Mike Stephens of Amberley asks :-

What causes autism?

Pat Tuohy, of the Ministry of Health's Population Health Directorate, responded.

Autism is a rare developmental condition associated with a range of behavioural and intellectual disabilities in children and adults. It is one of a group of related conditions called autism spectrum disorders (ASD) which include ‘pervasive developmental disorders’(PDD), classical autism, and Asperger syndrome.

People with ASD share a common set of impairments which include: understanding and use of verbal and non-verbal communication, understanding social behaviour, difficulties in thinking and behaving flexibly restricted, obsessional or repetitive activities.

There is altered brain morphology in most people with ASD, such as frontal lobe thickening, cerebellar reduction and changes in the amygdala. There may also be cellular and axonal abnormalities and lack of “mirror cells”, which are believed to assist the brain to mirror other’s facial expressions and therefore infer language signals and emotions of others.

There are a range of views about the cause of autism. It was once considered to be caused by cold and distant mothering practices (The refrigerator mother), however this view has been supplanted by a more physically based theories of cause.

Some of the current theories are that autism is caused by viruses, possibly measles virus or vaccine, by exposure to environmental toxins (mercury), a polygenic neurological condition caused by defects in brain structure and/or function, is a socially constructed condition which represents a variant of the normal range of the human condition, or an extreme of the normal range of “maleness”

The dominant medical view is that autism is a congenital neurological condition with a polygenic cause. There is about a 90 per cent concordance in identical twins, and a 35 times increase in prevalence in siblings of a person with autism. The male to female ratio is about 4 to1 for classic autism and 9 to1 for asperger syndrome

The evidence for the polygenic nature is strong, as many studies have shown that some people with autism have many tiny gene mutations, and chromosomes 7 and 11 are implicated. However in most cases the changes to the genome are widespread and inconsistent, suggesting that autism may be the final common path for many different genetic abnormalities. This fits the clinical picture but makes it hard to explain the genetics.

Barron-Cohen, a well known but controversial autism researcher has characterized autism as a phenotypic expression of “extreme maleness”, indicating that dysregulation of hormonal imprinting of the fetal brain may be a cause.