Brain functioning and brain chemistry are the two strongest arguments for a medical basis for ADHD. Studies have consistently shown that individuals with ADHD have abnormal brain functioning, or irregular levels of brain chemicals, most generally dopamine, a chemical involved with movement, mood, attention and learning abilities.
Abnormal transmission of this key chemical causes many of the common symptoms of ADHD, both Inattentive Type and Hyperactive Type. The findings of some studies have also demonstrated delayed brain development in certain brain areas of children with ADHD. This developmental delay is the medical basis for ADHD symptoms which relate to social development and maturity.
Similarly, brain scan studies using MRI (magnetic resonance imaging) technology have revealed that children with ADHD – Primarily Hyperactive Type have two and half times the normal amount of the brain chemical glutamate and abnormally low amounts of the brain chemical GABA compared to neurotypical children.
The primary function of glutamate is to transmit data related to cognition, memory and learning. As a stimulant it helps trigger responses as information is received by the human brain. For example, when someone sees someone they know, glutamate assists in the recognition process and in determining the person's immediate response, such as joy and excitement.
Because glutamate is a stimulant, an overabundance of this chemical in the human brain contributes to the hyperactivity common among children with ADHD. At the same time, a reduced amount of the chemical GABA, which is responsible for inhibitory functioning, is the cause for impulsivity among children with ADHD Primarily Hyperactive Type.
In September 2010, the medical journal The Lancet reported on the findings of a study conducted by the Medical Research Council (MRC) Centre in Neuropsychiatric Genetics and Genomics and Department of Psychological Medicine and Neurology, Cardiff University, Wales, UK. The study concluded that ADHD is a brain development disorder with a genetic basis, or in medical terms, a genetic neurodevelopmental disorder. While previous studies had shown a correlation between parents with ADHD and children with ADHD, this study identified specific copy number variants (CNVs) in the DNA analysis of individuals with the disorder. These CNVs are areas where there are missing or duplicate segments within the DNA strands. These genetic indicators are similar to those for other brain disorders including schizophrenia and autism.
In addition and as hinted above, ADHD tends to run in families. Some studies have shown that fathers who had ADHD in their youth nearly always have children with ADHD. Similarly if one twin has ADHD, the other twin is 75% more likely to have it.
In addition to brain functioning and genetics, environmental factors also come into play. Adults with ADHD are more prone to addictive behavior, such as drug and alcohol abuse, and pregnant women with ADHD may continue to use these toxic substances during their pregnancies. Studies have demonstrated that children whose mothers smoked tobacco or consumed alcohol or drugs during pregnancy are at increased risk of having ADHD. However, it is difficult to determine whether this is truly based on fetal exposure to drugs or alcohol, or further supports the genetic basis of ADHD. Other environmental concerns include exposure to toxic substances such as lead in early childhood.
In summary, the evidence of the medical basis for ADHD is ever more convincing with each new published study.