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Prevalence of Pervasive Developmental Disorders in the British Nationwide Survey of Child Mental Health
Prevalence of Pervasive Developmental Disorders in the British Nationwide Survey of Child Mental Health ERIC FOMBONNE, F.R.C.PSYCH., HELEN SIMMONS, M.R.C.PSYCH., TAMSIN FORD, M.R.C.PSYCH., HOWARD MELTZER, PH.D., AND ROBERT GOODMAN, F.R.C.PSYCH.
ABSTRACT Objective: The prevalence of pervasive developmental disorders (PDD) is not well established and needs monitoring. The prevalence of PDD in the 1999 nationwide British survey of child and adolescent mental health was investigated. Method: A randomized, stratified sample of children (N = 12,529) aged 5 to 15 years was generated from the Child Benefit Register. Trained interviewers interviewed parents and youths aged 11 or older with a standardized diagnostic interview (Development and Well-Being Assessment), and questionnaire data (Strengths and Difficulties Questionnaire) were obtained from teachers and parents, who also completed self-report measures of psychological distress. Final diagnostic determination was achieved by a team of experienced clinicians using all data sources. Results: A total of 10,438 (83%) interviews were conducted. There were 2 girls with Rett syndrome (weighted prevalence: 3.8/10,000 girls) and 27 children with other PDD (weighted prevalence: 26.1/10,000). Compared with children with a psychiatric disorder other than PDD, social but not behavioral problems were more frequent in the PDD group. Parents of children with PDD had higher rates of psychological distress than those from the two comparison groups. Conclusions: Consistent with other recent surveys, PDD rates are higher than those reported 30 years ago. The burden associated with PDD is very high. J. Am. Acad. Child Adolesc. Psychiatry, 2001, 40(7):820–827. Key Words: epidemiology, autism, Rett syndrome, pervasive developmental disorder, prevalence.
Since the first study conducted in the mid-1960s in England (Lotter, 1966), 28 epidemiological surveys of autistic disorders have been conducted in various countries and published in English (Fombonne, in press). However, there is still some uncertainty (Fombonne, 1999a) and even controversy (Fombonne, 1996, in press; Gillberg and Wing, 1999) about the prevalence of autistic disorder and of other subtypes of autistic spectrum conditions such as Asperger’s disorder and other variants of pervasive developmental disorders (PDD) that fall short of diagnostic criteria for autistic disorder (i.e., PDD-not otherwise specified). Accepted February 20, 2001. Dr. Fombonne, Dr. Simmons, Dr. Ford, and Professor Goodman are with the Institute of Psychiatry, Department of Child and Adolescent Psychiatry, King’s College London; Dr. Meltzer is with the Office of National Statistics, London. The survey was funded by the British Department of Health and carried out by the Office of National Statistics. Reprint requests to Dr. Fombonne, Child and Adolescent Psychiatry, Institute of Psychiatry, Denmark Hill, London SE5 8AF, England; e-mail: e.fombonne@ iop.kcl.ac.uk. 0890-8567/01/4007–0820䉷2001 by the American Academy of Child and Adolescent Psychiatry.
Autism surveys have all relied on the same methodology to identify cases in populations, i.e., using a two- or multiple-phase design with initial screening phase(s) followed by more in-depth diagnostic phases to determine final caseness status. Thus no survey has used random, stratified, or more complex sampling strategies. Moreover, all surveys have been conducted in areas or regions chosen for convenience reasons. While there is no a priori reason to believe that rates should vary substantially across regions within a country, factors such as uneven level of service development and differential awareness by local lay and professional public might explain some betweenarea variation in prevalence estimates. Thus, extrapolation from local survey estimates to national figures should be regarded with some caution. The purpose of this article is to report on the findings for PDD from a recent nationwide survey of child and adolescent mental health conducted on a nationally representative sample of children and adolescents in Britain. Specifically, the objectives of the study were (1) to estimate the prevalence rate for PDD and (2) to describe the correlates of PDD in terms of associated behavioral and medical symptoms and global impact on functioning.
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METHOD Sample The parents of each child living in the United Kingdom are entitled to receive child benefits unless the child is under the care of social services. The centralized computerized records from the Child Benefit Register (CBR) were used as a sampling frame to select children aged 5 to 15 throughout England, Wales, and Scotland. Families with no detailed postal code in the database and those subjected to a current revision of their record (e.g., death of the child, changing address) were excluded. The total number finally included in the sampling frame was 6,422,202, representing an estimated 90% coverage of the whole population. The sampling frame was further stratified by Regional Health Authority, and within that, by sociodemographic groupings. Postal sectors were then selected at random within that frame with a probability proportional to the size of the sector. To facilitate the logistics of the survey, a few areas were oversampled but proper weights were introduced in the analysis to adjust for unequal sampling probabilities. From the final list of 475 postal sectors selected from the 8,265 sectors covering the whole country, the CBR was instructed to sample 30 children within each postal sector and a letter was subsequently sent to the families by the CBR on behalf of the Office of National Statistics survey team. Of the 14,250 families contacted, 931 parents (6.5%) chose to opt out by calling the CBR and an additional 790 addresses (5.5%) were found to be incorrect. This left a sample of 12,529 children eligible for interview. Instruments The diagnostic interview used in this survey is a new instrument (Development and Well-Being Assessment; DAWBA) combining the features of structured and semistructured interviews (Goodman et al., 2000). The DAWBA is administered by trained nonclinician interviewers who interview parents about psychiatric symptoms and their functional impact. Skip rules and screening questions allow for a reduction of administration time. Positive symptoms are followed up by openended questions and supplementary prompts, and parental descriptions are entered verbatim (but not rated) by the interviewer in a computer. A teacher questionnaire on common symptoms and their impact is included with space for additional comments. A DAWBA module for direct interview of subjects is available for 11- to 16-year-old youths. Most common psychiatric disorders are covered by the DAWBA. Diagnoses are subsequently generated by computer algorithms based on DSM-IV (American Psychiatric Association, 1994) and ICD-10 (World Health Organization, 1993), using the information from all available informants. These computer-generated summary sheets and all interview data are subsequently reviewed by experienced clinicians who confirm or overturn computer-generated diagnoses. The reliability of clinical diagnosis was assessed with two clinicians who independently rated 500 randomly chosen subjects. The κ statistic for chance-corrected agreement for the presence or absence of any DSM-IV diagnosis was 0.86 (95% confidence interval [CI]: 0.78–0.95). Although the main focus of the survey was on common child psychiatric disorders, screening questions were introduced in the DAWBA to detect early developmental abnormalities. More specifically, parents were asked the following three questions: “In his/her first three years of life, was there anything that seriously worried you about (i) how s/he got on with other people? (ii) the way his/her speech developed? and (iii) any odd rituals or unusual habits that were very hard to interrupt?” Positive answers to any of these questions were followed by the question, “Has this/have these now cleared up completely?” Then, detailed parental descriptions of the child’s past and current levels of social and communicative behaviors and play/activities, results from previous assessments, and schooling history were obtained and diagnoses of PDD were sub-
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sequently derived from all available data. All cases given a diagnosis of PDD (n = 31) by the survey team were independently reviewed by a child and adolescent psychiatrist with expertise in the diagnosis of PDD (E.F.). Direct subject interviews were carried out with four of the eight subjects aged 11 or older, and teacher data were missing for three cases. Other survey instruments used in this analysis were the parent, teacher, and youth (for children aged 11 or older) extended versions of the Strengths and Difficulties Questionnaire (SDQ) (Goodman, 1997, 1999). The SDQ is a brief screening measure that inquires about psychiatric symptoms and positive attributes and assesses the impact of the psychiatric symptoms in terms of chronicity, distress, social impairment, and burden for others. In addition to the total score, five subscores are derived which tap emotional, conduct and hyperactive behaviors, prosocial behaviors, and peer relationships. The behavior and the impact scores have been shown to have excellent reliability and validity (Goodman, 1997, 1999). In addition, parents reported on their child’s general health and completed a 12-item self-report measure which assesses common psychological symptoms for the current period (General Health Questionnaire; GHQ-12) (Goldberg and Williams, 1988). Mental Age Survey children of all ages were administered the British Picture Vocabulary Scale (BPVS-II) (Dunn et al., 1997) and reading and spelling tests from the British Ability Scales (Elliott et al., 1996). In addition, teachers were also asked to estimate the mental age of the child in terms of his/her overall intellectual and scholastic ability. The child’s mental age was then estimated either with a ratio IQ obtained by dividing the teacher estimate of the child’s mental age