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Updated: Sep 1, 2023

What is ADHD?

Attention deficit disorder with or without hyperactivity/impulsivity (ADHD) is a neurodevelopmental disorder of neurobiological origin that causes difficulties in controlling and organizing one's behaviors: the child exhibits inattentive (lack of concentration, distraction), hyperactive or impulsive behaviors.

These behaviors are observed more often than in other children of the same age and in several environments in which the child lives (e.g., school, social or family).

What causes ADHD?

The causes of ADHD are not yet known, but it is known that genetics and environment play an important role.

To date, there is no test that can diagnose ADHD. There is no blood test, genetic test, psychological test, or brain imaging that can be used to determine if a child has ADHD or is at risk for ADHD.

However, it is known that ADHD is a complex neurobiological disorder. Several factors clearly contribute to its onset.

Genetics and ADHD

ADHD is hereditary: genes strongly influence the development of this disorder.

  1. If a parent has ADHD, it is likely that one or more of their children will also develop ADHD.

  2. If a child has ADHD, it is likely that one of their siblings will also develop ADHD.

  • The heritability or genetic influence of ADHD is about 75%. This means that ADHD is passed down almost like eye color or height.

  • ADHD is not attributable to a single gene. Rather, it appears that certain genes increase a person's risk of developing the condition. Researchers believe that several different genes are involved in the development of ADHD.

  • Many of these genes are associated with neurotransmitters and proteins that carry messages in the brain.

Researchers are now trying to understand :

  • How such genes combine;

  • How genes interact with environmental factors to influence behavior;

  • How genes interact with treatment.

Environmental Factors and ADHD

  • Non-shared environmental factors (such as complications during pregnancy, at birth, or shortly after birth) contribute to the development of ADHD in 10-25% of cases.

  • Shared environmental factors (such as the location of the family home) contribute very little to the development of ADHD.

ADHD and complications during pregnancy, at birth and soon after birth

Problems that arise during pregnancy, at birth, or soon afterwards apparently increase the risk of ADHD. They include:

  • Exposure to nicotine and alcohol or high maternal stress levels in utero;

  • Prematurity;

  • Prolonged lack of oxygen at birth;

  • Low birth weight;

  • Head trauma.

ADHD and brain function

At this time, brain imaging cannot be used to diagnose ADHD. This technology is only used for research purposes.

However, imaging advances will be made in the future that will provide information about the brain structure and function of people with ADHD.

These advances will potentially contribute to the diagnosis of this disorder. Recent studies based on these new techniques reveal brain differences in people with ADHD.

It is highly likely that these abnormalities contribute to the behavioral symptoms and cognitive problems that these individuals exhibit.

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