What type of pulmonary fibrosis is characterized by reticulations with subpleural sparing in the mid to upper lung?

Fibrotic Hypersensitivity Pneumonitis: The Type of Pulmonary Fibrosis with Reticulations and Subpleural Sparing in Mid to Upper Lungs

Fibrotic hypersensitivity pneumonitis (fHP) is the type of pulmonary fibrosis characterized by reticulations with subpleural sparing in the mid to upper lung zones. This distinctive radiological pattern helps differentiate it from other types of pulmonary fibrosis, particularly idiopathic pulmonary fibrosis (IPF).

Radiological Features of Fibrotic Hypersensitivity Pneumonitis

Key Distinguishing Features

• Distribution pattern: Mid to upper lung predominance 1
• Subpleural sparing: Relative preservation of the immediate subpleural regions 1
• Reticulation pattern: Irregular linear opacities forming a net-like pattern 2
• Additional features:
  • Traction bronchiectasis
  • Patchy ground-glass opacities
  • Possible mosaic attenuation (three-density pattern) 1
  • Relative sparing of costophrenic angles 1

Comparison with Other Fibrotic Lung Diseases

Fibrotic HP vs. IPF

• IPF typically shows:
  • Basal and subpleural predominance 1
  • Honeycombing (in definite UIP pattern) 1
  • No subpleural sparing 1
  • No peribronchovascular extension 1

Fibrotic HP vs. NSIP

• While both can show subpleural sparing, NSIP typically has:
  • More extensive ground-glass opacities 1, 3
  • More uniform appearance 1
  • Less upper lung predominance 3
  • Peribronchovascular distribution may be seen in both 3

Diagnostic Approach

HRCT Patterns in Fibrotic HP

1. Typical HP pattern: Requires both parenchymal infiltration (ground-glass opacities, mosaic attenuation) and small airway disease features (ill-defined centrilobular nodules, air trapping) in a diffuse distribution 1

2. Compatible with HP pattern: Less specific patterns including uniform ground-glass opacities, airspace consolidation, or lung cysts 1

3. UIP-like pattern with HP features: Some cases of fibrotic HP may present with a UIP-like pattern but with additional features suggesting HP, such as:

   • Mid to upper lung involvement
   • Peribronchovascular extension
   • Subpleural sparing
   • Relative sparing of costophrenic angles 1

Histopathological Correlation

Fibrotic HP histologically shows:

• Bronchiolocentric cellular and fibrosing interstitial pneumonia 1
• Patchy cellular interstitial infiltrate 1
• Poorly formed granulomas 1
• Variable fibrosis that may mimic UIP in advanced cases 1

Clinical Implications and Management

Prognostic Significance

• Fibrotic HP has a variable prognosis, with some cases progressing to end-stage fibrosis 4
• The presence of a UIP-like pattern in HP is associated with worse outcomes 5

Treatment Approach

• Primary intervention: Antigen identification and avoidance 4
• Pharmacological therapy:
  • Immunosuppressive therapy (corticosteroids, other immunosuppressants) 4
  • Antifibrotic agents may be considered in progressive disease 4, 5

Common Pitfalls in Diagnosis

1. Misdiagnosis as IPF: When upper lung involvement and subpleural sparing are subtle or overlooked 2

2. Overlooking exposure history: Failure to identify potential antigens that could trigger HP 1

3. Confusing with other patterns: NSIP can also show subpleural sparing but typically has more ground-glass opacities and less upper lung predominance 3

4. Incomplete radiological assessment: Not evaluating the full distribution pattern or missing subtle features like the three-density pattern 1

In summary, when evaluating pulmonary fibrosis with reticulations showing subpleural sparing in the mid to upper lungs, fibrotic hypersensitivity pneumonitis should be the primary diagnostic consideration. This pattern is distinctly different from the basal and peripheral predominant pattern seen in IPF and has important implications for treatment approach and prognosis.