by the age of the child or by the standard score derived from the BPVS. Within the PDD group, teacher’s IQ ratios and BPVS IQs were available for 23 and 18 children, respectively; the correlation between these two estimates was very high (r = 0.863; p < .001) on 16 subjects with both estimates available. Thus either measure was used to estimate intellectual level in children with PDD. For three subjects without a formal assessment, a clinical estimate was obtained on the basis of a review of all interview data. In one case, the teacher estimate of normal functioning was downgraded as the review of clinical data suggested a fair degree of mental retardation. Data Collection Procedure Lay interviewers (approximately 300) regularly involved in the Office of National Statistics surveys were used to collect survey data. Special attempts were made to trace families whose addresses or names had changed because of various circumstances. Interviewer training emphasized the need to obtain respondents’ descriptions of any problems and concerns in their own words, facilitating this with openended prompts and recording the answers verbatim. Interviewers completed the face-to-face interview with the main caregiver (90% mothers) first, and permission was subsequently sought to ask questions of the sampled child. Children aged 11 to 15 had a private faceto-face interview and also completed directly on a laptop computer a questionnaire containing more sensitive questions about smoking, alcohol, and drug experiences. At the end of the interview, parents were asked to nominate a teacher who knew their child well; the SDQ was subsequently mailed to this teacher. A reminder letter was sent to teachers who did not respond. Additional details about the methodology of the survey are available elsewhere (Meltzer et al., 2000). Statistical Analyses Two weights were devised to adjust for unequal probability of postal sector selection in the sampling frame and to compensate for differential response rate by region. Then, adjustment weights were used to correct for nonresponse bias, with the age and sex distribution of survey respondents being equated to the population structure as determined from the
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1991 census. All prevalence estimates were calculated with the weighted sample, and asymptotic 95% CIs were obtained with the software Stata version 6 (Stata Corporation, 1997). The remaining analyses were conducted on unweighted data as further analyses of the survey showed small design effects on most estimates. A conventional p value of .05 was retained as the level of statistical significance. RESULTS
Of the 12,529 eligible children, interviews were conducted with 10,438 subjects (83%). Failure to interview was due to either noncontact (2%) or refusal (15%). Within cooperating families, child interviews were completed in 95.3% of the cases. The teacher response rate was 80.3% for the whole sample and 89.7% in the PDD group (χ21 = 1.16; not significant). Participation was in general lower within the London metropolitan area (approximately 70%), but appropriate weights corrected for the possible resulting bias. No data were available on nonrespondents either for global psychopathology or for developmental disorders; it is therefore impossible to evaluate the extent to which nonparticipation in this survey might have been associated with higher rates of psychiatric morbidity, and more specifically with the presence or absence of a child with a PDD in the family. Of the 10,438 subjects included in the survey, 9,455 had no psychiatric diagnosis and 952 received a psychiatric diagnosis other than PDD (including 232 who had a DSM-IV diagnosis of attention-deficit/hyperactivity disorder). A total of 31 children initially received one of the PDD diagnoses from the survey team. The review of all records by the child and adolescent psychiatrist confirmed the initial diagnostic evaluation on 29 of these 31 cases (93.5% agreement). Two cases were finally excluded after discussion and subsequently included in the psychiatric comparison group. One girl with equivocal current symptom patterns had been exposed to serious physical abuse before being placed in foster care, and data were missing on her early development. One boy described as having had “autistic tendencies” had made substantial improvements in social functioning according to parental reports; as teacher data were missing, a conservative decision was made to exclude this case. Of the 29 children with a PDD, 3 (10.3%) were in residential schools. The characteristics of children with and without PDD are summarized in Table 1. There was a significant difference (χ24 = 38.4; p < .0001) in age distribution between the three groups: children with PDD were younger and children with another psychiatric disorder were older than normal controls. A marked male preponderance was 822
observed for children with either PDD or other psychiatric disorders, with a male-female ratio of 4.8:1 for the PDD group. Social class distribution also differed between the three groups (χ28 = 85.6; p < .0001). Compared with normal controls, the social class of the PDD sample was skewed toward higher social classes although, because of the small sample size, the comparison fell just short of statistical significance (χ24 = 8.0; p = .09). By contrast, social classes in the psychiatric control group showed a shift in the direction of lower social classes compared with both normal controls (χ24 = 76.8; p < .001) and children with PDD (χ24 = 24.3; p < .001). Cases were distributed evenly in the country, and there was no difference between the three groups for the region of residence (χ210 = 7.56; not significant). No difference was found for ethnicity. Family size, however, differed significantly across the three groups (χ26 = 35.1; p < .0001): children with PDD came from smaller families and psychiatric controls from larger families when compared with normal controls. In sum, children with PDD differed from the other children by their male preponderance, their younger age, and a tendency to come from smaller families and families with higher socioeconomic status. Among the 29 children with a PDD, 2 girls had Rett syndrome, both with moderate to severe mental retardaTABLE 1 Sample Characteristics Normal Controls (n = 9,455) Age, % 5–7 yr 8–11 yr 12–15 yr Gender, % male Social class, % I II III (nm) III (m) IV–V Ethnicity, % white Family size, % 1 child 2 children 3 children 4+ children
Children With a Psychiatric Disorder PDD (n = 954) (n = 29)
28.9 37.7 33.4 48.8
22.8 34.4 42.8 60.1
41.4 34.5 24.1 82.8
8.2 33.2 12.2 26.2 20.2 91.3
4.0 23.9 14.8 28.0 29.3 92.3
20.7 37.9 13.8 17.2 10.3 89.7
20.7 45.9 23.1 10.3
22.6 39.0 22.7 15.6
20.7 55.2 13.8 10.3
pa
.0001 .0001
.0001 NS .0001
Note: PDD = pervasive developmental disorders; nm = nonmanual; m = manual; NS = not significant. a 2 χ tests.
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tion. In this survey, the weighted prevalence of Rett syndrome was 3.5/10,000 girls (95% CI: 0–8.5/10,000). Attempts to subtype the remaining 27 children with PDD variants were found to be unreliable owing to the limitation of the survey data, and in particular to the lack of detailed early developmental history. However, there was no case of childhood disintegrative disorder and Asperger’s disorder was reported in five children (all boys). About half the sample had mental retardation (48.3%; and 44.4% without the two with Rett syndrome) according to our combined estimate. The weighted prevalence of PDD, excluding Rett syndrome, was 26.1/ 10,000 (95% CI: 16.2–36.0/10,000). The prevalence of PDD was not different among the younger children aged 5 to 9 (30.7/10,000; 95% CI: 15.1–46.3/10,000) compared with the older children (22.1/10,000; 95% CI: 9.6–34.6/10,000). The prevalence of all types of PDD, including the two cases of Rett syndrome, was 27.9/10,000 (95% CI : 17.7–38.0/10,000).
Table 2 presents rates of developmental and health problems as reported by the parents. To limit the number of comparisons, problems occurring in less than 10% of the PDD sample are not reported. However, it is worth mentioning that in the PDD group, parents reported one case of fragile X (3.7% of 27 subjects) and no case of cerebral palsy, Down syndrome, or other medical disorders known to be associated with autism (in particular, tuberous sclerosis and congenital rubella). In keeping with their developmental delays, children with PDD had significantly higher rates of motor coordination, bladder control, and bowel control problems than children with or without a psychiatric disorder. They also had significantly higher rates of epilepsy than children in both comparison groups. Of the eight remaining health items, seven comparisons yielded nonsignificant differences in children with PDD compared with the other two groups, with the exception of a marginal increase in “stomach problems” in children with PDD compared with normally developing
TABLE 2 Parent Reports of Health and Developmental Problems Normal Controls (n = 9,455) (%)
Psychiatric Controls (n = 954) (%)
PDD (n = 29) (%)
Bed-wetting
4.0
11.2
27.6
Soiling pants
0.9
3.1
20.7
Coordination difficulties
1.6
7.4
48.3
Asthma
15.1
20.3
13.8
Eczema
12.3
13.8
10.3
Glue ear, otitis media, or hearing grommets
4.5
6.8
10.3
Stomach or digestive problems or abdominal pains
5.6
10.2
13.8
Epilepsy
0.4
2.2
13.8
Food allergies
3.3
5.6
20.7
Other allergies
5.5
6.5
13.8
Eye problems
10.2
14.0
10.3
3.9
6.6
10.3
Hearing problems
ORa
95% CI
p
7.1 2.4 24.3 7.5 47.9 9.9 0.84 0.59 0.83 0.70 2.1 1.3 3.15 1.64 32.7 9.2 7.6 5.1 2.8 2.8 1.11 0.77 2.7 1.4
3.0–16.6 1.03–5.76 9.4–62.8 2.74–20.6 22.5–102.3 4.55–21.7 0.29–2.43 0.20–1.72 0.25–2.75 0.21–2.36 0.63–07.1 0.37–4.42 1.09–9.14 0.55–4.9 10.7–99.9 2.8–30.6 3.06–18.9 1.94–13.36 0.97–8.15 0.93–8.5 0.33–3.7 0.23–2.62 0.81–9.0 0.41–4.82
.001 .043 .001 .001 .001 .001 NS NS NS NS NS NS .034 NS .001 .001 .001 .001 NS NS NS NS NS NS
Note: PDD = pervasive developmental disorders; OR = odds ratio; CI = confidence interval; NS = not significant. a All odds ratios are adjusted for age and gender. The first line refers to comparisons of children with PDD to normal controls, the second line to comparisons of children with PDD to psychiatric controls.
