Clinical and histological effects of Bronchial Thermoplasty in severe asthma

Background: Bronchial Thermoplasty (BT) has been shown to improve asthma control and symptoms, and reduce exacerbation frequency and rescue medication use. Small studies have shown reduced airway smooth muscle (ASM) mass following BT, but understanding of the mechanism of action of BT is incomplete. Aims: To investigate the clinical, physiological and histological effects of BT in severe asthma. Methods: 34 severe asthma patients undergoing BT as part of clinical care were recruited from four UK centres. Asthma questionnaires and lung function were assessed before, 6 weeks after, and 6 months after treatment. 19 patients underwent endobronchial biopsy at the time of BT procedures, which were examined for ASM and epithelial changes. Results: ACQ6 and AQLQ improved significantly 6 weeks after BT (mean change -0.55, p=0.03; mean change 0.57, p=0.02 respectively), but this was not maintained at 6 months. Median (interquartile range IQR) ASM% reduced from 13.8% (7.7-16.9) at baseline to 5.8% (2.1-12.1) at first follow-up (p=0.03), although marginally increased to 6.6% (3.1-14.1) at second follow-up (p=0.1 baseline to second follow-up). Median (IQR) epithelial integrity improved from 28.9% (15.1-42.7) at baseline to 43.4% (28.8-52.7) at first follow-up (p=0.004), and was mostly maintained at 42.4% (16-68) at second follow-up (p=0.06 baseline to second follow-up). There was no change in lung function. Conclusion: BT improves ACQ6 and AQLQ over baseline in severe asthma but does not affect lung function. ASM% initially reduces in response to BT but subsequently increases again. Epithelial integrity increases following BT, and this effect is mostly maintained beyond the end of treatment.