Lung function measurements in children - copsac

wheeze compared with controls (162). This supports the belief that RSV is the cause of
respiratory illness in infancy.
A more recent publication based on 8,280 twin pairs showed that a model in which
asthma “causes” RSV hospitalization fitted significantly better than a model in which
RSV hospitalization “causes” asthma (163). On the other hand Wu and colleagues
concluded in another recent paper that they have shown strong evidence for causal
relationship of winter viruses and early childhood asthma (figure 8) (118). In the light of
these two particular papers Kuehni et al. discussed the causal link between RSV
infection and asthma in an editorial (164). The editorial concluded the new data were
more in favor of the hypothesis that the association between RSV and asthma is due to
shared predisposition rather than to a causal effect of RSV. However, the authors
pointed out that the asthma phenotype was poorly defined in both studies. The GINA
guidelines have defined three categories: transient early wheezing, persistent early-onset
wheezing and late-onset wheezing/asthma (86).
Cohort studies have suggested that the association between RSV infections and later
wheeze wanes with time (age) (96, 99). Huge epidemiological studies have power but
difficulty getting certain asthma outcomes (e.g. lung function measurements), therefore
it is also important to supplement with smaller clinical studies such as this twin study.
We found no differential effect from severity of RSV infection on asthma and allergy 7
years after the infection. This may suggest that some undisclosed environmental factor
instead could be responsible for the different severities of RSV infection. A study of the
whole twin cohort showed that the severity of RSV infection was determined partly by
genetic factors (16%); family environment accounted for 73% and nonshared
environment for 11% of the individual susceptibility to develop severe respiratory
syncytial virus infection (115). The nature of such exposure is unknown and surprising
in monozygotic twins, maybe this study material (subgroup) was unique at some level.
Scientists have discussed the theory of epigenetic, which refers to heritable changes in
phenotype (appearance) or gene expression caused by mechanisms other than changes
in the underlying DNA sequence. Monozygotic twin siblings frequently present many
phenotypic differences, such as their susceptibility to disease. Recent studies suggest
that phenotypic discordance between monozygotic twins is to some extent due to
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