Asthma is the most common and important chronic non-infectious disease in childhood and its prevalence has increased in recent years.1 However, information on childhood asthma prevalence research covering Portuguese young children is scarce and has not been compiled. Therefore, as part of the INAIRCHILD project,2 this letter aimed to review the major scientific findings of the most recently published papers on childhood asthma prevalence in Portuguese preschool aged children.

Articles published from 2005 to date were identified in the following on-line databases: Science Direct, Scopus, PubMed, and Google Scholar. Only studies in press or fully published (in English and/or Portuguese) were analyzed. Although 8 studies were found published in the last 10 years on the assessment of childhood asthma prevalence in Portuguese young children, only two of them focused on preschool aged children.

Thus, this letter reviews those only two published papers, one recently published3 and the other one published 5 years ago,4 concerning study design, methodologies used, prevalence rates and risk factors.

Table 1 summarizes the main characteristics of the two studies reviewed. Both of these studies were cross-sectional and they were conducted in big urban areas near the coast, although the study by Branco et al.3 was also conducted in rural areas away from the coast (Bragança district) providing comparisons between different geographical, social and demographic contexts. Different study population sizes were considered, but the aim was the same. Different methodologies were used to recruit the study population and to obtain health information, although in both ISAAC-derived questionnaires were used, which provided estimates of asthma prevalence based on symptoms and asthma previously diagnosed. However, the comparison of asthma prevalence results between the two studies is difficult due to different criteria used for asthma diagnosis, namely: (i) combination of answers given by the patient about respiratory symptoms and the physician's best knowledge of the patient's asthma status4 and (ii) wheezing and dyspnea simultaneously mentioned in the absence of upper respiratory infections.3 In fact, de Sousa et al.4 assumed they had found a lower prevalence for preschool age than expected due to diagnostic problems (the diagnostic procedures were based on a combination of the answers given by the patient on respiratory symptoms and the physician's best knowledge of the patient's asthma status). Thus, the difference between the asthma prevalence values found in the two studies should be interpreted with caution, because it could possibly be due to different methodologies used and different criteria considered for asthma diagnosis. Moreover, other factors like children's ages and environmental contexts were also different, which could have contributed to differences in the prevalence results.

In these two studies medical exams, like exhaled nitric oxide or spirometry, to confirm asthma diagnosis were not performed. Although not very common in preschool children, spirometry seems to be feasible in that age.5 It has been used worldwide to confirm asthmatic symptoms reported in questionnaires,6 which is very important to increase the robustness of the conclusions.

Risk factors for childhood asthma prevalence, namely demographic, environmental, psychosocial and clinical risk factors, were only studied in Branco et al.3 which concluded that living in a non-rural location, being male and having an asthmatic parent were risk factors for childhood asthma development in Portuguese preschool children. However, other important risk factors for childhood asthma development already reported in the literature for primary school aged children were not considered, namely exposure to air pollution.7

These findings led to the conclusion that asthma studies on Portuguese preschool children are rare and limited due to methodological constraints, namely obtaining precise health information and performing medical exams to confirm symptoms. More studies are needed, particularly in preschool age children living both in urban and rural areas. Those studies should include medical exams to confirm asthma symptoms. The study of lifestyle risk factors must be associated to others already studied in primary school children.