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children but not with psychiatric controls. Moreover, rates of problems were at similar levels, suggesting that the lack of statistical differences was not accounted for by the low statistical power attributable to the small sample size in the PDD group. The only significant difference applied to the parental report of food allergies, which was significantly higher in the PDD group than in both comparison groups. Further analyses did not indicate that those PDD children with food allergies had more severe hyperactivity or mental retardation. The rate of food allergies was 8.2% in the psychiatric controls with an attention-deficit/ hyperactivity disorder diagnosis compared with 18.5% in the PDD group (excluding the two girls with Rett syndrome), and a repeated comparison between these two subgroups led to a much attenuated association (odds ratio = 2.6; 95% CI: 0.9–7.8; p = .084); moreover, when the impact score (see below) was further introduced as a covariate in the model, no difference remained between PDD and hyperactive children (odds ratio = 1.7; 95% CI: 0.5–5.5; p = .4). In general, the estimates of the odds ratios for all analyses decreased in magnitude when the
comparison used the psychiatric group rather than the normal controls, suggesting that the former group might be more appropriate to establish the specificity of associations with correlates. Behavioral deviance scores as measured by the SDQ completed independently by parents and teachers are presented in Table 3. On the parent SDQ, children with PDD obtained more deviant mean scores than normal controls on all scales. When compared with psychiatric controls, children with PDD also had significantly higher total scores; however, these two groups were similar for their scores of emotional and conduct symptoms, suggesting that the higher total scores of the children with PDD were not accounted for by their behavioral problems; rather, the differences were attributable to a large extent to their social difficulties as illustrated by the fact that the largest mean differences were found for the prosocial and peer relationship subscores. The same pattern of results emerged with the teacher SDQ. Children with PDD were markedly different from normally developing children; however, differences with children having other
TABLE 3 Children’s Behavioral Difficulties and Parental Self-Report of Psychological Distress
SDQ Total score Parent Teacher Emotional subscore Parent Teacher Conduct subscore Parent Teacher Hyperactivity subscore Parent Teacher Peer relationships subscore Parent Teacher Prosocial subscore Parent Teacher Impact subscore Parent Teacher Parental self-report (GHQ-12) Total score
NC (n = 9,455)
PC (n = 954)
PDD (n = 29)
Post Hoc Scheffé Tests a
7.59 5.73
16.28 14.29
21.71 14.35
PDD>PC>NC PDD,PC>NC
1.72 1.27
3.69 2.79
4.45 3.15
PDD,PC>NC PDD,PC>NC
1.39 0.68
3.59 2.99
3.36 1.46
PDD,PC>NC PC>PDD,NC
3.18 2.57
6.02 6.00
8.18 5.38
PDD>PC>NC PDD,PC>NC
1.31 1.21
2.98 2.90
5.71 4.35
PDD>PC>NC PDD>PC>NC
8.71 7.46
7.67 5.35
4.25 4.31
PDD
0.20 0.26
2.01 1.70
4.79 2.85
PDD>PC>NC PDD,PC>NC
1.63
3.47
4.07
PDD,PC>NC
Note: NC = normal controls; PC = psychiatric controls; PDD = pervasive developmental disorders; SDQ = Strengths and Difficulties Questionnaire; GHQ-12 = General Health Questionnaire. a All F tests significant at p < .001.
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psychiatric disorders had a more distinctive pattern, with prosocial and peer subscores indicating more deviance in the children with PDD and conduct mean scores (and to a lesser extent hyperactive subscores) indicating less deviance in the children with PDD. The impact scores of the parent SDQ (5 items; range 0–10) and of the teacher SDQ (3 items; range 0–6) were used to assess overall impairment in the child’s functioning. Impact scores for both informants were much higher in children with PDD than in normal and psychiatric control children and significantly so for three of the four comparisons (Table 3). Self-report scores on the GHQ showed similar levels of psychological distress in parents of children with PDD and with other non-PDD diagnoses (Table 3). Within the PDD group, GHQ scores were significantly related to total SDQ scores (Spearman r = 0.397; p < .05) but not to IQ estimates. DISCUSSION Study Limitations
First, although the participation rate was high, about a fifth of selected households could not be included. There is evidence from previous child psychiatric surveys that rates of psychopathology are higher among nonrespondents (Fombonne, 1994; Rutter et al., 1970), which leads to a biased estimate of prevalence. When data on nonrespondents are available, proper weights can be devised to control for this potential bias. In this household survey, however, no data were available on nonrespondents and therefore prevalence estimations could not compensate for nonparticipation in the survey. It is difficult to assess whether or not the resulting estimates are biased. In most previous autism surveys, refusal rates by parents of children with a PDD tended in fact to be very low, with a median figure of 6% in 10 surveys in which these data were available (Fombonne, in press). Thus if the participation rate was differentially influenced by the presence of a child with PDD within the household, our prevalence estimates might indeed be inflated rather than underestimated. However, it is not possible to assess the magnitude and direction of potential bias in the resulting rates. Second, children under the care of local authorities were not eligible for inclusion in the survey; this increases the possibility that prevalence rates might be underestimated. Whether or not this was the case cannot be assessed from available data; however, an ongoing survey of such children will shed light on this issue in the near future. J . A M . A C A D . C H I L D A D O L E S C . P S YC H I AT RY, 4 0 : 7 , J U LY 2 0 0 1
Third, cases were identified with data collected through structured standardized interviews administered by nonclinician interviewers and primarily designed for assessing common psychiatric disorders. Nevertheless, the inclusion of specific questions for early developmental concerns, the combination of open-ended questions and of parental descriptions, and reliance on clinical judgment for final case determination provided a satisfactory procedure to assess PDD in this survey. However, reliable differentiation between subtypes of PDD could not be achieved, limiting our estimations to combined rates of autistic spectrum disorders. Fourth, the final number of cases identified was small, although not very different from the median number of 51 cases per survey found in 27 previous epidemiological investigations (Fombonne, in press). The precision of the resulting prevalence estimates was modest, precluding meaningfully estimating rates in subgroups; also, the assessment of correlates was performed with necessarily suboptimal statistical power, although this was compensated to a degree by the large sample sizes available in both comparison groups. This feature should nevertheless be borne in mind, particularly when assessing nonsignificant results hereafter. Prevalence Findings
The prevalence of Rett syndrome in this survey was remarkably similar to one previous U.K. estimate (Asthana et al., 1990) and consistent with the figure of approximately 1/10,000 girls reported in other studies (Chakrabarti and Fombonne, in press; Hagberg and Hagberg, 1997). The prevalence of 26.1/10,000 reported for other PDD is consistent with a recent best estimate of 22/10,000 for combined PDD derived from a quantitative analysis of surveys of autism (Fombonne, in press). Three recent U.K. surveys have yielded estimates which were either lower (10.1/10,000: Taylor et al., 1999) or higher (57.9/ 10,000: Baird et al., 2000; 62.6/10,000: Chakrabarti and Fombonne, in press) than in this survey. The survey with the lowest rate relied on administrative records, whereas the two surveys with the highest rates used an intense, multiphase screening procedure for case identification. Conducted at the same time in the same country, this 6-fold variation in rates illustrates well the major impact on estimates of methodological factors and the inherent difficulties of comparisons across studies. Unlike all other epidemiological surveys, however, our estimate is based on a sample representative of the whole country. As such, it is less susceptible to bias due to local or regional 825
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differences in case ascertainment and it can be extrapolated to Great Britain more confidently. Our estimates were similar among younger and older subjects in our survey. Sample-size limitations precluded us from performing more fine-tuned comparisons across successive birth cohorts to test hypotheses about secular changes in rates of autism, a method that could be used in larger studies (Fombonne et al., 1997). Associated Features
Our sample contained a somewhat elevated proportion of high-functioning subjects and of boys. These features may or may not be correlated since higher male-female ratios are typically found among high-functioning subjects with autism in clinical (Lord et al., 1982) and epidemiological (Fombonne, 1999a) studies. It is interesting that similar high male-female ratios and proportions of highfunctioning subjects were reported in other recent British surveys (Baird et al., 2000; Chakrabarti and Fombonne, in press). The crudeness of the intellectual assessments should, however, be kept in mind as should the possibility that these unusual characteristics might simply correspond to sample fluctuations. The major medical correlates were, however, in keeping with other surveys with a low proportion of PDD children with fragile X disorder (Chakrabarti and Fombonne, in press; Fombonne, in press) and no cases of other severe medical disorders usually occurring in autism in small (<10%) proportions (Fombonne, in press; Rutter et al., 1994). The rate of epilepsy was also in keeping with other surveys, but it should be acknowledged that this represents an underestimate of the lifetime risk of epilepsy because most subjects in the survey have not yet gone through the period at highest risk of seizures (Deykin and MacMahon, 1979; Volkmar and Nelson, 1990). Somatic Symptoms
In light of recent controversies (Fombonne, 1999b, 2001b; Kawashima et al., 2000; Wakefield et al., 1998; Whiteley et al., 2000), symptoms reflecting potential dysfunction of the immune or gut systems were of particular interest even though they were reported by parents only. Asthma and eczema were not more frequent in the PDD group compared with both comparison groups. Similarly, rates of ear infections were not significantly increased in the PDD group, contrary to earlier reports (Konstantareas and Homatidis, 1987). Abdominal pains were not significantly increased in frequency when compared with a large comparison group of children with a psychiatric disorder. The rates of stomach or digestive problems remained at 826
