Twins and asthma

Editor—Strachan 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 The unpublished figures are based on 12 024 twins and 2015 singletons.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.