Honeycombing describes small adjacent subpleural cystic structures in the context of established pulmonary fibrosis with destruction of lung architecture and traction bronchiolectasis. A single layer of cysts is now considered sufficient to apply this descriptor in the correct context ¹¹.

Terminology

The terms microcystic honeycombing and macrocystic honeycombing are sometimes used. Exuberant honeycombing is a feature of connective tissue disease.

Pathology

The typical histological correlate of honeycombing is advanced traction bronchiolectasis surrounded by alveolar collapse ⁹. Microscopic honeycombing does not necessarily correlate with CT honeycombing ¹¹. Pathologists regard honeycomb cysts as collapsed cystic secondary lobules lined by respiratory epithelium ¹².

Radiographic features

CT

Honeycombing and traction bronchiolectasis can be difficult to distinguish - not surprising since traction bronchiolectasis is thought to progress to honeycombing. Honeycomb cysts are usually similar in size, <5mm (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.

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

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. Even 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.