Causes/pathogenesis

Environmental

Asthma results from an interaction between different environmental and genetic factors. The environmental influences begin during pregnancy: allergic sensitisation has been described before birth, and several studies have demonstrated reduced lung function in newborn infants of smoking mothers compared to those of nonsmoking mothers. Smoking increases the risk of both asthma and poorer lung function throughout childhood. All children should have the right to an environment free from tobacco smoke products both before and after birth.

Lifestyle changes have been linked to the increased prevalence of asthma, and especially allergic asthma. Studies from Russian and Finnish Karelen show a much higher prevalence of asthma and allergic diseases in the Finnish population compared with the Russian.

Respiratory virus infections are the major cause of acute bronchiolitis in infancy and of acute asthma attacks among older asthmatic children (more on childhood viral infections can be found in chapter 16). 1.5–2% of all children are hospitalised due to respiratory syncytial virus (RSV) bronchiolitis during the first 2 years of life, and approximately 60% of these children later develop asthma. At 13 years of age, more than 40% of children hospitalised in infancy with acute RSV bronchiolitis still have symptoms and bronchial hyperresponsiveness. From 2 years of age, rhinovirus infections are the most frequent precipitators of acute asthma. With modern techniques of virus diagnosis (e.g. those based on PCR), approximately 65% of all asthma attacks in schoolchildren have been reported to be due to rhinovirus infection, and when all types of virus infections are included it has been estimated that 85% of acute asthma attacks are precipitated by respiratory virus infections.

From 2 years of age and especially during school years, inhalant allergy becomes increasingly important for childhood asthma. Approximately 60% of all school-aged asthmatic children are allergic. The most important allergens vary according to climate, but in all European countries animal dander is among the most frequent allergens in asthma. In a warm and humid climate, house dust mites and moulds are also of major importance, and, depending upon climate, the seasonal allergens (birch, grass and mugwort pollen) play a role. Allergen exposure may cause acute asthma exacerbations, and even in the absence of an exacerbation, may increase airway inflammation and bronchial hyperresponsiveness.

Allergens may be encountered both outdoors and indoors, and house dust mites and animal dander are particularly important perennial indoor allergens. Occupational agents play a minor role during childhood, but several types of allergy may influence the choice of education in relationship to later working life. Kindergartens and schools are the working environment of children, and the need for a healthy indoor environment in such institutions should be emphasised. Special consideration should be given to the increased risk of respiratory infections, especially in kindergartens. In schools, precautions may be taken to reduce allergen exposure for allergic asthmatic children. Emphasis should also be put upon mastering exercise-induced asthma in gymnastic lessons and physical training.

Genetic

Asthma, and one of its major causes, allergy, have strong hereditary traits. During recent years, much effort has been put into genetic family studies in order to identify genetic markers. A large number of markers with possible relationships to asthma and airway inflammation have already been identified, but these vary between populations. There has also been increased focus upon epigenetics: the finding that environmental influences may cause DNA methylation and histone formation, and thus change and inactivate the influence of specific genes, has given insight into how the environment may interact with genes, and has shown that this interaction may even be transferred from mother to child.

Furthermore, hereditary traits have been found to influence the response to asthmatic drugs. Examples include β2-receptor sensitivity and responsiveness to inhibitors of leukotriene synthesis.

Exercise

Throughout childhood, but increasingly during school age, exercise is an important cause of asthma exacerbations (exercise-induced asthma). It has been reported that 30% of all asthmatic children suffer from restriction of physical activity and it is very important to teach asthmatic children to master exercise, by education, advice related to ‘warming up’ and medical treatment.

See the entire Childhood asthma Chapter