• Register
  • Login
  • Bookmark this page!
Skip Navigation LinksHome Page    >    Autism - Pervasive Developmental Disorder (PDD)
 
 
Pregnancy links...
Health links...
Genetics links...
Medical links...
Other links...
 

Autism - Pervasive Developmental Disorder (PDD)

Clinical types

A number of clinical types with different names come under this heading. It appears that this is a group of disorders whose causes are diverse, but which have not been adequately characterized. They include:
  • Classical (infantile) autism.

  • Asperger syndrome.

  • A number of other diseases that are characterized by a clinical picture similar to autism or having autistic traits.

Clinical signs

Classical autism (infantile autism): The characteristics manifested by these children include social and communicational adjustment disorders, limited, stereotypical fields of interest, and developmental disorders that appear in the first three years of life. In most cases, the child behaves abnormally even in infancy (before the age of one year).

Asperger syndrome: The signs in this condition are similar to those of infantile autism, but milder. The disease occurs and can be diagnosed in childhood.

There are a number of diseases characterized by a clinical picture similar to autism or that have autistic traits: These include fragile X syndrome, a test for which should be carried out in all such cases, and other syndromes with or without mental retardation. These include chromosome disorders such as a duplication of an area below the centromere on the long arm of chromosome 15, etc. Other conditions such as phenylketonuria should be investigated. However, it must be ascertained that the mother herself does not have phenylketonuria, since such mothers can give birth to children with both physical and mental problems unless she adheres strictly to a special diet throughout pregnancy. The mother's status regarding phenylketonuria can be ascertained by evaluating the phenylalanine/tyrosine ratio. Psychiatric disorders are also sometimes included in the differential diagnosis. All these conditions must be considered by the geneticist.

Inheritance pattern

The pattern of inheritance is still not clear, and apparently not all cases are hereditary. According to the results of several studies, it appears that genetic factors cause around 80% - 90% of all cases of autism.

In addition, there are cases in which environmental factors during pregnancy and/or birth are associated with abnormal development of the brain center responsible for autism.

The genetic factors are probably heterogeneous - an analysis of various studies has shown that more than 12 genes are implicated in autism, to the extent that in different families the causative gene can be completely different.
The general opinion is that, at least in some cases, transmission is multifactorial, i.e. there can be more than 1 gene involved and the environment exerts an influence as well. In other cases transmission is by autosomal recessive inheritance - in these cases one gene with reduced penetrance is involved, and this is influenced by different environmental conditions. This is supported by the occurrence of further cases of autism of different types in siblings.

Penetrance

This is not precisely known and probably differs in different families. The disease is four times more frequent in males.

Associated features that can be demonstrated by imagery

Usually there are no defects that can be identified by ultrasound examination during pregnancy.

What is the risk of recurrence in a subsequent pregnancy?

In cases where the affected individual is the only one in the family: The frequency of childhood autism is 1 in 2,000. Mild autism (based on the severity of the definition used) is of course more frequent, approaching 1%.

The risk for siblings to be affected is 75 times higher than the regular risk for the general population. The total risk for siblings is approximately 5% in the case of autism defined by classical criteria. The percentage is slightly higher if the fetus in the following pregnancy is a male. Distantly related family members have no significant additional risk for having an affected child.

If there are two affected siblings: The risk of having a third affected child is approximately 12% - 25%. More distant relatives do not have a significant additional risk for having an affected child. As mentioned, the frequency of this condition in the general population is approximately 1% depending on the definition used.


Basics: Autism-Symptoms


Molecular genetic information

The gene for the disease

The genes that cause autism have yet to be found. According to mathematical/genetic estimates, there are probably more than 10 different genes that in different combinations can cause this condition. Different families probably carry different abnormal genes and different mutations. In addition environmental conditions also affect the expression of the disease. The complexity of the causes of autism is probably the reason that the genes have not yet been found.

The key for identifying genetic defects in diseases with genetic heterogeneity such as autism involves finding families with multiple cases of PDD and autism. Research groups are therefore concentrating on and are interested in such families at the present time.

The locations of the possible genes

Studies conducted in recent years have raised the question that genes in a number of sites in the genome, particularly 2q, 7q, 16p, and 19p, may be implicated. Recently, evidence has been found in some patients that there is a genetic defect in chromosome 15, in the region 15q11-q13. According to a number of researchers, however, this is only responsible for approximately 15% of all cases of autism. In these studies the gene itself has not been identified, and even if it is present and a mutation in it is associated with autism, it does not cause the condition by itself, but only together with environmental influences. Therefore, until the picture becomes clearer, it is not practical to examine families with only one case of autism/PDD.

Genetic testing

Diagnostic testing

There are to date no clinical genetic tests for diagnosing this condition. Only in families with multiple cases is it possible to carry out investigations that may lead to the possibility of performing practical tests in some families in the future, initially by indirect tests (linkage analysis), and in the more distant future, once the gene itself has been found, by identifying the mutation or mutations. This will be done in cooperation with a genetic institute. See information sheet: Indirect testing for genetic markers in a family that has one or several patients - when there are a number of different genes that can each cause the disease - the gene, or most of the genes not having been located / identified / mapped - multifactorial diseases.

Carrier testing

This cannot usually be performed, unless a specific syndrome such as fragile X syndrome, etc. is found. See diagnostic testing above.

Fetal testing

This cannot usually be performed, unless a specific syndrome such as fragile X syndrome, etc. is found. See diagnostic testing above.
 
Haven't found what you're looking for? Search for it here:   
Comments (0)





Leave a Comment
Name
Email (will not be published)
Website (optional)
Subject
Your Comment

Line and paragraph breaks are automatic. Some HTML allowed: <a href="" title="">, <b>, <i>, <strike>

Please type the characters you see in the picture:


All content Copyright © 2009-2011, Genetics of Pregnancy Encyclopedia Corporation. All rights reserved.