Introduction We present a series of cases referred to our severe asthma clinic that symptomatically appear like asthma, yet physiology demonstrated one of the more uncommon types of severe Small Airway Disease (SAD), including: Allergic Bronchopulmonary Aspergillosis (ABPA), Obliterative Bronchiolitis (OB), follicular bronchiolitis and Diffuse Idiopathic Pulmonary Neuroendocrine Cell Hyperplasia (DIPNECH). These diseases represent a diagnostic challenge. We have also reviewed cases of tracheomalacia and horseshoe lung, which in our report shared similar physiology to SAD.

Methods Our findings in these SAD patients are compared with randomly identified cases of genuine severe asthma.

Results ABPA, OB, follicular bronchiolitis and DIPNECH shared similar physiology in this project and are compared together as Non-Asthmatic SAD (NASAD). When compared to asthmatic SAD (ASAD), NASAD demonstrated a lower FEF_25–75% (NASAD range 4–26%, median 9%, 7 out of 8 < 18% v. ASAD range 8–32%, median 26.5%, 1 out of 6 < 18%) and a higher RV (NASAD range 74–203%, median 153%, 5 out of 8 > 120% v. ASAD range 66–121%, median 97.5%, 2 out of 6 > 120%). All patients, except one, demonstrated normal gas transfers strongly suggesting an absence of emphysema. HRCT alone did not provide any diagnostic benefit. Only asthma demonstrated significant and consistent bronchodilator reversibility. Tracheomalacia mimicked SAD, demonstrating an inconsistent pattern of FEF_25–75% and a small degree of variation of RV around normal. Horseshoe lung demonstrated similar physiology to NASAD.

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Abstract S40 Figure 1

Conclusions Clinicians need to consider the above mentioned diseases as an alternative diagnosis to emphysema and asthma in those with a characteristically SAD flow volume loop, a high RV and normal gas transfers.