Tuesday, 1 July 2014

Mercury exposure and autism or ADHD meta-analysed

The paper by Yoshimasu and colleagues [1] is the talking point today, and their assertion following meta-analysis that: "Moderate adverse effects were observed only between environmental inorganic or organic mercury exposures and ASD/ADHD".

Eruption... @ Wikipedia 
For clarity, ASD means autism spectrum disorder and ADHD refers to attention-deficit hyperactivity disorder. Inorganic and organic mercury refer to some of the different forms of mercury. Meta-analysis, as I've said before, is all about collating results from various studies and coming up with some consensus viewpoint. As per the author's comment though: "the number of epidemiological studies on this issue was limited and still at an early stage".

Still, inorganic mercury sourced from air pollution was significantly associated with autism risk based on studies like this one from Palmer and colleagues [2] and this one from, er, Palmer and colleagues [3]. Methylmercury derived from maternal fish consumption was associated with offspring ADHD as per the study by Sagiv and colleagues [4] for example.

When it came to that most contentious of topics - vaccination and autism - Yoshimasu et al concluded that vaccine sources of mercury (defined as clinical exposure) "did not show any material associations with an increased risk of ASD or ADHD" based on their analysis of the available data. Rather as discussed, it seems that other environmental sources of the stuff were where further investigations might be indicated.

I know mention of the word 'mercury' and autism (outside of the film Mercury Rising) has the ability to furrow brows and heat up discussion in some circles. I don't intend to get caught up in this debate with this post simply because I'm not an expert on such matters. I can direct you to the analysis by Taylor and colleagues (see here) who recently concluded that there was similarly no evidence for a link between the vaccine sources of mercury and autism risk as part of their meta-analysis of [some of] the vaccination and autism literature. I might also drop in the findings of Maglione and colleagues [5] at this point too. The even more recent paper by Hooker and colleagues [6] (open-access) might also be something to have a look at because of the overlap in studies they and Taylor examined, yet their drawing a slightly opposing conclusion (bearing in mind the conflicts of interest reported both in their data and for the authors themselves).

Acknowledging the controversy about mercury and autism, I do think science has more to do in this area. I think back to the various studies which have indicated a higher functional biological burden of heavy metals [7] like mercury identified in some cases of autism (see here and see here for example). The Wright study [8] in particular, hinting at outliers - "results appear to be influenced strongly by a small number of extreme values in the ASD and special school group" - suggests that mercury if it is an issue, is probably not going to be a universal factor for everyone with autism, bearing in mind this study only took a snapshot of mercury levels and did not look at the possibility of early exposures [9] for example. Combined with other issues such as geographical location [10] one might assume that further investigations should perhaps be focused on specific groups on the autism (and ADHD) spectrum to try and answer the questions of why mercury levels are elevated, any implications from such elevations and what can be done to reduce elevated levels?

And just before I go, the FDA guidance on fish consumption during pregnancy and for the young has recently changed to reflect the fact that (a) fish are actually pretty nutritious and (b) not every type of fish is teeming with mercury.

Of course we already have a grand tradition of eating fish here in Blighty... and long may it continue.

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[1] Yoshimasu K. et al. A meta-analysis of the evidence on the impact of prenatal and early infancy exposures to mercury on autism and attention deficit/hyperactivity disorder in the childhood. Neurotoxicology. 2014 Jun 18. pii: S0161-813X(14)00098-9.

[2] Palmer RF. et al. Environmental mercury release, special education rates, and autism disorder: an ecological study of Texas. Health & Place. 2006; 12: 203-209.

[3] Palmer RF. et al. Proximity to point sources of environmental mercury release as a predictor of autism prevalence. Health & Place. 2009; 15: 18-24.

[4] Sagiv SK. et al. Prenatal Exposure to Mercury and Fish Consumption During Pregnancy and Attention-Deficit/Hyperactivity Disorder–Related Behavior in Children. Arch Pediatr Adolesc Med. 2012 Dec;166(12):1123-31.

[5] Maglione MA. et al. Safety of Vaccines Used for Routine Immunization of US Children: A Systematic Review. Pediatrics. 2014. 1 July.

[6] Hooker B. et al. Methodological Issues and Evidence of Malfeasance in Research Purporting to Show Thimerosal in Vaccines Is Safe. BioMed Research International. 2014; 247218.

[7] Gorini F. et al. The Role of Heavy Metal Pollution in Neurobehavioral Disorders: a Focus on Autism. Review Journal of Autism and Developmental Disorders. 2014. June 27.

[8] Wright B. et al. A comparison of urinary mercury between children with autism spectrum disorders and control children. PLoS One. 2012;7(2):e29547.

[9] Adams JB. et al. Mercury, lead, and zinc in baby teeth of children with autism versus controls. J Toxicol Environ Health A. 2007 Jun;70(12):1046-51.

[10] Ando T. et al. Mercury distribution in seawater of Kagoshima Bay near the active Volcano, Mt. Sakurajima in Japan. Bull Environ Contam Toxicol. 2010 Apr;84(4):477-81.

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ResearchBlogging.org Yoshimasu K, Kiyohara C, Takemura S, & Nakai K (2014). A meta-analysis of the evidence on the impact of prenatal and early infancy exposures to mercury on autism and attention deficit/hyperactivity disorder in the childhood. Neurotoxicology PMID: 24952233