Attention-deficit/hyperactivity disorder, commonly known as ADHD, is a genetic condition, say scientists who have provided compelling evidence. The study, funded by the Wellcome Trust, is published in the latest issue of The Lancet. The researchers, from the Medical Research Council (MRC) Centre in Neuropsychiatric Genetics and Genomics and Department of Psychological Medicine and Neurology, Cardiff University, Wales, UK, conclude in their article that ADHD is a brain development disorder.

ADHD makes children fidgety, agitated, impulsive and easily distracted. Experts say approximately 1 in every 50 children is affected by ADHD. Although a number of factors have suggested genetics probably plays a major role, ADHD has frequently been attributed to a diet with too much sugar and poor upbringing.

Previous studies had shown that a parent with ADHD is more likely to have a child who also has the condition compared to a parent without ADHD - a factor which points to some likely genetic association. If an identical twin has ADHD, there is a 75% chance that the other twin will also have it.

The authors explain that ADHD has no cure, but behavioural interventions and pharmacological therapy can help treat the symptoms.

In this latest study, researchers analysed the DNA of 366 children who had ADHD and 1,047 children who did not have the condition (controls).

The DNA analyses revealed that those with ADHD were more likely to have missing DNA segments, or segments of DNA that were duplicated compared to the controls. The authors explain that this kind of genetic variation is more widespread in brain disorders. In a communiqué, The Lancet writes:

Thus this new study provides the first direct evidence that ADHD is a neurodevelopmental disorder.

Significant overlap was also detected between the segments (copy number variations, or CNVs), as well as those associated with schizophrenia and autism.

Although ADHD and autism are classed entirely separately, some overlap does exist between the two in terms of learning difficulties and symptoms. This study suggests there may also be some biological overlap between the two conditions, especially an overlap on chromosome 16 which had been implicated in schizophrenia and some other psychiatric disorders and spans a number of genes, including one we know that is vital for brain development.

Lead investigator Professor Anita Thapar, said:

We hope that these findings will help overcome the stigma associated with ADHD. Too often, people dismiss ADHD as being down to bad parenting or poor diet. As a clinician, it was clear to me that this was unlikely to be the case. Now we can say with confidence that ADHD is a genetic disease and that the brains of children with this condition develop differently to those of other children.

Author Dr Nigel Williams, said:

Children with ADHD have a significantly higher rate of missing or duplicated DNA segments compared to other children and we have seen a clear genetic link between these segments and other brain disorders. These findings give us tantalising clues to the changes that can lead to ADHD.

Third author, Dr Kate Langley, explained:

ADHD is not caused by a single genetic change, but is likely caused by a number of genetic changes, including CNVs, interacting with as yet unidentified environmental factors. Screening children for the CNVs that we have identified will not help diagnose their condition. We already have very rigorous clinical assessments to do just that.

These findings should help eliminate the stigma and misunderstanding that surrounds ADHD, the scientists say.

This study will help people see ADHD as a neurodevelopmental disorder, like autism, instead of a behavioural problem.

Professor Thapar said:

Genetics gives us a window into the biology of the brain. In the future these findings will help unravel the biological basis of ADHD which in turn will help develop new and more effective treatments.

Dr Peter H Burbach, Department of Neuroscience and Pharmacology, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, Netherlands, in a Comment also published in The Lancet, said:

The first gains beyond today's study might be initial insights into the pathogenesis and neurobiology of brain development as influenced by these genetic variants. This knowledge will eventually enter the clinic and might affect the way people think about and treat neurodevelopmental disorders by accounting for the biological consequence of the specific patient's genotype.

A person with ADHD finds it much more difficult to focus on something without being distracted. They have greater difficulty in controlling what they are doing or saying and are less able to control how much physical activity is appropriate for a particular situation compared to somebody without ADHD. In other words, a person with ADHD is much more impulsive and restless.

Health care professionals may use any of the following terms when describing a child (or adult) who is overactive and has difficulty concentrating - attention deficit, attention deficit hyperactivity disorder, hyperkinetic disorder, hyperactivity.

North Americans commonly use the terms ADD (attention deficit disorder) or ADHD (attention deficit hyperactivity disorder). In the UK hyperkinetic disorder is the official term - however, ADD and ADHD have become widely used.

ADHD in children is completely different from normal childhood excited and boisterous behavior. Many children, especially very young ones, are inattentive and restless without necessarily being affected by ADHD.

Click here to read about ADHD in more detail.

"Rare chromosomal deletions and duplications in attention-deficit hyperactivity disorder: a genome-wide analysis"
Dr Nigel M Williams PhD, Irina Zaharieva BSc, Andrew Martin BSc, Kate Langley PhD, Kiran Mantripragada PhD, Ragnheidur Fossdal PhD, Hreinn Stefansson PhD, Kari Stefansson MD, Pall Magnusson MD, Olafur O Gudmundsson MD, Omar Gustafsson PhD, Prof Peter Holmans PhD, Prof Michael J Owen MD, Prof Michael O'Donovan MD, Prof Anita Thapar MD
The Lancet, Early Online Publication, 30 September 2010
doi:10.1016/S0140-6736(10)61109-9

Written by Christian Nordqvist