Small airways dysfunction: associated factors, clinical aspects and new perspectives.

Scientific interest in small airways emerged in the 1960s. However, despite this early recognition, they long remained a "silent" area of the lung. In this nonsystematic narrative mini-review, we evaluated the different spirometry thresholds used to define small airways dysfunction (SAD), comparing them with other diagnostic techniques. We then examined the various factors associated with SAD, focusing on SAD as defined by spirometry.Maximal mid-expiratory flow, especially below the lower limit of normal (LLN), appears to be the most accurate and reproducible spirometric measure to assess SAD. However, no gold-standard spirometric definition has been endorsed by respiratory academic societies, and forced expiratory volume in 3 s or 6 s <LLN or combined forced expiratory flow could be alternative criteria. Impulse oscillometry or computed tomography scanning represent possible alternative assessment methods, with limitations regarding reproducibility.Globally, prevalence of SAD varies between regions. Air pollution appears to be associated with a higher prevalence of SAD, particularly particulate matter with aerodynamic diameter <2.5 µm. Both active and passive smoking are associated with SAD. SAD is also strongly associated with COPD and the risk of developing it, making SAD a marker of accelerated decline in lung function. Several occupational exposures are associated with SAD, including exposure to organic and inorganic dust. A history of lung infections, especially tuberculosis, is also associated with SAD. Advanced age may also be an associated factor, although the association depends on the criteria used to define SAD. Cardiac events have been associated to SAD, which might explain the association between SAD and higher mortality risk.By integrating these different associations in clinical assessment, it would be possible to screen more reliably and preventively for SAD. This would ensure closer surveillance, which is justified given the strong association with COPD.