Investigating Why Autism Happens

Autism

Discover the science behind why autism happens, exploring genetic, environmental factors, and brain structure.

Steven Zauderer

April 25, 2024

9 min read

min read

Understanding Autism

Autism is a complex neurodevelopmental disorder that is characterized by challenges with social skills, repetitive behaviors, speech, and nonverbal communication. The question of why autism happens is a multifaceted one, with research pointing to a combination of brain overgrowth, genetic and environmental factors.

Brain Overgrowth in Autism

In children with autism, abnormal brain overgrowth has been observed during the first two years of life. The most significant overgrowth is seen in cerebral, cerebellar, and limbic structures crucial for cognitive, social, emotional, and language functions by 2-4 years of age. This excessive growth is followed by abnormally slow or arrested growth.

The exact reason for this overgrowth and its relationship to the development of autism is still under investigation. However, it's clear that these early changes in brain development play a significant role in the manifestation of autism symptoms.

Genetic and Environmental Factors

The causes of autism are multifaceted, with both genetic and environmental factors playing a role. Genetic causes of autism have been identified through significant advances in the field of genetics. However, the search for environmental factors that contribute to autism risk is still ongoing. One area that has been neglected is the study of interactions between genes and environmental factors.

Various pregnancy and birth complications, such as preterm birth, low birth weight, maternal diabetes, and high blood pressure during pregnancy, are well-established environmental risk factors for autism, although the underlying mechanisms are not fully understood [3]. Infections, serious illnesses like influenza, and hospitalizations during pregnancy, as well as autoimmune diseases in mothers, are linked to an increased risk of autism in children, indicating the role of the maternal immune system in autism risk.

Furthermore, exposure to the drug valproate during pregnancy is known to increase the risk of autism, as well as various birth defects, highlighting a clear environmental risk factor for autism [3]. Evidence also suggests that exposure to air pollution during gestation or early life increases a child's risk of autism, although the specific components of air pollution involved remain uncertain.

While ongoing research continues to uncover more about why autism happens, understanding the interplay of brain overgrowth and genetic and environmental factors is crucial. This broad understanding helps to shape the best interventions and support for those with autism and their families.

Neurobiological Mechanisms

Unraveling 'why autism happens' involves a deep dive into the neurobiological mechanisms that underpin this complex condition. This includes examining neuroimaging studies, understanding abnormal brain connectivity, and observing changes in brain structure.

Neuroimaging Studies

Neuroimaging studies play a critical role in revealing the differences in brain structure and function between individuals with Autism Spectrum Disorder (ASD) and typically developing controls. Structural MRI studies have shown abnormalities in gray and white matter with regional brain differences. These studies provide a detailed view of the brain, allowing researchers to identify specific regions where these abnormalities occur.

Abnormal Brain Connectivity

People with autism exhibit alterations in white matter, the bundles of long neuron fibers that connect different brain regions. For instance, changes in the corpus callosum, a key white matter tract connecting the brain's hemispheres, are linked to an increased likelihood of autism traits. These alterations in brain connectivity can impact how different regions of the brain communicate, which may contribute to the symptoms seen in ASD.

Brain Structure Changes

Abnormal brain overgrowth has been observed in children with autism during the first 2 years of life. The most deviant overgrowth is observed in cerebral, cerebellar, and limbic structures crucial for cognitive, social, emotional, and language functions by 2-4 years of age. This excessive growth is followed by abnormally slow or arrested growth.

Volumetric and morphometric brain studies have revealed an enlarged brain volume in younger individuals with ASD, followed by arrested growth and a possible decline in volumetric capacity after around 10-15 years of age. Additionally, some children later diagnosed with autism exhibit unusually fast growth in specific brain regions in infancy, such as the cortex [4].

Moreover, autistic people have decreased amounts of brain tissue in parts of the cerebellum, a structure at the base of the skull that plays a role in cognition and social interaction.

Understanding these neurobiological mechanisms is crucial in unlocking the puzzle of why autism happens. Continuing research in this area could potentially lead to more effective interventions and treatments for ASD in the future.

Environmental Risk Factors

Various environmental factors have been identified that potentially contribute to the development of autism. Below, we explore some of these factors, including pregnancy and birth complications, the maternal immune system, and exposure to certain drugs and air pollution.

Pregnancy and Birth Complications

Various complications during pregnancy and birth, such as preterm birth, low birth weight, maternal diabetes, and high blood pressure during pregnancy, are well-established environmental risk factors for autism. However, the underlying mechanisms linking these complications to autism are not fully understood. More research is needed to unravel these complex relationships and develop more effective preventive strategies Spectrum News.

Risk Factor	Associated Risk
Preterm Birth	Increased Autism Risk
Low Birth Weight	Increased Autism Risk
Maternal Diabetes	Increased Autism Risk
High Blood Pressure During Pregnancy	Increased Autism Risk

Maternal Immune System

The maternal immune system may also play a role in autism risk. Infections, serious illnesses like influenza, hospitalizations during pregnancy, and autoimmune diseases in mothers have been linked to an increased risk of autism in children. This suggests that the maternal immune system and its responses could influence the development of autism Spectrum News.

Risk Factor	Associated Risk
Infections During Pregnancy	Increased Autism Risk
Serious Illnesses During Pregnancy	Increased Autism Risk
Autoimmune Diseases in Mothers	Increased Autism Risk

Drug Exposure and Air Pollution

Certain environmental exposures have also been linked to autism. For example, exposure to the drug valproate during pregnancy is known to increase the risk of autism, as well as various birth defects. This highlights the significance of drug exposure as a clear environmental risk factor for autism Spectrum News.