low levels, and there was no report by parents that any child had inflammatory bowel disease or Crohn’s disease; this finding is consistent with the lack of a demonstrated association between these bowel disorders and autism in large epidemiological and clinical series (Fombonne, 1998). Food allergies were the only symptom that was significantly elevated in the PDD group. The precise meaning of this finding is difficult to explore further without corroborative medical evidence of such symptoms, and replication in larger samples is needed to establish its validity. It is worth noting, however, that this symptom was particularly endorsed by parents of children with neurodevelopmental disorders, who might be more prone to seek such explanations; furthermore, adjusting for the impact of the disorder removed the difference between PDD and hyperactive children completely. Behavior Problems and Family Distress
Although the SDQs are not measures designed for use with autism samples, they provided a useful additional dimensional assessment of behavioral problems as seen in different contexts. It was clear that the total scores reflected well the severity of these pervasive disturbances of development. Furthermore, the distinctive pattern of subscores suggested that both at home and at school, children with PDD were not more emotionally or behaviorally disturbed than their peers with other psychiatric disorders, with the exception of hyperactive symptoms reported by parents. By contrast, children with PDD stood out as having very deviant social scores as could have been predicted from the nature of their disorders. In line with several studies which explored levels of stress and burden on parents of children with PDD (Holroyd and McArthur, 1976; Konstantareas and Homatidis, 1991; Wolf et al., 1989), parents of our sample reported high levels of distress and burden. The epidemiological nature of this investigation provides strong support to previous findings in clinical samples, and the results confirm the high level of needs of this group of families. Study Strengths
Two particular strengths of this investigation must be underscored. First, this is the first survey of autism to rely on a stratified random sample representative of a population. Consequently, the findings have a greater potential for generalization than do most other epidemiological inquiries. Second, capitalizing on the survey design, we could compare children with PDD with two comparison groups of large sample sizes, including one of children J . A M . A C A D . C H I L D A D O L E S C . P S YC H I AT RY, 4 0 : 7 , J U LY 2 0 0 1
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with psychiatric disorders. Even though the sample of PDD children was small, the survey design allowed us to perform meaningful comparisons on a range of symptoms and impairment scores. This comparative approach has been generally lacking in other epidemiological surveys of autism. The estimate derived from this survey is consistent with the higher prevalence rates reported recently in several surveys (Baird et al., 2000; Centers for Disease Control, 2000; Chakrabarti and Fombonne, in press; Kadesjo et al., 1999). Equally, as outlined above, methodological factors have an important impact on survey estimates, which calls for some caution about their interpretation and comparisons. In particular, increased prevalence rates in recent surveys cannot be interpreted as indicative of a secular increase in the incidence of autism and its variants unless case definition and case-finding methods are held strictly constant in comparing rates across studies and over time, a target that is impossible to reach with the existing literature (Fombonne, 2001a, in press).
American Psychiatric Association (1994), Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV). Washington, DC: American Psychiatric Association Asthana JC, Sinha S, Haslam JS, Kingston HM (1990), Survey of adolescents with severe intellectual handicap. Arch Dis Child 65:1133–1136 Baird G, Charman T, Baron-Cohen S et al. (2000), A screening instrument for autism at 18 months of age: a 6-year follow-up study. J Am Acad Child Adolesc Psychiatry 39:694–702 Centers for Disease Control and Prevention (2000), Prevalence of Autism in Brick Township, New Jersey, 1998: Community Report (http://www.cdc.gov/ nceh/cddh/dd/rpttoc.htm) Chakrabarti S, Fombonne E (in press), Pervasive developmental disorders among preschool children in Staffordshire (England). JAMA Deykin EY, MacMahon B (1979), The incidence of seizures among children with autistic symptoms. Am J Psychiatry 136:1310–1312 Dunn LM, Whetton C, Burley J (1997), The British Picture Vocabulary Scale, Second Edition Testbook. Windsor, England: NFER Elliott C, Smith P, McCulloch K (1996), British Ability Scales, Second Edition: Administration and Scoring Manual. Windsor, England: NFER Fombonne E (1994), The Chartres study, I: prevalence of psychiatric disorders among French school-aged children. Br J Psychiatry 164:69–79 Fombonne E (1996), Is the prevalence of autism increasing? J Autism Dev Disord 6:673–676 Fombonne E (1998), Inflammatory bowel disease and autism. Lancet 351:955 Fombonne E (1999a), The epidemiology of autism: a review. Psychol Med 29:769–786
Fombonne E (1999b), Are measles infections or measles immunizations linked to autism? J Autism Dev Disord 29:349–350 Fombonne E (2001a), Is there an epidemic of autism? Pediatrics 107:411–413 Fombonne E (2001b), Presentation to Immunization Safety Review Committee. New studies: March 8. Washington, DC: Institute of Medicine Fombonne E (in press), Epidemiological findings in autism and related pervasive developmental disorders. In: Educating Children With Autism, Lord C, ed. Washington, DC: National Academy of Sciences Press Fombonne E, du Mazaubrun C, Cans C, Grandjean H (1997), Autism and associated medical disorders in a large French epidemiological sample. J Am Acad Child Adolesc Psychiatry 36:1561–1569 Gillberg C, Wing L (1999), Autism: not an extremely rare disorder. Acta Psychiatr Scand 99:399–406 Goldberg D, Williams PA (1988), User’s Guide to the General Health Questionnaire. Windsor, England: NFER-NELSON Goodman R (1997), The Strengths and Difficulties Questionnaire: a research note. J Child Psychol Psychiatry 38:581–586 Goodman R (1999), The extended version of the Strengths and Difficulties Questionnaire as a guide to child psychiatric caseness and consequent burden. J Child Psychol Psychiatry 40:791–799 Goodman R, Ford T, Richards H, Gatward R, Meltzer H (2000), The Development and Well-Being Assessment: description and initial validation of an integrated assessment of child and adolescent psychopathology. J Child Psychol Psychiatry 41:645–655 Hagberg B, Hagberg G (1997), Rett syndrome: epidemiology and geographical variability. Eur Child Adolesc Psychiatry 6:5–7 Holroyd J, McArthur D (1976), Mental retardation and stress on the parents: a contrast between Down syndrome and childhood autism. Am J Ment Defic 80:431–436 Kadesjo B, Gillberg C, Hagberg B (1999), Autism and Asperger syndrome in seven-year-old children: a total population study. J Autism Dev Disord 29:327–331 Kawashima H, Mori T, Kashiwagi Y, Takekuma K, Hoshika A, Wakefield A (2000), Detection and sequencing of measles virus from peripheral mononuclear cells from patients with inflammatory bowel disease and autism. Dig Dis Sci 45:723–729 Konstantareas MM, Homatidis S (1987), Ear infections in autistic and normal children. J Autism Dev Disord 17:585–594 Konstantareas MM, Homatidis S (1991), Effects of developmental disorder on parents: theoretical and applied considerations. Psychiatr Clin North Am 14:183–198 Lord C, Schopler E, Revecki D (1982), Sex differences in autism. J Autism Dev Disord 12:317–330 Lotter V (1966), Epidemiology of autistic conditions in young children, I: prevalence. Soc Psychiatry 1:124–137 Meltzer H, Gatward R, Goodman R, Ford T (2000), The Mental Health of Children and Adolescents in Great Britain. London: Stationery Office Rutter M, Bailey A, Bolton P, Le Couteur A (1994), Autism and known medical conditions: myth and substance. J Child Psychol Psychiatry 35:311–322 Rutter M, Tizard J, Whitmore K (1970), Education, Health and Behaviour. London: Longmans Stata Corporation (1997), Statistic/Data Analysis. College Station, TX: Stata Press Taylor B, Miller E, Farrington CP et al. (1999), Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association. Lancet 353:2026–2029 Volkmar F, Nelson D (1990), Seizure disorders in autism. J Am Acad Child Adolesc Psychiatry 29:127–129 Wakefield A, Murch S, Anthony A et al. (1998), Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. Lancet 351:637–641 Whiteley P, Rodgers J, Shattock P (2000), MMR and autism (commentary). Autism 4:207–211 Wolf LC, Noh S, Fisman SN, Speechley M (1989), Psychological effects of parenting stress on parents of autistic children. J Autism Dev Disord 19:157–166 World Health Organization (1993), The ICD-10 Classification of Mental and Behavioural Disorders: Diagnostic Criteria for Research. Geneva: World Health Organization
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Without having to sustain the claim of an epidemic, the consistency of recent estimates toward much higher prevalence figures than those reported 30 years ago should nevertheless be seen as sufficient to draw the attention to a large group of children and their families who have important service needs. Clinicians should be reminded that distress in families with a child suffering from any PDD is very high and that to alleviate it is a major treatment goal. REFERENCES
ABSTRACT Objective: The prevalence of pervasive developmental disorders (PDD) is not well established and needs monitoring. The prevalence of PDD in the 1999 nationwide British survey of child and adolescent mental health was investigated. Method: A randomized, stratified sample of children (N = 12,529) aged 5 to 15 years was generated from the Child Benefit Register. Trained interviewers interviewed parents and youths aged 11 or older with a standardized diagnostic interview (Development and Well-Being Assessment), and questionnaire data (Strengths and Difficulties Questionnaire) were obtained from teachers and parents, who also completed self-report measures of psychological distress. Final diagnostic determination was achieved by a team of experienced clinicians using all data sources. Results: A total of 10,438 (83%) interviews were conducted. There were 2 girls with Rett syndrome (weighted prevalence: 3.8/10,000 girls) and 27 children with other PDD (weighted prevalence: 26.1/10,000). Compared with children with a psychiatric disorder other than PDD, social but not behavioral problems were more frequent in the PDD group. Parents of children with PDD had higher rates of psychological distress than those from the two comparison groups. Conclusions: Consistent with other recent surveys, PDD rates are higher than those reported 30 years ago. The burden associated with PDD is very high. J. Am. Acad. Child Adolesc. Psychiatry, 2001, 40(7):820–827. Key Words: epidemiology, autism, Rett syndrome, pervasive developmental disorder, prevalence.
