Increased bronchial responsiveness to exercise as a risk factor for symptomatic asthma: findings from a longitudinal population study of children and adolescents
The prevalence of asthma seems to be on the increase and risk factors are not well-established. To investigate the relationship between asymptomatic increased bronchial responsiveness to exercise (BRE) and subsequent development of asthma, we studied a population sample of children and adolescents, aged 7-17 yrs at enrollment, examined twice 6 yrs apart (54 and 68%, respectively, of the original sample participated in the two examinations). Complete information was available for 383 subjects (191 males and 192 females). Case history and questionnaire data were obtained and were used to assess the presence or absence of symptomatic asthma. Exercise testing, performed at enrolment only, consisted of steady running on a 10% sloping treadmill for 6 min in a climate chamber. BRE was recorded as percentage fall in forced expiratory volume in one second (FEV1) from pre-exercise FEV1, within 15 min after exercise. Twenty subjects (5%) had asthma at enrolment, whereas 58 subjects (15%) had current asthma (i.e. symptoms within the preceding 12 months) at the time of follow-up, of whom 40 were asymptomatic at enrolment (new asthma). In general, increased BRE at enrolment was significantly associated with symptomatic asthma at follow-up. Analysing the data for subjects who were asymptomatic at enrolment (n=363), showed that new asthma at follow-up was not predicted by increased BRE at enrolment, whether BRE was evaluated as a continuous (R2=0.06, p=0.13) or dichotomous (cut-off: BRE =10% decline in FEV1; odds ratio (OR) 0.8 (0.4-1.2); p=0.11) variable. However, the data do not exclude the possibility that asymptomatic subjects with BRE >15% have an increased risk for subsequent development of asthma. The findings of the present study suggest that increased bronchial responsiveness to exercise in asymptomatic children and adolescents is not a strong predictor for subsequent development of symptomatic asthma. Further studies, preferably of larger samples, are needed to confirm these findings.