Autism not on the rise -- correct diagnoses are
By Liat Clark
The study, published in the British Medical Journal, compared phone interview data from interviews with parents of 19?,993 twins, with national patient records of 1,?078,?975 children
A study looking at autism prevalence in Sweden over a ten-year period has concluded that although clinical diagnoses have risen, cases have in fact remained stable. It puts the increase down to "administrative changes" that have impacted how those on the spectrum are diagnosed and registered.
The study, published in the British Medical Journal, is an interesting one in context of continuing concerns that autism diagnoses have inexplicably risen in the past few decades. Studies have in the past investigated the potential reasons behind those rises, from environmental impacts to the highly controversial and widely debunked 1998 Andrew Wakefield paper claiming a link between the MMR vaccine and autism. "From the 1970s and onwards the reported prevalence of autism spectrum disorder has increased substantially," write the study authors. "The condition was considered rare, affecting fewer than 0.05 percent of the population, but it is now generally agreed that the lifetime prevalence is at least 1 percent in both young people and adults." In some parts of the world, that figure is even higher. The authors continue: "Despite the increase in reported prevalence of autism spectrum disorder, there is no direct evidence that this corresponds to an increase in the prevalence of the autism phenotype -- that is, the symptoms on which the diagnostic criteria are based."
The Swedish investigation -- a collaboration between the University of Gothenburg, the Karolinska Institutet in Stockholm and Mount Sinai School of Medicine in New York -- attributed the rise more generally to the broadening of diagnostic criteria, increasing awareness leading to other disorders presenting under the spectrum (studies show a corresponding drop in learning disability diagnoses in the late 90s and early 00s) and availability of services (i.e. more autism specialists gaining more referrals may lead to an increase in diagnoses).
The Swedish researchers sought to help clarify a key issue that leaves questions hanging over historical autism spectrum diagnoses: whether changing trends in evaluating symptoms are in fact responsible for the increased diagnoses.
To find out, they analysed patterns in records going back a decade, from the Child and Adolescent Twin Study in Sweden and the Swedish national patient register. The former began in 2004, and saw parents of all twins born in the country from 1992 asked to take part. The current paper included all twins whose parents took part in the lengthy autism and ADHD phone interviews, some 19 ,993 twins born between 1993 and 2002. The patient register, on the other hand, features inpatient and outpatient data on psychiatric care, including clinical diagnoses. The study authors looked at 1, 078, 975 records on the register, from children born between 1993 and 2002.
The two rich datasets offer two very different things -- a clinician's diagnosis, and a parent's description of their child's behaviour, language and more. (The latter was made to a layperson, and data could be used to retroactively work out how many children presented undiagnosed autism spectrum disorders). The Swedish team looked for a significant contrast between these two datasets -- they did not find one. The symptoms described by parents were reflected in the clinical patient data, when the researchers looked deeper. According to diagnoses included on the patient register, autism spectrum disorder had an average prevalence in the population of 0.42 percent, ranging from 0.23 percent to 0.6 percent. The twin study showed prevalence of 0.95 percent on average, ranging from 0.52 percent to 1.59 percent -- but the levels stabilised.
This is key -- if undiagnosed symptoms are stable, while diagnosed cases rise, it suggests there was no striking increase in cases of autism spectrum disorder; just an increase in correct diagnoses. The patient register was effectively just catching up with the parent interview data, which actually presented at the same frequency as today's autism prevalence percentage -- about 1 percent.
The answer, the study authors conclude, is that administrative changes are responsible for the upward trend in autism cases over the ten-year period. "We believe that our findings indicate that the prevalence of autism spectrum disorder is not increasing in childhood," they write. As a result, they argue, the "allocation of specific resources to study 'the epidemic of autism' should not be allowed to spiral out of proportion". The authors warn that other developmental disorders could be overlooked in the healthcare system if there is a continued emphasis on autism. "Research and clinical practice need to refocus on the child's overall clinical situation and to acknowledge that autism is but one of the many Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations."
The authors argue that although there are some limitations as to the overall accuracy of their study, the huge sample sizes and removal of bias by using phone interview data rather than purely clinical examinations, outweighs these downsides.
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