Small airways in asthma

Click here to listen to Professor Leif Bjermer (Sweden) discuss the importance of small airway inflammation in asthma pathogenesis and its clinical correlates.

The small airways (<2mm diameter) account for the vast majority (98.9%) of lung volume¹ (Figure 1) and play a large part in the pathogenesis of asthma.² The reactivity of the small airways to both specific and nonspecific allergens is increased in asthmatic patients compared to normal subjects³ and in mild asthma, peripheral resistance can be several times higher than in normal subjects.^4,5 Small airway inflammation is also associated with nocturnal asthma.⁶

Figure 1: The small airways make up the majority of lung volume

Most clinical measures of asthma detect inflammation and airway obstruction in the larger airways. However, the inflammatory processes associated with the small airways may be more severe⁷ (Figure 2) and these airways also have an increased contractile response compared to larger airways.⁸ Read more on Asthma pathophysiology.

A number of pathological changes occur in the small airways of asthma patients which further illustrate the involvement of this lung region in the disease. These changes are described below.

Large numbers of immune cells and inflammatory mediators may be present in the small airways and eosinophils in particular are more numerous in the small airways than in the large airways of asthma patients⁹ (Figure 2).

Figure 2: Immunocytochemical cell markers in airways <2 mm and >2 mm in diameter from patients with asthma⁷

Airway remodelling is characteristic in the large airways of asthma patients, but this effect is not limited to the bronchi and bronchioles. Chronic inflammation also thickens the walls of the small airways^2,10 (Figure 3).

Figure 3: Pathological findings in the small airways and distal lung compartment of asthma patients²

Inflammation and structural remodelling in the small airways can have a large impact on lung function because these small airways account for the vast majority of lung volume (Figure 1). Therefore assessment of small airway function is important in the diagnosis of asthma. Read more on Diagnosing asthma.

To learn more about how to reach and treat the small airways with Alvesco^®, please see the Targeting small airways section.

 References:

1. Virchow JC. [Asthma--a small airway disease: concepts and evidence]. Pneumologie 2009;63 Suppl 2:S96-101.
2. Contoli M, Bousquet J, Fabbri LM, et al. The small airways and distal lung compartment in asthma and COPD: a time for reappraisal. Allergy 2010;65:141-151.
3. Wagner EM, Bleecker ER, Permutt S, et al. Direct assessment of small airways reactivity in human subjects. Am J Respir Crit Care Med 1998;157:447-452.
4. Hyde DM, Hamid Q, Irvin CG. Anatomy, pathology, and physiology of the tracheobronchial tree: emphasis on the distal airways. J Allergy Clin Immunol 2009;124:S72-7.
5. Wagner EM, Liu MC, Weinmann GG, et al. Peripheral lung resistance in normal and asthmatic subjects. Am Rev Respir Dis 1990;141:584-8.
6. Kraft M, Pak J, Martin RJ, et al. Distal lung dysfunction at night in nocturnal asthma. Am J Respir Crit Care Med 2001;163:1551-6.
7. Hamid Q, Song Y, Kotsimbos TC, et al. Inflammation of small airways in asthma. J Allergy Clin Immunol 1997;100:44-51.
8. Mitchell HW, Cvetkovski R, Sparrow MP, et al. Concurrent measurement of smooth muscle shortening, lumen narrowing and flow to acetylcholine in large and small porcine bronchi. Eur Respir J 1998;12:1053-1061.
9. Carroll N, Cooke C, James A. The distribution of eosinophils and lymphocytes in the large and small airways of asthmatics. Eur Respir J 1997;10:292-300.
10. Carroll N, Elliot J, Morton A, et al. The structure of large and small airways in nonfatal and fatal asthma. Am Rev Respir Dis 1993;147:405-10.