Autism Causes

By Drs. David Niesel and Norbert Herzog

A diagnosis of autism spectrum disorder or ASD in a child can be devastating for parents. Scientists have been seeking the cause of ASD for decades but it still remains elusive. ASD is a group of neurodevelopmental disorders that result in a wide range of symptoms, disabilities and skills. The official categorization of ASD includes three primary diagnoses: Autistic Disorder, Pervasive Developmental Delays - Not Otherwise Specified (PDD-NOS) and Asperger syndrome. The diagnosis of Autistic Disorder must include impairments in three cardinal domains: communication abnormalities, impaired social functions and repetitive behaviors and restricted interests. Autistic Disorder is more on the severe end of the spectrum with impairments in all three domains. Autistics exhibit marked problems with social interactions, they have delayed and abnormal communications development with as many as 50% not speaking at all. Behaviorally, those affected can be insistent in maintaining routines, resist change and can engage in repetitive behaviors such as hand flapping or rocking.

People with PDD-NOS have some but not all of the symptoms of AD or another Pervasive Developmental Disorder. While those with Asperger's Syndrome have problems with social interaction and respond unusually to their environment much like individuals with autism. However, they do not have the communication and cognitive deficits. They can exhibit unusual idiosyncratic interests in very limited things, for example train schedules, snakes or the weather.

So what causes autism? Twin studies have revealed that there is a strong genetic component to autism. If one identical twin is affected with Autistic Disorder there is a 36-95% chance that the other twin will also be affected. While the risk for fraternal twins is 5-20% which is also the chance of a couple with an autistic child having a second child with autism. The second clue that there is a genetic component is that there are other known genetic disorders in which there are higher autism rates including Downs Syndrome, Turner Syndrome and duplications in chromosome 15.

The search for the gene(s) involved has been underway for many years. There are close to 800 genes that have been found altered in people with autism. It appears that different genes or combinations of altered genes lead to autism in different people. No two autistic individuals are likely to have the same genetic alterations leading. It has been determined that 5-10% of autistic individuals have acquired new mutations not present in their parents' genes. Gene alterations may couple with environmental exposures in causing autism. For example, gestational diabetes, folic acid deficiency at conception, in utero exposure to certain drugs, living near a city with air pollution all increase the risk for autism. There is also some evidence to suggest that the gut microbiome plays an important role in development and disruptions in the normal flora may also contribute to the development of ASD.

Scientists are working hard to understand the complex genetic and environmental factors that are involved in ASD. Unfortunately, the causes of ASD are very heterogeneous. One fact is irrefutable, autism is not caused by vaccinations.

Medical Discovery News is hosted by professors Norbert Herzog at Quinnipiac University, and David Niesel of the University of Texas Medical Branch. Learn more at