Since the first study conducted in the mid-1960s in England (Lotter, 1966), 28 epidemiological surveys of autistic disorders have been conducted in various countries and published in English (Fombonne, in press). However, there is still some uncertainty (Fombonne, 1999a) and even controversy (Fombonne, 1996, in press; Gillberg and Wing, 1999) about the prevalence of autistic disorder and of other subtypes of autistic spectrum conditions such as Asperger’s disorder and other variants of pervasive developmental disorders (PDD) that fall short of diagnostic criteria for autistic disorder (i.e., PDD-not otherwise specified). Accepted February 20, 2001. Dr. Fombonne, Dr. Simmons, Dr. Ford, and Professor Goodman are with the Institute of Psychiatry, Department of Child and Adolescent Psychiatry, King’s College London; Dr. Meltzer is with the Office of National Statistics, London. The survey was funded by the British Department of Health and carried out by the Office of National Statistics. Reprint requests to Dr. Fombonne, Child and Adolescent Psychiatry, Institute of Psychiatry, Denmark Hill, London SE5 8AF, England; e-mail: e.fombonne@ iop.kcl.ac.uk. 0890-8567/01/4007–0820䉷2001 by the American Academy of Child and Adolescent Psychiatry.
Autism surveys have all relied on the same methodology to identify cases in populations, i.e., using a two- or multiple-phase design with initial screening phase(s) followed by more in-depth diagnostic phases to determine final caseness status. Thus no survey has used random, stratified, or more complex sampling strategies. Moreover, all surveys have been conducted in areas or regions chosen for convenience reasons. While there is no a priori reason to believe that rates should vary substantially across regions within a country, factors such as uneven level of service development and differential awareness by local lay and professional public might explain some betweenarea variation in prevalence estimates. Thus, extrapolation from local survey estimates to national figures should be regarded with some caution. The purpose of this article is to report on the findings for PDD from a recent nationwide survey of child and adolescent mental health conducted on a nationally representative sample of children and adolescents in Britain. Specifically, the objectives of the study were (1) to estimate the prevalence rate for PDD and (2) to describe the correlates of PDD in terms of associated behavioral and medical symptoms and global impact on functioning.
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METHOD Sample The parents of each child living in the United Kingdom are entitled to receive child benefits unless the child is under the care of social services. The centralized computerized records from the Child Benefit Register (CBR) were used as a sampling frame to select children aged 5 to 15 throughout England, Wales, and Scotland. Families with no detailed postal code in the database and those subjected to a current revision of their record (e.g., death of the child, changing address) were excluded. The total number finally included in the sampling frame was 6,422,202, representing an estimated 90% coverage of the whole population. The sampling frame was further stratified by Regional Health Authority, and within that, by sociodemographic groupings. Postal sectors were then selected at random within that frame with a probability proportional to the size of the sector. To facilitate the logistics of the survey, a few areas were oversampled but proper weights were introduced in the analysis to adjust for unequal sampling probabilities. From the final list of 475 postal sectors selected from the 8,265 sectors covering the whole country, the CBR was instructed to sample 30 children within each postal sector and a letter was subsequently sent to the families by the CBR on behalf of the Office of National Statistics survey team. Of the 14,250 families contacted, 931 parents (6.5%) chose to opt out by calling the CBR and an additional 790 addresses (5.5%) were found to be incorrect. This left a sample of 12,529 children eligible for interview. Instruments The diagnostic interview used in this survey is a new instrument (Development and Well-Being Assessment; DAWBA) combining the features of structured and semistructured interviews (Goodman et al., 2000). The DAWBA is administered by trained nonclinician interviewers who interview parents about psychiatric symptoms and their functional impact. Skip rules and screening questions allow for a reduction of administration time. Positive symptoms are followed up by openended questions and supplementary prompts, and parental descriptions are entered verbatim (but not rated) by the interviewer in a computer. A teacher questionnaire on common symptoms and their impact is included with space for additional comments. A DAWBA module for direct interview of subjects is available for 11- to 16-year-old youths. Most common psychiatric disorders are covered by the DAWBA. Diagnoses are subsequently generated by computer algorithms based on DSM-IV (American Psychiatric Association, 1994) and ICD-10 (World Health Organization, 1993), using the information from all available informants. These computer-generated summary sheets and all interview data are subsequently reviewed by experienced clinicians who confirm or overturn computer-generated diagnoses. The reliability of clinical diagnosis was assessed with two clinicians who independently rated 500 randomly chosen subjects. The κ statistic for chance-corrected agreement for the presence or absence of any DSM-IV diagnosis was 0.86 (95% confidence interval [CI]: 0.78–0.95). Although the main focus of the survey was on common child psychiatric disorders, screening questions were introduced in the DAWBA to detect early developmental abnormalities. More specifically, parents were asked the following three questions: “In his/her first three years of life, was there anything that seriously worried you about (i) how s/he got on with other people? (ii) the way his/her speech developed? and (iii) any odd rituals or unusual habits that were very hard to interrupt?” Positive answers to any of these questions were followed by the question, “Has this/have these now cleared up completely?” Then, detailed parental descriptions of the child’s past and current levels of social and communicative behaviors and play/activities, results from previous assessments, and schooling history were obtained and diagnoses of PDD were sub-
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sequently derived from all available data. All cases given a diagnosis of PDD (n = 31) by the survey team were independently reviewed by a child and adolescent psychiatrist with expertise in the diagnosis of PDD (E.F.). Direct subject interviews were carried out with four of the eight subjects aged 11 or older, and teacher data were missing for three cases. Other survey instruments used in this analysis were the parent, teacher, and youth (for children aged 11 or older) extended versions of the Strengths and Difficulties Questionnaire (SDQ) (Goodman, 1997, 1999). The SDQ is a brief screening measure that inquires about psychiatric symptoms and positive attributes and assesses the impact of the psychiatric symptoms in terms of chronicity, distress, social impairment, and burden for others. In addition to the total score, five subscores are derived which tap emotional, conduct and hyperactive behaviors, prosocial behaviors, and peer relationships. The behavior and the impact scores have been shown to have excellent reliability and validity (Goodman, 1997, 1999). In addition, parents reported on their child’s general health and completed a 12-item self-report measure which assesses common psychological symptoms for the current period (General Health Questionnaire; GHQ-12) (Goldberg and Williams, 1988). Mental Age Survey children of all ages were administered the British Picture Vocabulary Scale (BPVS-II) (Dunn et al., 1997) and reading and spelling tests from the British Ability Scales (Elliott et al., 1996). In addition, teachers were also asked to estimate the mental age of the child in terms of his/her overall intellectual and scholastic ability. The child’s mental age was then estimated either with a ratio IQ obtained by dividing the teacher estimate of the child’s mental age by the age of the child or by the standard score derived from the BPVS. Within the PDD group, teacher’s IQ ratios and BPVS IQs were available for 23 and 18 children, respectively; the correlation between these two estimates was very high (r = 0.863; p < .001) on 16 subjects with both estimates available. Thus either measure was used to estimate intellectual level in children with PDD. For three subjects without a formal assessment, a clinical estimate was obtained on the basis of a review of all interview data. In one case, the teacher estimate of normal functioning was downgraded as the review of clinical data suggested a fair degree of mental retardation. Data Collection Procedure Lay interviewers (approximately 300) regularly involved in the Office of National Statistics surveys were used to collect survey data. Special attempts were made to trace families whose addresses or names had changed because of various circumstances. Interviewer training emphasized the need to obtain respondents’ descriptions of any problems and concerns in their own words, facilitating this with openended prompts and recording the answers verbatim. Interviewers completed the face-to-face interview with the main caregiver (90% mothers) first, and permission was subsequently sought to ask questions of the sampled child. Children aged 11 to 15 had a private faceto-face interview and also completed directly on a laptop computer a questionnaire containing more sensitive questions about smoking, alcohol, and drug experiences. At the end of the interview, parents were asked to nominate a teacher who knew their child well; the SDQ was subsequently mailed to this teacher. A reminder letter was sent to teachers who did not respond. Additional details about the methodology of the survey are available elsewhere (Meltzer et al., 2000). Statistical Analyses Two weights were devised to adjust for unequal probability of postal sector selection in the sampling frame and to compensate for differential response rate by region. Then, adjustment weights were used to correct for nonresponse bias, with the age and sex distribution of survey respondents being equated to the population structure as determined from the
