The strange case of ICD code F84.8

We all make mistakes. I do however consider myself to be quite a consientious person; checking and re-checking various things in my life (I am one of those people who has to make sure that the gas cooker is turned off every evening before bed, even though I have no history of ever leaving it on).

I did however make a recent mistake on a reply post I made over at autism.about.com run by Lisa Jo Rudy on the topic of 'Atypical autism or PDD-NOS'. The discussion thread was all about labels and diagnoses; to which I posted, what I thought was an interesting point; a reply which mentioned the ICD-10 criteria for autism spectrum conditions which contained a typo. I was corrected, I might add, by someone with sharper eyes than I.

Getting back to my post: I mentioned those peculiar codes of the ICD-10 autism criteria: F84.8 Other Pervasive Developmental Disorder and F84.9 Pervasive Developmental Disorder, unspecified. It has always been a little bit of a mystery to me as to why ICD-10 contains both these two codings. Maybe I should back up a little and show readers what codings are available in ICD-10 pertient to pervasive developmental disorders:

• F84.0 = childhood autism. 
• F84.1 = atypical autism (including some sub-categories for areas of atypicaility). 
• F84.2 = Rett syndrome. 
• F84.3 = other childhood disintegrative disorder. 
• F84.4 = overactive disorder(?). 
• F84.5 = Asperger syndrome.

Then our mystery F84.8 and F84.9.

F84.9 PDD unspecified has a description saying that it is a residual category for PDD where there is either inadequate information or contradictory findings regarding diagnosis. F84.8 has no description at all. No description but present - the strange case of F84.8.

If we assume that F84.1 (atypical autism) is equivalent to PDD-NOS (or Autism Spectrum Disorder in the UK) and F84.9 is a catch-all for autism or PDD-NOS where some issue/s are present which do not make diagnosis as clear-cut, we are still left questioning what fits into F84.8.

Looking at the research where F84.8 is mentioned I'm afraid I am none the wiser. This paper, published in BMC Pediatrics on the early detection of autism spectrum conditions in the UK makes reference to the coding. The authors imply that F84.1 is correctly used to determine atypical autism but then go on to say that semantic pragmatic language disorder is also covered under either F84.1 or F84.8 or F84.9. This paper also makes reference to the F84.8 and F84.9 codings but again I don't see the distinction.

I assume that, at the time of the ICD-10 planning, there was some logic as to how these codings would be used. Was F84.8 the 'overspill of the overspill' coding?

To quote from Sir Arthur's greatest invention: "when you have eliminated the impossible, whatever remains, however improbable must be the truth".