This week, we discussed a case of a 76 yo F with progressive lower lung predominant reticulations with PFTs showing isolated reduction in DLco and demonstrating exertional hypoxemia. Differential based on her HRCT was chronic hypersensitivity pneumonitis (cHP) versus idiopathic pulmonary fibrosis (IPF). Ultimately, based on the presence of air trapping we felt the HRCT was inconsistent with usual interstitial pneumonia (UIP) pattern.
Why does it matter that we distinguish between IPF and other ILDs that result in a UIP pattern?
A UIP HRCT and histopathologic pattern is the hallmark of idiopathic pulmonary fibrosis (IPF). However, other ILDs (CTD-ILD and cHP) are associated with UIP pattern. Management and prognosis of these conditions are different than for IPF. So consequential are these differences that often a surgical lung biopsy (SLB) is performed to cinch the diagnosis!
Reminder – what defines a UIP HRCT pattern?
- Subpleural and basal predominant, heterogenous, often asymmetric
- Honeycombing +- traction bronchiectasis
- Superimposed with a reticular pattern, relatively mild GGO
- Irregular thickening of interlobular septa
Coronal view of HRCT demonstrating UIP pattern of fibrosis, with subpleural/basilar predominant honeycombing & traction bronchiectasis
By contrast, a UIP pattern with the presence of 3+ lobes of air trapping on HRCT noted to be “inconsistent with UIP” based on 2011 ATS/ERS/JRS/ALAT guidelines and is commonly associated with cHP. HP should be considered when fibrosis and honeycomb cysts predominate in the upper/mid lungs, mosaic attenuation/three density/“head cheese” sign present, or when fibrosis appears diffuse in axial plane
inspiratory (left) and expiratory (right) coronal views of HRCT with UIP pattern due to chronic hypersensitivity pneumonitis. Lobular air trapping (geographic hyperlucent regions) confirmed on expiratory imaging.
By the way – what is the “head cheese” sign?
The headcheese sign is a mixed infiltrative and obstructive process usually associated with bronchiolitis. It is very specific but not pathognomonic for hypersensitivity pneumonitis.
Headcheese IRL…ew
Inspiratory/expiratory cuts showing 3 distinct attenuations
Are air trapping and IPF mutually exclusive?
A 2018 single-center retrospective study looked at patients with UIP and “inconsistent with UIP” HRCT with specific attention to presence and characteristics of air trapping. Among enrollees were patients for whom IPF was the final histopathologic diagnosis although HRCT findings suggested against UIP.
They found:
- Qualitative and quantitative air trapping was common in patients with UIP pattern, whether IPF (41-45%) or non-IPF (30-49%) ILD
- Upper lobe air trapping was rare in IPF and more commonly associated with cHP (11/13 cases)
Takeaways
- IPF (the most commonly encountered ILD) is associated with a UIP HRCT and histopathologic pattern
- UIP HRCT pattern consists of basilar/subpleural fibrosis, traction bronchiectasis and honeycombing with relative paucity of groundglass opacities
- Expiratory air trapping, an HRCT feature commonly associated with cHP, is inconsistent with UIP when present in >3 secondary lobules
- A recent study suggests that upper lobe predominance of air trapping is the feature most suggestive against IPF
- The 2022 ATS/ERS/JRS/ALAT guidelines do not include an update on this subject
Sources: