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doi: 10.1111/ppe.12109

177

Letter to the Editor

Risk Assessment for Autism Spectrum Disorders by Representative Database Tomoyuki Kawada Department of Hygiene and Public Health, Nippon Medical School, Tokyo, Japan

Nilsen et al. clearly presented no self-selection bias in their large pregnancy cohort [Norwegian Mother and Child Cohort Study and the Autism Birth Cohort (ABC) study] by comparing with nationwide registry data [the Medical Birth Registry of Norway (MBRN)].1 Although the ethnicity and sampling time of the baseline data were limited, I deeply appreciate their validation study for stable risk assessment in autism spectrum disorders (ASDs). The authors selected seven prenatal and perinatal exposures for their analysis such as primipara pregnancy (no, yes), prenatal folic acid use (no, yes), prenatal smoking (no, yes), low birthweight (no, yes), preterm birth (no, yes), offspring sex (female, male), and caesarean section history (no, yes). I have some concerns for their study. First, I agree that there were no significant differences in each ratio of adjusted odds ratio between the ABC study and the MBRN study. But there were some discrepancies in statistical significance. For example, in their subanalysis by selecting singleton pregnancies, preventive effect of prenatal folic acid use and risk of prenatal smoking on ASDs in the MBRN study. In contrast, these significant differences were not observed in the ABC study. Are these caused by the difference in statistical power, or are there any reasons? As a second concern, I suppose that their baseline data can be applied for their cohort study. During the follow-up of ASDs incidence, are there any possibilities to include environmental factors such as air pollution? Volk et al. recently conducted case–control studies in subjects aged from 2 to 5 years to know the effect of air pollution on ASDs.2,3 There are several

Correspondence: Tomoyuki Kawada, Department of Hygiene and Public Health, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-Ku, Tokyo 113-8602, Japan. E-mail: [email protected]

© 2014 John Wiley & Sons Ltd Paediatric and Perinatal Epidemiology, 2014, 28, 177

indicators of air pollution such as suspended particulate matter, nitrogen dioxide, and ozone. In addition, these are closely related to socio-economic factors. If there is a limitation for using individual air pollution measures, the environmental monitoring measures can be substituted as aggregated data. Although the number of baseline data is satisfactory, the occurrence of ASDs is not frequent. Cumulative incidence of ASDs in the ABC study was 0.26% in their table 1. Nilsen et al. used logistic regression analysis with modification to avoid the loss of statistical power. As the authors clarified the representativeness of their database, I strongly recommend them conducting an intensive follow-up study to clarify the risk factors for ASDs.

References 1 Nilsen RM, Surén P, Gunnes N, Alsaker ER, Bresnahan M, Hirtz D, et al. Analysis of self-selection bias in a population-based cohort study of autism spectrum disorders. Paediatric and Perinatal Epidemiology 2013; 27:553–563. 2 Volk H, Lurmann F, Penfold B, Hertz-Picciotto I, McConnell R. Traffic related air pollution, particulate matter, and autism risk. JAMA Psychiatry 2013; 70:71–77. 3 Volk HE, Kerin T, Lurmann F, Hertz-Picciotto I, McConnell R, Campbell DB. Autism spectrum disorder: interaction of air pollution with the MET receptor tyrosine kinase gene. Epidemiology (Cambridge, Mass.) 2014; 25:44–47.

Risk assessment for autism spectrum disorders by representative database.

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