The Consequences and Challenges of Developmental Mercury Exposure

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Translations: Spanish

What Thimerosal Does to the Developing Brain

In the United States, neurodevelopmental disorders have been on the rise for several decades. In 1976, about one child in thirty was learning-disabled, but by 2013, it was roughly one in six.^1,2 Similarly, one in one thousand children had a diagnosed autism spectrum disorder (ASD) in 1988 versus about one in forty-five in 2013³ and one in thirty-six as of 2016.⁴ Comparable trends have been observed for attention-deficit/hyperactivity disorder (ADHD)—from one in eighteen children in 1996 to one in eight by 2012⁵—and also for once-rare tic disorders. By 2012, up to 46 percent of school children had experienced tics during their lifetime, making it the most common movement disorder.⁶

These disorders have plagued our society and children—with increasing numbers of affected children now entering adulthood. Yet these individuals, their families and even their medical providers are usually unaware of the fact that their difficulties may have resulted from infant or fetal exposure to the mercury-containing vaccine preservative thimerosal, an exposure that dramatically increased in the 1990s.

AXONS AND THIMEROSAL
Within the brain, nerve cells called neurons are the basic building blocks of the nervous system. Neurons connect to one another, forming a network of communication (see Figure 1). Neurons send and receive information using tiny electrical and chemical signals, thereby allowing a person to perceive, think and understand the world.

A typical neuron has a cell body, an axon (a long, slender, thread-like projection that sends information) and dendrites (shorter branches that receive information). Although a neuron may have many dendrites, it has only one axon (see Figure 2).

Axons can vary in length from approximately one millimeter to one meter. Short-range axons, which are required for short-range communications within a single brain region, only need to span short distances.

Long-range axons are required for long-range neurons that aim to link distinct brain regions, and they sometimes must span long distances. For example, a long-range axon may extend from the frontal lobe (the front part of the brain) to the cerebellum in the back part of the brain. Long-range axons are predominantly used for sensory processing, for attention and for putting thoughts together from different areas of the brain.

Unfortunately, in neurons exposed to toxicants such as the mercury in thimerosal, the structural building blocks of the axon (molecules called tubulin) are disassembled, causing the axon to disintegrate. Long-range axons—the ones that connect different parts of the brain—are especially vulnerable to these toxic exposures.

Neurons have a limited capacity to regenerate their axons, but for many reasons (some known and some unknown) the brain loses much of its ability to regenerate itself early during the developmental period. Postnatal, mature neuronal axons can only regenerate for very short distances, regardless of the original length of the axon. The shorter the distance between the regeneration site and its target, the more successful the regeneration of the axon.

Regeneration is particularly difficult for long-range axons. As the brain tries to regenerate after a loss of long-range axons, the regeneration process often results simply in an increase in the number of short axons. Studies show that when long-range connections decrease or are lost, short-range connections increase. Importantly, the opposite occurs in normal brain development. As the normal brain develops and matures over the years, its connectivity shifts from local to more global processing—that is, from short-range to long-range connectivity.

Coincident with increased thimerosal exposure, three neurodevelopmental disorders dramatically increased in prevalence beginning in the 1990s—ASD, ADHD and tic disorder. Research suggests that children who developed one of the three disorders following exposure to thimerosal may have lost a critical number of their long-range axons.^7,8 These studies indicate that their brains try to compensate by sprouting short-range axons, and brain connectivity shifts from long-range to short-range.7 (Interestingly, in learning delay, also found to be associated with thimerosal exposure, research has revealed a loss of long-range brain connectivity, but without a concomitant increase in short-range axons.)

SIDE ROADS VERSUS SUPERHIGHWAYS
When brain connectivity shifts from long-range to short-range in individuals with thimerosal-induced ASD, ADHD or a tic disorder, a sensation or thought has to jump from one short-range neuron to the next and the next and the next, instead of traveling quickly down a single axon of a long-range neuron. This may occur, for example, with sensory impulses such as visual or auditory processing (see Figure 3).

 

Whereas long-range axons can be compared to superhighways, short-range axons are like side roads with lots of stop lights. You can get where you need to go on a side road, but it can be more complicated, time-consuming and exhausting. In the brain, this convoluted routing can result in a drastically decreased processing speed. It also requires more energy, which can be tiring. As a result, the brain may be better at localized processing than global processing. This may make it harder to pay attention and process sensory information, and easier to get obsessed about small things.

When localized processing dominates, it also can lead to disruptions in attention, sensory processing and other big-picture thinking, as well as poor judgment and perspective. Although most children exposed to thimerosal retain high intelligence, their brains must work harder because their processing is more complicated and attention requires more effort.

IMPLICATIONS
Observed differences across individuals with ASD, ADHD or tic disorder are mainly in severity, with some variation in the areas of the brain most affected. This can depend on when the exposure occurred within the developmental process as well as on individual susceptibility. Importantly, the severity of a given disorder correlates with the severity of abnormal connectivity. In other words, the worse the long-range under-connectivity and short-range over-connectivity, the worse the severity of the disorder.

It is critical to recognize, therefore, that exposure to thimerosal during fetal and infant development can result in significant brain changes. ASD, ADHD and tic disorder all reveal similar changes to brain structure, showing long-range under-connectivity and short-range over-connectivity.⁷ This evidence suggests that the three disorders and possibly other neurodevelopmental disorders all fall within the broader category of connectivity spectrum disorders, which can result from neural long-range under-connectivity and short-range over-connectivity.⁸ An understanding of these brain changes may help individuals and their families better cope with the resulting challenges.

REFERENCES
1. Boyle CA, Boulet S, Schieve LA et al. Trends in the prevalence of developmental disabilities in U.S. children, 1997-2008. Pediatrics 2011;127: 1034-1042.
2. Campbell AW. Vaccines: both sides of the same coin. Altern Ther Health Med 2015;21: 8-10.
3. Zablotsky B, Black LI, Maenner MJ, Schieve LA, Blumberg SJ. Estimated prevalence of autism and other developmental disabilities following questionnaire changes in the 2014 National Health Interview Survey. National Health Statistics Reports, No. 87, November 2015. http://www.cdc.gov/nchs/data/nhsr/nhsr087.pdf.
4. Zablotsky B, Black LI, Blumberg SJ. Estimated prevalence of children with diagnosed developmental disabilities in the United States, 2014–2016. NCHS Data Brief, No. 91, November 2017. https://www.cdc.gov/nchs/data/databriefs/db291.pdf.
5. Child Trends DataBank. ADHD: indicators on children and youth. http://www.childtrends.org/wp-content/uploads/2012/07/76_ADHD.pdf.
6. Cubo E. Review of prevalence studies of tic disorders: methodological caveats. Tremor Other Hyperkinet Mov (NY) 2012;2.
7. Kern JK, Geier DA, King PG, Sykes LK, Mehta JA, Geier MR. Shared brain connectivity issues, symptoms, and comorbidities in autism spectrum disorder, attention deficit/hyperactivity disorder, and Tourette syndrome. Brain Connect 2015;5: 321-335.
8. Geier DA, Kern JK, Homme KG, Geier MR. Abnormal brain connectivity spectrum disorders following thimerosal administration: a prospective longitudinal case-control assessment of medical records in the Vaccine Safety Datalink. Dose Response 2017;15: 1559325817690849.

This article appeared in Wise Traditions in Food, Farming and the Healing Arts, the quarterly magazine of the Weston A. Price Foundation, Spring 2018.

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