The Epidemiology of Asperger Syndrome: A Total Population Study
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The Epidemiology of Asperger Syndrome: A Total Population Study

© Cambridge University Press, 1993 This article was originally published in The Journal of Child Psychology and Psychiatry and Allied Disciplines, Vol. 34, No. 8, pp. 1327-1350, November 1993. It is provided to Asperger Syndrome Coalition of the United States, Inc. for use on its website with the express written permission of Cambridge University Press. It may be downloaded and printed for personal reference, but not otherwise copied, altered in any way or transmitted to others (unless explicitly stated otherwise) without the written permission of Cambridge University Press. Hypertext links to other Web locations do not constitute any endorsement or authorization by Cambridge University Press.

Abstract:
This paper describes a total population study of Asperger syndrome using a two-stage procedure. All school children in an outer G–teborg borough were screened. Final case selection based on clinical work-up showed a minimum prevalence of 3.6 per 1,000 children (7-16 years of age) using Gillberg and Gillberg's criteria and a male to female ratio of 4:1. Including suspected and possible Asperger's syndrome cases, the prevalence rose to 7.1 per 1,000 children and the male:female ratio dropped to 2.3:1. These findings are discussed as they relate to previously published results in the field and to findings obtained using Szatmari et al.'s and ICD-10 draft criteria for the disorder. 

 

Introduction
Since Lorna Wing's vivid clinical account of "Asperger's syndrome" appeared in 1981 (Wing, 1981), about 50 articles have been published on the subject. However, the epidemiology of Asperger syndrome (AS) has never been specifically studied.

Considering that the 10th revision of the International Classification of Diseases (ICD-10) (World Health Organization, 1992) includes "Asperger's syndrome" as a subcategory of Pervasive Developmental Disorders, obtaining reliable prevalence estimates becomes mandatory.

A review of indirect data from neighboring fields revealed that a minimum prevalence in children who attend normal schools would be 2.6 per 1,000 children, i.e. several times higher than that reported for autism (Gillberg & Gillberg, 1989). The rate among mentally retarded children appears to be similar (Gillberg, Persson, Grufman & Themnr, 1986). Still, given the relatively low prevalence of mental retardation, AS cases recruited from this subpopulation add little to the total population prevalence. The available data originate from surveys not particularly geared to examining AS prevalence. The object of the present study was to provide reliable epidemiological data on AS. 
 
Defining AS
The definition of a "case" presents difficulties in all prevalence studies on childhood psychiatric disorders (Schwartz Gould, Wunsch-Hitzig & Dohrenwend, 1981), and AS is no exception. The main reason for this is that we lack a true "gold standard" (Szatmari, 1989). Furthermore, there is no universal agreement on diagnostic characteristics (Szatmari, 1991). Asperger's own descriptions are penetrating but not sufficiently systematic (Asperger, 1944). His frame of reference was Bleuler's typology (i.e. "autistic psychopathy") that is out of keeping with current diagnostic manuals. Wing, in her pioneer paper, did not explicitly spell out which symptoms had to be present for a diagnosis to be made, though more specified characteristics are presented in a recently published paper (Wing, 1991). Other authors, such as Gillberg and Gillberg (1989), and Szatmari, Brenner and Nagy (1989) from Asperger's and Wing's work, clinical experience and comparative studies, have proposed operationalized diagnostic criteria. In several other publications (Bosch, 1970; Van Krevelen, 1971; Wolff & Barlow, 1979; Wolff & Chick, 1980; Nagy & Szatmari, 1986; Kerbeshian & Burd, 1986; Rutter & Schopler, 1987; Kay & Kolvin, 1987; Bowman, 1988; Frith, 1989; Tantam, 1988a, b, 1991; Goodman, 1989; Bishop, 1989; Baron-Cohen, 1988; Kerbeshian, Burd & Fisher, 1990; Green, 1990; Howlin, 1991; Cox, 1991; Wolff, 1991a, b; Wolff, Townsend, McGuire & Weeks, 1991; Gillberg, 1992) the delineation of the syndrome vis-ý-vis autism, schizoid personality and schizotypal personality disorder, Tourette syndrome, semantic-pragmatic language disorder and obsessive-compulsive disorder has been discussed. The criteria of the Gillbergs (1989, 1991) and Szatmari et al. (1989) have been elaborated with the purpose of making them compatible with current diagnostic manuals. The proposals of these two groups have much in common, but differ in that the Gillbergs underline the children's obsessional and narrow pattern of interest and Szatmari et al. highlight their social isolation. In short, Szatmari et al.'s criteria appear to be slightly more in line with "the passive" and the Gillbergs' criteria with "the active but odd" in Wing's typology of autism spectrum disorders (Wing & Gould, 1979; Wing & Attwood, 1987).

The ICD-10 draft criteria for AS (WHO, 1990) are similar to those of the Gillbergs, but with one important exception. In the ICD-10 "the term Asperger's syndrome proposes that there is a group of individuals who have a disorder of social development similar to that found in infantile autism, but with a pattern of early language development that appears grossly normal" (Cox, 1991). Accordingly, criteria of abnormalities in verbal communication are not included. However, by not including operationalised criteria of abnormalities, such as odd prosody and semantic-pragmatic problems, the ICD-10 criteria exclude features that other authors in the field find important and characteristic of AS (Asperger, 1944; Wing, 1981, 1991). Current diagnostic criteria for AS are outlined in Tables 1-3.

