Pathological abnormalities in the peripheral membranous bronchioles less than 2 mm in diameter (i.e. small airways) is found in asthma patients across a spectrum of severity including the 'difficult-to-treat phenotype'.
So far no unanimously accepted method is available for small airway abnormality detection. Recent pharmaceutical developments have led to inhaler devices with smaller aerosol particles and systemic biologic treatments, enabling therapeutic drug delivery to the distal lung regions.
Small airway disease in asthma: pathophysiological and diagnostic considerations. Contoli M, Santus P, Papi A. Curr Opin Pulm Med. 2015 Jan;21(1):68-73.
Small-airway disease in asthma: pharmacological considerations. Usmani OS. Curr Opin Pulm Med. 2015 Jan;21(1):55-67.