Autism Spectrum Disorder

A token stands in the middle of a wooden square, three others stand outside.Click to enlarge
Autism can occur in many different manifestations
Source: radachynskyi / fotolia.com

Autism is a developmental disorder which can show very different manifestations. It is assumed that genetic causes contribute to a large extent to the development of autism. Do environmental factors also play a role?

Table of Contents

Autism is seen as a developmental disorder of the central nervous system and grouped into three diagnostic categories: early childhood autism, Asperger’s syndrome and atypical autism. However, since the types overlap and mild forms also occur, the term ASD, autism spectrum disorders, is used as a generic term. The forms of autism cannot be determined by biomarkers but rather by patient observation based on certain diagnostic criteria.

Early childhood autism manifests itself before the age of 3 in either absent or delayed speech development, accompanied by a possible intelligence development disorder. In children with Asperger's Syndrome, speech and intelligence development is not impaired. Commonly seen in all forms of autism are the more or less pronounced problems in social interaction, behaviour and habits. For patients diagnosed with severe early childhood autism it is impossible to lead an independent adult life, whereas milder forms of atypical autism may be barely noticeable.

 

 

How often does autism occur in Germany?

No figures are available on the frequency of autism in Germany. Globally a prevalence of 0.6 % – 1% of the population is estimated. Autism affects four times as many boys as girls. Especially in the US, the hypothesis of a significant increase in the incidence of autism in the population has been voiced over the last decade but this hypothesis is still controversial. The increase might be caused not only by environmental and lifestyle conditions but also by new and broader diagnostic criteria as well as a higher awareness of parents and physicians. In recent years many studies, for example SEED (Study to Explore Early Development, www.cdc.gov/ncbddd/autism/seed.html) are being carried out to investigate the influence of lifestyle and environmental factors in the development of autism.

 

Which risk factors are known?

Autism occurs in some families more frequently, therefore genetic factors or mechanisms are likely to be one of the major causes. This is supported by the fact that the prevalence of autism in twins who grow up together is much higher in identical twins compared to non-identical twins. Gene environment interactions might also play a role which means that environmental factors might alter gene regulation (epigenetics).

The vulnerable window, the time period in which humans are particularly vulnerable to developmental damage and to adverse health effects of environmental stress, is during pregnancy and early childhood. Various associated risk factors are under discussion, these include the use of certain medications such as paracetamol, low birth weight, advanced maternal age, vitamin deficiency (especially vitamin D and folic acid in the first few months of pregnancy), stress and infections during pregnancy or infancy. The hypothesis that infant vaccines with mercury-containing serum might play a role has been refuted. However, this hypothesis led in general to more focus on environmental chemicals.

In particular, chemicals that affect the child's developing brain, as well as those that affect the endocrine or immune system, may be relevant. These include heavy metals such as lead and mercury, pesticides such as organophosphates, flame retardants, polychlorinated biphenyls and phthalates and some volatile organic compounds (VOCs) such as trichlorethylene and styrene. Studies investigating correlations between exposure to these chemicals and autism often do not lead to consistent results because not all exposure factors are considered or even known. In addition, the specific vulnerable window (e. g. month of pregnancy) has to be taken into account. This also applies for example to air pollution caused by road traffic which might play a role in the first few months of life.

In a large-scale EU project called MiND (http://mind-project.eu/) the researchers are investigating the genetic causes of autism and the gene-environment interaction. It is to be hoped that this research will provide concrete indications to reduce risks for pregnant women and for people who wish to have children.

 

 

Reviews

Kalkbrenner AE, Schmidt RJ, Penlesky AC: Environmental chemical exposures and autism spectrum disorders: a review of the epidemiological evidence. Curr. Probl. Peditr. Adolesc. Health Care 44, 10 (2014) 277-318.

Lyall K, Schmidt RJ, Hertz-Picciotto I: Maternal lifestyle and environmental risk factors for autism spectrum disorders. Int. J. Epidem. (2014) 443-464.

Sealey LA, Hughes BW, Sriskanda AN, Guest JR, Gibson AD, Johnson-Williams L: Environmental factors in the development of autism spectrum disorders. Environ. Internat. 88 (2016) 288-298.

Mühlendahl KE: Environment in autism spectrum disorder – a review. Umwelt, Hygiene, Arbeitsmedizin 22, 4 (2017) 175-180. Im Internet unter: https://www.ecomed-umweltmedizin.de/archiv/umweltmedizin-hygiene-arbeitsmedizin-band-22-nr-4-2017