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There is not a single cause alone that can be attributed to the development of ADHD. Rather, ADHD is a complex disorder caused by a combination of different types of risk factors including genetic, psychosocial, environmental, and biological. Current thinking is that several risk factors must coalesce and act together in order for a child to have ADHD. Specific risk factors include:

• Genetic - Studies have shown that 25% of close relatives of children with ADHD also have ADHD, whereas, the occurrence in the general population is approximately 8%. In addition, studies of identical twins indicate a strong genetic component, with 92% of identical twins and 33% of fraternal twins also having ADHD. Adoptive relatives of children with ADHD have less of a likelihood of having ADHD than biological relatives.

• Environmental - smoking during pregnancy and alcohol/substance abuse have been linked to ADHD. Infants who have experienced lead poisoning (not so common anymore) also have a high risk for developing ADHD. Some researchers are of the opinion that there is a link between ADHD and excessive exposure to TV at a very young age. A history of brain injury has also been linked to ADHD.

• Psychosocial - ADHD has been linked to children who grow up in homes where they are exposed to severe marital strife, domestic violence, paternal criminality, maternal mental disorder, large family size, or foster placement. In general, the more adversity found in a child's life, the higher the risk for development of ADHD.

• Biological - intensive investigation into the biological basis of ADHD continues to progress. Some of the data that has accumulated shows the following:

  • There are 3 circuits that form a network in the front of the brain called the frontal subcortical circuits and they form the anatomic basis of executive (reasoning ability), social, and motivated behaviors. Dysfunction of these circuits results in disinhibition (reduced control of activity), less effective executive function, and apathy (reduction of attention). Imaging studies have shown that there are changes to this area of the brain in children with ADHD. It is interesting to note that these networks form the neurobiological basis of many other psychiatric disorders including obsessive compulsive disorder, bipolar disorder, substance abuse and others - conditions that are frequently seen together with ADHD.
  • Studies have shown that many children with ADHD have small volume reductions in the frontal subcortical circuit area of the brain.
  • In addition, an abnormality in functioning between the dopamine and norepinephrine neurotransmitter systems is thought to result in reduced functioning of the frontal cortex and certain deeper brain structures. This results in impaired attention, reduced alertness and vigilance, and impairment of executive thinking such as abstract reasoning, planning, and memory. The involvement of these neurotransmitters is supported by the response of ADHD to certain types of medications.

• Birth Complications - presence of birth complications has been linked in small studies to ADHD, including toxemia (toxins in the baby's bloodstream), eclampsia (a dangerous complication that results from pregnancy-induced hypertension), long labor, fetal distress, maternal age, and fetal weight less than 1,000 grams at birth. It is thought that several of these complications can lead to hypoxia (reduced oxygen supply) in the fetus which may have an effect on the basal ganglia in the developing fetus (a brain structure related to the frontal-subcortical units mentioned above) and implicated in ADHD. The basal ganglia may be particularly affected by hypoxia since it is metabolically one of the most active structures in the brain.