STUDY SUMMARY

We conducted a large study over eight successive years of California births to examine changes in the observed rate of autism among children who are clients of the Department of Developmental Services and their Regional Centers (DDS/RC). We sought to evaluate the degree to which improvements in detection and changes in diagnostic practices contribute to changes in the rate of autism for DDS/RC clients..

Children born between 1987-1994 with a diagnosis of autism (with or without mental retardation (MR)) were identified from the electronic client files maintained by DDS/RC. To evaluate the role of diagnostic substitution, we also identified children with a diagnosis of unexplained MR without autism. A total of 5,038 children with a diagnosis of full-syndrome autism (autistic disorder) and 11,114 children with a diagnosis of unexplained MR without autism were identified from 4,590,333 California births.

Over the eight birth years, the prevalence rate of autism for children enrolled with DDS/RC increased from 5.8 to 14.9 per 10,000, an absolute change of 9.1 per 10,000. During the same period, the prevalence rate of unexplained MR without autism for children enrolled with DDS/RC decreased from 28.8 to 19.5 per 10,000, an absolute change of 9.3 per 10,000. These changes in rate were not influenced by changes in demographic characteristics such as maternal age, race/ethnicity, maternal education, child gender or whether twins or multiple births.

These data suggest that improvements in detection and changes in diagnostic practices may account for much of the increase in autistic disorder observed among DDS/RC clients. Whether there has also been a true increase is not known. These data clearly demonstrate that autism is much more common than previously believed.

QUESTIONS AND ANSWERS

What proportion of children with autism received services from the DDS/RC system? How complete is your study in counting children with autism?

Our best estimate is that at least 75-80% of children with full autistic disorder are enrolled with the DDS/RC system and are counted in our study. In general, children with milder forms of autism spectrum disorders (such as atypical autism, Asperger's syndrome, and PDD (Pervasive Developmental Disorder)) are not provided with services in the DDS/RC system.

Why didn't you include children who are not DDS/RC clients in your study?

We had no way to identify children outside of the DDS/RC system for this study. We now have a large grant from the Federal Centers for Disease Control and Prevention (CDC) that will give us the resources to include additional children for a large part of the state. In the next few years we will be able get a fuller picture of what the true rate of autistic spectrum disorders is for California children.

The study suggests that what looks like an increase in the rate of autism may be largely explained by a decrease in the use of a diagnosis of MR without autism. Is this plausible?

We think it is. The diagnosis available to us in this study was the one recorded in the electronic file for each child. Typically, this electronically-recorded diagnosis is established at initial determination of eligibility for services. Many of these children are quite young and it may not be possible to know the full diagnosis when they initially become enrolled for services. In recent years, better diagnostic instruments for use with young children have been developed, making it likely that autism will be increasingly recognized in young children and get recorded in the electronic file as the initial enrollment diagnosis.

But is it possible that the rate of mental retardation without autism may have actually decreased? Could prenatal diagnosis and elective terminations mean there is a true decrease in MR?

Our research focused only on mental retardation that is not explained by Down Syndrome or other recognized causes and is not detectable by prenatal diagnosis. Thus, elective terminations are not an explanation for the decrease in MR that we observe. We can think of no other reason that mental retardation may actually be decreasing.

Could at least part of the increase in autism be explained by families with an affected child moving to California to take advantage of the services offered by DDS/RC?

Our study was limited to children born in California. The increase we observe is among Califonia births.

Could at least part of the increase be explained by an increase in children with milder forms of autism or PDD?

Children with mild forms of autism (sometimes called atypical autism, Asperger's syndrome, PDD) have not traditionally been served through the DDS/RC system. There are some indications that this may have changed in more recent years in some areas of the state and this may explain some of the increase in autism we see in the DDS/RC population. Our study could not determine if there is an increase of these children in the general population.

Is the increase in autism greater for any particular subgroups in the population?

Over the eight birth years, we see similar patterns of increase across the range of maternal age and education, race/ethnicity, for boys and girls, and for singleton births and twins and other multiple births. The increase is clearly not confined to any particular groups in our California population.

Are there other studies with similar results?

Studies in general support our finding that autism is much more common in the population than previously believed. California has a unique system of services for children with developmental disabilities that makes it possible for us to do large scale analyses like this, comparing autism and MR diagnoses within one service system. Comparable analyses that look for trends over time in one population have not been done before. However, recent studies conducted in a number of different places show higher rates of autism than was found in earlier studies conducted elsewhere.

What are the implications of this research?

Our study documents that autism is, unfortunately, a common disorder. It is important that children receive early attention and appropriate interventions to help them perform to their maximum ability. Money for research is essential to identify the best interventions, to identify causal factors, to identify ways of preventing autism, and to accurately count the numbers of children with autism in our population.