The Complexity in Assessing the Overlap Between ADHD and ASD – a Review of Clinical Methods and the Neurobiological Evidence

Background: Attention Deficit Hyperactivity Disorder (ADHD) and autistic spectrum disorders (ASD) are early onset neurodevelopmental disorders of childhood. Both are sexually dimorphic with males affected four times more frequently than females. Although DSM-IV-TR recognizes that over activity and inattention are frequent in ASD, but a diagnosis of ADHD is not made if ASD is present. This view has been challenged and there has been considerable debate of late concerning co-morbidity between ASD and ADHD. This topic is of scientific and clinical interest because it raises important questions about possible underlying mechanisms between ASD and ADHD. A better understanding of overlap between ADHD and ASD could indicate directions for further studies to find shared genetic vulnerabilities. 
Objectives: to identify, review and analyse research findings, conclusions and methodologies on the overlap between ASD and ADHD. This paper also aims to discuss the limitations to the approaches utilised to date and to make suggestions for future research.
Methods: electronic databases and hand searches were made of the literature concerning clinical overlap, methods of assessing it and the underlying neuropsychological deficits.
Results: 23 relevant studies were identified, reviewed and included in this paper. Majority of the studies reviewed paid little or no attention to possible artifacts in the detection of co-morbidity between ASD and ADHD (for example referral and screening/surveillance biases, overlapping diagnostic criteria). This could explain remaining uncertainty about the extent and the nature of co-morbidity and disparate findings reported. Moreover majority of studies failed to address a key question:  whether co-morbidity between ADHD and ASD actually exists, is artifactual or simply represents overlap of symptoms between the two conditions.  On another hand what one might be conceiving as overlap between ADHD and ASD might actually be a reflection of common underlying anatomical irregularities in these two neurodevelopmental disorders. For example, the shared brain abnormalities between the ADHD and ASD groups of an increase in grey matter in the left inferior parietal/postcentral gyrus may underlie shared cognitive deficits in these groups, such as response inhibition in the absence of repetitive cues. The executive dysfunction theories of autism and ADHD hold that deficits in executive function account for the defining behavioural features of these two disorders. Both children with ADHD and children with autism show deficits in verbal fluency, and spatial working memory. A deficit in response inhibition is consistently reported in ADHD, and a number of studies have also described impaired response inhibition in autism.
Conclusions: most studies suggesting co-occurrence of ADHD and ASD have been based on relatively small and highly selected clinical samples, which can be prone to referral bias and failed to address other possible artifacts in the detection of co-morbidity. A heterogeneous approach to defining psychiatric co-morbidity was observed. In order to determine whether symptoms of ADHD and autism naturally cluster in children from the general population, it is important to examine these associations in larger, population-based samples using best possible methods.