Respiratory syncytial virus and other respiratory tract viruses lead to common colds in most infants, whereas a minority develop acute severe bronchiolitis often requiring hospitalization. We hypothesized that such an excessive response to respiratory tract viral infection is caused by host factors reflected in pre-existing increased bronchial responsiveness.

We sought to compare bronchial responsiveness and lung function in 1-month-old neonates who later develop acute severe bronchiolitis with those who do not.

We measured infant lung function (n = 402) and bronchial responsiveness to methacholine (n = 363) using the raised-volume rapid thoracoabdominal compression technique before any respiratory symptoms in 1-month-old neonates from the Copenhagen Prospective Study of Asthma in Childhood birth cohort born to mothers with asthma. The children were prospectively monitored for respiratory symptoms and given a diagnosis of acute severe bronchiolitis according to a fixed algorithm.

Thirty-four (8.5%) infants had acute severe bronchiolitis before 2 years of age, 21 (62%) were hospitalized, and 23 (67%) of the cases were associated with respiratory syncytial virus. Children who later had acute severe bronchiolitis irrespective of viral species had a 2.5-fold increased responsiveness to methacholine (provocative dose of methacholine producing a 15% decrease in transcutaneous oxygen pressure [PD₁₅]) at age 1 month compared with control subjects (median PD₁₅ in cases vs control subjects, 0.13 vs 0.33 μmol; P = .01), whereas differences in baseline airflow were not significant for forced expiratory volume at 0.5 seconds (mean z score for cases vs control subjects, −0.18 vs −0.01; P = .36) and forced expiratory flow at 50% of forced vital capacity (mean z score for cases vs control subjects, −0.37 vs −0.09; P = .13).

Bronchial hyperresponsiveness in at-risk neonates precedes acute severe bronchiolitis in response to infections with respiratory tract virus.