Does Motor Skill Competence Distinguish Subtypes of Pervasive Developmental Disorder in a 'High-Functioning' Population of Children?

Background:

Motor skills deficits are not core characteristics of Asperger syndrome as defined by DSM-IV-TR, but they are often regarded as being closely associated with the syndrome in the form of clumsiness or dyspraxia. No specific motor skill deficits are described in terms of the diagnostic criteria for autism or PDD-NOS, or their associated characteristics.

Objectives:

We aimed to examine the prevalence of reported delays in motor development, current clumsiness and dyspraxia, in relation to independent clinical diagnoses in a large heterogeneous sample of ‘high functioning’ children with Asperger syndrome, autism, or PDD-NOS.

Methods:

Data from parental report were analysed for a sample of 987 children (4-16 years, mean age 9.0 yrs) from clinics in London, UK, and Tampere, Finland, which had been subject to comprehensive and standardized autism assessments (including ADI algorithm, ADOS). All subjects had normal-range intelligence. Diagnoses were defined according to DSM-IV-TR criteria, and comprised autism (N=199), Asperger syndrome (N=159), PDD-NOS (N=255), and clinical controls without ASD (N=374). Measures of motor competence included developmental motor milestones, current fine and gross motor skills, and a dyspraxia/clumsiness index. Domains were derived by principal components analysis of a validated parent-report scale.

Results:

Ages at which key motor skills were achieved were significantly later in those diagnosed with autism than in Asperger syndrome, after controlling for IQ. No mean difference was found between the PDD-NOS and Asperger syndrome groups on this variable, but both were significantly delayed relative to controls. The three PDD groups did not differ significantly in terms of mean verbal IQ, nor did they differ in this respect from the controls.

In terms of both fine and gross motor skills, children within the broad category of PDD performed significantly less well than clinical controls. There was no mean difference in any measure of motor competence between children with a clinical diagnosis of autism, and those with a clinical diagnosis of Asperger syndrome, although both groups were significantly more impaired than those with a diagnosis of PDD-NOS.

On the measure of dyspraxia/clumsiness children with autism had significantly more severe symptoms than those with Asperger syndrome. Those with PDD-NOS were similar to the Asperger syndrome group and both the latter groups were more significantly impaired than normal controls.

Conclusions:

A delay in onset of language skills, a defining criterion for autism, is associated with delay in the onset of motor milestones too. By middle childhood, there is no significant difference between children with autism and those with Asperger syndrome in terms of fine or gross motor competence. On a measure of dyspraxia/clumsiness, children with autism are more impaired than those with Asperger syndrome. Accordingly, we found no evidence to substantiate the widely-held assumption that motor clumsiness, or indeed any other measure of motor impairment, is characteristic of children who meet criteria for the diagnosis of Asperger syndrome, compared to those with ‘high-functioning’ autism.