Purpose of review: In 2009, an expert panel met to propose a WHO definition of asthma severity and control, and criteria for describing exacerbations and their severity, which would be globally applicable. This review addresses their findings in the context of recent literature, and assesses the usefulness of these definitions in children in particular.
Recent findings: Severe asthma was defined by the level of current clinical control and risks as: 'Uncontrolled asthma which can result in risk of frequent severe exacerbations (or death) and/or adverse reactions to medications and/or chronic morbidity (including impaired lung function or reduced lung growth in children)'. Severe asthma includes three groups, with different public health messages and challenges: untreated severe asthma, due to undiagnosed asthma or unavailability of therapy, difficult-to-treat severe asthma (due to adherence issues, inappropriate or incorrect use of medicines, environmental triggers or co-morbidity), and treatment-resistant severe asthma, including asthma for which control is not achieved despite the highest level of recommended treatment or asthma which is controlled only with the highest level of recommended treatment.
Summary: These definitions will enable more precise measurement of the burden of severe childhood asthma globally. International collaborations in epidemiological and mechanistic studies, and randomized controlled trials of treatment, will be facilitated. However, both pathophysiology and severity are influenced by a number of factors with wide global variation; international comparisons should be interpreted with caution.