In the present study we applied the diagnostic criteria for AS outlined by Gillberg and Gillberg (1989) and elaborated in Gillberg (1991), since, at the time of embarking on the study, these were the only published criteria available to us. Also, we report the result of applying Szatmari et al.'s (1989) and the ICD-10 (WHO, 1990) diagnostic criteria for the disorder.

  

Table 1. Asperger syndrome: Gillberg and Gillberg's (1989)
diagnostic criteria elaborated 
   
 

1.  Social impairment (extreme egocentricity)
     (at least two of the following): 
          - Inability to interact with peers 
          - Lack of desire to interact with peers
          - Lack of appreciation of social cues 
          - Socially and emotionally inappropriate behavior

2.  Narrow interest (at least one of the following): 
          - Exclusion of other activities 
          - Repetitive adherence 
          - More rote than meaning

3.  Repetitive routines (at least one of the following):
         - On self, in aspects of life
         - On others

4.  Speech and language peculiarities (at least three of the following): 
         - Delayed development
         - Superficially perfect expressive language
         - Formal pedantic language 
         - Odd prosody, peculiar voice characteristics
         - Impairment of comprehension including misinterpretation of literal/implied meanings

5.  Nonverbal communication problems (at least one of the following): 
         - Limited use of gestures 
         - Clumsy/gauche body language 
         - Limited facial expression 
         - Inappropriate expression 
         - Peculiar, stiff gaze

6.  Motor clumsiness:
        Poor performance on neuro-developmental examination 
 

 
Table 2. Asperger's syndrome: Szatmari et al.'s (1989)
diagnostic criteria 
    
1.  Solitary - two of: 
          - No close friends 
          - Avoids others
          - No interest in making friends
          - A loner

2.  Impaired Social Interaction - one of:
          - Approaches others only to have own needs met
          - A clumsy social approach 
          - One-sided responses to peers
          - Difficulty sensing feelings of others 
          - Detached from feelings of others

3.  Impaired Nonverbal Communication - one of:
         - Limited facial expression 
         - Unable to read emotion from facial expressions of child 
         - Unable to give messages with eyes
         - Does not look at others 
         - Does not use hands to express oneself 
         - Gestures are large and clumsy
         - Comes too close to others

4.  Odd Speech - two of:
         - Abnormalities in inflection 
         - Talks too much
         - Talks too little 
         - Lack of cohesion to conversation
         - Idiosyncratic use of words 
         - Repetitive patterns of speech

5.  Does not meet DSM-III-R criteria for:
         - Autistic disorder 
 
 
 
Table 3. Asperger's syndrome: ICD-10 (1990)
diagnostic criteria 
    
  1. A lack of any clinically significant general delay in language or cognitive development. Diagnosis requires that single words should have developed by two years of age or earlier and that communicative phrases be used by three years of age or earlier. Self-help skills, adaptive behavior and curiosity about the environment during the first three years should be at a level consistent with normal intellectual development. However, motor milestones may be somewhat delayed and motor clumsiness is usual (although not a necessary feature). Isolated special skills, often related to abnormal preoccupations, are common, but are not required for diagnosis.  
  2. Qualitative impairments in reciprocal social interaction (criteria as for autism). Diagnosis requires demonstrable abnormalities in at least three out of the following five areas:
    1. failure adequately to use eye-to-eye gaze, facial expression, body posture and gesture to regulate social interaction;
    2. failure to develop (in a manner appropriate to mental age, and despite ample opportunities) peer relationships that involve a mutual sharing of interests, activities and emotions;
    3. rarely seeking and using other people for comfort and affection at times of stress or distress and/or offering comfort and affection to others when they are showing distress or unhappiness;
    4. lack of shared enjoyment in terms of vicarious pleasure in other people's happiness and/or a spontaneous seeking to share their own enjoyment through joint involvement with others;
    5. a lack of socio-emotional reciprocity as shown by an impaired or deviant response to other people's emotions; and/or lack of modulation of behavior according to social context, and/or a weak integration of social, emotional and communicative behaviors  
  3. Restricted, repetitive, and stereotyped patterns of behavior, interests and activities (criteria as for autism; however it would be less usual for these to include either motor mannerisms or preoccupations with part-objects or nonfunctional elements of play materials). Diagnosis requires demonstrable abnormalities in at least two out of the following six areas:
    1. an encompassing preoccupation with stereotyped and restricted patterns of interest;
    2. specific attachments to unusual objects;
    3. apparently compulsive adherence to specific, nonfunctional, routines or rituals;
    4. stereotyped and repetitive motor mannerisms that involve either hand/finger flapping or twisting, or complex whole body movements;
    5. preoccupation with part-objects or nonfunctional elements of play materials (such as their odor, the feel of their surface, or the noise/vibration that they generate);
    6. distress over changes in small, nonfunctional, details of the environment

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