background adhd and autism spectrum disorders (asd) are both severely impairing neurodevelopmental disorders with a prevalence of around 4-5% and 1.6%, respectively. The disorders share a strong overlap in symptomatology. About 60% of patients with adhd show symptoms of asd and the vast majority of asd patients also fulfil diagnostic criteria for adhd.
aim To review and discuss the empirical evidence for shared versus distinct aetiological factors and shared versus distinct neurobiologic and cognitive endophenotypes of adhd and asd.
methods Relevant empirical and review studies will be identified in PubMed and their findings presented.
result Recent quantitative genetic twin and family studies provide evidence that adhd and asd share genetic underpinnings. In addition, genetic linkage analyses have revealed overlapping genomic regions (such as 5p13, 15q13- 15, 16p13, 17p11) implicated in both disorders. In terms of cognitive functioning, executive functions (ef), motor functioning and social cognition are impaired in both adhd and asd, with quantitative rather than qualitative differences between adhd and asd. Thus ef and social deficits proved to be more widespread and severe in asd than in adhd. Structural variation in brain regions relevant to these three neuropsychological domains has also been implicated in adhd and asd. The prefrontal cortex and frontostriatal white matter connections are of particular importance to ef and have been found altered in patients with adhd and those with asd. Similarly, the cerebellum plays a major role in the fine tuning of motor movements and is reduced in size in both adhd and asd patients. Furthermore, structures in the network that subserves social cognition (orbitofrontal cortex, superior temporal gyrus and amygdala) are related to both adhd and asd. Discrepant findings that await further resolution are that brain size is overall smaller in adhd, and, at least at younger age, larger in asd.
discussion For long, adhd and asd has been studied in separate and independent research traditions. Recent interest in the symptomatic, genetic and neurobiologic overlap between adhd and asd may stimulate to formulate new hypotheses on the very nature of these neurodevelopmental disorders of brain, cognition and behavior. One of these hypotheses is that both disorders are in essence due to alterations in the connectivity of the brain at both the molecularcellular level and the neural system level.