Friday, October 07, 2005

Reviewing the Autism Prevalence (The Epidemiology: Part 3)

The question as to why deviation and variation exist at all can be partly accounted by chance, but more so by other factors and specifically the changes in diagnostic criteria.

During that time span 1966-2001, we progressed from individual researchers criteria for autism such as Treffort (1970), to DSM-III criteria used by Sugiyama & Abe (1989). Finally, we came to the DSM-IV criteria used by Bertrand et al. (2001). Gernsbacher et al. (2005) also mentions that better awareness of Autism Spectrum Disorders and the fact that new diagnostic categories take time to be fully utilized, account for some of the change over the years and more precise diagnostic instruments are now used as well.

This is could account for the rise we see over time. If only the specific discoverers of Autism Spectrum Disorders and those who were familiar with their work used the term of Autism, the bulk of mental health professionals would not be likely to apply that term to a particular person.

In the DSM-III (1980) their were only two categories, but this set down in stone, what the criteria for diagnosing Autism Spectrum Disorders were. Other mental health professionals could more efficiently learn and use the criteria for diagnosis. The 1980 decade correlates to a jump in the prevalence in the 1980s. In DSM-IV (1994) the 1990 decade correlates to another jump. In the DSM-IV, there are five categories and a more permissive diagnostic structure (Gernsbacher et al., 2005).

Although correlation is not causation, it is possible to conclude that the change in the diagnostic criteria is responsible for the increase in the prevalence of Autism Spectrum Disorders as a whole and also of Autistic Disorder Specifically. Put more basically; Autism is what you define it as. The more permissive the diagnostic structure, the more persons whom the term will apply to.

The mental health epidemiology has been conducted in multiple countries; including Sweden (Kadesjo, Gillberg, & Hagberg, 1999), Finland (Kielinen, Linna, & Moilanen, 2000), The United States (Treffort, 1970), Japan (Sugiyama & Abe, 1989), Germany (Steinhausen, Gobel, Breinlinger, & Wohlloben, 1986) France (Ciadella, & Mamelle, 1989), The United Kingdom (Webb, Lobo, Hervas, Scourfield, & Fraser, 1997), and Iceland (Magnusson, & Saemundsen, 2001).

It has been noted that the source used to diagnose effects the prevalence rate. The DSM-IV Autistic Disorder is more permissive than is the DSM-III equivalent Infantile Autism (Gernsbacher et al., 2005). In addition nearly three fourths of the present day Autism Spectrum Disorder diagnoses are not Autistic Disorder (Chakrabarti, & Fombonne, 2001). Only one of these variants was present in the DSM-III. The change in diagnosis patterns in Autistic Disorder and in the PDD’s in general can be traced to changes made from the DSM-III to DSM-IV.

However such statements have been disputed. A good example of the problem is given by the Medical Investigation of Neurodevelopmental Disorder (MIND) Institute which was commissioned by the California Legislature to determine if the changes in Autism prevalence could be caused the loosened diagnostic criteria. The MIND Institute concluded that the increase in Autism could not be explained by the loosening of the criteria for Pervasive Developmental Disorder (p. 5, M.I.N.D. Institute, 2002). The study consisted of studying two cohorts. The first was born between 1983-1985. The second was born between 1993-1995. Both sets were assessed using DSM-IV criteria. The authors found that both groups, former and latter, met criteria. Since both cohorts met criteria they were then compared for prevalence. The cohort born in 1993-1995 had far more. Ergo, the authors concluded that the increase was not due to diagnostic changes (p. 5, M.I.N.D. Institute, 2002).

A problem is noted by (Gernsbacher et al., 2005) in that the earlier cohort would have been diagnosed under DSM-III criteria which were stricter. The DSM-IV criteria were loose enough to encompass both cohorts. That is unlikely to be said for the DSM-III criteria. (Gernsbacher et al., 2005) note the faulty logic used here and provide an excellent additional example based on theoretical male height measurement.

In addition, I will provide an example based on pebbles and a strainer. The DSM-III is a strainer which a number of pebbles fell through. The DSM-IV is a strainer with bigger holes compared to the DSM-III strainer. More pebbles are going to fall through. Furthermore, if you put all of the pebbles that fell through the DSM-III strainer into the DSM-IV strainer, of course they will go through. It is impossible to conclude whether change occurred because of diagnostic changes based on such design logic.


Same as in the previous post “Reviewing the Autism Prevalence (The Epidemiology: Part 2)”


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