Parents all ask how common are Autism Spectrum Disorders (ASDs) and why are they
being diagnosed so often. Frequency information about prevalence is difficult to
discuss due to changing criteria and diagnostic categories. The most recent
numbers from the Center for Disease Control states a prevalence of 1:140 for
girls and 1/70 for boys. Prevalence rates have increased as subthreshold
criteria for inclusion have been promoted. The inclusion of milder cases has
been criticized for being too inclusive and leading to over diagnosis. Although
this is clearly important, there are other numerous factors including general
public awareness and diagnostic substitution where a child with a prior
diagnosis is rediagnosed as having an ASD.
The term idiopathic ASD is used for children who meet criteria for an ASD but do not have an associated medical condition known to cause ASD’s. The term secondary ASD refers to cases with an identifiable syndrome or medical disorder. Some of these conditions include the following: Fragile X syndrome, Neurocutaneous disorders,
Phenylketonuria (PKU), Fetal alcohol syndrome, Angelman syndrome and Rett
ASDs are biologically based and highly heritable. It has been very difficult to determine causation due to genetic complexity and the large variability of presentation. It is likely that multiple genes are involved and it is unclear whether other genetic and environmental influences play a role. Such environmental factors may represent an intrauterine stressor that effects fetal brain development.
Numerous factors have been determined that effect prevalence. these include a prior sib having been diagnosed as having an ASD and advanced parental age. It is clear the etiology is multifactorial with a variety of genetic and environmental factors playing a role.
ASDs are much more common in boys than girls. Male to female ratios ranging from 2:1 to 6:1 have been cited. The ratios are even higher if only cases of high
functioning autism and Asperger Syndrome are included. The reason for this male
predominance is not known.
The risk of postnatal factors including immunizations have been the focus of many public discussions. Studies have consistently supported the lack of association between thimerosal containing vaccines and ASDs. Yet, due to publicity and fear many parents continue to feel their child’s ASD was caused by vaccine exposure and many parents continue to avoid and refuse vaccines due to fear of their child developing an ASD.
Brain changes found in those with autism suggest pathology arising
during the pregnancy. Studies have also shown brain volume changes as well as
brain structure variability. Functional MRI studies suggest processing
differences including differences in gaze fixation, facial recognition and
variable deficits in imitation, empathy and language.