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1991 census. All prevalence estimates were calculated with the weighted sample, and asymptotic 95% CIs were obtained with the software Stata version 6 (Stata Corporation, 1997). The remaining analyses were conducted on unweighted data as further analyses of the survey showed small design effects on most estimates. A conventional p value of .05 was retained as the level of statistical significance. RESULTS
Of the 12,529 eligible children, interviews were conducted with 10,438 subjects (83%). Failure to interview was due to either noncontact (2%) or refusal (15%). Within cooperating families, child interviews were completed in 95.3% of the cases. The teacher response rate was 80.3% for the whole sample and 89.7% in the PDD group (χ21 = 1.16; not significant). Participation was in general lower within the London metropolitan area (approximately 70%), but appropriate weights corrected for the possible resulting bias. No data were available on nonrespondents either for global psychopathology or for developmental disorders; it is therefore impossible to evaluate the extent to which nonparticipation in this survey might have been associated with higher rates of psychiatric morbidity, and more specifically with the presence or absence of a child with a PDD in the family. Of the 10,438 subjects included in the survey, 9,455 had no psychiatric diagnosis and 952 received a psychiatric diagnosis other than PDD (including 232 who had a DSM-IV diagnosis of attention-deficit/hyperactivity disorder). A total of 31 children initially received one of the PDD diagnoses from the survey team. The review of all records by the child and adolescent psychiatrist confirmed the initial diagnostic evaluation on 29 of these 31 cases (93.5% agreement). Two cases were finally excluded after discussion and subsequently included in the psychiatric comparison group. One girl with equivocal current symptom patterns had been exposed to serious physical abuse before being placed in foster care, and data were missing on her early development. One boy described as having had “autistic tendencies” had made substantial improvements in social functioning according to parental reports; as teacher data were missing, a conservative decision was made to exclude this case. Of the 29 children with a PDD, 3 (10.3%) were in residential schools. The characteristics of children with and without PDD are summarized in Table 1. There was a significant difference (χ24 = 38.4; p < .0001) in age distribution between the three groups: children with PDD were younger and children with another psychiatric disorder were older than normal controls. A marked male preponderance was 822
observed for children with either PDD or other psychiatric disorders, with a male-female ratio of 4.8:1 for the PDD group. Social class distribution also differed between the three groups (χ28 = 85.6; p < .0001). Compared with normal controls, the social class of the PDD sample was skewed toward higher social classes although, because of the small sample size, the comparison fell just short of statistical significance (χ24 = 8.0; p = .09). By contrast, social classes in the psychiatric control group showed a shift in the direction of lower social classes compared with both normal controls (χ24 = 76.8; p < .001) and children with PDD (χ24 = 24.3; p < .001). Cases were distributed evenly in the country, and there was no difference between the three groups for the region of residence (χ210 = 7.56; not significant). No difference was found for ethnicity. Family size, however, differed significantly across the three groups (χ26 = 35.1; p < .0001): children with PDD came from smaller families and psychiatric controls from larger families when compared with normal controls. In sum, children with PDD differed from the other children by their male preponderance, their younger age, and a tendency to come from smaller families and families with higher socioeconomic status. Among the 29 children with a PDD, 2 girls had Rett syndrome, both with moderate to severe mental retardaTABLE 1 Sample Characteristics Normal Controls (n = 9,455) Age, % 5–7 yr 8–11 yr 12–15 yr Gender, % male Social class, % I II III (nm) III (m) IV–V Ethnicity, % white Family size, % 1 child 2 children 3 children 4+ children
Children With a Psychiatric Disorder PDD (n = 954) (n = 29)
28.9 37.7 33.4 48.8
22.8 34.4 42.8 60.1
41.4 34.5 24.1 82.8
8.2 33.2 12.2 26.2 20.2 91.3
4.0 23.9 14.8 28.0 29.3 92.3
20.7 37.9 13.8 17.2 10.3 89.7
20.7 45.9 23.1 10.3
22.6 39.0 22.7 15.6
20.7 55.2 13.8 10.3
pa
.0001 .0001
.0001 NS .0001
Note: PDD = pervasive developmental disorders; nm = nonmanual; m = manual; NS = not significant. a 2 χ tests.
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tion. In this survey, the weighted prevalence of Rett syndrome was 3.5/10,000 girls (95% CI: 0–8.5/10,000). Attempts to subtype the remaining 27 children with PDD variants were found to be unreliable owing to the limitation of the survey data, and in particular to the lack of detailed early developmental history. However, there was no case of childhood disintegrative disorder and Asperger’s disorder was reported in five children (all boys). About half the sample had mental retardation (48.3%; and 44.4% without the two with Rett syndrome) according to our combined estimate. The weighted prevalence of PDD, excluding Rett syndrome, was 26.1/ 10,000 (95% CI: 16.2–36.0/10,000). The prevalence of PDD was not different among the younger children aged 5 to 9 (30.7/10,000; 95% CI: 15.1–46.3/10,000) compared with the older children (22.1/10,000; 95% CI: 9.6–34.6/10,000). The prevalence of all types of PDD, including the two cases of Rett syndrome, was 27.9/10,000 (95% CI : 17.7–38.0/10,000).
Table 2 presents rates of developmental and health problems as reported by the parents. To limit the number of comparisons, problems occurring in less than 10% of the PDD sample are not reported. However, it is worth mentioning that in the PDD group, parents reported one case of fragile X (3.7% of 27 subjects) and no case of cerebral palsy, Down syndrome, or other medical disorders known to be associated with autism (in particular, tuberous sclerosis and congenital rubella). In keeping with their developmental delays, children with PDD had significantly higher rates of motor coordination, bladder control, and bowel control problems than children with or without a psychiatric disorder. They also had significantly higher rates of epilepsy than children in both comparison groups. Of the eight remaining health items, seven comparisons yielded nonsignificant differences in children with PDD compared with the other two groups, with the exception of a marginal increase in “stomach problems” in children with PDD compared with normally developing
TABLE 2 Parent Reports of Health and Developmental Problems Normal Controls (n = 9,455) (%)
Psychiatric Controls (n = 954) (%)
PDD (n = 29) (%)
Bed-wetting
4.0
11.2
27.6
Soiling pants
0.9
3.1
20.7
Coordination difficulties
1.6
7.4
48.3
Asthma
15.1
20.3
13.8
Eczema
12.3
13.8
10.3
Glue ear, otitis media, or hearing grommets
4.5
6.8
10.3
Stomach or digestive problems or abdominal pains
5.6
10.2
13.8
Epilepsy
0.4
2.2
13.8
Food allergies
3.3
5.6
20.7
Other allergies
5.5
6.5
13.8
Eye problems
10.2
14.0
10.3
3.9
6.6
10.3
Hearing problems
ORa
95% CI
p
7.1 2.4 24.3 7.5 47.9 9.9 0.84 0.59 0.83 0.70 2.1 1.3 3.15 1.64 32.7 9.2 7.6 5.1 2.8 2.8 1.11 0.77 2.7 1.4
3.0–16.6 1.03–5.76 9.4–62.8 2.74–20.6 22.5–102.3 4.55–21.7 0.29–2.43 0.20–1.72 0.25–2.75 0.21–2.36 0.63–07.1 0.37–4.42 1.09–9.14 0.55–4.9 10.7–99.9 2.8–30.6 3.06–18.9 1.94–13.36 0.97–8.15 0.93–8.5 0.33–3.7 0.23–2.62 0.81–9.0 0.41–4.82
.001 .043 .001 .001 .001 .001 NS NS NS NS NS NS .034 NS .001 .001 .001 .001 NS NS NS NS NS NS
Note: PDD = pervasive developmental disorders; OR = odds ratio; CI = confidence interval; NS = not significant. a All odds ratios are adjusted for age and gender. The first line refers to comparisons of children with PDD to normal controls, the second line to comparisons of children with PDD to psychiatric controls.
