Attention deficit hyperactivity disorder: Causes of ADHD

Nobody knows what causes ADHD, but experts do know that it has a strong genetic component. In addition, they think the genes that control the levels of certain chemicals in the brain - called neurotransmitters - seem to be different in those with ADHD.

In some cases, though, there is no genetic link to ADHD. Nevertheless, this common behavioural disorder is diagnosed in children whose mothers smoked or drank alcohol during pregnancy. It is also diagnosed in children whose mothers had difficult pregnancies.

Babies with low birth weight may have an increased risk of ADHD. The same is true for children who have had head injuries, particularly an injury to the frontal lobe. Young children who are exposed to lead or other environmental toxins early in life may also have a higher risk of ADHD.

ADHD always begins in childhood even if it is not actually diagnosed until adulthood. That means adults who are newly diagnosed have actually had ADHD for years. In addition, research shows that 30%-70% of children with ADHD continue to have symptoms of the disorder when they become adults.

What is the genetic connection to ADHD?

In 2010, UK scientists said they found the first direct evidence that Attention Deficit Hyperactivity Disorder (ADHD) is a genetic condition. They say their findings provide proof that ADHD cannot be blamed on bad parenting or poor diet and hope that the research removes the stigma associated with the condition.

Scientists at the MRC Centre in Neuropsychiatric Genetics and Genomics and Department of Psychological Medicine and Neurology, Cardiff University, and colleagues, set out to confirm previous studies that suggested genetics lay behind the development of ADHD.

They analysed the DNA of 366 children with ADHD and 1,047 without the condition. They found that children with ADHD were more likely to have chunks of DNA segments which are either duplicated or missing than participants who did not have the condition. Writing in The Lancet, they said this type of genetic variation is found to be more common in brain disorders, therefore providing the first direct evidence that ADHD is a neurodevelopmental disorder.

What brain changes occur with ADHD?

Studies show that children and adults with ADHD tend to have abnormal functioning of brain chemicals known as neurotransmitters. There also tends to be abnormal functioning in the nerve pathways that regulate behaviour. In addition, children with ADHD may have certain parts of the brain that are smaller or less active than is the case in children who don’t have ADHD.

Recent studies show that the brain chemical, dopamine, may play a role in ADHD. This carries signals between nerves in the brain and is linked to many functions, including movement, sleep, mood, attention and learning.

One dopamine study focused on the genetics of ADHD - specifically, on a particular variation of the DRD4 gene, which is associated with a dopamine receptor in the brain. The study showed children with ADHD are more likely to have this variation of the DRD4 gene than other children. Interestingly, not all children with ADHD had the DRD4 gene variation, but those who did generally had higher IQ scores than other children with ADHD. Plus, the gene variation was most common in children whose ADHD improved over time.

Another dopamine study involving adults with ADHD showed this group had a sluggish dopamine system and helped explain why stimulant ADHD medications such as methylphenidate can be beneficial. Such drugs are thought to increase dopamine by strengthening the weak dopamine signals in the brain. As drugs of abuse, such as   nicotine and cocaine, also temporarily increase brain dopamine activity, the study authors hypothesised that the decreased dopamine activity associated with ADHD may help explain why people with ADHD may have a greater risk of drug abuse.