Reduced risk of hospital admission for childhood asthma among Scottish twins: record linkage study

Sirs,

Strachan et al (23nd September) had found an interesting difference
between hospital admissions for childhood asthma between twins and
singletons. They conclude that there is reduced risk of asthma among
twins.

We have reported that there was no significant difference in
questionnaire based asthma diagnoses between singletons and twins aged 30-
59 years among Finnish Twin Cohort, but did not give figures (1). The
Finnish Twin Cohort is a population-based sample of like-sexed adult twins
pairs selected from the Central Population Registry of Finland as pairs of
individuals with the same birth date and sex, as well as the same surname
and local community of birth. These selection criteria captured also some
unrelated individuals. The unpublished figures are based on 12024 twins
and 2015 singletons. Age standardized asthma prevalences were 2.1 (95%CI:
1.6-2.5) for male twins and 2.1 (1.1-3.0) for male singletons. For women
prevalences were 2.1 (1.7-2.5) and 2.5 (1.4-3.5) respectively. These
findings are in accordance with studies among children and young adults
from Finland (2) and other Nordic countries (3,4). The reported
occurrences of asthma in these twin studies correspond to those found in
general populations. Although limitations arise because of interstudy
differences (e.g. age range, definition of asthma and study period) these
studies supports our findings among older adults.

We suggest that the reported differences in hospital admission rates
may also be caused by different use of medical care among twins and
singletons. Mothers of twins are on average older than mothers of
singletons and they usually have also other children. Thus they have more
experience about children and their diseases. It is also probable that,
when one twin has been in hospital for asthma, parents don’t seek help
with his/her twin sibling so easily, because the situation is already
familiar and even medication may be available at home.

Sincerely,

Elisa Huovinen, M.D.

Jaakko Kaprio, M.D.

Professor of Public Health

1. Huovinen E, Kaprio J, Laitinen L et al. Incidence and prevalence
of asthma among adult Finnish men and women of the Finnish Twin Cohort
from 1975 to 1990, and their relation to hay fever and chronic bronchitis.
Chest 1999;115: 928-936.

2. Räsänen M, Laitinen T, Kaprio J et al . Hay fever, asthma and number of
older siblings - a twin study. Clin Exp Allergy 1997;27:515-518

3. Skadhauge LR, Christensen K, Kyvik KO et al. Genetic and environmental
influence on asthma: a population-based study of 11,688 Danish twin pairs.
Eur Respir J 1999;13:8-14.

4. Harris JR, Magnus P, Samuelsen SO et al. No evidence for effects of
family environment on asthma. A retrospective study of Norwegian twins. Am
J Respir Crit Care Med 1997;156:43-49.