Diffuse cystic lung disease of unexplained cause with coexistent small airway disease: a possible causal relationship?

Diffuse "true" cystic lung disease is rare, and the specificity of high-resolution computed tomography (HRCT) has reduced the need for biopsy. We present 5 patients with similar clinical and HRCT features of cystic lung disease that were sufficiently atypical to warrant surgical lung biopsies that showed coexistent small airway diseases (SAD). There were 4 female patients and 1 male patient with a mean age of 43 years. All were never smokers. Four had symptoms such as dyspnea (1), cough (2), or both (1). HRCTs showed variably sized thin-walled cystic airspaces without zonal distribution, some with prominent vessels in their walls. One case was unilateral. Surgical lung biopsy showed cystic change comprising localized loss of alveolar density with coexistent SADs [chronic bronchiolitis (n = 2), eosinophilic bronchiolitis, probable asthma (n = 1), and diffuse idiopathic neuroendocrine cell hyperplasia (n = 2)]. Two patients who were tested for Birt-Hogg-Dube-related gene mutations proved negative, and all lacked other features of Birt-Hogg-Dube. We hypothesize that chronic damage to small airways may lead to cystic degeneration in a minority of patients. Precedents in relation to Sjogren syndrome and hypersensitivity pneumonitis raise the possibility of a causal association between pathologies in these 2 anatomic compartments, although HRCT data in relation to common SADs indicate that this would be a rare phenomenon. The driving factor remains unknown.