Introduction
Wheezing related to lower respiratory tract infections (LRTI) is common in infancy, with approximately one in every three children experiencing at least one wheezing episode during the first three years of life.1,2 The first wheezing episode in infants aged under 24 months has traditionally been called bronchiolitis, although the age limit of bronchiolitis in most European countries is 12 months.3 Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis in wheezing infants younger than 12 months, whereas rhinovirus is common in those wheezing child aged 12-24 months.3
Previous birth cohort and post-bronchiolitis studies reported that bronchiolitis and wheezing in early childhood may have long-term effects on respiratory health.4,5 Wheezing and asthma symptoms, although common after bronchiolitis at preschool age, usually improve at school age. However, after puberty, the symptoms may recur, even in those without any symptom recurrence after bronchiolitis.4 Long-term follow-up studies demonstrated an increased risk of asthma and impaired lung function continuing until adulthood after viral wheezing in early childhood.6-9
The factors that influence the development of asthma are complex and remain, despite of active research, poorly understood.10 A familial history of asthma, especially in the mother, exposure to tobacco smoke in infancy and the presence of atopic dermatitis in early life were found to be common risk factors for asthma after wheezing in the first two years of life in both birth cohort and post-bronchiolitis studies.4,11 In young children, specific laboratory markers, such as eosinophilia and a high immunoglobulin E (IgE) level, as well as early-life sensitization to airborne allergens confirmed by skin prick tests (SPTs) or by measuring specific IgE to airborne allergens in serum samples, have been documented as predictors of asthma in later life in hospital-based follow-up studies.12
At preschool age, an asthma diagnosis is usually based on typical symptoms and asthma-predictive risk factors.13Therefore, a diagnosis of asthma in early childhood may be more uncertain than in later years (i.e. school age) when airflow obstruction can be confirmed by lung function tests.13
We followed up a cohort of 100 children aged less than 24 months at the time of hospitalization for viral wheezing episodes in Kuopio University Children’s Hospital, Finland in 1992–1993 14 to early adulthood. The aim of this study was to evaluate early childhood risk factors for asthma in later life (i.e. aged 17–20 years) in this prospectively followed cohort. An additional aim was to describe the evolution on asthma from infancy through to early adulthood after hospitalization for viral wheezing in early childhood.