In a recent issue of the The Pyschologist (Vol 27, No. 3), Angelica Ronald presents an interesting review of Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD) in light of the DSM-5.
Many clinical and child psychologists will diagnose a child with more than one condition, known as comorbidity. This is often true for the co-occurrence of Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD). The statistics suggest that '30 per cent to 80 per cent of individuals with ASD also meet the criteria for ADHD, and 20 per cent to 50 per cent of individuals with ADHD also meet the criteria for an ASD (Rommelse et al., 2010)'. Interestingly enough, the DSM-IV did not allow for dual diagnoses of ADHD and ASD, whereas the new edition (the DSM-5) does allow for ASD-ADHD comorbidity.
Shared genetic and environmental roots
ASD and ADHD are both developmental conditions that more frequently occur in males and include social functioning problems, yet their behavioural characteristics are distinct. 'We know from twin studies that ASD and ADHD are two of the most highly heritable behavioural conditions (Ronald & Hoekstra, 2011). [...] What has become apparent in the last 5 years from converging findings from large twin studies, in Sweden, Australia, the US and the UK, is that ASD and ADHD share a considerable degree of genetic influence (Ronald, Edelson et al., 2010; Ronald et al., 2008; Taylor et al., 2013)'. Moreover, the findings 'are consistent whether categorical diagnoses or quantitative trait measures are used. This suggests that the link between ASD and ADHD is partly due to a common genetic pathway underlying both conditions'.
Arguably, the genetic link is obvious; what else would explain such high overlap? It is likely, however, that environmental risk factors play a role, as neither ASD nor ADHD is 100 per cent heritable. 'Traits related to both ASD and ADHD have been associated with prenatal maternal stress (Ronald, Hickey & Whitehouse, 2011), and both conditions have been associated with pre-, peri- and postnatal complications (Kolevzon et al., 2007; Ronald, HappÃ© et al., 2010; Thapar et al., 2013)'.
Interacting traits and symptoms
It is interesting that the core symptoms of ASD are caused by largely distinct genetic and environmental causes (Dworzynski et al., 2009; Ronald et al., 2006; Ronald, Larsson et al., 2011), whereas the individual symptoms of ADHD have considerable overlap in their genetic roots (McLoughlin et al., 2007). Most research to date has consisted of cross-sectional studies on the relationship between ASD and ADHD, but there is also a need for longitudinal studies due to the developmental nature of these conditions.
Although there seems to be considerable evidence from twin studies of an overlap between ASD and ADHD, for the time being there is good reason to consider both as distinct conditions. Some environmental risk factors appear to be specific to ADHD (maternal smoking during pregnancy) or ASD (higher paternal age) (Gabis et al., 2010). Due to DSM-5 diagnostic changes, researchers will have a better opportunity to examine how the specific disorders interact and exist either independently or comorbidly.
An emphasis should be placed on the particular symptoms for each disorder. As more research is undertaken, it is hoped that the relationship between ASD and ADHD will be better understood and lead to improved interventions that will benefit individuals with these conditions.
Assessements for ASD and ADHD diagnosis are available from Hogrefe.
ADHD assessment: Conners 3