[The key role of chest CT in severe asthma phenotyping]

Before initiating biological therapy, chest CT is indicated as a means of initial evaluation of severe asthma. While its primary role is to identify potential differential diagnoses, it concomitantly provides information effectively contributing to asthma phenotyping. In routine practice, analysis focuses on bronchial wall thickening, of which the extent correlates with disease severity and degree of bronchial obstruction. In cases of severe asthma, absent wall thickening is unusual, and may prompt a search for either an alternative diagnosis, or potential comorbidities such as rhinosinusitis, which may be symptomatic. Assessment of expiratory air trapping is useful in the event of fixed obstructive ventilatory impairment, of which the extent correlates with the degree of bronchial obstruction. Pathobiology of mucus plugs, a phenotyping tool regarding T2 inflammation and a marker of severity, has recently drawn increased interest. The density of these plugs should be analysed, given that spontaneous hyperattenuation, most often within dilated bronchi, facilitates establishment of the diagnosis of allergic bronchopulmonary aspergillosis. To sum up, investigation of bronchial wall thickening, expiratory air trapping, and mucus plugs represent potential means of assessing treatment response.