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Re: Determining the true prevalence of Autistic Spectrum Disorders

Dear Editor,

We read with interest the January edition of Archives, containing three thought-provoking articles on the Autistic Spectrum. The article by Harrison et al (1), which described the two source “capture-recapture” technique as a means of determining “true” prevalence, provided an enlightened alternative to using conventional, “passive” ascertainment of point prevalence. We have conducted an 18 month retrospective audit of our social communication clinic for children in Enfield. The primary aim was to compare our clinic with the recommendations set out by the National Autism Plan (2); however it also raised issues regarding:

i) the difficulties in obtaining accurate prevalence and incidence estimates and

ii) the impact of changes in child health surveillance policy.

Cases were identified from the Enfield social communication clinic referral dataset for the referral period January 2004-June 2005. 124 children were referred during this time period and Sixty-six children were found to have fulfilled the criteria for Autistic Spectrum Disorder (using either ICD-10 or DSM IV). The birth rate in Enfield is approximately 4000/year. The age-related incidence of ASD was calculated looking at chronological years. In the age groups 2-3yrs and 3-4 yrs, for which 40 cases were diagnosed over an 18 month period, we found an age- specific incidence of 33.2 per 10 000 children per year. This is considerably higher than the incidence rate of 8.3 per 10 000 reported by Powell et al (3) who looked at pre-school children being diagnosed in the West Midlands between 1991-1996. Published prevalence estimates are however highly variable, as demonstrated by the recent systematic review by Williams et al4, ranging from 3.3-121 per 10 000 children, largely dependent on diagnostic criteria, age of assessment and case-load location.

Enfield’s Pupil Level Annual School Census records from 2005 reveal that the LEA was aware of 255 children known to have a diagnosis of ASD (point prevalence of 46 per 10 000). Capture and Recapture methodology applied to SEN data and our records over a longer period of time would be expected to give a truer picture of ASD incidence and prevalence.

We were surprised by the Harrison et al (1) Scottish data where only 5% of cases were less than four years of age. 60% of the children diagnosed in our social communication clinic fell within the 2-4 years age group. We speculate that this is primarily due to our referral patterns (Figure 1).

We found that 26% of our referrals came from Health Visitors following the 2 year check. This suggests that routine surveillance is making a significant contribution to the early detection of autistic spectrum disorders in Enfield. This has been documented by other investigators. Tebruegge et al (5) conducted a retrospective study of Year 4 pupils attending 74 schools in Maidstone which found that 68% of children diagnosed with ASD had been referred for further assessment as a consequence of routine 2year/3.5 year checks. The uptake rate of the 2 Year check offered by Health Visitors in Enfield is around 80 % (PIMS Data 2001) and is a good opportunity for communication difficulties to be picked up in a culturally diverse area where questionnaire based assessments are less effective. Unfortunately the extent of this contribution to Enfield will only be clear when the current local practice is discontinued.

There are clearly barriers to sharing database information across agencies but the ability to do so would help ascertain not only the “true” prevalence of ASD but also local demographic needs, enabling comprehensive planning and provision of services.

L Perera, R Vijeratnam, E Bolland.

Department of Community Paediatrics, Enfield PCT.

Correspondence to: Dr RS Vijeratnam (Rajan.Vijeratnam@enfield.nhs.uk)

Competing interests: none

References:

1) Harrison MJ et al. Prevalence of autistic spectrum disorders in Lothian, Scotland:an estimate using the “capture-recapture” technique. Arch Dis Child 2006;91:16-19

2) National Autism Plan for Children, March 2003.Published by the National Autistic Society for NIASA in collaboration with The Royal College of Psychiatrists,The Royal College of Paediatrics and Child Health (RCPCH) and the All Party Parliamentary Group on Autism (APPGA). Downloadable from www.nas.org.uk

3) Powell JE, et al. Changes in the incidence of childhood autism and other autistic spectrum disorders in preschool children from two areas of the West Midlands, UK. Dev Med Child Neurol. 2000;42:624–628

4) Williams JG, Higgins JPT, Brayne CEG. Systematic review of prevalence studies of autism spectrum disorders. Arch Dis Child 2006;91:8- 15

5) Tebruegge M, Nandini V, Ritchie J. Does routine child health surveillance contribute to the early detection of children with pervasive developmental disorders? An epidemiological study in Kent, UK. BMC Pediatrics 2004, 4:4 Published online March 2004 www.biomedcentral.com/bmcpediatr

Figure 1:Source of referalls to SCC