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Autism cases level off

 

Clinical bottom line

There is a levelling off in the number of autism cases since the early 1990s. Neither this nor the previous increase was related to any change in the rate of MMR vaccination. MMR vaccination was mentioned by a small proportion of all parents (less than 2%) as a possible trigger, but that increased after the 1997 MMR scare. Age at diagnosis has decreased, suggesting changes in diagnostic practice.


Reference


R Lingam et al. Prevalence of autism and parentally reported triggers in a north east London population. Archives of Diseases in Childhood 2003 88: 666-670.


Study

This was an update on a previous study (Bandolier 86-4) examining the prevalence of autism in the UK in north east London where there was a computerised record of children with autistic spectrum disorders. This was augmented by approaching community paediatricians, special needs schools, and local child psychiatry teams for additional cases. The latest review included record identified in 2000.

The diagnosis was that made by the principal clinician, using the date as the first mention of autism in any clinical record or letter for date of provisional diagnosis. Records were also examined for any mention of regression, parental concern, and final diagnosis, and the dates of all these events related to immunisation with MMR.

Results

There were 567 children with autistic spectrum disorder born between 1979 and 1998. Using a denominator of the children aged 5-14 years in the area, at the end of 2000 the prevalence of autistic spectrum disorder in children aged 5-14 was 19 per 10,000 children.

The median age of regression, parental concern and both provisional and final diagnosis were much higher in those with Asperger's syndrome than those with childhood or atypical autism (Table 1). There was some evidence of a reduction in age of diagnosis over time. Provisional diagnosis was made at 3.5 years in 1985 but 3.0 years by 1995 for childhood and atypical autism. For Asperger's syndrome, provisional diagnosis was made at 6.0 years in 1985 but 4.9 years by 1995.

Table 1: Major events

 
Diagnosis
Childhood autism
Atypical autism
Asperger's syndrome
Number
278
195
94
Male (%)
83
81
86
Regression (%)
27
23
2
Bowel problems (%)
18
16
10
Median age (months) at:
Regression
18
20
40
Parental concern
18
22
28
Provisional diagnosis
35
44
74
Final diagnosis
40
51
97

 

In 44 of 106 children with childhood or atypical autism a specific trigger was mentioned as a possible cause. These were:

After excluding unvaccinated cases and those vaccinated when aged over 24 months, MMR was mentioned as a trigger in 6/30 (20%) after August 19997 when the MMR theory of a link with autism was first publicised, and in 2/46 (4%) before August 1997.

The trend in diagnosis flattened after about 1991 (Figure 1), at about 2.6 per 1000 live births for childhood and atypical autism.

Figure 1: Annual cases in a population



Comment

There is a levelling off in the number of autism cases since the early 1990s. Neither this nor the previous increase was related to any change in the rate of MMR vaccination. MMR vaccination was mentioned by a small proportion of all parents (less than 2%) as a possible trigger, but that increased after the 1997 MMR scare. Age at diagnosis has decreased, suggesting changes in diagnostic practice.