Autism Spectrum Disorder Diagnosis Prevalence Continues To Increase

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According to an article in the Morbidity and Mortality Weekly Report, a publication of the U.S. Centers for Disease Control and Prevention (CDC), the prevalence of autism spectrum disorder (ASD) is higher than ever [1]. The CDC periodically surveys the prevalence of ASD, looking during each surveillance period at the percentage of 8-year-old children who have current ASD diagnoses. In 2000, a survey of children born in 1992 revealed that one in 150 had ASD. These numbers have been steadily climbing, such that the most recent survey — completed in 2008 and surveying children born in 2000 — indicates that one in 88 children has ASD. The numbers are even more disturbing for male children, who have an ASD prevalence of one in 54.

Autism is still incompletely understood by doctors and scientists. It’s characterized by social impairment and stereotypical, often repetitive, behaviors. Unfortunately, because there are as yet no biological markers that physicians can use to definitively diagnose ASD — currently, researchers have only been able to identify general locations in the genome that contribute to autism susceptibility — the prevalence is difficult to assess with absolute certainty. This likely at least partially explains the increase in ASD frequency in recent decades; the diagnosis of autism, which was once limited to a rather narrow group of behaviors, has expanded into an umbrella that encompasses a wide variety of traits (hence the change in the name of the diagnosis from autism to autism spectrum disorder). The CDC acknowledges in their recent report that the extent to which increases in ASD diagnoses are due to actual increased prevalence of the disorder is unknown [1].

Despite the fact that autism isn’t completely understood, however, researchers are learning more about it and the physical characteristics that accompany it. For instance, boys appear to be more affected than girls because sex hormones in males interact differently with a gene called RAR-related orphan receptor A (RORA) [2], which is implicated in ASD [3].

Children with ASD appear to have significantly more prefrontal neurons (brain cells in a particular area of the brain responsible for social behavior, reasoning, and parts of communication) than children without ASD [4]. These changes begin before birth, and researchers suspect that autism is due to a combination of genetic and environmental factors that affect the developing fetus. There is no indication that postnatal events cause autism or interact with pre-autistic genes to produce the disorder.

Interestingly, recent data suggests that the environment influences autism susceptibility more than previously thought; in twins with autism, environmental factors accounted for 55-58% of ASD, while genetic heritability accounted for only 37-38% [5]. Other data suggest that in autistic individuals, rare, spontaneous mutations occur at abnormally high rates [6]. The increasing prevalence of ASD diagnosis underscores the importance of continued scientific research into the disorder and its environmental and genetic causes.

References

1. CDC. Prevalence of autism spectrum disorders – autism and developmental disabilities monitoring network, 14 sites, United States, 2008. MMWR Surveill Summ. 2012 Mar 30;61(3):1-19.
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2. Sarachana et al. Sex Hormones in Autism: Androgens and Estrogens Differentially and Reciprocally Regulate RORA, a Novel Candidate Gene for Autism. PLoS One. 2011 Feb 16;6(2):e17116.
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3. Nguyen et al. Global methylation profiling of lymphoblastoid cell lines reveals epigenetic contributions to autism spectrum disorders and a novel autism candidate gene, RORA, whose protein product is reduced in autistic brain. FASEB J. 2010 Aug;24(8):3036-51. Epub 2010 Apr 7.
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4. Courchesne et al. Neuron number and size in prefrontal cortex of children with autism. JAMA. 2011 Nov 9;306(18):2001-10.
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5. Hallmayer et al. Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism. Arch Gen Psychiatry. 2011 Jul 4. [Epub ahead of print]
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6. Sanders et al. Multiple Recurrent De Novo CNVs, Including Duplications of the 7q11.23 Williams Syndrome Region, Are Strongly Associated with Autism. Neuron. 2011 Jun 9;70(5):863-85.
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