**4. Pathogenesis of common symptoms in obstructive bronchopathies**

The mechanisms by which the symptoms of asthma occur in preschool children do not differ from those that cause the process at any age. The permeability of the bronchial lumen is maintained by a neuro-chemical mechanism, but airway calibre may be affected either by an imbalance in this mechanism or by the intervention of certain cells and their biochemical mediators involved in the inflammatory reaction, usually triggered in childhood by an allergic reaction or an unfavourable home environment. In the obstruction of the wider bronchi, the dominant mechanism is the constrictor one, of neuro-chemical cause, whereas in the peripheral bronchi (small airways), inflammation is the greatest cause, through cellular mediators that are also constrictors.

BHR and inflammation are the pathogenetic basis of asthma. BHR is usually present already in the newborns of atopic families, although it is true that certain exogenous harmful agents can increase it. Similarly, the regular or intense exposure to these pollutants and also viral infections can cause broncholability in non-predisposed children. This increased bronchola‐ bility occurs as the result of inflammation that such elements, including allergens, produce in the bronchial mucous membrane. These facts are demonstrated in adults and children from school age, but in preschool children under five, the immediate influence of these exogenous factors is less certain. The injured bronchial epithelium is restored after the aggression that leads to crises of dyspnoea or wheezing, and the permanence of the injury can depend on the intensity of the aggression and the repetition of the same, leading to more or less severe and repeated symptoms. Therefore, there is doubt whether the inflammatory reaction is establish‐ ed from the onset of the first symptoms or whether that permanence occurs after the recurrence of the crisis or after the most serious crises, on which the therapeutic approach may depend. Various conventional methods are used to study bronchial inflammatory reactions, although in younger children, it is not always easy to conduct them. The most common are the following:

Direct methods: (1) Cell study from sputum obtained by bronchoalveolar lavage (BAL), induced sputum, forced cough or aspiration. (2) Bronchial biopsy.

Indirect, non-invasive methods: (1) Exhaled nitric oxide measurement. (2) Exhaled breath condensate: evaluation of several measurements of the inflammation. (3) Blood eosinophilia assessment and eosinophil cationic protein (ECP) in serum levels.

The inflammatory reaction is a key event in the pathogenesis of asthma, as it is evident in adults and children from school age. It seems certain that inflammation is present even in the first episodes of dyspnoea in children who later go on to develop allergic asthma, but there is no unanimity in the recognition of this fact. In the work of Maclennan et al. [14] in preschool children with episodes of severe dyspnoea repeated between 2 and 12 times in the previous year (it is known that from the third episode, asthma can be diagnosed in young children), it is found that a percentage of them had an increased serum IgE levels although it is not significant in relation to a control group.

It is needed to know whether the persistence of inflammation and its intensity corresponds with the frequency and severity of the crisis, that is, if the mucosal lesions are restored after mild episodes that occur at long intervals of time, with implications in the therapeutic approach aimed at preventing recurrences.
