Asthma phenotypes in children
Although the term “phenotype” was proposed as early as the beginning of the 20th century, the process of identifying asthma phenotypes and creating classification criteria for asthma on this basis was not entirely straightforward.
Asthma phenotypes in children
Problems of pathophysiological heterogeneity and clinical diversity of various forms of bronchial asthma (BA) have been actively developed by domestic scientists and are reflected to a greater or lesser extent in various classifications, and the current stage of BA study is marked by the inclusion of the concept of heterogeneity directly in the definition of the disease: “Asthma is a heterogeneous disease, is characterized, as a rule, by chronic inflammation of the respiratory tract.
Phenotyping of the disease and associated endotyping becomes a key factor in the choice of the optimal therapy for asthma, allows to provide a differentiated approach to treatment, development of individual therapy schemes for the disease and effective algorithms for the use of drugs.
The phenotype (from the Greek word phainotip – I show, I find) is considered as a set of characteristics inherent in an individual at a certain stage of development, a kind of “removal” of genetic information towards environmental factors; these are clinically observed patterns, clinical and morphological findings, and a response to treatment. A phenotype is formed on the basis of a genotype mediated by a number of external environmental factors, which is what we see and may be able to change. The endotype describes the key pathogenetic mechanisms for a particular asthma phenotype.
Age is one of the main factors that form the asthma phenotype and includes pathophysiological events, exposure of allergens and triggers, and changes in the natural course of the disease. Important aspects of heterogeneity of childhood asthma and its course in different age groups have been reflected in modern clinical guidelines.
Age periodization in clinical guidelines for children’s asthma is based on the peculiarities of the disease and is based on practical convenience, with the following age groups being distinguished: infants (0–2 years old), preschool children (3–5 years old), schoolchildren (6–12 years old), adolescents. It should be noted that there is no official international definition of the exact age for adolescents. The term “adolescents” is not mentioned in international conventions, but the United Nations recognizes adolescents as persons aged 10 to 19, i.e. those whose age is limited to the second decade of life. Modern science defines adolescence by country, region, culture and nationality, and sex (10–12 to 17–18 years). The majority of works on age grouping indicate that the teenage age of boys lasts from 13 to 16 years, and of girls – from 12 to 15 years; the younger teenage age is 11–13 years; the older teenage age is 14–15 years, as well as the teenage age from 17 to 21 years for boys and from 16 to 20 years for girls, i.e. the time limits of the teenage age (17–21 years) are also differentiated by sex. Girls and girls enter these periods of development a year earlier and complete them earlier. This is due to the impact of gender on the intensity of growth and development. Each of these periods of development has its own characteristics. The proposed age periodization is based on the biological principle: this period covers the period from the beginning of puberty to the moment when the young body acquires the ability to effectively reproduce (transition, or puberty).
The development of perceptions of the changing characteristics of asthma in different age periods is reflected in the harmonized report of the Association of Allergists Practitioners (PRACTALL). The document notes that different BA phenotypes can be determined based on the age of the child and the triggering factors of the disease. Recognition of these different phenotypes and severity of the disease can help to better evaluate prognosis and therapy strategies . The approach to the determination of AD phenotypes in children over 2 years of age, as outlined in the harmonized report, is illustrated in Figure 6. The predominance of a particular phenotype is useful for solving diagnosis and treatment problems, but the phenotypes can be combined to produce overlapping phenomena.
The principles of differentiation of bronchial asthma phenotypes according to these guidelines apply to pre–school and school–age children and include a virusesinduced phenotype; exercise induced asthma; allergic (allergen–induced) asthma phenotype; and bronchial asthma of unknown etiology. The most frequent and visible phenotype among older children is allergic (atopic) bronchial asthma. The absence of a specific allergic trigger may indicate a phenotype of non–allergic bronchial asthma. However, clinicians should be cautious about the likelihood of this phenotype, as failure to identify an allergic trigger phenomenon may only be an indication that it has not been detected.
Asthma phenotyping problems in adolescents have long been the focus of attention, but so far there is no complete understanding of the formation and stability of phenotypes, especially in the absence of longitudinal studies. There are no stated distinctive features of phenotypes in terms of severity or duration of the disease. The phenotype covers the clinically significant properties of the disease, but does not necessarily relate these characteristics to the etiology and pathophysiology of the disease.
Phenotypes of AD differ in severity, treatment efficacy, risk of adverse outcomes, risk of progressive loss of lung function and formation of irreversible bronchial obstruction. The notion of asthma as a single disease is becoming a thing of the past, and more and more researchers see asthma as a clinical syndrome that requires not only phenotypic detail, but also an analysis of age dynamics, assessment of evolution, and prognosis of disease progression.