Asthma is a common condition in children and adolescents. Recently, the Global Asthma Network (GAN) reported an asthma prevalence of 12.8% for children and adolescents [1]. Although most children and adolescents have mild-moderate asthma, there is a group of severe asthmatics whose evolution is associated with greater morbidity, mortality, health costs, and persistence of the disease into adulthood [2, 3]. A recent meta-analysis conducted in Europe reported a prevalence of severe asthma in children and adolescents of 3% with no significant differences by gender [4]. In 2014, American Thoracic Society/European Respiratory Society (ATS/ERS) consensus was published, defining severe asthma for children, adolescents, and adults. ATS/ERS consider severe asthma as a condition requiring high doses of inhaled corticosteroids (ICS), plus a second controller drug or use of systemic corticosteroids [5]. The latest update of the Global Initiative for Asthma (GINA) proposes a retrospective classification to define the severity of asthma, which considers the treatment necessary to control symptoms and exacerbations for at least two or three months, allowing the identification of patients with asthma refractory to treatment with high doses of ICS plus long-acting bronchodilators, and who could benefit from biological therapy [6]. The term problematic severe asthma has gained acceptance in recent years, as it allows for a personalized clinical approach. Problematic severe asthma remains symptomatic despite maximum therapy and can be divided into severe treatment-resistant asthma (STRA), difficult asthma (DA), and refractory DA (RDA) [7]. DA can have multiple causes that can coexist, including poor adherence to treatment and/or poor inhalation technique, the presence of untreated comorbidities, exposure to allergens or environmental pollutants, and even psychological stress [8]. When all modifiable factors in DA have been identified and controlled and the patients remain symptomatic nonetheless, we are in the presence of STRA, which is a type of asthma resistant to corticosteroids due to variable mechanisms and which requires further study [9]. Figure 1 shows the categories of problematic severe asthma.

The rapid advancements of precision medicine in pediatric asthma have allowed us to identify phenotypes, endotypes, genetic markers, and biomarkers that have helped us better understand the mechanisms of severe asthma [10]. This has also allowed the advancement of personalized therapy in severe pediatric asthma and the increasingly safe and effective incorporation of biological drugs [11]. This article aims to analyze the evidence in the diagnosis of severe asthma in children and adolescents in recent years.

A review of the scientific evidence on severe pediatric asthma between October and November 2024 was conducted. Studies from the last five years were collected from different scientific databases (Medline, Web of Science, EBSCO Host, Science Direct, and SCOPUS databases). Two categories were used for the search: i) pediatric or childhood and ii) severe or difficult-to-treat asthma. The articles found were divided into five groups: genetic markers, biomarkers, lung function, radiological techniques, and bronchoscopy (BC).

