Autism

31 July 2019

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What is autism?

There are two characteristics are typically of autism spectrum disorders (ASD) these are; difficulty with social interaction and obsessive behaviors. Basically people with autism struggle with relationships and often defy social convention during conversations. An example would be asking a person with a prosthetic leg, “What happened to your leg?”. There is no menace in the question, they just don’t understand why their question may be inappropriate or even offensive to someone. Some people with ASD may not talk at all. The second characteristic is fixation or obsession. This is an intense interest in a particular topic ranging from trains to cats. Or it can manifest in behaviors such as constantly washing their hand or having very particular daily routines and eating habits. There are a number of secondary symptoms associated with ASD and these can include hyperactivity, aggression, self-injury, anxiety and major depression among others. The key word in autism spectrum disorder is spectrum meaning people vary widely in the number and types of symptoms they experience. In an individual any of the symptoms can range from mild to severe. So what causes these traits to develop? What is the biological reason for ASD? That is the million dollar question!

The brain biology behind autism

ASD is classified as a neurodevelopmental disorder this means the development of the nervous system and brain is interrupted which can result in functional and morphological irregularity. The disruption in brain development is caused by abnormal cell division and apoptosis (cell death). Increased neuronal inflammation has also been shown to contribute to the disruption of developmental processes. There is accumulating evidence to suggest that mirror neurons are responsible for some of the pathologies seen in ASD. Mirror neurons are brain cells that are activated when you perform a specific action. These cells are also activated when you see someone else performing that same task. These neurons allow us to understand the actions of others and helps us process emotional states and other social cognitive functions such as empathy. Recent research seems to suggest impairment in these mirror neurons may contribute to some of the social and cognitive impairments seen in ASD. Interestingly, the synapses in the brain – the points at which cells talk to each other - are also thought to be involved in ASD. There needs to be a balanced development of excitatory and inhibitory synapse. This is where cells either pass on a message or stop that message from reaching other cells. This balance is crucial for allowing the brain to process sensory and cognitive information correctly. Imbalances in the development of these synaptic networks contribute to many neurodevelopmental disorders including ASD.

Although each of these various aspects of brain development has been shown to contribute to ASD it is critical to remember that that ASD is a spectrum. This means there isn’t one single pathology or set of symptoms that is seen in every individual with ASD. Therefore scientists have yet to pinpoint a specific gene, cell type or part of the brain that is specifically responsible for causing ASD.

What causes autism?

ASD is a highly heritable disorder, this means that there are genetic factors that can be passed from parent to child that cause ASD. Researchers know that ASD is heritable from the information they have gathered from twin and family studies where they can track whether ASD is passed on from parent to child.
There is no single gene that has been associated with autism, except in rare disorders such as fragile X syndrome and tuberous sclerosis both of which are on the autism spectrum. There are numerous genes that have been found to contribute to ASD. Together the unique combination of these genes in each person contributes to the development of the unique symptoms seen in each case. These genes often code for proteins that are made in the brain and help it to function or develop. Genes are not the only factor that contributes to the development of ASD, there is evidence to suggest numerous environmental factors also play a role in the development of these disorders. Factors such as alcohol consumption, tobacco smoking, and drug use during pregnancy have all been linked to abnormal brain development in children that are similar to the abnormalities associated with ASD. Malnutrition such as lack of vitamin D has been shown to cause morphological and functional changes during child brain development typically seen in ASD. It is critical to note that no one single environmental factor has been proven to cause ASD and a combination of several factors are likely to increase the likelihood of ASD developing in children.

ASD is a complex disorder with numerous factors that influence the severity and they array of symptoms seen in an individual. It is neurodevelopmental disorder that affects the development of the brain which affect both function and connectivity. This result in individuals with ASD having difficulty in understanding emotion, intention, thoughts and the mental states of others.


References

  1. Fakhoury, M. Autistic spectrum disorders: A review of clinical features, theories and diagnosis. International Journal of Developmental Neuroscience 43, 70-77 (2015).
  2. de la Torre-Ubieta, L., Won, H., Stein, J. L. & Geschwind, D. H. Advancing the understanding of autism disease mechanisms through genetics. Nature medicine 22, 345 (2016).
  3. Matson, J. L., Wilkins, J. & Gonzalez, M. Early identification and diagnosis in autism spectrum disorders in young children and infants: How early is too early? Research in Autism Spectrum Disorders 2, 75-84 (2008).
  4. Boyd, B. A., Odom, S. L., Humphreys, B. P. & Sam, A. M. Infants and toddlers with autism spectrum disorder: Early identification and early intervention. Journal of Early Intervention 32, 75-98 (2010).
  5. Kral, T. V., Eriksen, W. T., Souders, M. C. & Pinto-Martin, J. A. Eating behaviors, diet quality, and gastrointestinal symptoms in children with autism spectrum disorders: a brief review. Journal of pediatric nursing 28, 548-556 (2013).