Bronchiolitis is an internationally recognised diagnosis that is commonly applied to children across healthcare systems. In the UK, 2% of all infants are admitted to paediatric hospitals each year with a diagnosis of bronchiolitis, typically over 6–8 weeks winter peak period. Globally, bronchiolitis is just as common though seasons are more diffuse and variable in tropical climates. For such a common disease, there remain significant gaps in our understanding of its pathophysiology and also its inter-relationship with other common lower respiratory tract disease in young children. This lack of data is now restraining progress in the development of management and therapeutics for the preschool age group who are most affected by lower respiratory tract disease (both for infections, ie, respiratory syncytial virus (RSV) and for the response to infection, ie, wheeze phenotypes). So the study by Dumas et al in Thorax is welcome, as they attempt to stratify subgroups of patients with a clinical diagnosis of bronchiolitis.1

Yet, despite being a common condition in young children, it has proven challenging to develop an international consensus on what precisely defines bronchiolitis from a clinical perspective and how this differs from other common lower respiratory conditions. Most paediatricians consider that they would instantly recognise a case, but agreement may be less consistent when clinicians are less experienced, children are older, have clinical risk factors or present outside the peak RSV season. In many ways this reflects gaps in our current knowledge about lower respiratory tract disease in young children, as the diagnosis of bronchiolitis is intended to be the clinical representation of typical histological features within the bronchioles of those affected: oedematous bronchioles filled with necrotic epithelium and neutrophils, bound together by mucous.2 As a histological review of only the most severe cases of bronchiolitis (resulting in death) have informed this …