Asperger syndrome is an uncommon condition, but probably more common than classic autism (the only published population study estimated prevalence at 36 per 10 000 children for Asperger syndrome and 5 per 10 000 for autism (Ehlers & Gillberg, 1993)). Misdiagnosis or delayed diagnosis of this disorder is a serious problem, and the average age at diagnosis is several years later than for autism (Gillberg, 1989). Obviously, this can be traumatic for individuals and families; furthermore, the most effective intervention programmes begin early, and establishing management strategies at an early age can minimise later behavioural problems (Howlin, 1998).

In 1944 Hans Asperger described a condition he termed autistic psychopathy, characterised by problems in social integration and non-verbal communication associated with idiosyncratic verbal communication and an egocentric preoccupation with unusual and circumscribed interests. Patients with this condition had difficulties with empathy and intuition and had a tendency to intellectualisation. They were also clumsy (50–90% had motor coordination problems), found it hard to take part in team sports and exhibited behavioural difficulties including aggression and being victims of bullying. Asperger did not provide diagnostic criteria for this condition and it remained obscure until a review article by Lorna Wing in 1981.

Wing renewed interest in the condition, which she renamed Asperger syndrome, and described the following difficulties in the first 2 years of life of children with the condition:

  1. a lack of normal interest and pleasure in people around them;
  2. a reduction in the quality and quantity of babbling;
  3. a significant reduction in shared interests;
  4. a significant reduction in the wish to communicate verbally or non-verbally;
  5. a delay in speech acquisition and impoverishment of content;
  6. no imaginative play or imaginative play confined to one or two rigid patterns.

A number of authors have subsequently suggested diagnostic criteria, but the six proposed by Gillberg (1991) are, arguably, closest to Asperger’s original description of the syndrome (Box 1). Inclusion of the syndrome in both international diagnostic systems (ICD–10 (World Health Organization, 1992) and DSM–IV (American Psychiatric Association, 1994)) has resulted in broad clinical recognition of the diagnosis, but also in confusion. Asperger syndrome has been renamed Asperger disorder in DSM–IV, and the criteria of both differ from Gillberg’s criteria and Asperger’s original description of the syndrome.

The DSM–IV diagnosis is based on impairment of social interaction and the presence of stereotypical or repetitive behaviours (Box 2). Diagnosis requires that the impairment is clinically significant, occurs before 3 years of age and excludes clinically significant delay in language, cognition or other skills. The ICD–10 research criteria (World Health Organization, 1993) are virtually identical. By excluding speech and language difficulties, the DSM definition of Asperger disorder is narrower than Asperger syndrome as defined by Wing or Gillberg and would exclude some of the original cases described by Hans Asperger. As the vast majority of persons with Asperger disorder/syndrome do have speech and language abnormalities it was hoped that future text revisions of DSM–IV might correct this anomaly. Indeed, the recent DSM–IV–TR guides that the occurrence of “no clinically significant delays in language does not imply that individuals with Asperger Disorder have no problems with communication” (American Psychiatric Association, 2000, p. 80). Examples given include unusual verbosity or subtle abnormalities of social communication (such as turn-taking in conversation). We feel that DSM–IV–TR draws attention to the issue, but underestimates the language difficulty involved. This paper examines the differential diagnosis of Asperger syndrome (Wing, 1981) and disorder (American Psychiatric Association, 1994), beginning with the more common and finish with the less common causes of diagnostic confusion.