Editorials

Autistic spectrum disorders

Autism seems to be on the increase. This at least is the feeling of many professionals in the field of child development in Britain, who believe that in recent years they have been seeing more children with autistic spectrum disorders. Autism was first characterised in 1943 by Kanner,¹ who described a group of children with an unusual pattern of behaviour present from birth or before 30 months. He called this "early infantile autism." His essential criteria for diagnosis were social aloofness and elaborate repetitive routines.² Subsequent epidemiological studies have shown that autism is not, as Kanner first thought, a unique and separate condition occurring in children of otherwise normal development but that it is closely related to a range of developmental disorders.^{3 4} These include Asperger's syndrome,⁵ a condition characterised by borderline or normal IQ; social isolation or naive, inappropriate social interaction; intensive interest in only one or two subjects; a narrow, repetitive life style; limited or inappropriate intonation and body language; and poor motor coordination.

Autistic spectrum disorders share a triad of impaired social interaction, communication, and imagination, associated with a rigid, repetitive pattern of behaviour. Onset is usually at birth or during the first three years of life, but problems can begin later in childhood. The triad can be recognised at all levels of intelligence and can occur alone or together with any other physical or psychological disorder.^{3 4} Under the Inter national Classification of Diseases, the whole range is termed pervasive developmental disorders, but in Britain parents understand and prefer the term autistic spectrum disorders.

Estimates of prevalence are tentative

Only two published studies have attempted to examine the prevalence of all autistic spectrum disorders, one concentrating mainly on children with intelligence quotients (IQs) under 70 and the other on those with IQs of 70 or above. The first study included children aged under 15 years, born between 1956 and 1970, and living in the former borough of Camberwell in inner London who were attending any kind of special school or receiving special educational help.³ Nine per cent of children with an IQ below 70 had autistic spectrum disorders. The proportion in the total population was 22 in 10000 children, and, because of the method of selection, only two in 10000 of these children had IQs above 70. These children with higher IQs had the behavioural features of Asperger's syndrome.⁵

The second study examined children aged 7-16 years, born in 1975-83, living in Torslanda (a mainly middle class borough of Gothenburg, Sweden), and attending mainstream schools.⁶ At least 71 in 10000 children had disorders in the autistic spectrum and IQs of 70 or above, and at least half of these children showed criteria for Asperger's syndrome.

Both studies involved population screening followed by intensive examination and diagnosis of selected individuals by means of specially designed schedules for eliciting and recording information relevant to specified diagnostic criteria. The demographic differences between the areas are unlikely to have had a major effect on the prevalence; earlier studies in Gothenburg of children with IQs below 70 gave similar rates for the triad of impairments to those found in Camberwell.⁷ It is therefore permissible to combine findings from the two studies to give a tentative estimate for the prevalence of autistic spectrum disorders of 91 per 10000 children aged under 16 of any level of IQ. Both studies underlined the difficulties of defining the borderlines of the autistic spectrum and of subgroups within it. Diagnosis depends on recognising the complex patterns of behaviour revealed in the developmental history and current clinical picture. This requires training and experience, and even then there is much scope for individual interpretation of the criteria.

Specialist referrals have raised awareness

In 1993 I reviewed 16 studies of the epidemiology of "typical autism," defined in various ways, carried out in Europe, the United States, Canada, and Japan.⁸ The prevalence varied from 3.3 to 16 per 10000. There was no evidence of an increase in prevalence over time. In studies where the authors strictly applied Kanner's original criteria, the observed rates were closely similar. There was some evidence from British and Swedish studies of higher proportions, even of Kanner's autism, among children of parents who had emigrated to Europe from "exotic" countries; one possible explanation might be lack of immunity to viruses in the host country of the type that can cause intrauterine infections.⁴ However, immigration cannot account for the fact that professionals from areas where there are no or few immigrants also feel that numbers are increasing. There are no published studies of prevalence among later descendants of immigrants.

Another factor that may contribute to the impression that autism is increasing is a change in referral practices in Britain. Before the 1970s, most children with learning disabilities were admitted to special schools, training centres, or institutions without being referred for specialist diagnosis of the developmental disorder. Nowadays, referral for expert medical diagnosis and treatment is the rule rather than the exception, resulting in greater awareness and interest in such conditions. Psychiatrists in adult mental illness are also beginning to recognise that some people they see have autistic spectrum disorders, especially Asperger's syndrome.

Thus there is no firm evidence for or against a general rise in the prevalence of "typical autism" or other autistic spectrum disorders. The impression that there is a rise could be due to a change in referral patterns, widening of diagnostic criteria for typical autism (which are difficult to apply with precision anyway), and increased awareness of the varied manifestations of disorders in the autistic spectrum (especially those associated with higher IQ). On the other hand, there might be real changes in prevalence, locally or nationally, due to temporary or permanent factors. Some recent research on typical autism suggests that complex genetic factors may have a major role in its aetiology.⁹ However, in a minority of cases, mostly comprising people who are severely disabled, other physical causes may be implicated.^{3 4} It is possible that there are interactions between genetic susceptibility and other physical factors. There is also some evidence that mothers of children with typical autism are of higher than average maternal age.¹⁰ If this is a real association, changes towards later childbirth¹¹ might affect the prevalence of typical autism and possibly other autistic spectrum disorders.

Counting the numbers of children diagnosed in clinics is not an appropriate method of investigating changes in prevalence. Methods of early detection now being explored, based on observations of the development of joint referencing and pretend play in 18 month old children,¹² might facilitate the serial studies of incidence and prevalence that would be needed to demonstrate changes in prevalence in years to come.

Consultant psychiatrist Centre for Social and Communication Disorders, Bromley, Kent BR2 9HT

Lorna Wing 

1. Kanner L. Autistic disturbances of affective contact. Nervous Child 1943;2:217-50.
2. Kanner L, Eisenberg L. Early infantile autism, 1943-1955. Am J Orthopsychiatry 1956;26:55-65.
3. Wing L, Gould J. Severe impairments of social interaction and associated abnormalities in children: epidemiology and classification. J Autism Dev Disord 1979;9:11-29. [Medline]
4. Gillberg C. The Emmanuel Miller Memorial Lecture 1991. Autism and autistic-like conditions: subclasses among disorders of empathy. J Child Psychol Psychiatry 1992;33:813-42. [Medline]
5. Wing L. Asperger's syndrome: a clinical account. Psychol Med 1981;11:115-29. [Medline]
6. Ehlers S, Gillberg C. The epidemiology of Asperger syndrome. A total population study. J Child Psychol Psychiatry 1993;34:1327-50. [Medline]
7. Gillberg C, Persson E, Grufman M, Themner U. Psychiatric disorders in mildly and severely mentally retarded urban children and adolescents: epidemiological aspects. Br J Psychiatry 1986;149:68-74. [Abstract/Free Full Text]
8. Wing L. The definition and prevalence of autism: a review. European Child and Adolescent Psychiatry 1993;2:61-74.
9. Bolton P, MacDonald H, Pickles A, Rios P, Goode S, Crowson M, et al. A case-control family history study of autism. J Child Psychol Psychiatry 1994;35:877-900. [Medline]
10. Gillberg C. Maternal age and infantile autism. J Autism Dev Disord 1980;10:293-8. [Medline]
11. Gosden R, Rutherford A. Delayed childbearing. BMJ 1995;311:1585-6. [Free Full Text]
12. Baron-Cohen S, Cox A, Baird G, Swettenham J, Nightingale N, Morgan K, et al. Psychological markers in the detection of autism in infancy in a large population. Br J Psychiatry (in press).