The study of the human genome has allowed the discovery of new genes involved in pathogenesis, interaction with the environment, and response to treatment in pediatric severe asthma [12]. Genome-wide association studies (GWAS) performed in children and adolescents with asthma of different ethnicities have demonstrated the association between single nucleotide polymorphisms (SNPs) or novel genetic loci with severe asthma exacerbations [13–15] and severe early-onset asthma [16, 17]. An investigation conducted in children and adolescents with asthma demonstrated that PRKG1 gene expression is related to variability in response to short-acting β₂-adrenergic receptor agonists (SABAs) in certain populations such as African Americans or Latinos/Hispanics [18]. Another similar study found that the expression of the DNAH5 gene (related to the movement of respiratory cilia) is associated with the variability of the bronchodilator response (BDR) in Puerto Rican children and adolescents [19]. The relationship between genetics and the response to ICS has also been the subject of study in recent years. New age-dependent genetic polymorphisms associated with the response to ICS have been identified in adult and pediatric asthmatic patients, such as THSD4, HIVEP2, DPP10, and HDAC9, which would be related to asthma susceptibility, severity, and exacerbations [20]. In the peripheral blood mononuclear cells (PBMCs) of children and adolescents with difficult-to-control asthma, decreased expression of the alpha corticosteroid receptor and a lower in vitro corticosteroid response have been found than in easily controlled asthmatics [21]. The microRNAs (miRNAs) are small RNAs (20–24 nucleotides) involved in the pathogenesis of some chronic pediatric diseases such as asthma [22]. Intercellular communication of miRNAs is carried out through exosomes, which are small extracellular vesicles that allow miRNAs to participate in the processes of inflammation, hyperreactivity, and airway remodeling in asthma [23]. The CACNA2D3 and WNT5A intergenic region has been associated with exacerbations in children and young adults, despite a history of treatment with ICS [24]. It has also been shown that blood miRNAs in children and adolescents with asthma are associated with a poorer response to treatment with ICS [25]. Another study in children and adults created a polygenic machine learning predictive model for corticosteroid response, which could provide more information for clinical decision-making [26]. A miRNA sequencing study in children and adolescents with asthma identified two circulating miRNAs (miR-1246 and miR-200b-3p) that were associated with the prediction of BDR and that were also replicated in cohorts of adult asthmatics, which would help to establish differences in response to treatment [27]. Elevated levels of two miRNAs (miR‐378a‐3p and miR‐144‐3p) have also been found in children and adolescents in patients with increased use of rescue bronchodilators, a condition that is associated with uncontrolled asthma with more symptoms [28]. In another study, five miRNAs (miR-532-3p, 296-5p, 766-3p, 7-5p, and 451b) were found to be associated with severe exacerbations of pediatric asthma and severe exacerbations in chronic obstructive pulmonary disease (COPD) in adults, possibly indicating that there are common genomic mechanisms between both conditions [29]. A study with miRNAs such as miR-210-3p has been negatively associated with regulatory T cells (Tregs), which are cells that participate in maintaining the balance of the allergic response. In addition, miR-210-3p proved to be a genetic marker that could be used to predict the severity of pediatric asthma [30]. In children with severe asthma, gene regulatory enhancers in leukocytes have been identified as different from those in mild asthmatics [31]. Epigenome-wide association study (EWAS) in asthma allows medics to analyze the interaction between genetics, environment, and response to treatment. Recently the DNA methylation (DNAm) in blood cells, nasal and bronchial epithelium has been related to the variability of the BDR in children and adolescents with moderate to severe asthma [32–34]. A study that analyzed the rs4986791 polymorphism of the toll-like receptors (TLRs) showed that pediatric patients with moderate asthma and heterozygous genotype more commonly developed severe asthma than patients with homozygous genotype [35]. In adolescents with severe asthma, the interleukin-10 (IL-10) polymorphisms rs3024498 were associated with poor control and IL-17 polymorphism rs3819024 genes with lower BDR [36]. Finally, the +2044G>A polymorphism of IL-13 gene was associated with the severity of pediatric asthma [37].

With the advancement of precision medicine, the use of biomarkers is attaining increasing importance in the identification of endotypes (type 2 and non-type 2), prediction, and monitoring of response to new treatments (e.g., biological) in severe pediatric asthma [38, 39].

The usefulness of lung function in severe asthma monitoring is fundamental. In an observational cohort of children and adolescents with asthma, it was shown that males with recurrent exacerbations have a high risk of a lower forced expiratory volume in 1 second (FEV₁) across childhood [56]. A study conducted in children and adolescents compared lung function in severe and non-severe asthmatics and found that the maximum response to bronchodilators found more frequently in severe asthma was associated with more asthma exacerbations, hospitalizations, worse control, and decreased lung function at one year of follow-up [57]. A retrospective study comparing children with severe asthma, non-severe asthma, and healthy children showed that the FEV₁/forced vital capacity (FVC) ratio was significantly decreased and the lung clearance index (LCI) significantly higher in severe asthmatics at the start of treatment, after a year of treatment, most severe asthmatics had significantly improved spirometric abnormalities, but the LCI remained elevated, reflecting the inhomogeneity of ventilation in this group of patients [58]. LCI was found elevated in 20% of children with severe asthma and was negatively correlated with FEV₁ z-score. Severe asthmatics had significantly higher mean LCI than mild-moderate asthmatics and healthy controls [59]. Furthermore, it has been identified that within children with problematic severe asthma, the LCI of those with STRA is significantly higher than those with DA or healthy controls, which could suggest that the LCI could be a marker of STRA and the 65% of children with STRA registered a decrease in LCI after treatment with parenteral triamcinolone, which was only consistent with FeNO, but not with traditional spirometry parameters [60]. Table 1 summarizes the clinical utility of genetic markers, biomarkers, and lung function in severe pediatric asthma.

