Endobronchial ultrasound and the assessment of airway wall thickness in asthma

Whilst high resolution computer tomography (HRCT) scanning lends support to the notion that the airways are thickened in asthma, this technique is limited by its sensitivity of measurement, inter-observer variability and necessity for radiation exposure. This is the first work to describe endobronchial ultrasound (EBUS) using a radial probe instead under direct vision into the airways during fibre-optic bronchoscopy to measure total airway wall thickness (AWT) in vivo. The technique was validated in an in vitro model (sheep airway) and in vivo by comparison with HRCT measurements in mild/moderate asthmatic and healthy volunteer subjects. In the asthmatic group these findings were related to PC20 histamine, as a measure of BHR, post-bronchodilator FEV, reversibility to β2 agonists, and duration of asthma. Endobronchial biopsies were assessed for histological markers of airway remodelling – reticular basement membrane thickness (RbMt), submucosal collagen and proteoglycan deposition. Inter and intraobserver variability using EBUS to measure AWT were good and were in agreement with those by HRCT. Increased AWT was found in asthmatics but contrary to that predicted by mathematical models it was inversely correlated with BHR and reversibility to β2 agonists. Although RbMt, submucosal collagens I, III, V and proteoglycans perlican, biglycan, decorin, fibronectin were increased in asthmatic subjects over controls, no correlations were found with AWT. I suggest that increased AWT in asthmatics leads to ‘stiffening’ of the airways which opposes excessive bronchoconstriction and is therefore beneficial in protecting the airways from closure.