To avoid confusion, the term "autistic spectrum disorders" should only be used as the collective term for a group of defined disorders

Autism is a severe neurodevelopmental disorder associated with considerable personal suffering, parental burden and community cost. In recent reports, the prevalence of autism has varied widely from five to 67 cases per 10 000 children, an increase compared with the 3.5–4.5 cases per 10 000 children reported in the 1970s.1,2 This apparent prevalence increase has raised considerable public concern, particularly as it has been temporally linked to the introduction of the measles–mumps–rubella (MMR) vaccine. However, recent epidemiological investigations found no causal link between autism and the MMR vaccine.3,4 The increase in prevalence might indicate a true increase in incidence, but most of the increase can be accounted for by changes in case-finding methods and diagnostic criteria, and by differences in sample sizes, and the age range and intellectual ability of the populations studied.3 The increase in numbers identified has led to a corresponding increase in demand for services.

The predominant international approach to diagnosis used by the ICD-10 classification of mental and behavioural disorders5 and the Diagnostic and statistical manual of mental disorders, 4th edition [DSM-IV],6 groups autistic conditions in the category pervasive developmental disorder (PDD) and specifies criteria for the subtypes autistic disorder, Asperger's disorder and atypical autism (PDD – not otherwise specified [PDD-NOS]) (Box). Recently, some confusion has been introduced, as there is a general move away from the term PDD towards the term autistic spectrum disorder (ASD), which has been used in at least four different ways.

1. To refer to a broader group of conditions sharing a "triad of impairments" in social interaction, verbal and non-verbal communication and imagination. Wing7 introduced the term ASD for this purpose. These conditions include the PDDs, but also disorders of empathy and deficits in attention, motor control and perception.8 The term ASD implies a continuum of disturbance in each of these three domains and has led to empirical studies of the continuum of social reciprocity which has implications for testing the clinical validity of categorical diagnostic subtypes of ASD.9,10 Contemporary genetic studies of autism provide evidence of a broad phenotype of social disability, anxiety, depression and unusual personality traits, and a complex interaction of several genes.11 It is not known whether specific patterns of genetic abnormality or the interaction of other risk factors with a common genetic predisposition operate to produce different developmental outcomes, such as severe language disability. Ultimately, genetic studies will determine the validity of categorical diagnoses. In the meantime, for this broad phenotype construct of ASD to have better research and clinical utility each subtype requires international consensus on the discriminating diagnostic criteria, and reliable methods to measure the continuum of each domain.

2. To describe a continuum of intellectual ability among children with PDD — from normal IQ levels (functioning at a high level) through to severe levels of intellectual disability. Some argue that autism is a single-spectrum condition, with the observed differences resulting from different levels of intellectual ability, and that subtypes (eg, Asperger's disorder) are not empirically justified.12 Other studies have used the criterion of significant delay in language development to differentiate children with autism who function at a high level from those with Asperger's disorder. They have found higher levels of psychopathology in children with Asperger's disorder,13 and significant differences in executive function and motor planning between these two groups.14

3. As a description of symptom severity. For example, the Department of Education and Training in Victoria uses the Childhood Autism Rating Scale15 completed by clinicians to provide a cumulative score on the severity of some autistic symptoms to assist in determining funding for education aides. Applying the concept of severity to a disorder that has multiple diagnostic criteria is problematic. For example, a child with autism might have relatively better developed language skills and only a few untroublesome rituals, but may have severe social withdrawal. Can a concept of overall severity be meaningfully applied to such a child and might it overshadow the recognition of a potentially treatable comorbid condition such as anxiety, depression, or attention deficit hyperactivity symptoms? Children with autism who function at a high level are referred to by some as having mild symptoms,12 even though there is evidence that they are often handicapped by high levels of psychopathology.13

4. As a developmental concept. Autistic spectrum is used to describe how skills, such as language, might improve relatively over time so that some children with autism might move from being less able to being more able.16

Use of the term ASD is likely to persist, but to avoid confusion it should be confined to the collective term for a group of defined disorders. Wing used it to promote access to services otherwise denied to young people with autism functioning at a high level.7 This problem still exists, for example in Victoria. Such children do not meet the criteria for funding for an education aide, even though they have a range of severe social, emotional and behavioural problems. Should the inclusion criteria for services be broadened to include the full range of social, emotional and behavioural needs, or, to contain costs, should they be confined to intellectual ability?

Behavioural and educational approaches to treatment have received the best empirical support.17 Therefore, allocation of increased funding to support a flexible range of behavioural, educational and family support programs, based on a comprehensive assessment of the diagnostic and cognitive profile and the emotional and behavioural needs of all young people with a PDD, is likely to prove the most economical use of resources in the long term.