Clinical patterns and natural history of asthma.

Childhood asthma typically begins in infancy with a respiratory syncytial virus (RSV) infection. Although the majority of infants become infected with RSV, lower respiratory illness develops in only about 20%. About 25% to 50% of those subsequently experience recurrent acute asthma from viral respiratory infections (VRI). Children younger than 5 years have a high frequency of VRI and have the highest frequency of hospitalization for asthma of any age group. In a 35-year study of the natural history of asthma, 20% of 7-year-old children were found to have asthma, but most had only episodic illness with VRI. The majority of those children improved with age, but a substantial minority continued to have recurrent episodes as adults, generally induced by VRI or exercise. Persistent asthma developed in only a few. Children who had symptoms of asthma without VRI were more likely to continue having frequent episodic or chronic asthma as adults. Despite generally suboptimal treatment during the 35 years of the study, forced expiratory volume at one second did not deteriorate over time; it remained normal in children who had only episodic asthma, and it was consistently low in the children with severe, persistent asthma.