Recognition of Small Airways Obstruction in Asthma and COPD - The Road Less Travelled

Abstract

The small airways, once regarded as the silent zone in the air conducting system of the lungs are now known to be one of the initial sites of involvement in diseases like asthma and Chronic Obstructive Pulmonary Disease (COPD). Identification of the involvement of distal airways in these diseases is often difficult to assess, clinically as well as by conventional pulmonary function tests and therefore, usually remains undiscovered in early stages. Early recognition of their involvement in asthma and COPD and timely management may reduce long term morbidity in these conditions. This article aims to highlight the relatively lesser recognized facts about small airways involvement in asthma and COPD and role of imaging and newer modalities for detection.

Keywords: High resolution computed tomography; Mosaic attenuation; Pulmonary function tests.

[Table/Fig-1]:

Contrast enhanced computed tomogram of the thorax showing; a) bronchiolar wall thickening and bronchiolectasis (arrows) representing the small airway involvement and; b) multiple subtle linear branching opacities in the periphery (arrowheads) seen as a direct sign of small airway disease.

[Table/Fig-3]:

High resolution CT thorax showing sub pleural branching structures represent mucus filled small airways tree- in- bud appearance (arrows).

[Table/Fig-4]:

a) HRCT thorax of a patient with COPD displaying focal areas of hyperlucency suggestive of air trapping in the right lung (arrowheads) with linear and branching small peripherally located opacities (arrow) representing distal airways obstruction; b) Non contrast CT in a patient with severe COPD displaying diffuse involvement of small airways due to which the mosaic pattern is not appreciated and there is diffuse hypoattenuation of the lung (arrows). Also noted are the direct signs of nodular and linear branching opacities on the right side (dashed arrow); c) Bronchial wall thickening noted in the same patient.

[Table/Fig-5]:

HRCT thorax images: a) Expiratory scan of an asthmatic patient showing segmental areas of air-trapping (arrow) seen as hyperlucent areas bilaterally; b) Same patient having lobular areas of air trapping on the left side also (arrow). A few scattered centrilobular nodules peripherally (dashed arrow) representing small airway obstruction; c) Expiratory scans of a poorly-controlled asthmatic patient showing patchy lobular areas of air trapping (arrow). Note the thin linear branching opacities seen in the periphery representing obliteration of small airways (arrowheads).

[Table/Fig-6]:

HRCT thorax showing bilateral bronchiectasis (dashed arrow) with bronchiolectasis with peripheral branching and linear densities (arrows) and associated emphysema (arrowheads) in the left lung suggestive of small airway involvement in a patient with COPD.