Has the ADHD 'phenotype' become more common over the past decade?

It's a question that continues to prominently figure in ADHD (attention-deficit hyperactivity disorder) circles: Are there more people out there 'developing' ADHD, or is the quite rapid increase in cases being diagnosed due to various other 'artificial' factors?

Far be it from me to come down on either side with any great certainty - as per a similar question being asked with autism in mind - but the findings reported by Mina Rydell and colleagues [1] do provide some important discussion on this question. Authors concluded that: "the increased rates of clinically diagnosed ADHD might reflect changes in diagnostic and treatment practices of ADHD, administrative changes in reporting diagnoses, greater awareness of ADHD, better access to healthcare, or current overdiagnosis, rather than an increase in the ADHD phenotype." But...

The evidence for their conclusion came from their use of an instrument called the A-TAC (Autism-Tics, ADHD and other Comorbidities inventory) and its application to a study initiative called the
Child and Adolescent Twin Study in Sweden (CATSS). Indeed mentioning about a similar question being posed in autism research circles, I'll direct you to a recent blog post where A-TAC and CATSS led researchers - some of the same researchers as on the Rydell paper - to conclude that the autism of today might not necessarily be the same as autism of yesteryear (see here).

As well as assessing ADHD symptoms with the A-TAC (not to be confused with the ATEC) for some 20,000 twins between 2004 and 2014, researchers also "modeled the lifetime prevalence of diagnostic-level and subthreshold ADHD with logistic regression" using data from the National Patient Register (NPR) in Sweden.

Results: well, as I've already indicated, there seemed to be quite an influence of those other 'artificial' factors noted from the Rydell paper. So: "The prevalence of diagnostic-level ADHD based on parent ratings did not differ significantly over time from 2004 to 2014." This refers to data garnered through the CATSS using the A-TAC. By contrast: "Clinically diagnosed ADHD increased more than fivefold from 2004 to 2014" based on data from the NPR. You can perhaps see the difference between the two databases.

Of course the debate is not entirely closed as to what might be going on with the ADHD phenotype and the rise in cases being diagnosed. The authors for example, do report that they found "small increases in normal and subthreshold variations of ADHD-like traits" which could indicate that more children are starting to show more ADHD-related traits as time passed. Harking also back to other previous research by some of the authors [2], I wonder if further investigations into this area might take into account variables such as physical activity / inactivity as being something to consider, in light of other evidence (see here)? I don 't doubt there will be other considerations too (see here and see here and see here)...

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[1] Rydell M. et al. Has the attention deficit hyperactivity disorder phenotype become more common in children between 2004 and 2014? Trends over 10 years from a Swedish general population sample. J Child Psychol Psychiatry. 2018 Feb 27.

[2] Rommel AS. et al. Is Physical Activity Causally Associated With Symptoms of Attention-Deficit/Hyperactivity Disorder? J Am Acad Child Adolesc Psychiatry. 2015 Jul;54(7):565-70.

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Does a diagnosis of autism today mean the same as a diagnosis of autism decades ago?

I'm pretty sure that the findings reported by Olof Arvidsson and colleagues [1] are going to be a talking point for some time to come. Examining whether: "A decrease over time in the number of autism symptoms required for a clinical autism diagnosis would partly help explain" the increase in the prevalence of autism "reported to have increased worldwide", the authors relied on data from the parents of several thousand twins alongside other resources to look at this important issue.

There is no doubt that there has been a considerable increase in the numbers of people being diagnosed with autism (see here) stretching across nations (see here). People have previously used the term 'epidemic' to describe the increase but that's not strictly accurate given that autism is not an infectious disease like flu for example. And despite some very forthright assertions about 'knowing the causes' from various quarters, the reason(s) for the increase are likely complex and probably not always the same in different parts of the world (see here for example)...

The Child and Adolescent Twin Study in Sweden (CATSS) and the Swedish National Patient Register (NPR) were the data sources included for study by Arvidsson and colleagues. CATSS is a telephone-based study where parents of all twins born in Sweden are asked to participate. Those who responded to administration of the Autism-Tics, ADHD and other Comorbidities inventory (A-TAC) were included in the present study (N~28,000 between 1992 and 2007).  Alongside, participants diagnosed with autism or with an autism spectrum disorder (ASD) were identified in the NPR (N=430) and whittled down to some 270 children based on being born between 1992 and 2007.

The study aims were three-fold: "(a) detect whether the number of ASD symptoms in clinically diagnosed ASD cases had decreased over time, (b) analyze whether any changes might be associated with age at diagnosis and (c) investigate whether individuals diagnosed with ASD in childhood would show variation in the amount of coexisting non-ASD problems and of dysfunction and suffering over time."

Results: it looks like there were some important trends going through the years based on the presented data. Based on the A-TAC data defining an "autism symptom score", there was a steady decline in said mean autism symptom score over the years 1992-1993 to 2002. In every single year banding, the mean autism symptom score decreased, denoting fewer symptoms/features to be present. The authors concluded that: "Over time, considerably fewer autism symptoms seemed to be required for a clinical diagnosis of autism."

And there's more. When age at diagnosis was divided up to form two groups - those diagnosed between 1-6 years of age, and those diagnosed later, between 7-12 years of age - for those diagnosed between 7-12 years of age the "mean autism symptoms score in the A-TAC decreased for 8.96 in 2004-2005 to 4.63 in 2014." No such significant change was seen in the 1-6 years of age at diagnosis grouping. The authors conclude that it could be this 7-12 year old at diagnosis grouping that seem to be driving the 'fewer symptoms needed for a diagnosis' finding, given that their older age at diagnosis potentially implies a less obvious autistic presentation or "with impairments stemming from 'comorbid problems." All of this is set within the context that registered ASD diagnoses went up and up and up during the periods being analysed.

There's lots to take in from this research but I'll start with a quote from the authors: "the observed decrease in autism 'symptom load' in diagnosed individuals does not necessarily correspond to a decrease in the need for interventions and societal support." This was quite evident when authors looked at the data on comorbidity and "dysfunction/suffering scores" (yes, some people with autism do suffer as a result of their symptoms) that remained fairly constant throughout. Indeed, added to this, the authors talk about how there may be a "change in perception of what it is that is disabling" in terms of schools for example, requiring more "cooperation in groups and self-reflecting abilities." Overall symptom severity might be decreasing but the environment *might* be getting tougher in different ways?

But it's difficult to brush these findings under the carpet and how they relate to the question titling this post: Does a diagnosis of autism today mean the same as a diagnosis of autism decades ago? On the basis of these findings, one would have to say perhaps not (in the general quantitative sense). More study is required however including on how 'female autism' *might* influence some of the patterns (see here) discussed by Arvidsson et al, alongside the idea that age and maturation might influence the often quite fluid presentation of autism [2]. I'm also minded to reiterate that 'autism-related dimensions' are not always 'autism-specific dimensions' (see here) and the important studies suggesting that the presentation of autistic traits does not always merit a clinical diagnosis of autism (see here).

And bearing in mind the tweet snapshot picture accompanying this post, the paper by Sebastian Lundström and colleagues [3] is here and well worth a read...

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[1] Arvidsson O. et al. Secular changes in the symptom level of clinically diagnosed autism. J Child Psychol Psychiatry. 2018 Jan 29.

[2] Lever AG. & Geurts HM. Is Older Age Associated with Higher Self- and Other-Rated ASD Characteristics? J Autism Dev Disord. 2018 Jan 18.

[3] Lundström S. et al. Autism phenotype versus registered diagnosis in Swedish children: prevalence trends over 10 years in general population samples. BMJ. 2015 Apr 28;350:h1961.

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