Five per cent of all school children in Denmark show symptoms of ADHD. For adults, it is around three per cent. ADHD is a neurodevelopmental disorder with onset in childhood. Two-thirds of children who are diagnosed with ADHD continue to have ADHD as adults. In other cases, ADHD is not diagnosed until adulthood.

Researchers from the national psychiatry project iPSYCH have studied the genetic differences between people diagnosed during childhood and people diagnosed as adults.

"We’ve found that the genetic architecture differs depending on how old you are when you get an ADHD diagnosis," says Associate Professor Ditte Demontis who is behind the study.

Less hyperactivity in adults

Approximately 74 per cent of the risk of being diagnosed with ADHD is caused by genetics. The genetics that cause ADHD are 'polygenic', which means that ADHD is caused by several genetic variants in the genome, each of which contributes slightly to the risk of developing the disease. Genetic architecture is the overall term for all variants in the genome that contribute to ADHD.

In the new study, researchers analysed the genetic architecture of people diagnosed with ADHD as children and people diagnosed with ADHD as adults. By comparing these results with the results of other large-scale genetic studies of autism and depression, the researchers discovered that the genetic architecture in children diagnosed with ADHD overlaps with autism significantly more than the genetic architecture of people diagnosed as adults. 

For individuals diagnosed with ADHD as adults, on the other hand, the genetic architecture overlaps with the genetics of depression to a much higher degree than those who are diagnosed as children. This means that people diagnosed with ADHD as adults have an increased risk of depression due in part to genetic risk factors.

The researchers also found that the genetic architecture of people diagnosed with ADHD as adults had a lower load of genetic variants involved in hyperactivity and inattention issues than people who are diagnosed with ADHD during childhood.

"In other words, people who are diagnosed with ADHD as adults are generally less genetically predisposed to be hyperactive and inattentive. This result may help to explain why the time of diagnosis occurred later in life for this particular group of people with ADHD," explains Ditte Demontis.

Overall, these results suggest that there are differences in the underlying genetic architecture of ADHD depending on when you are diagnosed. The results of the study provide new information on which illnesses you have an increased genetic risk of developing depending on when in life you receive your ADHD diagnosis.

The results have been published in Nature Genetics.

Background for the results

The study is a genome-wide associations study

The study is funded by the Lundbeck Foundation

The scientific article can be found here https://www.nature.com/articles/s41588-022-01143-7

Contact

Associate Professor Ditte Demontis
Department of Biomedicine, Aarhus University
ditte@biomed.au.dk
Direct tel.: +4528539746