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children but not with psychiatric controls. Moreover, rates of problems were at similar levels, suggesting that the lack of statistical differences was not accounted for by the low statistical power attributable to the small sample size in the PDD group. The only significant difference applied to the parental report of food allergies, which was significantly higher in the PDD group than in both comparison groups. Further analyses did not indicate that those PDD children with food allergies had more severe hyperactivity or mental retardation. The rate of food allergies was 8.2% in the psychiatric controls with an attention-deficit/ hyperactivity disorder diagnosis compared with 18.5% in the PDD group (excluding the two girls with Rett syndrome), and a repeated comparison between these two subgroups led to a much attenuated association (odds ratio = 2.6; 95% CI: 0.9–7.8; p = .084); moreover, when the impact score (see below) was further introduced as a covariate in the model, no difference remained between PDD and hyperactive children (odds ratio = 1.7; 95% CI: 0.5–5.5; p = .4). In general, the estimates of the odds ratios for all analyses decreased in magnitude when the
comparison used the psychiatric group rather than the normal controls, suggesting that the former group might be more appropriate to establish the specificity of associations with correlates. Behavioral deviance scores as measured by the SDQ completed independently by parents and teachers are presented in Table 3. On the parent SDQ, children with PDD obtained more deviant mean scores than normal controls on all scales. When compared with psychiatric controls, children with PDD also had significantly higher total scores; however, these two groups were similar for their scores of emotional and conduct symptoms, suggesting that the higher total scores of the children with PDD were not accounted for by their behavioral problems; rather, the differences were attributable to a large extent to their social difficulties as illustrated by the fact that the largest mean differences were found for the prosocial and peer relationship subscores. The same pattern of results emerged with the teacher SDQ. Children with PDD were markedly different from normally developing children; however, differences with children having other
TABLE 3 Children’s Behavioral Difficulties and Parental Self-Report of Psychological Distress
SDQ Total score Parent Teacher Emotional subscore Parent Teacher Conduct subscore Parent Teacher Hyperactivity subscore Parent Teacher Peer relationships subscore Parent Teacher Prosocial subscore Parent Teacher Impact subscore Parent Teacher Parental self-report (GHQ-12) Total score
NC (n = 9,455)
PC (n = 954)
PDD (n = 29)
Post Hoc Scheffé Tests a
7.59 5.73
16.28 14.29
21.71 14.35
PDD>PC>NC PDD,PC>NC
1.72 1.27
3.69 2.79
4.45 3.15
PDD,PC>NC PDD,PC>NC
1.39 0.68
3.59 2.99
3.36 1.46
PDD,PC>NC PC>PDD,NC
3.18 2.57
6.02 6.00
8.18 5.38
PDD>PC>NC PDD,PC>NC
1.31 1.21
2.98 2.90
5.71 4.35
PDD>PC>NC PDD>PC>NC
8.71 7.46
7.67 5.35
4.25 4.31
PDD
0.20 0.26
2.01 1.70
4.79 2.85
PDD>PC>NC PDD,PC>NC
1.63
3.47
4.07
PDD,PC>NC
Note: NC = normal controls; PC = psychiatric controls; PDD = pervasive developmental disorders; SDQ = Strengths and Difficulties Questionnaire; GHQ-12 = General Health Questionnaire. a All F tests significant at p < .001.
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psychiatric disorders had a more distinctive pattern, with prosocial and peer subscores indicating more deviance in the children with PDD and conduct mean scores (and to a lesser extent hyperactive subscores) indicating less deviance in the children with PDD. The impact scores of the parent SDQ (5 items; range 0–10) and of the teacher SDQ (3 items; range 0–6) were used to assess overall impairment in the child’s functioning. Impact scores for both informants were much higher in children with PDD than in normal and psychiatric control children and significantly so for three of the four comparisons (Table 3). Self-report scores on the GHQ showed similar levels of psychological distress in parents of children with PDD and with other non-PDD diagnoses (Table 3). Within the PDD group, GHQ scores were significantly related to total SDQ scores (Spearman r = 0.397; p < .05) but not to IQ estimates. DISCUSSION Study Limitations
First, although the participation rate was high, about a fifth of selected households could not be included. There is evidence from previous child psychiatric surveys that rates of psychopathology are higher among nonrespondents (Fombonne, 1994; Rutter et al., 1970), which leads to a biased estimate of prevalence. When data on nonrespondents are available, proper weights can be devised to control for this potential bias. In this household survey, however, no data were available on nonrespondents and therefore prevalence estimations could not compensate for nonparticipation in the survey. It is difficult to assess whether or not the resulting estimates are biased. In most previous autism surveys, refusal rates by parents of children with a PDD tended in fact to be very low, with a median figure of 6% in 10 surveys in which these data were available (Fombonne, in press). Thus if the participation rate was differentially influenced by the presence of a child with PDD within the household, our prevalence estimates might indeed be inflated rather than underestimated. However, it is not possible to assess the magnitude and direction of potential bias in the resulting rates. Second, children under the care of local authorities were not eligible for inclusion in the survey; this increases the possibility that prevalence rates might be underestimated. Whether or not this was the case cannot be assessed from available data; however, an ongoing survey of such children will shed light on this issue in the near future. J . A M . A C A D . C H I L D A D O L E S C . P S YC H I AT RY, 4 0 : 7 , J U LY 2 0 0 1
Third, cases were identified with data collected through structured standardized interviews administered by nonclinician interviewers and primarily designed for assessing common psychiatric disorders. Nevertheless, the inclusion of specific questions for early developmental concerns, the combination of open-ended questions and of parental descriptions, and reliance on clinical judgment for final case determination provided a satisfactory procedure to assess PDD in this survey. However, reliable differentiation between subtypes of PDD could not be achieved, limiting our estimations to combined rates of autistic spectrum disorders. Fourth, the final number of cases identified was small, although not very different from the median number of 51 cases per survey found in 27 previous epidemiological investigations (Fombonne, in press). The precision of the resulting prevalence estimates was modest, precluding meaningfully estimating rates in subgroups; also, the assessment of correlates was performed with necessarily suboptimal statistical power, although this was compensated to a degree by the large sample sizes available in both comparison groups. This feature should nevertheless be borne in mind, particularly when assessing nonsignificant results hereafter. Prevalence Findings
The prevalence of Rett syndrome in this survey was remarkably similar to one previous U.K. estimate (Asthana et al., 1990) and consistent with the figure of approximately 1/10,000 girls reported in other studies (Chakrabarti and Fombonne, in press; Hagberg and Hagberg, 1997). The prevalence of 26.1/10,000 reported for other PDD is consistent with a recent best estimate of 22/10,000 for combined PDD derived from a quantitative analysis of surveys of autism (Fombonne, in press). Three recent U.K. surveys have yielded estimates which were either lower (10.1/10,000: Taylor et al., 1999) or higher (57.9/ 10,000: Baird et al., 2000; 62.6/10,000: Chakrabarti and Fombonne, in press) than in this survey. The survey with the lowest rate relied on administrative records, whereas the two surveys with the highest rates used an intense, multiphase screening procedure for case identification. Conducted at the same time in the same country, this 6-fold variation in rates illustrates well the major impact on estimates of methodological factors and the inherent difficulties of comparisons across studies. Unlike all other epidemiological surveys, however, our estimate is based on a sample representative of the whole country. As such, it is less susceptible to bias due to local or regional 825
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differences in case ascertainment and it can be extrapolated to Great Britain more confidently. Our estimates were similar among younger and older subjects in our survey. Sample-size limitations precluded us from performing more fine-tuned comparisons across successive birth cohorts to test hypotheses about secular changes in rates of autism, a method that could be used in larger studies (Fombonne et al., 1997). Associated Features
