Honeycombing describes small adjacent subpleural cystic structures in the context of established pulmonary fibrosis with destruction of lung parenchyma and loss of architecture ¹¹. A single layer of cysts is now considered sufficient to apply this descriptor if other signs of pulmonary fibrosis are present ¹¹.

Terminology

The terms microcystic honeycombing and macrocystic honeycombing are sometimes used but without a clear definition. The initial stages of honeycombing are perceptible only on microscopy. Cysts progressively enlarge, becoming visible on CT ¹¹. Exuberant honeycombing refers to numerous large adjacent cysts and is a feature of connective tissue disease ¹³.

Pathology

The typical histological correlate of honeycombing is advanced traction bronchiolectasis surrounded by alveolar collapse ⁹. Pathologists regard honeycomb cysts as collapsed cystic secondary lobules lined by respiratory epithelium ¹². Cysts are typically connected to airways by a slit-like orifice and flap which suggests the role of a check- valve mechanism in cyst growth. Rapid pre-terminal cyst growth has been reported with autopsy correlation ¹⁴.

Radiographic features

CT

Honeycombing and traction bronchiolectasis can be difficult to distinguish, but this is not surprising since traction bronchiolectasis is thought to progress to honeycombing. Honeycomb cysts are usually similar in size, <5 mm (range 3-25 mm), clustered in a row or stacked and with shared walls. The walls are relatively thick, around 1-3 mm. They are first seen adjacent to the pleura, usually in the basal lower lobes in areas of dense fibrosis ¹⁰. The cysts enlarge over time and acute pre-terminal enlargement has been described in one patient with rapid deccline in pulmonary function and autopsy features supporting a check-valve mechanism

History and etymology

The term “honeycomb lung” is thought to have originated in the 19^th century in Germany and is thought to have first appeared in 1949 in a study by N Oswald and T Parkinson ⁵.

Differential diagnosis

• traction bronchiolectasis

• in some situations

  • paraseptal emphysema (single layer of larger thin-walled juxta-pleural cyst-like spaces in non-fibrotic lung with upper lobe predominance) ¹⁰

  • airspace enlargement with fibrosis/smoking-related interstitial fibrosis (larger multi-layered or confluent thin-walled cysts, or cysts separated by normal lung which often spare the subpleural lung and lower zones) ¹⁰

  • paracicatricial emphysema ³

• airspace consolidation in the presence of pulmonary emphysema ⁸

Practical points

• using the term "honeycombing" implies substantial pulmonary fibrosis is present, so it should be used with care ¹¹

• differentiation of honeycombing from emphysematous spaces with peripheral fibrosis can be difficult and the two conditions may coexist; the distribution is helpful, honeycombing is posterior lower zone predominant whereas emphysema with fibrosis is upper zone predominant ¹²

• pathologists may find the diagnosis of honeycombing difficult as it can be difficult to distinguish from traction bronchiectasis; a clear histological definition would facilitate imaging evaluation ¹²