title: Causal direction between respiratory syncytial virus bronchiolitis and asthma studied in monozygotic twins.
abstract: BACKGROUND
  Respiratory syncytial virus (RSV) bronchiolitis has been associated with later development of asthma, wheezing, abnormal pulmonary function, and sensitization. Our aim was to determine the differential effect within monozygotic (MZ) twin pairs discordant for severe RSV bronchiolitis in infancy on the subsequent development of asthma, pulmonary function, and allergy.
  
  
  METHODS
  Thirty-seven MZ twin pairs discordant for RSV hospitalization in infancy (mean age 10.6 months) were compared at the mean age of 7.6 years for lung function, bronchial responsiveness, fractional exhaled nitric oxide (Feno), asthma diagnosis, use of asthma medication, and skin prick test to common inhalant allergens.
  
  
  RESULTS
  There were no differences within MZ twin pairs discordant for RSV hospitalization in infancy with respect to pulmonary function, Feno, asthma prevalence, asthma medication use, or sensitization (P > .1 for all comparisons).
  
  
  CONCLUSIONS
  We found no differential effect from severity of RSV infection on the development of asthma and allergy in MZ twin pairs discordant for RSV hospitalization in infancy. This argues against a specific effect of severe RSV infection in the development of asthma and allergy. Because of the small sample size, this study must be considered as a hypothesis-generating study.
authors.affiliation: Danish Pediatric Asthma Center, Copenhagen University Hospital, Gentofte, L213 Ledreborg alle 34, DK-2820 Copenhagen, Denmark.
authors.forename: Porntiva
authors.initials: P
authors.lastname: Poorisrisak
authors.forename: Liselotte Brydensholt
authors.initials: LB
authors.lastname: Halkjaer
authors.forename: Simon Francis
authors.initials: SF
authors.lastname: Thomsen
authors.forename: Lone Graff
authors.initials: LG
authors.lastname: Stensballe
authors.forename: Kirsten Ohm
authors.initials: KO
authors.lastname: Kyvik
authors.forename: Axel
authors.initials: A
authors.lastname: Skytthe
authors.forename: Peter Oluf
authors.initials: PO
authors.lastname: Schioetz
authors.forename: Hans
authors.initials: H
authors.lastname: Bisgaard
mesh_terms: D001249:Asthma / Q000175:diagnosis / Q000209:etiology*; D001990:Bronchiolitis, Viral / Q000150:complications*; D002648:Child; D002675:Child, Preschool; D004200:Diseases in Twins; D005260:Female; D006801:Humans; D006967:Hypersensitivity / Q000175:diagnosis / Q000209:etiology; D008297:Male; D012042:Registries; D012129:Respiratory Function Tests; D018357:Respiratory Syncytial Virus Infections / Q000150:complications*; D018113:Respiratory Syncytial Virus, Human; D014430:Twins, Monozygotic