ADHD is a hereditary disorder. The field of medical genetics examines the clustering of disorders in families. Identical twins share 100% of their genes. If one twin has ADHD, the other has a 90% chance of having ADHD. Non-identical twins and other siblings who share less common genetic material, have a 30-40% chance of having ADHD. Compare this with the 7% risk of the general population.
Sophisticated technology has led to the expansion of the field of molecular genetics, which allows us to search for the susceptible areas on specific genes. At present there is strong evidence implicating receptor sites for the chemical messenger dopamine on chromosomes 11 and 5.
Advances in the field of brain imaging allow us to track brain functioning in different areas of the brain. Individuals with ADHD show decreased functioning in the frontal lobes with overloading in the areas that collect auditory and visual inputs compared to those individuals without the disorder. These effects are reversed with the use of stimulant medication. The brain imaging studies correlate with research into the neuropsychology of the disorder, which demonstrates frontal lobe and other deficits.
The frontal lobe, connections to the basal ganglia and the cerebellum are the structures implicated in the disorder. Research identifies the balance between the chemical messengers noradrenaline and dopamine as the culprits and all effective medications target these areas.
(Dr Lesley Carew, Child Psychiatrist)