Sunday, December 11, 2005

Autism Epidemic Revisited

I am ambivalent about writing an post, since there has been so much blogbuzz on the topic.  Much of the controversy has to do with two things: the notion that the incidence of autism is increasing, and the notion that mercury in vaccines has contributed to the increased incidence of the disorder.  The mercury issue has been pretty well hashed out by Orac, Paul, Skeptico, Autism Diva, and others...too numerous to count.  The "autism epidemic," likewise, had led to an epidemic of writing -- most of it bad.  A Google search on "autism epidemic" (with the quotes) turns up over 66,000 hits.  

And now I've come across another one.  It isn't new.  It was published in July 2005 on Medscape.  (Medscape articles require registration, which is free.  It is a bit of a nuisance, but Medscape is a pretty good resource, so it is worth the trouble.)  I mention this article because it may be one of the better articles on the question of whether the incidence of autism is increasing.  Furthermore, it illustrates some good general points about one of my favorite topics: .  

Publication Logo Autism "Epidemic?" A Newsmaker Interview With Morton Ann Gernsbacher, PhD, And Craig J. Newschaffer, PhD

Laurie Barclay, MD
July 15, 2005 

Editor's Note: Despite heightened media attention on the autism "epidemic," a report published in the July issue of Current Directions in Psychological Science offers three arguments against a true increase in autism prevalence. These include changes in diagnostic criteria for autism, with current criteria being more inclusive than when the diagnosis was first defined in the 1940s; methodological flaws in an unpublished California study widely cited as showing dramatically increased prevalence; and problems in using the U.S. Department of Education's annual "child count" data.

To find out more about this issue and its clinical implications, Medscape's Laurie Barclay interviewed lead author Morton Ann Gernsbacher, PhD, a Vilas Research Professor, the Sir Frederic Bartlett Professor at the University of Wisconsin-Madison, and President-Elect of the American Psychological Society.

For an alternate viewpoint, Dr. Barclay also interviewed Craig J. Newschaffer, PhD, an associate professor of epidemiology at the Center for Autism and Developmental Disabilities, Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. Dr. Newschaffer is lead author of a study using cohort curves to suggest that autism prevalence has been increasing with time, as reported in the March issue of Pediatrics. [...]

The article provides a nice balance of viewpoints, all from persons with decent qualifications.  Reading it is a good exercise in critical thinking.  Reading it correctly requires good analytical skills.  To illustrate:
Medscape: What are the significant changes in diagnostic criteria for autism between 1980 and 1994?

Dr. Gernsbacher:
Whereas the 1980 DSM-III entry required satisfying six mandatory criteria, the more recent 1994 DSM-IV offers 16 optional criteria, only half of which need to be met. Moreover, the severe phrasing of the 1980 mandatory criteria contrasts with the more inclusive phrasing of the 1994 optional criteria. For instance, to qualify for a diagnosis according to the 1980 criteria, an individual needed to exhibit ''a pervasive lack of responsiveness to other people." In contrast, according to 1994 criteria, an individual must demonstrate only ''a lack of spontaneous seeking to share.... achievements with other people'' and peer relationships less sophisticated than would be predicted by the individual's developmental level. The 1980 mandatory criteria of ''gross deficits in language development'' and ''if speech is present, peculiar speech patterns such as immediate and delayed echolalia, metaphorical language, pronominal reversal'' were replaced by the 1994 options of difficulty ''sustain[ing] a conversation'' or ''lack of varied ...social imitative play." ''Bizarre responses to various aspects of the environment'' became ''persistent preoccupation with parts of objects."

Furthermore, whereas the earlier 1980 (DSM-III) entry comprised only two diagnostic categories (infantile autism and childhood onset pervasive developmental disorder), the more recent 1994 (DSM-IV) entry comprises five. [...]
You will see a lot of people debating this point, without really knowing the details.  Some people will say that the increase in reported rates of autism "couldn't" be explained by changes in diagnostic methodology or criteria.  But it is clear -- once you understand all the verbiage, that is -- that the changes in diagnostic criteria were substantial.  Not only that, but the diagnostic categories were broadened, such that milder cases now qualify for a diagnosis.  These are really important points.  The point I am making here is this: you have to know the details, if you are going to make an informed comment on the subject.  This is not one of those areas where an intuitive guess about the numbers will have any validity; you can't say the numbers are just too big to be explained by one factor, and expect anyone to find that persuasive.

Another point is illustrated by the article, one that has nothing to do with autism.  Specifically, skepticism is not merely oppositional thinking; a good skeptic is comfortable saying "I don't know," when the evidence is not sufficient to draw a firm conclusion.  Similarly, a good skeptic is comfortable making a tentative decision.  Of course, when one makes a tentative decision, is is necessary to remember that the decision is not based on solid evidence.  Then, one can avoid the error of establishing complex, important decision trees that do not have solid roots.
Medscape: Why should we be cautious about this label ["epidemic"], given changes in diagnostic criteria and in heightened awareness and recognition of this condition?

Dr. Newschaffer: [...] I believe that there currently is little strong evidence supporting either hypothesis (real risk versus diagnostic bias) and that proponents of one versus another hypothesis seem to hold their view based mainly on the basis of beliefs that are fallacious – either that the increase has been so large [that] some of it has to be real, or that the heritable component of autism is so large [that] the increase over time must be due to diagnostic changes.
Notice the phrase: "little strong evidence supporting either hypothesis."   In other words, she just plain does not know, and will not say, which hypothesis is correct.  If the evidence is not there, it is not there.  Deal with it.  Just don't try to browbeat others into believing something that is not supported by sufficient evidence, merely because you have decided to make a tentative decision on the subject.

(Note: The Rest of the Story/Corpus Callosum has moved. Visit the new site here.)
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Just a few things:

1. The twin studies clearly indicate that there is a strong genetic component. They also indicate that even though twins ostensibly share the same environment, one identical twin with autism does not guarantee that the other twin will have it. This is true regardless of the criteria used for autism diagnosis.

2. There are autism clusters.

Both indicate that in addition to the genetic component there is an environmental component. Clusters therefore epidemic.

Whether you believe that it comes from the plume of a volcano or the point of a needle, there is an epidemic.
Clusters do not prove an epidemic. There could be non-epidemic-causing triggers at play.

Autism is not 100% heritable, but neither is homosexuality or left-handedness or giftedness.

Based on changes in the characteristics of the autistic population over time, I'm quite sure at this point that there is no epidemic.
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