Sample characteristics
Medical notes for 318 subjects were obtained. They varied in quality and exhaustiveness. For some children, GP records included several consultant reports, speech and language assessments, and educational psychology reports. For other children, the information available was scanty, with sometimes the only available data consisting of one, or a few, letters between the GPs and consultants. A high proportion of records had missing data on parental age, socio-economic status, and detailed psychometric assessment of the child and therefore the frequencies of these variables are not described here. Of the 318 children whose medical forms were obtained, the raters confirmed a diagnosis of PDD in 294 children (92.5%). Compared to children with a confirmed PDD diagnosis, children for whom the diagnosis was not confirmed (n = 24) had significantly fewer PDD symptoms (2.1 vs 6.2; p < .001), higher language level (phrase speech: 80% vs 45%; p=.051), and more frequent parental concern arising for the first time after the age of 3 years (20% vs 2.9%; p=.024). No significant differences were found with respect to gender, birth year, presence of epilepsy or regression or in the average age at first diagnosis in the GPRD database.
The main characteristics of the 294 children with a confirmed diagnosis of PDD are shown in Table
1. The male/female ratio was 4.25:1. A third of the children had no phrase speech when language level was recorded (at a mean age of 7.9 years). About a third of children had estimated intellectual skills falling into the normal range. 55 (19%) children showed clear evidence of regression and loss of acquired skills, and a further 34 had a developmental pattern consistent with an uneven and slow rate of acquisition of new skills as they grew up. The rate of regression and of epilepsy (18%) are consistent with those described in other surveys of autistic children. The mean number and pattern of DSM-IV symptoms was consistent with the diagnostic concepts of autism, especially as symptoms of social deficits appeared to be reported more frequently (Table
1). The computer-based algorithm identified 237 (80.6%) of the 294 cases as having a PDD.
Table 1
Characteristics of 294 Children with PDD
Male | | 238 | 81.0 |
Diagnosis subtype | Autism | 180 | 61.2 |
| PDD NOS | 19 | 6.5 |
| Asperger | 18 | 6.1 |
| Unknown | 77 | 26.2 |
Birth year | 1973 – 1977 | 19 | 6.5 |
| 1978 – 1982 | 31 | 10.7 |
| 1983 – 1987 | 65 | 22.3 |
| 1988 – 1992 | 115 | 39.5 |
| 1993 – 1997 | 61 | 21.0 |
Past or current epilepsy | | 53 | 18.0 |
Overall level of language1
| Mute | 45 | 16.9 |
| Single words | 52 | 19.5 |
| Few phrases | 50 | 18.7 |
| Phrase speech | 120 | 44.9 |
Estimated intellectual functioning | Normal IQ | 89 | 34.5 |
| Mild MR | 99 | 38.4 |
| Moderate MR | 49 | 19.0 |
| Severe MR | 21 | 8.1 |
Age of parental recognition (months) | 0 – 12 | 82 | 39.6 |
| 13 – 24 | 94 | 45.4 |
| 25 – 36 | 25 | 12.1 |
| 36+ | 6 | 2.9 |
Regression | | 55 | 19.0 |
| N | Mean | SD |
Age at first GPRD diagnosis (years) | 278 | 6.25 | 4.52 |
Age at first letter (years) | 282 | 4.50 | 3.84 |
Age at last report on note (years) | 289 | 10.66 | 5.65 |
Mean number of DSM-IV PDD symptoms | | | |
–Total | 294 | 6.23 | 4.17 |
–Language/communication | 294 | 1.66 | 1.43 |
–Social interaction | 294 | 2.53 | 1.82 |
–Repetitive behaviours | 294 | 2.05 | 1.76 |
It was possible to allocate a more specific diagnosis to 217 of the 294 children with PDD. This was autistic disorder in 180 children (82.9%), Asperger Disorder (AD) in 18 children (8.3%), and PDDNOS (Pervasive Developmental Disorder Not Otherwise Specified) in 19 children (8.8%). The confidence level in the diagnostic subtype was generally high (high in 67.1%; medium in 19.7%, and low in 13.1%). In the remaining 77 children (26.2%), the quality of the data did not allow for the diagnosis of a specific PDD subtype. A comparison of the PDD children with and without a more specific diagnosis showed that children without a PDD subtype were comparable to children with autism with respect to language level and intellectual functioning but closer to the children with either PDDNOS or AD with respect to age at first electronic diagnosis and rate of regression. Compared to both other groups, they had significantly fewer PDD symptoms, most certainly reflecting the poorest quality of the notes that precisely precluded a final sub-typing to be attained by raters.
