Asthma increases risk of ADHD?

Asthma increasing the risk of ADHD? With a title like that derived from the paper by Mu-Hong Chen and colleagues* I couldn't resist posting an entry about it. Indeed the paper has one or two of the elements that I've come to love over my couple of years of blogging; in that we have two seemingly disparate conditions - one physiological, one behavioural - yet within the confines of the old 'correlation does not equal causation' quote, some possibility of a connection.

Breathe @ Wikipedia  

ADHD - Attention Deficit Hyperactivity Disorder - has been on my blogging radar a few times in the past few months. Not only with regards to the numbers (see here) which seem to be going up all the time** but also with some rather curious associations in mind, such as solar intensity (see here). Stay with me on this one.

The Chen paper is an interesting one based on a surprisingly large body of research supportive of some link between the two conditions***. Their specific take on this issue was to look at the temporal relationship between asthma and ADHD, which they did by means of a case - control study. Trawling a national database, which is something they are seemingly rather good at in Taiwan (see here) they came up with a very nice sample size of children with asthma (n=2294) and controls (n=9176) and looked at how many kids were subsequently diagnosed with ADHD.

The results:
"Children with asthma had a higher incidence of developing ADHD (7% vs. 4.6%, p < .001)".

OK the difference in ADHD rates might not appear startling but bear in mind the numbers of cases and controls that they looked at. Indeed the overall risk - hazard ratio - after controlling for various other factors was 1.31 (95% CI: 1.07-1.59).

Browsing through older posts on this blog it appears I've talked about some of Chen's work previously with an entry on allergic and autoimmune conditions being more readily diagnosed in cases of autism (see here). This tells me that this research group have some interest in the area of physiology and behaviour being linked and have the means to look at some quite big samples to test the correlations.

So with correlation put to the test and potentially something found, the next questions: how and why? Given how close and cosy ADHD seems to be getting to diagnoses like autism - see the Rao & Landa study**** and accompanying press - on top of all that ESSENCE chatter (see here), one might speculate at potentially similar mechanisms being involved. Indeed, when I wrote about asthma and autism a while back (see here) a few themes emerged based on familial genetics and some overlap for immune function which may be relevant. More recent speculations on things like the hygiene hypothesis (and see here) become even more relevant when it comes to conditions like asthma. I wish I could give a more definitive answer about any connection but it's all rather complicated. One last thought to mention is that rather interesting study by Barbara Stewart (see here) on some anatomical differences found in the airways of children with autism. I'm no expert on the physiology of the lungs and asthma but wonder whether this might also be relevant (or not) to cases of ADHD too.

And finally(!) another quote from the Chen study: "Further studies are required to investigate whether the prompt treatment of asthma and comorbid allergic diseases could prevent the development of ADHD or decrease ADHD symptoms". Food for thought?

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* Chen MH. et al. Asthma and attention-deficit/hyperactivity disorder: a nationwide population-based prospective cohort study. J Child Psychol Psychiatry. June 2013.

** Getahun D. et al. Recent trends in childhood attention-deficit/hyperactivity disorder. JAMA Pediatr. 2013; 167: 282-288.

*** Mogensen N. et al. Association between childhood asthma and ADHD symptoms in adolescence--a prospective population-based twin study. Allergy. 2011; 66: 1224-1230.

**** Rao PA. & Landa RJ. Association between severity of behavioral phenotype and comorbid attention deficit hyperactivity disorder symptoms in children with autism spectrum disorders. Autism. June 2013.

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Chen MH, Su TP, Chen YS, Hsu JW, Huang KL, Chang WH, Chen TJ, & Bai YM (2013). Asthma and attention-deficit/hyperactivity disorder: a nationwide population-based prospective cohort study. Journal of child psychology and psychiatry, and allied disciplines PMID: 23730913