What Causes Autism Spectrum Disorder?

Research provides clues to the cause(s) of autism.

Posted Sep 12, 2018

The rapidly increasing prevalence of autism spectrum disorder (ASD) makes finding out what causes ASD even more vital. According to the Centers for Disease Control and Prevention (CDC), ASD now afflicts 1 in 59 children (and 1 in 37 boys). By contrast, 1 in 150 children were affected in the year 2000.  Other studies suggest that current prevalence may be even higher.

The quick and easy answer to the cause of autism is, “We don’t know.” However, we do have more understanding than this, although the answer is complex. The current understanding of ASD is that there are multiple causes of ASD, not a singular cause of all cases. 

We currently know of many genetic disorders that have ASD associated with them: fragile X syndrome, tuberous sclerosis, Down syndrome, Rett syndrome, and neurofibromatosis.  The knowledge of these genetic disorders and their association with autism has led many scientists to believe that the cause of ASD is primarily genetic.  The heritability of ASD was estimated by scientists to be about 90%.  But, at present, the known genetic disorders that increase the risk of ASD make up at most 25% of cases of ASD. (1)

In that investigations into the genetic causes of ASD have been less fruitful than expected, some researchers have turned to studying potential environmental causes.  As such more recent studies have shown the heritability of ASD to be between 37–50%.(1), making the environmental causes of ASD more significant than the genetic ones.  However, it should be noted that any illness is caused by the complex interaction of genes with the environment.  Each person’s cause of ASD may have a different degree to which genetics and the environment play a role.

The environmental causes of ASD have remained somewhat elusive, and proving causality may be impossible.  However, researchers have found some significant correlations of environmental contributants with ASD.  We know that there is an increased risk for ASD in children born to mothers over 35 and to fathers over 40 years of age.  Other risk factors are closer spacing of pregnancies, prematurity, low birth weight, and being a first-born child.  An infection during pregnancy or a prolonged fever also increases the risk for having a child with ASD.  Certain other exposures during pregnancy have been correlated with ASD as well: rubella infection, first-trimester exposure to thalidomide, valproic acid (Depakote for seizures or bipolar disorder), terbutaline use to stop premature labor, and untreated phenylketonuria. (2)

There has been growing concern and evidence for an association of ASD with pesticides, phthalates (found in plastics), perfumes, air pollution, other chemicals, and electromagnetic fields.  Further, experts have already known that mercury, lead, arsenic, polychlorinated biphenyls (PCBs), and toluene are toxic to humans and cause neurodevelopmental disorders. Thus, it would not be surprising if we found that a chemical or combination of chemicals did cause or dramatically increase the risk of ASD. About 85,000 chemicals have been manufactured in the U.S. and of these, 2,800 are used in high volumes with little information about their developmental toxicity.(3)

Several studies have shown that those with ASD have changes in genes that are involved in detoxifying the body from harmful chemicals. This would make those persons more at risk of toxicity and adverse effects from these chemicals. One review of the literature regarding environmental toxicants found 92% of 37 studies showed an association between ASD and environmental exposures such as pesticides, phthalates, PCBs, solvents, toxic waste sites, heavy metals, and air pollutants. The strongest evidence in the studies appeared to be with pesticides and air pollution. Exposure to toxicants in the water supply or mothers’ eating of fish (increasing mercury exposure) was not supported as an increased risk for ASD. It should be noted that these studies show a correlation and do not prove causality.(3)

Pesticides kill “pests” by damaging the pests' nervous system. Thus, it would not be unreasonable if pesticides did have harmful effects on the human nervous system despite not causing death.  Higher levels of organophosphate insecticides have been found in pregnant women whose children later developed ASD.  Also, higher levels of organophosphates in the umbilical cord blood were associated with ASD symptoms in early childhood.  These pesticides have been found to be related to problems in memory, motor coordination, cognitive development, and visuospatial performance.  Since organophosphates were more recently banned for household use, pyrethroids or pyrethrin have been more commonly used as pesticides.  Metabolites of pyrethrins have been found in 70% of adults.  It is clear that these pesticides are pervasive in our environment with little safety data.  But the data that we do have is concerning.  More high-quality studies are needed to evaluate the safety of pesticides used for both agricultural and residential purposes.(4)

