The causes of ADHD have been explored increasingly over the past few decades, however still remain quite poorly understood. In order to understand the research it is important to understand that just because research may find a consistent relationship between two factors, doesn’t mean that one factor caused the other. Unfortunately this is the state of most human research, where we cannot inflict upon humans in an ethical manner certain factors to see if it develops into ADHD. Animal studies can shed some light on these factors, however the human condition is far more complex. Some of the most interesting research coming from ADHD research at the moment is in brain scanning, particularly functional brain scanning that can look at the brain in real time whilst performing tasks of attention. This research is trying to pinpoint regions of the brain and aspects of neurophysiology involved in creating the symptoms of ADHD. An analysis of qEEG in different subtypes of ADHD may pinpoint whether certain neurophysiological profiles can be used to predict and diagnose ADHD in a more concrete and objective manner than a list of symptoms.

Genetics Causes

Anyone working with families with ADHD can clearly see that there is a strong genetic influence. This is supported by the scientific research. Research has shown that if a person has ADHD, 25% of first-degree relatives are also likely to have ADHD, placing them at around 5x increased risk than the non-ADHD population. Twin studies show even strong evidence with identical twins 70-90% likely to also be diagnosed with ADHD. Non-identical twin studies show an incidence of around 30%, only slightly higher than that of a first degree relative. What was interesting from these genetic studies was that family environment played almost no role in the incidence of ADHD, which means that bad parenting cannot be used as a mean of blame for parents with children with ADHD.

Brain-based Causes of ADHD

There has been a long held belief going back hundreds of years that ADHD is related to some level of brain-based dysfunction. Going back hundreds of years there have been many noteable similarities between people with ADHD and people who have sustained brain injuries to their frontal lobes. The frontal lobes have long been known to be associated with executive functioning and behavioural control. People with frontal lobe damage have been shown to have issues with impulse control and executive functioning, leading to researchers suggesting that people with ADHD may have suffered some early insults to the frontal lobe, even in utero. Many researchers suggested this may be due to either a blow to the head, or even an infection like encephalitis or meningitis. However, as researchers looked into this it was evident that the vast majority of people with ADHD did not have any of these infections or insults, and whilst pregnancy &/or birth complications were more prevalent in people with ADHD, they couldn’t certainly account for the majority. As scans have shown that few people actually have any tissue damage, alternative causes to brain disruption needed to be looked into.

Brain Development in ADHD

Causes of abnormal brain development

Environmental factors such as drugs consumed during pregnancy can create abnormal brain development that have symptoms seen in ADHD. Studies into cigarettes and alcohol use during pregnancy have shown increased risk of an ADHD diagnosis. It is however noted that this association may not just be due to the substances causing the issues, but there may genetic factors contributing both to increased drinking and increased risk of inheritance. Exposure to lead can also place people at higher risk for ADHD, however again this is only a minor factor in the overall cause of ADHD as lead exposure is relatively rare these days and ADHD rates certainly are not dropping.

Types of abnormal brain development

Several meta-analyses have been done looking at the development of brain regions in people with ADHD and the following is a summary of these studies:

1. 5 brain regions are smaller in people with ADHD: the cerebellum, the splenium (that connects the two hemispheres of the frontal lobes), the right side of the caudate nucleus, the right hemisphere in general and the frontal lobes.
2. General brain volume is smaller, the most significant being the caudate region of the basal ganglia.
3. Brain volume increases with age, with it around 2-3 years behind in maturation, with the gap closing by late teenage years. However this doesn’t mean the brain functioning becomes normal.
4. Prefrontal regions (especially on the right), several structures in the basal ganglia, the midline anterior cingulate cortex, and the cerebellum are not only smaller but less active.

Brain Activity in ADHD

Many studies have measured brain activity in people with ADHD, and consistently issues have been found in the frontal lobes. Research shows that on EEG there is an excess of slow wave activity (usually associated with sleep or drowsy states), particularly in the frontal lobes. When looking at fast wave activity, often associated with thinking and reasoning skills, people with ADHD typically have lower levels. Stimulant medications have in many children with ADHD been shown to suppress much of this slow wave and activity and activate the brain at higher levels. This is suggested to be the reason why stimulants can create anxiety and tics in some people on high levels of stimulants, as both anxiety and tic disorders have been associated with too much fast-wave. Neurofeedback protocols to inhibit slow wave and activate beta waves (fast wave) have been show in research since 1976 to be able to have affects similar to stimulants in enhancing attention, but without the negative side effects. After over 40 years of research into neurofeedback and ADHD, neurofeedback has become recognised as one of the major therapies in the field.

It is thought however that high levels of slow wave or low levels of fast-wave (particularly the theta/beta ratio) represents only one EEG subtype of ADHD, and that perhaps ADHD diagnosis could be better informed by looking specifically at what sort of EEG or brain activity abnormalities are present. For example, a study in 2012 found that a staggering 30% of their ADHD sample had epileptiform activity (whilst showing no outward signs of epilepsy).

Studies looking at blood flow in people with ADHD have also been conducted. Like the reduced brain activity on EEG in the frontal lobes, researchers have also found that people with ADHD have reduced blood flow in the frontal lobes as well as the caudate nucleus. Given the findings that the striatum (part of the caudate nucleus) is smaller in people with ADHD, and this structure in involved in attention and response inhibition, it is unsurprising that there is less blood flow and these regions are not functioning optimally. When these people are medicated it has been shown that blood flow increases to these regions, further supporting evidence of the importance of abnormal functioning of these regions in children and adults with ADHD.

Brain Chemistry in ADHD

Some researchers have suggested that people with ADHD may have differences in brain neurotransmitters. The brain cells, known as neurons, communicate both via electrical firing and chemical interactions between neurons. Neurotransmitters are the chemical means by which neurons communicate. Unfortunately these hypotheses are hard to test and it isn’t exactly ethical to stick needles into the brains of children and adults with ADHD and test for these neurotransmitters. Most evidence is based on animal studies. For example, when dopamine is reduced in animals these animals can become quite hyperactive, and stimulant medicates have been shown to help these animals. The minimal evidence that does exist seems to be potentially pointing towards dopamine and possibly norepinephrine.