ADHD causes are not fully understood, but many risk factors and correlations with other conditions have been found, including.^1,11,12

·      -  low birth weight, prematurity, or the mother experiencing stressful events during pregnancy;

·       - foetal exposure to alcohol, cigarettes, medications, lead, manganese;

·     - low socio-economic status;

·     -  hostile parental care;

·      - genetics (heritability is estimated at 70-80%);

·     - how environmental factors influence how the genome synthesises products and affects activity;

·     -  brain injury, epilepsy

·     -  genetic variants and genome changes

There is a complex neurobiology and functional magnetic resonance imaging (fMRI) and other imaging scans indicates brain structure and neurone connectivity may differ in ADHD compared to the general population. The brain may also mature later with ADHD.^6,11

Neurotransmission changes in ADHD may involve dysregulation of monoaminergic systems, mainly dopaminergic and noradrenergic signalling pathways, involving specific receptor sub-types in the brain and neurotransmitter carriers. Serotonin (5-hydroxytryptamine or 5-HT), glutamate, and GABA are also likely to be involved. Effects may be particularly prominent during very intense activity such as stressful situations or very low activity such as when sleepy.¹¹

The gut microbiota affecting the gut-brain axis is also implicated. Studies have found higher levels of certain bacteria species Enterococcus, Bifidobacterium, and Odoribacter and lower levels of Faecalibacterium in people with ADHD. Gut bacteria are involved in aspects such as GABA and dopamine signalling as well as inflammatory response. ¹¹