Phthalates are chemicals that make plastics flexible and harder to break.  They are used in hundreds of products such as detergents, lubricating oils, vinyl flooring, adhesives, automotive plastics, raincoats, and personal-care products including nail polish, soaps, shampoos, and hairsprays.  They are also found in 75% of cosmetics.  Phthalates are found in inflatable toys and some children toys.  One is exposed to phthalates by eating and drinking products that have come into contact with phthalates.  Young children are at greatest risk as they frequently put their hands in their mouths especially after touching plastic toys.(4) According to the CDC, the outcome of exposure to even low levels of phthalates is not known.  I know, this is shocking!

Phthalates have been implicated in behavioral problems, ASD, ADHD, obesity, social deficits, thyroid dysfunction, and conduct problems.  In animals, they have been linked to low birth weight, infertility, and abnormalities in male genitalia. One study found a doubling of ASD in children who had polyvinyl chloride (PVC) flooring in the parents' or children’s rooms.  PVC flooring is a significant source of airborne phthalates. (4)

Bisphenol A (BPA) is a type of phthalate that is found in plastic drink bottles, can liners, plastic food wrapping, baby bottles, and more.  It has estrogen properties and disrupts the normal functioning of the endocrine system.  It has also been linked to diabetes and obesity.  BPA was banned in bottles due to health concerns, but there is some evidence that the replacement for BPA, known as BPB, is just as harmful.(4)

To protect you and your children from phthalates, you should reduce the use of products containing phthalates.  When possible, choose products that are phthalate-free.  Limit the number of baby-care products you use. Avoid products with fragrances.  The Campaign for Safe Cosmetics website keeps a list of safe products.  Use glass and stainless steel products when possible instead of plastic when preparing or storing food. If you must use plastic, choose those that state “phthalate-free” or those marked as # 1,2, 4 or 5, which are considered safer. 

Never microwave food in plastic containers or put plasticware in the dishwasher.  High temperatures cause leaching of phthalates into food.  Do not use canned formula, but instead use powdered formula or breastfeed.  Do not buy PVC products especially if used in teethers.  Avoid inhaling phthalates.  Do not paint unless in well a ventilated room and remove your child from the room until fumes have dissipated.  You can also buy paint without dibutyl phthalate.  Choose nonvinyl shower curtains and raincoats.  Clean frequently as phthalates end up in the dust in your home. Do not use air fresheners as all have phthalates in them. If you use perfume, spray it on your clothes and not on your skin. (4)

Lastly, vaccines have been implicated in causing ASD. This history is long and involved, but suffice it to say that many very high quality studies have disproven any link between vaccines and ASD.  In response to parental concerns, a 2013 study was done, showing no link between the number of vaccines given to a child and ASD. A 2015 study found that the MMR vaccine was not associated with ASD even in those at greater risk of ASD, including siblings of those with ASD.(5)

References

1.  Sandin S, Lichtenstein P, Kuja-Halkola R, Larsson H, Hultman CM, Reichenberg A.  The familial risk of autism.  JAMA. 2014; 311 (17): 1170-1777

2. Kuzniewicz MW, Wi S, Qian Y, Walsh EM, Armstrong MA, Croen LA.  Prevalence and neonatal factors associated with autism spectrum disorders in preterm infants.  J Pediatr.  2014; 164(1):  20-25

3.  DA Rossignol, SJ Genius, RE Frye.  Environmental toxicants and autism spectrum disordes:  a systematic review.  Transl Psychiatry.  2104: 4: 1-23.

4.  Lyall K, Schmidt RJ, Hertz-Picciotto I.  Maternal lifestyle and environmental risk factors for autism spectrum disorders.  Int J Epidemiol. 2014; 43(2) 443-464.

5.  Harrington JW, Allen K.  The clinician's guide to autism.  Pediatr Rev.  2014; 35(2): 62-77