Please note, this post is just the result of personally driven research over the past couple of days. It is not the informed opinion of someone who works with or lives with children who have either Fragile X or Rett syndrome. I am very open to being corrected if I have made any factual errors. I saw researching, writing and sharing this post as an educational opportunity. Please feel free to accordingly further my education.
The criteria for identifying autism spectrum disorder (ASD), refer to symptoms rather than cause. So there are a variety of causes (some known, many unknown), that will lead to an ASD diagnosis. With the looming changes to the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR to DSM-5), there has been increased awareness and discussion of how known genetic causes – such as Fragile X syndrome and Rett syndrome – affect the accuracy of an ASD diagnosis.
The proposals in DSM-5, recommend the removal of Retts from the diagnostic manual, and therefore, out of its current position as one of the five pervasive developmental disorders (PDDs) that form the “autism spectrum.” However, Fragile X, which is the leading known genetic cause of autism, would not be excluded from the spectrum in the DSM-5. Understanding why these two genetic conditions which lead to autism, will be treated differently in regards to autism in the DSM-5, sheds some light on the broader relationship between genetic causes of autism, and autism as a symptom-defined condition.
Like ASD, Rett syndrome can be diagnosed purely on symptom criteria. Unlike autism (in the broadest sense; the spectrum as a whole), the specific genetic mutation causing the symptoms of Retts, is known and can be identified in the individual. The rationale for the removal of Retts from the DSM-5 turns not strictly on the fact that the cause of the autism is known (otherwise, Fragile X and other genetic causes would also exclude an autism diagnosis). The rationale includes the fact that Rett patients often only have autistic symptoms for a brief period in early in childhood, so inclusion in the autism spectrum is not appropriate for most of these patients. The diagnosis of autism is usually highly stable, with only 7% falling off the spectrum on follow-up. (As a side point, it is interesting to note too that Retts almost exclusively affects girls, whereas autism is usually found in a male to female ratio of 4:1.)
Fragile X syndrome is not so closely tied to the symptoms that match autism, as Retts is. Whereas Retts syndrome is considered part of the autism spectrum, many people with Fragile X syndrome are not autistic, can exhibit other and distinct (sometimes severe) outcomes of the syndrome, and can be carriers (not exhibiting symptoms at all). About a third of children with Fragile X syndrome meet the criteria for ASD (though only between 2% and 6% of autistic children have Fragile X syndrome).
The proposed DSM-5 recommends clearly identifying if there is a known genetic cause of a person’s autism; explicitly including Fragile X as an example of such a cause. There are good reasons for this. Not just because the family and extended family need to know they might be carriers and how this can affect future children. But also because particular treatments and therapies better suit children with Fragile X (or other conditions), than the wider autistic population. There is ongoing research also specifically directed at Fragile X, which could specifically benefit those people whose autism is caused by it (it’s worth noting too that breakthroughs in such research may indeed also end up benefitting those with autism not caused by Fragile X).
So Fragile X and autism are separate diagnoses, which may also occur together. In the same way that a vast variety of other conditions can make it more likely a child exhibits autistic behaviours, yet falls short of meeting the criteria for autism.
Other than Rett syndrome, there are no other genetic conditions that automatically and explicitly fall under the autism spectrum umbrella in the DSM-IV. That is to say, there is no single and known genetic (or as yet, environmental) cause for the other categories; CDD, PDD-NOS, Aspergers, and (classic) autism.
(Those four remaining categories of autism spectrum disorder, would be essentially folded into the single reference term of “autism spectrum disorder” rather than “PDD” as they are currently know in the DSM-IV-TR. The criteria for that new all-encompassing category will be changed to accommodate and clarify what falls under its umbrella, and what does not. The exact details of that are far wider ranging that what I am covering in this post, though I have previously shared my personal opinion on the matter if you’re interested.)
As autism and its range of causes become better understood, it will be interesting to see what goes the way of Rett (as a separate, genetic condition that often presents differently from the majority of people on the spectrum), and what goes the way of Fragile X (an underlying condition that mostly does not present itself as autism). There are going to be (and already are) a raft of conditions that fall between and beyond those two genetic examples, but they are instructive in trying to get your head around the ongoing discourse about the relevance of autism causes, and about the future of the autism diagnostic category more generally.