'A diagnosis of autism or autism spectrum disorder (ASD) does not typically appear in a diagnostic vacuum'. I've said that sentence several times on this blog (see here for example) in line with how science has shown that many different labels (both behavioural and somatic) are over-represented when it comes to a diagnosis of autism. All very ESSENCE like (see here)...
Of the various over-represented comorbidity, attention-deficit hyperactivity disorder (ADHD) - either in symptoms or in diagnosis - is one of the more common ones (see here); something that has implications for screening (see here) and also management. The findings reported by Yi-Ling Chien and colleagues  (open-access) add something to the research looking at the possible hows-and-whys of ADHD appearing alongside autism with their focus on "unaffected siblings of probands with autism and Asperger syndrome (AS)." Such work ties into that observing 'unaffected by autism' does not necessarily mean 'symptom or trait-free' in the context of ideas such as the broader autism phenotype (BAP) (see here).
With the aim to "investigate the ADHD-related traits and attention performance in unaffected siblings of probands with autism and Asperger syndrome (AS), as well as the clinical correlates of ADHD-related traits" researchers concluded that generally, unaffected siblings (unaffected by autism) of those diagnosed with an ASD "were more hyperactive/impulsive and oppositional" than those so-called typically developing controls. The finding was based on the use of various questionnaires/schedules pertinent to both the "the core symptoms of DSM-IV ADHD" and also tests to "assess attention performance."
Of particular note was the observation of "more ADHD and oppositional traits in unaffected siblings of AS probands" when looking at subgroups on the autism spectrum. With caution, the authors suggest that such a finding may be evidence "that these traits might be a broader phenotype for AS." They also posit that "more severe ADHD-related symptoms in AS probands rather than autism probands suggest that these two subtypes may not be the same in their clinical expression regarding ADHD symptoms." In these days of plural autisms (see here), things seemingly get even more complicated when diagnostic subgroup x comorbidity is also thrown into the mix.
Although quite a bit more investigation is required in this area, there is at least one important point to take from the Chien work: unaffected siblings of those diagnosed with autism - particularly Asperger syndrome - may benefit from preferential clinical assessment for something like ADHD. I say that with the understanding that a diagnosis of ADHD has been linked to a heightened risk of various 'adverse' outcomes in the longer term (see here and see here) and again, minus any sweeping generalisations, specific interventions for ADHD can seemingly mitigate quite a bit of that excess risk (see here) and onward improve quality of life and more.
 Chien Y-L. et al. ADHD-related symptoms and attention profiles in the unaffected siblings of probands with autism spectrum disorder: focus on the subtypes of autism and Asperger’s disorder. Molecular Autism. 2017; 8: 3.