Letters

Twins and asthma

Difference in admission rates may be due to other factors

Author's reply

Difference in admission rates may be due to other factors

EDITORStrachan et al found an interesting difference between admissions to hospital for childhood asthma between twins and singletons.¹ They conclude that there is reduced risk of asthma among twins. We found no significant difference in questionnaire based asthma diagnoses between singletons and twins aged 30-59 years among the Finnish twin cohort, but we did not give figures.² The Finnish twin cohort is a population based sample of adult twin pairs of the same sex 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 also captured some unrelated individuals. The unpublished figures are based on 12 024 twins and 2015 singletons. Age standardised prevalences of asthma were 2.1 (95% confidence interval 1.6 to 2.5) for male twins and 2.1 (1.1 to 3.0) for male singletons. For women, prevalences were 2.1 (1.7 to 2.5) and 2.5 (1.4 to 3.5) respectively. These findings are in accordance with studies among children and young adults from Finland and other Nordic countries.^3-5 The reported occurrences of asthma in these twin studies correspond to those found in general populations. Although limitations arise because of interstudy differences (such as age range, definition of asthma, and study period) these studies support our findings among older adults.

The reported differences in rates of admission to hospital 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 also have other children. They therefore have more experience with children and their diseases. It is also probable that, when one twin has been in hospital for asthma, parents do not seek help for his or her twin sibling so easily because the situation is already familiar and medication may even be available at home.

Elisa Huovinen, researcher. 
elisa.huovinen{at}helsinki.fi Department of Public Health, University of Helsinki, FIN-00014 Helsinki, Finland

Jaakko Kaprio, professor of public health. 
jaakko.kaprio{at}helsinki.fi Department of Public Health and General Practice, University of Oulu, FIN-90014 Helsinki, Finland

1.	Strachan DP, Moran SE, McInnery K, Smalls M. Reduced risk of hospital admission for childhood asthma among Scottish twins: record linkage study. BMJ 2000; 321: 732-733[Free Full Text]. (23 September.)
2.	Huovinen E, Kaprio J, Laitinen LA, Koskenvuo M. 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[Abstract/Free Full Text].
3.	Räsänen M, Laitinen T, Kaprio J, Koskenvuo M, Laitinen LA. Hay fever, asthma and number of older siblingsa twin study. Clin Exp Allergy 1997; 27: 515-518[CrossRef][Medline].
4.	Skadhauge LR, Christensen K, Kyvik KO. Genetic and environmental influence on asthma: a population-based study of 11,688 Danish twin pairs. Eur Respir J 1999; 13: 8-14[Abstract].
5.	Harris JR, Magnus P, Samuelsen SO, Tambs K. 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[Abstract/Free Full Text].

Author's reply

EDITORHuovinen and Kaprio present useful data suggesting that among adults there is little difference in asthma prevalence between twins and singletons. There is growing recognition that wheezing illness in early childhood (when most hospital admissions for asthma occur in the United Kingdom) may be a different spectrum of disease to asthma in adulthood. This may explain the contrasting findings of our respective studies.

The three other studies cited by Huovinen and Kaprio did not compare twins directly with singletons by using a standard method of identifying asthma, but simply commented on the results from twin series in the context of local prevalence studies, which may have used different case definitions. The strength of our study and that of Bråbäck et al is that identical methods of case ascertainment were used among twins and singletons, leading to less misleading comparisons.¹

We alluded to the possibility that patterns of health service contact might have exaggerated the difference in admission rates between twins and singletons. However, the cumulative risk of admission for asthma by age 10 years among Scottish singletons born 1981-4 was only 2%, so it is unlikely that prior admission of the co-twin would influence asthma management in more than a small proportion of twins.

David Strachan, professor of epidemiology. 
Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE