Relative age and ADHD continued

"In a health service system with low prescribing rates for ADHD [attention-deficit hyperactivity disorder], a younger relative age is associated with an increased likelihood of receiving a clinical diagnosis of ADHD."

So said the findings published by Kapil Sayal and colleagues [1] (open-access available here) adding to an important body of peer-reviewed research evidence suggesting that younger relative age - relative age compared with other children in the same school year - does seem to be a factor in both receipt of a diagnosis of ADHD (see here) and use of medication purposed for ADHD (see here). I might also direct your attention to another paper also recently published by Sayal and colleagues [2] discussing ADHD in a more general sense.

The Sayal study represents yet another large population being used to analyse any such link; this time relying on "nationwide population-based registers to identify all Finnish children born between Jan 1, 1991, and Dec 31, 2004, who were diagnosed with ADHD from age 7 years onwards (age of starting school)." They found over 6000 children diagnosed with ADHD, and as well as looking at relative age compared to other children in the school year, also examined the important variable of year of diagnosis - "(1998-2003 vs 2004-11)" - to ascertain whether changes in diagnostic practices might be contributory to this issue.

As per the opening line, there was consistency in the Sayal findings with the other occasions that this issue has been examined. So: "Compared with the oldest children in the school year (ie, those born between January and April), the cumulative incidence of an ADHD diagnosis was greatest for the youngest children (ie, those born between September and December)." Sex/gender did not seem to show any differences in this trend. Also relevant: "This effect has increased in recent years."

If all that wasn't enough, the results published by Boland and colleagues [3] (open-access available here) also add to the data on relative age and ADHD by observing that: "Attention deficit hyperactivity disorder was the only identified relative age association" in their study utilising "electronic health record data from 6 sites representing 10.5 million individuals in 3 countries (United States, South Korea, and Taiwan)." The more general research brief for Boland et al to examine "Birth month and climate impact lifetime disease risk" took into account lots and lots of diagnoses not just ADHD concluding that: "children younger than their peers [are] experiencing greater ADHD risk."

Other coverage of the Sayal study (see here) caution about the 'applicability' of the results particularly in the context of school and ADHD here in Blighty. I agree that the study does not 'prove' relative age is directly linked to ADHD diagnosis (and/or medication use for ADHD) but in the context of this not being the first time that such a link has been made and the strength of the accumulating evidence, I find it difficult to suggest that any relationship is just a spurious one.

Finally, to quote from Sayal: "In terms of clinical and educational implications, these findings suggest that key adults (teachers and parents) might interpret behaviour differently of children who are younger than their classmates." Translation: one should be careful not to mis-classify 'developmental immaturity' with a diagnosis of ADHD [4] and that ideas discussing greater flexibility on school age start dates perhaps come into their own...

To close, lest we forget today of all days...

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[1] Sayal K. et al. Relative age within the school year and diagnosis of attention-deficit hyperactivity disorder: a nationwide population-based study. Lancet Psychiatry. 2017 Oct 9. pii: S2215-0366(17)30394-2.

[2] Sayal K. et al. ADHD in children and young people: prevalence, care pathways, and service provision. Lancet Psychiatry. 2017 Oct 9. pii: S2215-0366(17)30167-0.

[3] Boland MR. et al. Uncovering exposures responsible for birth season - disease effects: a global study. J Am Med Inform Assoc. 2017 Sep 28.

[4] Efron D. The role of schools in the diagnosis of ADHD. Lancet Psychiatry. 2017 Oct 9. pii: S2215-0366(17)30406-6.

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Relative age and ADHD

"ADHD children may just be immature, research suggests".

So went the recent BBC headline with reference to the findings reported by Mu-Hong Chen and colleagues [1] (open-access) and the idea that: "Relative age, as an indicator of neurocognitive maturity, is crucial in the risk of being diagnosed with ADHD [attention-deficit hyperactivity disorder] and receiving ADHD medication among children and adolescents."

Chen et al are not unfamiliar names discussed on this blog (see here for example) in light of the rise and rise of data emerging from the Taiwan National Health Insurance Research Database (NHIRD) covering all-manner of associations linked to diagnoses such as ADHD and autism. Indeed, not so long ago I was discussing the idea the atopic dermatitis *might* be implicated in one or other diagnosis (see here). This time around the NHIRD was used to test a different hypothesis: "those born in August are more likely to be diagnosed with ADHD and receive a prescription for ADHD treatment than those born in September."

An important detail first that many parents with a child born around late August / early September will probably have considered before their child starts school - "The cut-off birthdate for entry to school in Taiwan is August 31, and consequently, those born in August are typically the youngest in their grades." From a starting population of 1 million - "approximately 4.3% of the population of Taiwan" - who were randomly selected from the NHIRD, researchers whittled the numbers down to just under 400,000 aged 4-17 years who were "categorized on the basis of their birth month." The prevalence of ADHD and receipt of "a prescription for ADHD medication (methylphenidate or atomoxetine)" were examined by month, and various statistics were produced.

Results: an interesting pattern of diagnosis by birth month emerged. "The prevalence of subjects receiving ADHD diagnosis or medication increased with each birth month from September (1.8% and 1.2%) to August (2.9% and 2.1%)." In other words, those born in August were almost twice as likely to be diagnosed with ADHD as those born the previous September. This increased 'risk' mostly held true when analysis was also done on the basis of specific years starting 1997-1998 through to 2010-2011 and also crossed the genders.

The idea that relative immaturity compared to class peers might influence ADHD diagnosis is not a new one. Halldner and colleagues [2] reported on a smaller yet still pretty large cohort of children looking at ADHD diagnosis and medication treatment in Sweden. They reported that "ADHD diagnoses and medication treatment were both significantly more common in individuals born in November/December versus January/February" but also "no corresponding differences in parent- or self-reported ADHD symptoms by calendar birth month." Other authors [3] have arrived at similar conclusions including during recent research.

Some commentators have already cautioned about reading too much into the Chen results as a function of them being exclusively based on ADHD diagnosis and medication data without reference to important variables such as family history and environmental influences. I agree that further investigation is indicated before anyone heads down the 'it's all due to immaturity' route and perhaps over-simplifying what can often be a complicated diagnosis/presentation. This also perhaps ties into some important discussions on the increasing rate of ADHD being noted worldwide (see here).

That all being said, the idea that perceptions of classroom behaviour may not fully take into account age in comparison to peers perhaps suggests that further education and training should be offered to education providers in order to highlight this issue. That other more social factors such as level of economic deprivation (see here) may also influence decisions about using the label ADHD provides a timely reminder that psychiatric labels do not exist outside of social context.

Finally, readers might also entertain the idea that conception in the winter months leading to a late summer birth might also play a role in the results obtained and more than one environmental factor could play a role in the findings. That includes the continuing interest in the overlap between asthma and ADHD (see here). Science in this area might also learn a thing or two from some other recent research findings too...

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[1] Chen M-H. et al. Influence of Relative Age on Diagnosis and Treatment of Attention-Deficit Hyperactivity Disorder in Taiwanese Children. The Journal of Pediatrics. 2016. 10 March.

[2] Halldner L. et al. Relative immaturity and ADHD: findings from nationwide registers, parent- and self-reports. J Child Psychol Psychiatry. 2014 Aug;55(8):897-904.

[3] Elder TE. The importance of relative standards in ADHD diagnoses: evidence based on exact birth dates. J Health Econ. 2010 Sep;29(5):641-56.

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Chen, M., Lan, W., Bai, Y., Huang, K., Su, T., Tsai, S., Li, C., Lin, W., Chang, W., Pan, T., Chen, T., & Hsu, J. (2016). Influence of Relative Age on Diagnosis and Treatment of Attention-Deficit Hyperactivity Disorder in Taiwanese Children The Journal of Pediatrics DOI: 10.1016/j.jpeds.2016.02.012