRCT indeterminate for HP, regardless of the fibrosis pattern present.

Always obtain expiratory images. 2, 5 Air trapping may only be visible on expiratory CT and is a crucial diagnostic feature that can be missed on inspiratory images alone.

Do not diagnose IPF when centrilobular nodules are present. 6 This finding strongly suggests an alternative diagnosis, most commonly HP, and should prompt detailed exposure history and consideration of bronchoalveolar lavage.

CT findings alone cannot establish a definitive diagnosis of HP, especially in fibrotic cases. 2 Integration with exposure history (birds, mold, hot tubs, occupational antigens), BAL lymphocytosis (≥30%), and multidisciplinary discussion is essential for confident diagnosis 2, 7.