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Why are more children diagnosed with ADHD and ASD?

Erik Parner

Sektion for BiostatisticsAarhus University

A change in registration/recognition or a new environmental factor causing autism?

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Autism Spectrum Disorder

Follow-up until December 31, 2011.

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• Other psychiatic disorders – ADHD, Tourette’s syndrome, and obsessive-compulsive disorder (OCD) - has experienced a similar increase in number of diagnosed (Atladottir et al., 2007+2014).

• An earlier diagnosis explains some part of the increase. (Parner et al, 2008+2011).

• The heritability coefficient and sibling recurrence risk are contant over calendar time (Grønborg et al., 2013+2015).

• Change in diagnostic reporting practice can explain a large proportion of the increase in autism prevalence for children born 1980-1991 (Hansen et al, 2014).

Time trend findings

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Reporting practice and autism

Denmark changed from ICD-8 to ICD-10 in 1994 and added outpatients in the Danish Psychiatric Central Research Register in 1995.

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A thought experiment

Cohort A is a reference cohort in which we, for simplicity, assume that no changes in reporting practices occur; increases in prevalence are observed by age.

Cohort B is a later cohort where a change in reporting practices was implemented at age 10 years.

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An expected prevalence curve Bexp for cohort B is computed under the scenario of an increase in prevalence of the disorder over time (calendar effect) but assuming that no change in reporting practices took place.

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Change in disease diagnostic system explain 33%, inclusion of outpatients in the register 41%, and combined they explain 60% (=(89.8-41.7)/(89.8-9.4)) of the combined increase in autism prevalence.

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Quite interestingly, the explained proportions did not depend on•length of follow-up. •whether the child were born in 1980, or in 1991.

This within cohort analysis is only possible for individuals born prior to 1991.

Conclusion

Change in diagnostic reporting practice can explain a large proportion of the increase in autism prevalence for children born 1980-1991.