'Completing the set' used in the title of this post refers to the idea that a diagnosis of epilepsy rarely(?) seems to exist in some sort of diagnostic vacuum as per previous discussions whereby features of autism (see here) and dyspraxia / developmental coordination disorder (DCD) (see here) seem to be over-represented in cases of epilepsy.
This time around the focus was on attention-deficit hyperactivity disorder (ADHD) and the findings reported by Isabell Brikell and colleagues  suggesting that: "Individuals with epilepsy had a statistically significant increased risk of ADHD." Researchers arrived at their conclusions on the basis of examining some of those wonderful Scandinavian population registries that are providing all-manner of interesting details on possible trends and patterns in various areas: "We identified 1,899,654 individuals born between 1987 and 2006 via national Swedish registers..." Said data were actually used to look at "the familial coaggregation of epilepsy and ADHD and to estimate the contribution of genetic and environmental risk factors to their co-occurrence" but also served the purpose of looking at ADHD prevalence alongside epilepsy. Indeed it was also interesting to note the authors' conclusions about familial liability to the "cross-disorder overlap": "The genetic correlation was 0.21 (95% CI = 0.02-0.40) and explained 40% of the phenotypic correlation between epilepsy and ADHD, with the remaining variance largely explained by nonshared environmental factors." Mmm...
Such research - although requiring quite a bit more independent investigation  - follows an important trend in recent times observing how stand-alone developmental and/or psychiatric diagnoses often 'clump together' in seemingly at-risk patient groups. I've for example, talked about the important concept of ESSENCE - Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations  on this blog and how "co-existence with other conditions was the rule" (see here) rather than the minority perspective in the area of childhood psychiatry. Now we seem to be able to add ADHD to the list of comorbidity potentially over-represented alongside a diagnosis of epilepsy (bearing in mind that epilepsy covers quite a lot of diagnostic ground).
Mechanisms? Well, far be it from me to speculate too much, but an important starting point is the nature of epilepsy and how it affects brain function. It's not inconceivable that particular alterations to the functioning of the brain as a result of epilepsy (or even during some prodromal phase) might be enough to *induce* other behaviours/symptoms to be pronounced. Equally, one might subscribe to the the idea that changes to brain function due to other events or factors that may be connected to conditions such as autism or ADHD or DCD could be enough to induce the onset of epilepsy (this hypothesis draws support from the onset patterns typically seen in cases of autism and epilepsy). I don't doubt that relationships are likely to be complicated.
Much more needs to be done on this topic, not least in ensuring appropriate screening services when cases of epilepsy are diagnosed, particularly in childhood. With no medical or clinical advice given or intended (don't mess with epilepsy), I do wonder whether some of the peer-reviewed data talking about dietary changes being used to manage certain types of epilepsy also potentially impacting on presented symptoms of *some* other labels (see here) might also provide some clues as to potential shared mechanisms between epilepsy and other developmental/psychiatric labels?
 Brikell I. et al. Familial Liability to Epilepsy and Attention-Deficit/Hyperactivity Disorder: A Nationwide Cohort Study. Biol Psychiatry. 2017 Aug 12. pii: S0006-3223(17)31858-9.
 Caplan R. ADHD in Pediatric Epilepsy: Fact or Fiction? Epilepsy Curr. 2017 Mar-Apr;17(2):93-95.
 Gillberg C. The ESSENCE in child psychiatry: Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations. Res Dev Disabil. 2010 Nov-Dec;31(6):1543-51.