In recent years, the use of BC in severe pediatric asthma has had a resurgence, mainly to rule out alternative diagnoses, search for germs associated with the persistence of symptoms, or in situ measurement of inflammatory markers in patients undergoing treatment with high doses of steroids, or to identify candidates for the use of biologics [69]. In some centers where BC is routinely performed in severe pediatric asthma for diagnostic and therapeutic purposes, it has been found that persistent bacterial bronchitis, trachea or bronchomalacia, and anatomical abnormalities are the most frequently found findings [70]. When BAL is added to BC, the diagnostic yield of bacterial diseases increases, but it should be used with caution in patients with known tracheobronchomalacia or very symptomatic asthma, conditions in which worse tolerance to BAL has been reported [71]. A study in children and adolescents with severe asthma found 4 granulocytic patterns on BC with BAL: 52% paucigranulocytic, 22% mixed granulocytic, 15.9% isolated neutrophilic, and 9.5% isolated eosinophilic [72]. Patients with a mixed granulocytic or isolated eosinophilic pattern had greater clinical morbidity with greater use of systemic corticosteroids, whereas those with a paucigranulocytic or isolated neutrophilic pattern had less airflow limitation and less blood eosinophilia. Furthermore, patients with an isolated neutrophilic pattern had a greater number of bacterial pathogens isolated [72]. This last finding could be related to another investigation that BAL carried out in children and adolescents with severe asthma, finding that the predominantly neutrophilic group had proinflammatory neutrophils with greater survival and dysfunctional macrophages that contribute to perpetuating airway inflammation, deterioration of innate immunity, and susceptibility to infections [73]. BC with BAL has also helped to better understand bronchial inflammation at the molecular level. A study conducted in schoolchildren with severe asthma who used high doses of inhaled and often systemic corticosteroids showed that predominantly neutrophilic BAL is associated with a greater expression of a mixed pattern of cytokines and chemokines (Th1/Th2/Th17) in contrast to paucigranulocytic BAL, which was associated with a reduced expression of these cytokines/chemokines [74]. In schoolchildren and adolescents with severe asthma who underwent endobronchial biopsy, it was shown that both submucosal eosinophilic and neutrophilic infiltrations were associated with decreased lung function in spirometry [75]. EOs infiltration was associated with better BDR and neutrophilic infiltration was related to fixed obstruction. However, the presence of intraepithelial neutrophils was associated with better lung function, while bronchial CD8 + T cells were related to better asthma control and lower cytokine expression in sputum [75]. Ultimately, a study using BC with BAL in children and adolescents with asthma of varying severity found higher levels of thymic stromal lymphopoietin (TSLP) in severe asthma than in mild to moderate asthmatics. In the same study, the high TSLP group was found to have higher concentrations of cytokines (IL-5 and IL-1β), and lower average of the FEV₁/FVC ratio than the low TSLP group [76]. Table 2 summarizes the clinical utility of CT, MRI, and BC in severe pediatric asthma.

Genetic markers, biomarkers, lung function, radiological techniques, and BC are supportive tests and/or procedures for pediatric respiratory physicians to identify the endotype and/or phenotype of the child or adolescent with severe pediatric asthma.

Personalized medicine in severe pediatric asthma involves having the ability to understand the underlying mechanisms of the pathogenesis of each patient to initiate the most appropriate specific treatment for their disease. There are important limitations that must be considered when indicating these diagnostic studies in severe pediatric asthma. Firstly, the accessibility and cost of certain procedures are limited, requiring a rigorous selection process for eligible patients. The group of problematic severe asthmatics that have the greatest benefit from these studies is the STRA, which remains symptomatic despite maximum treatment and interventions in other modifiable factors. Likewise, another limitation that must be considered is the intricacy of carrying out the different studies. For example, performing a blood count to quantify EOs has a low degree of complexity, unlike others such as genetic markers, which require a specialized laboratory or MRI with hyperpolarized noble gases, which must have a protocol and trained personnel to administer the gas doses according to the expected lung capacity of each patient.

Despite all these limitations, diagnostic studies in severe pediatric asthma are useful in patients in whom it is necessary to know the phenotype/endotype, monitor disease activity, and select the treatment to reduce economic costs and/or adverse effects of unnecessary treatments. Ultimately, the development of diagnostic methods opens the way to improve the indication of existing therapies (e.g., biologicals) or the development of new therapies in the future.