The mean age at first parental concern regarding their child's development was 16.8 months (SD = 9.8) in 142 children where a precise age could be estimated. Age of first recognition of symptoms in medical records could be estimated in broad age bands in 207 subjects and occurred before age 3 years in 201 subjects (97.1%). Onset of first symptoms was also determined by the rater's judgment, based on the medical records, in 91 subjects and was 12 months (SD = 8.5). In 88 subjects where both a parental and a rater age of onset were available, the rater mean age of onset was significantly younger than the age at parental concern (12.1 months vs 13.3 months; paired t-test; p = 0.02). Finally, the presence of a PDD in a first-degree relative of the index child was reported in 7.8% of the sample, consistent with other surveys of PDDs [
4].
We compared children with an autistic disorder diagnosis with children with another PDD diagnosis (Table
2). The PDDNOS/AD group had significantly fewer language and intellectual impairments and were on average 2.3 years older than their autistic counterparts when recorded in the GPRD database. Regression was less often reported in the PDDNOS/AD group.
Table 2
Comparison of Children with Autism or other PDD (N = 217)
Male Sex | 146 | 81.1 | 31 | 83.8 | .703 |
Birth year | 1973 – 1977 | 11 | 6.2 | - | - | .217 |
| 1978 – 1982 | 15 | 8.4 | 3 | 8.3 | |
| 1983 – 1987 | 43 | 24.2 | 11 | 30.6 | |
| 1988 – 1992 | 68 | 38.2 | 18 | 50.0 | |
| 1993 – 1997 | 41 | 23.0 | 4 | 11.1 | |
Past or current epilepsy | 29 | 16.1 | 5 | 13.5 | .692 |
Overall level of language1
| Mute | 31 | 17.7 | - | - | .000 |
| Single words | 38 | 21.7 | 1 | 2.9 | |
| Few phrases | 39 | 22.3 | 5 | 14.7 | |
| Phrase speech | 67 | 38.3 | 28 | 82.4 | |
Estimated intellectual Functioning | Normal IQ | 42 | 25.3 | 27 | 81.8 | .000 |
| Mild MR | 78 | 47.0 | 4 | 12.1 | |
| Moderate MR | 34 | 20.5 | 2 | 6.1 | |
| Severe MR | 12 | 7.2 | - | - | |
Age of parental recognition (months) | 0 – 12 | 60 | 41.1 | 8 | 32.0 | .176 |
| 13 – 24 | 68 | 46.6 | 11 | 44.0 | |
| 25 – 36 | 16 | 11.0 | 4 | 16.0 | |
| 36+ | 2 | 1.4 | 2 | 8.0 | |
Regression2
| 43 | 24.0 | 3 | 8.3 | .036 |
Language delay | Yes probable/definite | 162 | 96.4 | 13 | 44.8 | .000 |
| Autism | PDD NOS & Asperger | P |
| N = 180 | N = 37 | |
| N | Mean | SD | N | Mean | SD | |
Age at first GPRD diagnosis (years) | 167 | 5.45 | 4.06 | 36 | 7.77 | 3.68 | .002 |
Age at first letter (years) | 174 | 3.45 | 2.48 | 36 | 5.08 | 3.39 | .009 |
Age at last report on note (years) | 176 | 10.50 | 5.61 | 36 | 10.51 | 4.28 | .992 |
Mean number of DSM-IV PDD symptoms | | | | | | | |
–Total | 180 | 7.67 | 3.92 | 37 | 7.24 | 3.81 | .543 |
–Language/communication | 180 | 2.14 | 1.41 | 37 | 1.62 | 1.28 | .040 |
–Social interaction | 180 | 3.04 | 1.76 | 37 | 3.24 | 1.57 | .525 |
–Repetitive behaviours | 180 | 2.49 | 1.73 | 37 | 2.38 | 2.06 | .732 |
Since regression and loss of skills is a clinical feature of potential interest for our main study, we examined further the clinical correlates of regression. As regression was infrequent in the PDDNOS/AD group and as these children were different from children with autism with respect to age at diagnosis and severity, we restricted this analysis to those children with a diagnosis of autistic disorder (Table
3). The regressive and non regressive groups differed with respect to language level and intellectual functioning where children with regression exhibited lower levels of functioning at the final assessment. They also had a significantly lower age at the first referral letter on file mentioning a developmental problem.