Furthermore, evidence suggests that exposure to air pollution during gestation or early life increases a child's risk of autism. However, the specific components of air pollution involved remain uncertain. Further research is needed to clarify these potential risk factors and explore their mechanistic links to autism Spectrum News.

Risk Factor	Associated Risk
Exposure to Valproate During Pregnancy	Increased Autism Risk
Exposure to Air Pollution During Gestation or Early Life	Increased Autism Risk

Understanding these environmental risk factors is crucial for developing effective preventive strategies and interventions for autism. However, it's also important to remember that autism is a complex condition influenced by a combination of genetic and environmental factors. Therefore, these risk factors should be considered as part of the broader puzzle of why autism happens.

Brain Structure in Autism

In the quest to understand why autism happens, researchers have taken a close look at the brain structures of individuals diagnosed with autism. Significant differences have been observed in key areas such as the hippocampus, cerebellum, and white matter.

Enlarged Hippocampus

One of the key findings in autism research is that children and adolescents with autism often have an enlarged hippocampus. The hippocampus, a crucial part of the brain, is responsible for forming and storing memories. However, it remains uncertain if this difference persists into adolescence and adulthood [4].

Age Group	Hippocampus Size
Children and Adolescents with Autism	Enlarged
Adults with Autism	Unknown

Cerebellum Differences

Additionally, autistic individuals display decreased amounts of brain tissue in parts of the cerebellum, a brain structure located at the base of the skull. Traditionally thought to coordinate movement, the cerebellum is now known to play a role in cognition and social interaction.

Brain Structure	Tissue Amount in Individuals with Autism
Cerebellum	Decreased

White Matter Alterations

Lastly, individuals with autism exhibit alterations in white matter, the bundles of long neuron fibers that connect different brain regions. Changes in the corpus callosum, a key white matter tract connecting the brain's hemispheres, are linked to an increased likelihood of autism traits.

Brain Structure	Alterations in Individuals with Autism
White Matter	Changes in Corpus Callosum

Such structural differences in the brain are crucial components of the puzzle of autism. By continuing to study these changes, researchers hope to gain a deeper understanding of why autism happens and how to develop more effective interventions and therapies.

Social Influence and Autism

Beyond the biological mechanisms and environmental factors, the role of societal influence in understanding why autism happens cannot be underestimated. Factors such as spatial clustering, information diffusion, and local social networks contribute significantly to the prevalence and diagnosis of autism.

Spatial Clustering

Spatial clustering refers to the phenomenon where cases of autism tend to cluster in specific geographic areas Source. This implies that children living very close to a child previously diagnosed with autism are more likely to be diagnosed with autism themselves. This observed increase in autism incidence due to spatial clustering suggests that the environment and social influences play an essential role in autism prevalence.

Information Diffusion

Information diffusion, or the spread of knowledge about autism, has played a significant role in the increased diagnosis of autism over the past two decades. With the widespread distribution of information about autism treatments and causes, awareness has increased, leading to more diagnoses Source. This shows the vital role that information diffusion plays in understanding why autism occurs and how it is diagnosed.

Local Social Networks

Local social networks, or the web of social interactions within a particular geographic area, can have a considerable impact on autism diagnosis. Knowledge about autism and strategies to secure resources for children with autism are likely to depend strongly on these social interactions Source.

The increase in prevalence, spatial clustering, and decreasing age of diagnosis of autism can be attributed to a social influence mechanism involving information diffusion through these local social networks. This highlights how societal factors can influence the understanding and diagnosis of autism.

In conclusion, while biological and environmental factors are integral to understanding why autism happens, societal influences also play a significant role. The interplay of spatial clustering, information diffusion, and local social networks provides a broader perspective on the complex nature of autism.

Genetic Factors in Autism

Research into the genetic elements that may contribute to why autism happens provides critical information about the underlying mechanisms of Autism Spectrum Disorder (ASD). These insights contribute to a better understanding of the complexity and heterogeneity of ASD.

Genetic Landscape of ASD

ASD is highly genetically heterogeneous, affecting an estimated 1 in 59 children. Hundreds of genes are thought to contribute to the disorder's symptoms, but these genes only account for about 10-20% of ASD cases. Even individuals with similar pathogenic variants may be diagnosed at different levels of the spectrum [5].

The genetic landscape of ASD encompasses a variety of genetic variations, including copy number variations, single nucleotide polymorphisms, and epigenetic alterations. These likely play a key role in modulating the phenotypic spectrum of ASD patients. The remaining risk for ASD may be influenced by environmental factors and epigenetic regulation of gene expression.

Role of Risk Genes

Large-scale genetic studies have revealed hundreds of risk genes for ASD. Certain genes involved in synapse formation and transcriptional regulation have shown reproducible hits. These genes are implicated in the formation of abnormal neural networks and disrupted synaptic circuitry in ASD.

Functions	Examples of Risk Genes
Synapse Formation	SHANK3, NRXN1, NLGN3
Transcriptional Regulation	CHD8, MECP2

Somatic Mosaicism and Sex-linked Modifiers

Somatic mosaicism, which results from post-zygotic DNA mutation, is increasingly recognized as crucial to various neurodevelopmental diseases, including autism. It is estimated to account for roughly 3-5% of simplex ASD cases and has been connected to abnormalities in synaptic function, cortical cytoarchitecture, and global gene expression.

Sex-linked modifiers may also have a role in the higher prevalence of ASD in males. Proposed genetic differences between males and females, as well as the differential effects of sex hormones, are potential mechanisms for sex-specific modulation of ASD risk.

Understanding the genetic factors in autism is a complex endeavor. These factors, along with environmental influences, contribute to the rich tapestry of ASD. Further research into these genetic factors is necessary to provide additional insights into why autism happens and how it can be better managed and potentially prevented.