We thank Harjutsalo and Tuomilehto (1) for their most informative comments. There can be no doubt that they have thoroughly investigated the potential association between type 1 diabetes and autism spectrum disorder (ASD) in a large population-based cohort of Finnish children and failed to find any suggestion of an association. Why might their data differ from ours?
Based on clinical experiences, we reported what had appeared to be a higher-than-expected prevalence of ASD in our clinic-based cohort of children with type 1 diabetes (2). Our report was submitted in the hope of stimulating further evaluation and discussion about possible links between these two common disorders of childhood. As discussed in our report, it is possible that our patient population is biased in that it is derived from a large tertiary care center rather than being population based as in the case of the Finnish analysis. Thus, our patient population may be more likely to suffer other serious chronic conditions in addition to type 1 diabetes than would a group receiving therapy in a more community-based diabetes program.
However, it is possible that the Finnish data mask the possible association between ASD and type 1 diabetes for one or more reasons. First, it is possible that ASD remains relatively underdiagnosed in their diabetic population. Second, it is more likely that there are significant differences between the two cohorts that make comparison difficult. For example, there are reports that suggest that the rising incidence of childhood type 1 diabetes is associated with reduced contributions of high-risk HLA haplotypes (3). These data suggest that, in a country with a very high incidence of type 1 diabetes such as Finland, the relative contribution of genetic susceptibility to the expression of the disorder is therefore diminished. If the relationship between ASD and type 1 diabetes is on the basis of shared genetic influences, then any possible relationship may be diluted out by the heavier contribution of environmental factors in high compared to medium or lower incidence countries.
The contribution of Harjutsalo and Tuomilehto is greatly appreciated in helping to facilitate a definitive answer to the question of disease association between type 1 diabetes and ASD. There may be other investigators willing to share their experiences on this issues.