Our sample contained a somewhat elevated proportion of high-functioning subjects and of boys. These features may or may not be correlated since higher male-female ratios are typically found among high-functioning subjects with autism in clinical (Lord et al., 1982) and epidemiological (Fombonne, 1999a) studies. It is interesting that similar high male-female ratios and proportions of highfunctioning subjects were reported in other recent British surveys (Baird et al., 2000; Chakrabarti and Fombonne, in press). The crudeness of the intellectual assessments should, however, be kept in mind as should the possibility that these unusual characteristics might simply correspond to sample fluctuations. The major medical correlates were, however, in keeping with other surveys with a low proportion of PDD children with fragile X disorder (Chakrabarti and Fombonne, in press; Fombonne, in press) and no cases of other severe medical disorders usually occurring in autism in small (<10%) proportions (Fombonne, in press; Rutter et al., 1994). The rate of epilepsy was also in keeping with other surveys, but it should be acknowledged that this represents an underestimate of the lifetime risk of epilepsy because most subjects in the survey have not yet gone through the period at highest risk of seizures (Deykin and MacMahon, 1979; Volkmar and Nelson, 1990). Somatic Symptoms
In light of recent controversies (Fombonne, 1999b, 2001b; Kawashima et al., 2000; Wakefield et al., 1998; Whiteley et al., 2000), symptoms reflecting potential dysfunction of the immune or gut systems were of particular interest even though they were reported by parents only. Asthma and eczema were not more frequent in the PDD group compared with both comparison groups. Similarly, rates of ear infections were not significantly increased in the PDD group, contrary to earlier reports (Konstantareas and Homatidis, 1987). Abdominal pains were not significantly increased in frequency when compared with a large comparison group of children with a psychiatric disorder. The rates of stomach or digestive problems remained at 826
low levels, and there was no report by parents that any child had inflammatory bowel disease or Crohn’s disease; this finding is consistent with the lack of a demonstrated association between these bowel disorders and autism in large epidemiological and clinical series (Fombonne, 1998). Food allergies were the only symptom that was significantly elevated in the PDD group. The precise meaning of this finding is difficult to explore further without corroborative medical evidence of such symptoms, and replication in larger samples is needed to establish its validity. It is worth noting, however, that this symptom was particularly endorsed by parents of children with neurodevelopmental disorders, who might be more prone to seek such explanations; furthermore, adjusting for the impact of the disorder removed the difference between PDD and hyperactive children completely. Behavior Problems and Family Distress
Although the SDQs are not measures designed for use with autism samples, they provided a useful additional dimensional assessment of behavioral problems as seen in different contexts. It was clear that the total scores reflected well the severity of these pervasive disturbances of development. Furthermore, the distinctive pattern of subscores suggested that both at home and at school, children with PDD were not more emotionally or behaviorally disturbed than their peers with other psychiatric disorders, with the exception of hyperactive symptoms reported by parents. By contrast, children with PDD stood out as having very deviant social scores as could have been predicted from the nature of their disorders. In line with several studies which explored levels of stress and burden on parents of children with PDD (Holroyd and McArthur, 1976; Konstantareas and Homatidis, 1991; Wolf et al., 1989), parents of our sample reported high levels of distress and burden. The epidemiological nature of this investigation provides strong support to previous findings in clinical samples, and the results confirm the high level of needs of this group of families. Study Strengths
Two particular strengths of this investigation must be underscored. First, this is the first survey of autism to rely on a stratified random sample representative of a population. Consequently, the findings have a greater potential for generalization than do most other epidemiological inquiries. Second, capitalizing on the survey design, we could compare children with PDD with two comparison groups of large sample sizes, including one of children J . A M . A C A D . C H I L D A D O L E S C . P S YC H I AT RY, 4 0 : 7 , J U LY 2 0 0 1
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with psychiatric disorders. Even though the sample of PDD children was small, the survey design allowed us to perform meaningful comparisons on a range of symptoms and impairment scores. This comparative approach has been generally lacking in other epidemiological surveys of autism. The estimate derived from this survey is consistent with the higher prevalence rates reported recently in several surveys (Baird et al., 2000; Centers for Disease Control, 2000; Chakrabarti and Fombonne, in press; Kadesjo et al., 1999). Equally, as outlined above, methodological factors have an important impact on survey estimates, which calls for some caution about their interpretation and comparisons. In particular, increased prevalence rates in recent surveys cannot be interpreted as indicative of a secular increase in the incidence of autism and its variants unless case definition and case-finding methods are held strictly constant in comparing rates across studies and over time, a target that is impossible to reach with the existing literature (Fombonne, 2001a, in press).
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Fombonne E (1999b), Are measles infections or measles immunizations linked to autism? J Autism Dev Disord 29:349–350 Fombonne E (2001a), Is there an epidemic of autism? Pediatrics 107:411–413 Fombonne E (2001b), Presentation to Immunization Safety Review Committee. New studies: March 8. Washington, DC: Institute of Medicine Fombonne E (in press), Epidemiological findings in autism and related pervasive developmental disorders. In: Educating Children With Autism, Lord C, ed. Washington, DC: National Academy of Sciences Press Fombonne E, du Mazaubrun C, Cans C, Grandjean H (1997), Autism and associated medical disorders in a large French epidemiological sample. J Am Acad Child Adolesc Psychiatry 36:1561–1569 Gillberg C, Wing L (1999), Autism: not an extremely rare disorder. Acta Psychiatr Scand 99:399–406 Goldberg D, Williams PA (1988), User’s Guide to the General Health Questionnaire. Windsor, England: NFER-NELSON Goodman R (1997), The Strengths and Difficulties Questionnaire: a research note. J Child Psychol Psychiatry 38:581–586 Goodman R (1999), The extended version of the Strengths and Difficulties Questionnaire as a guide to child psychiatric caseness and consequent burden. J Child Psychol Psychiatry 40:791–799 Goodman R, Ford T, Richards H, Gatward R, Meltzer H (2000), The Development and Well-Being Assessment: description and initial validation of an integrated assessment of child and adolescent psychopathology. J Child Psychol Psychiatry 41:645–655 Hagberg B, Hagberg G (1997), Rett syndrome: epidemiology and geographical variability. Eur Child Adolesc Psychiatry 6:5–7 Holroyd J, McArthur D (1976), Mental retardation and stress on the parents: a contrast between Down syndrome and childhood autism. Am J Ment Defic 80:431–436 Kadesjo B, Gillberg C, Hagberg B (1999), Autism and Asperger syndrome in seven-year-old children: a total population study. J Autism Dev Disord 29:327–331 Kawashima H, Mori T, Kashiwagi Y, Takekuma K, Hoshika A, Wakefield A (2000), Detection and sequencing of measles virus from peripheral mononuclear cells from patients with inflammatory bowel disease and autism. Dig Dis Sci 45:723–729 Konstantareas MM, Homatidis S (1987), Ear infections in autistic and normal children. J Autism Dev Disord 17:585–594 Konstantareas MM, Homatidis S (1991), Effects of developmental disorder on parents: theoretical and applied considerations. Psychiatr Clin North Am 14:183–198 Lord C, Schopler E, Revecki D (1982), Sex differences in autism. J Autism Dev Disord 12:317–330 Lotter V (1966), Epidemiology of autistic conditions in young children, I: prevalence. Soc Psychiatry 1:124–137 Meltzer H, Gatward R, Goodman R, Ford T (2000), The Mental Health of Children and Adolescents in Great Britain. London: Stationery Office Rutter M, Bailey A, Bolton P, Le Couteur A (1994), Autism and known medical conditions: myth and substance. J Child Psychol Psychiatry 35:311–322 Rutter M, Tizard J, Whitmore K (1970), Education, Health and Behaviour. London: Longmans Stata Corporation (1997), Statistic/Data Analysis. College Station, TX: Stata Press Taylor B, Miller E, Farrington CP et al. (1999), Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association. Lancet 353:2026–2029 Volkmar F, Nelson D (1990), Seizure disorders in autism. J Am Acad Child Adolesc Psychiatry 29:127–129 Wakefield A, Murch S, Anthony A et al. (1998), Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. Lancet 351:637–641 Whiteley P, Rodgers J, Shattock P (2000), MMR and autism (commentary). Autism 4:207–211 Wolf LC, Noh S, Fisman SN, Speechley M (1989), Psychological effects of parenting stress on parents of autistic children. J Autism Dev Disord 19:157–166 World Health Organization (1993), The ICD-10 Classification of Mental and Behavioural Disorders: Diagnostic Criteria for Research. Geneva: World Health Organization
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Clinical Implications
Without having to sustain the claim of an epidemic, the consistency of recent estimates toward much higher prevalence figures than those reported 30 years ago should nevertheless be seen as sufficient to draw the attention to a large group of children and their families who have important service needs. Clinicians should be reminded that distress in families with a child suffering from any PDD is very high and that to alleviate it is a major treatment goal. REFERENCES