Tuesday, April 18, 2006

No Evidence of "Epidemic" from Prevalence Studies

Given all the hype about the "autism epidemic" we are supposed to be experiencing, you would think there must be some evidence of it outside special education and administrative disability caseloads. Surprisingly, a review of epidemiological studies does not appear to support the existence of said epidemic.

I will list my findings below, trying to compare apples to apples. This is difficult because different studies don't use the same methodology, criteria and cohorts. Furthermore, while prevalence numbers from these studies are not as unreliable as those derived from administrative caseloads, their results are likely still affected by awareness, perceptions, willingness to diagnose, willingness to seek a diagnosis and willingness to accept a diagnosis.

Classic Autism

Table 1 lists what different studies have found regarding the prevalence of what I will call "classic autism". This is not intended to include other PDDs or "high functioning" autism. Not all studies use the same criteria to determine what "classic autism" is, however, but we will assume there is reasonable approximation between them.

Table 1: Studies on Prevalence of "Classic Autism".
StudyWhereCriteriaCohortPrevalence Per 10,000
Wing (1976)England, Wales, U.S. and Denmark?0-144-5
Hoshino (1982)Fukushima-ken, Japan?8-164.96
Gillberg (1984)Gothenburg, Nebraska?4-182
Steffenburg (1986)Swedish rural/urban?0-104.4
Matsuishi (1987)Kurume City, Japan?4-1215.5
Bryson (1988)Nova Scotia, CanadaThree characteristic symptoms?10
Tanohue (1988)Ibaraki, JapanDSM-III10-1613.9
Cialdella (1989)Rhone, FranceDSM-III5-95.1
Sugiyama (1989)Nagoya, Japan??13
Fombonne (1992)Four regions, France?12-164.9
Wignyosumarto (1992)Yogyakarta, IndonesiaBryson's scale plus CARS1-812
Sponheim (1998)NorwayICD-103-144-5
Irie (1999)Higashi-Osaka City, Japan?7-1210.4
Davidovitch (2001)Haifa, Israel?8-1210
Bertrand (2001)Brick Township, New JerseyDSM-IV3-1040
Lauritsen (2004)Denmark?0-911.8
Williams (2005)Two Australian StatesDSM-IV, ICD-100-144.3-5.5
Zhang (2006)Tianjin, China?2-611


I fail to see anything in Table 1 that should be cause for alarm, and that cannot be explained by differences in criteria and diagnostic practices.

It is also unclear where GenerationRescue got their 1 in 10,000 prevalence number for ASD in the 1970s. The concept of a broad autism spectrum did not even exist in the 1970s, which obviously means that no studies can corroborate that number.

There is a notable anomaly, however. The Brick Township study indicates there's a very high prevalence of Autistic Disorder in that area, at least 4 times what should be expected. This excludes other PDDs and Asperger's Syndrome. I believe this study needs to be replicated independently, and equivalence with other regional studies must be documented. It's quite possible researchers in this case had a bias for broader criteria. Use of DSM-IV might also be of significance. Some forms of "high functioning" autism are easily included in that definition.

The prevalence for all other PDDs and Asperger's syndrome in Brick Township was 27 per 10,000. This seems rather low and might be a reason for the discrepancy.

The numbers could also be affected by increasing recognition of autism in the population classified as having mental retardation. Shah (1982) indicated that 38% of the adult population with mental retardation had social impairments consistent with those of autism. Since about 0.9% of the general population is considered mentally retarded, labeling 38% of them as autistic would result in an autism prevalence of 34 per 10,000 from this population alone. As a point of reference, the current California DDS average for recognition of autism in the population with mental retardation is 7%.

Asperger's Syndrome

Asperger's syndrome was not recognized as a diagnosis until the early 1990s, so we don't have prevalence data before that.

Table 2: Studies on Prevalence of Asperger's Syndrome.
StudyWhereCriteriaCohortPrevalence Per 10,000
Ehlers (1993)Goteborg, Sweden?7-1636
Lauritsen (2004)Denmark?0-94.7
Gillberg (2006)Goteborg, Sweden?7-249.2


Even though there aren't many studies looking specifically at the prevalence of Asperger's syndrome, again, there's no good reason to suppose that it has increased in an alarming way. The great variability in results suggests that Asperger's syndrome is often undiagnosed or diagnosed as autism.

Full Spectrum

The idea of autism as a broad spectrum is fairly new as well. This means there are no studies on the prevalence of a full ASD spectrum before the 1990s. Table 3 lists most of the existing studies. Note that we are comparing apples and oranges to some extent because of differences in methodology. It's not clear which studies include Asperger's syndrome, for example.


Table 3: Studies on Prevalence of ASD.
StudyWhereCriteriaCohortPrevalence Per 10,000
Honda (1996)Yokohama, JapanICD-10521
Kielinen (2000)FinlandDSM-IV, ICD-105-720.7
Bertrand (2001)Brick Township, New JerseyDSM-IV3-1067
Scott (2002)Cambridgeshire, UK?5-1157
Webb (2003)Cardiff, UKICD-108-1220.2
Lingam (2003)North East LondonICD-9, ICD-105-1426
Yeargin-Allsopp (2003)Atlanta, GeorgiaDSM-IV3-1034
Icasiano (2004)Barwon, Australia?2-1739.2
Lauritsen (2004)Denmark?0-934.4
Chakrabarti (2005)Midlands, UK?4-658.7
Gillberg (2006)Goteborg, Sweden?7-2453.4


Again, there is nothing alarming in Table 3. Discrepancies could be explained by differences in criteria and diagnostic practices. The Brick Township study stands out, but not by much. The autism spectrum could easily be expanded beyond where it currently stands, resulting in an apparent prevalence increase.

Summary

A review of epidemiological studies on the prevalence of Classic Autism, Asperger's Syndrome and Autism Spectrum Disorders fails to show that an "epidemic" has occurred, despite all the hype. The Brick Township study is anomalous and should be replicated independently. In general, I would qualify these studies as messy since they make it very difficult to compare apples to apples. Methodologies that permit equivalence comparisons should be proposed and applied.

5 comments:

  1. Hi - I recently stumbled upon your blog and have really been enjoying your writing. Now about this literature examination...have you run any statistical analyses on these numbers? I'd be curious to see what exactly they'd say, if anything.

    ReplyDelete
  2. Thanks Tim. There are some reviews of the literature, but a proper analysis is basically impossible given the differences in methodology. They invariably end up comparing apples to oranges. You'll find conflicting conclusions as well.

    See for example:

    Kippes (2006)

    Williams (2006)

    Rutter (2005)

    Blaxill (2004)

    ReplyDelete
  3. I'll check those articles out, thanks. It's true that it would be difficult to analyze data without knowing methodologies, etc., but something revealing can come from some level of analysis. But I suppose that enough researchers have done pretty extensive reviews and meta-analyses, so I guess the work might be redundant. My first thought after reading your post was just that it's difficult to look at numbers and pull out or imply conclusions simply by sight, you know?

    ReplyDelete
  4. Right, but you can look at the numbers and see that the huge increases claimed by GenerationRescue and the like are not consistent with reality.

    ReplyDelete
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