Autism - Why the Increase?

Autism and autism spectrum disorder (ASD) are terms for a group of complex disorders of brain development, characterized by difficulties in social interaction, verbal and non-verbal communication and repetitive behaviors. There are multiple causes of ASD, although most are not yet known. The learning, thinking, and problem-solving abilities of people with ASD can range from gifted to severely challenged - ASD begins before the age of 3 and last throughout a person's life, although symptoms may improve over time.

The rise in the diagnosis of autism among young children has been rapid and alarming.  Broadening the diagnosis of the disease and increased surveillance may be a factor, but do not explain what has happened.  Prior to 1990 the estimates of autism prevalence were about 3 per 10,000.  In 2000 and 2002 the autism estimate was about 1 in 150 children. Two years later, 1 in 125 8-year-olds was believed to have autism. In 2006, the number grew to 1 in 110, and then the number went up to 1 in 88 based on 2008 data. Currently, one in 68 U.S. children has an autism spectrum disorder (ASD), according the Centers for Disease Control and Prevention. This newest estimate is based on the CDC's evaluation of health and educational records of all 8-year-old children in 11 states: Alabama, Wisconsin, Colorado, Missouri, Georgia, Arkansas, Arizona, Maryland, North Carolina, Utah and New Jersey. The incidence of autism ranged from a low of 1 in 175 children in Alabama to a high of 1 in 45 in New Jersey, according to the CDC. Children with autism continue to be overwhelmingly male. According to the new report, the CDC estimates 1 in 42 boys has autism, 4.5 times as many as girls (1 in 189).

Clearly, we are experiencing a huge, unprecedented problem.  Here is an abbreviated version of current thinking on causation.  The theory that mercury (thiomersal) in vaccines causes autism has been investigated extensively and discredited. The clinical symptoms of mercury poisoning differ significantly from those of autism. In addition, multiple population studies have found no association between thiomersal and autism, and rates of autism have continued to increase despite removal of thiomersal from vaccines.  

Studies of twins suggest that heritability is 0.7 for autism and as high as 0.9 for ASD, and siblings of those with autism are about 25 times more likely to be autistic than the general population. However, most of the mutations that increase autism risk have not been identified. A recent study involving 13 institutions around the world showed that mutation in ‘an autism specific gene’ could lead to autism marked by gastrointestinal disorders, and sleep disturbances.  Mutations in this gene would account for a very small number of cases, but the finding was the harbinger of discoveries to come. 

A recent study from Denmark (a small country with excellent statistical records) showed that both maternal and paternal age are associated with a greater risk of ASD in the offspring, depending on combinations of parental age categories. For mothers younger than 35 years, the risk of ASD increased with increasing father's age. For fathers younger than 35 years, the risk of ASD increased with increasing maternal age.

A study just reported from UC Davis showed that pregnant women who lived in close proximity to fields and farms where chemical pesticides were applied experienced a two-thirds increased risk of having a child with ASD or other developmental delay. The study examined associations between specific classes of pesticides, including organophosphates, pyrethroids and carbamates, applied during the study participants' pregnancies, and later diagnoses of autism and developmental delay in their offspring. This study validates the results of earlier research that has reported associations between having a child with autism and prenatal exposure to agricultural chemicals in California," said lead study author Janie F. Shelton, a UC Davis graduate student who now consults with the United Nations. "While we still must investigate whether certain sub-groups are more vulnerable to exposures to these compounds than others, the message is very clear: Women who are pregnant should take special care to avoid contact with agricultural chemicals whenever possible. In the early developmental gestational period, the brain is developing synapses, the spaces between neurons, where electrical impulses are turned into neurotransmitting chemicals that leap from one neuron to another to pass messages along. The formation of these junctions is really important and may well be where these pesticides are operating and affecting neurotransmission.”

A 2013 report from Harvard School of Public Health was the first large national study to examine links between autism and air pollution across the U.S. Exposure to diesel particulates, lead, manganese, mercury, methylene chloride and other pollutants are known to affect brain function and to affect the developing baby. Women in the U.S. exposed to high levels of air pollution while pregnant were up to twice as likely to have a child with autism as women who lived in areas with low pollution.

Writing this column has made me ever more concerned that our crowded, industrialized world has created conditions dangerous for human life, as well as the lives of other animals and plants. Changing directions will take enormous commitment and political will. Awareness of the damage we are doing to children should give us a powerful incentive to find new ways to live. 

Sadja Greenwood, MD, MPH  leave me a message, and I’ll answer you!