Table 3
Autistic Children with or without Regression (N = 1791)
Male Sex | 114 | 83.8 | 32 | 74.4 | .166 |
Birth year | 1973 – 1977 | 8 | 5.9 | 3 | 7.0 | .789(trend) |
| 1978 – 1982 | 12 | 8.9 | 3 | 7.0 | |
| 1983 – 1987 | 31 | 23.0 | 12 | 27.9 | |
| 1988 – 1992 | 56 | 41.5 | 12 | 27.9 | |
| 1993 – 1997 | 28 | 20.7 | 13 | 30.2 | |
Past or current epilepsy | 21 | 15.4 | 7 | 16.3 | .895 |
Overall level of language2
| Mute | 21 | 15.9 | 10 | 23.3 | .065 (trend) |
| Single words | 27 | 20.5 | 11 | 25.6 | |
| Few phrases | 28 | 21.2 | 11 | 25.6 | |
| Phrase speech | 56 | 42.4 | 11 | 25.6 | |
Estimated intellectual functioning | Normal IQ | 35 | 27.8 | 7 | 17.5 | .054 (trend) |
| Mild MR | 61 | 48.4 | 17 | 42.5 | |
| Moderate MR | 22 | 17.5 | 12 | 30.0 | |
| Severe MR | 8 | 6.3 | 4 | 10.0 | |
Age of parental recognition (months) | 0 – 12 | 45 | 43.3 | 15 | 35.7 | .897 (trend) |
| 13 – 24 | 45 | 43.3 | 23 | 54.8 | |
| 25 – 36 | 12 | 11.5 | 4 | 9.5 | |
| 36+ | 2 | 1.9 | - | - | |
Language delay | Yes probable/definite | 122 | 97.6 | 40 | 93.0 | .163 |
| Regression | |
| No (N = 136) | Yes (N = 43) | P |
| N | Mean | SD | N | Mean | SD | |
Age at first GPRD diagnosis (years) | 127 | 5.39 | 4.01 | 40 | 5.63 | 4.29 | .746 |
Age at first letter (years) | 133 | 3.66 | 2.75 | 41 | 2.76 | 1.02 | .002 |
Age at last report on note (years) | 134 | 10.61 | 5.65 | 42 | 10.15 | 5.56 | .646 |
Mean number of DSM-IV PDD symptoms | | | | | | | |
–Total | 136 | 7.51 | 3.94 | 43 | 8.35 | 3.67 | .221 |
–Language/communication | 136 | 2.08 | 1.43 | 43 | 2.37 | 1.29 | .236 |
–Social interaction | 136 | 3.04 | 1.66 | 43 | 3.14 | 2.02 | .738 |
–Repetitive behaviours | 136 | 2.40 | 1.73 | 43 | 2.84 | 1.68 | .144 |
Trends over time in clinical features that are known to indicate autism severity were also examined for the autism group (Table
4). Birth years were grouped into 5-year intervals. There was a significant trend for decreasing levels of mental retardation and for an increasing proportion of males, suggesting that clinical presentation became less severe over time. Age differences in the birth cohorts made the interpretation of trend for phrase speech and for epilepsy more difficult to interpret. Other clinical features, including regression, did not change significantly with time.
Table 4
Characteristics of 1781 Children with Autism over five year intervals
Male Sex | 8 | 72.7 | 12 | 80.0 | 32 | 74.4 | 55 | 80.9 | 38 | 92.7 | .058 |
Past or current epilepsy2
| 4 | 36.4 | 4 | 26.7 | 5 | 11.6 | 11 | 16.2 | 4 | 9.8 | .046 |
Phrase speech | 3 | 27.3 | 10 | 66.7 | 19 | 47.5 | 24 | 35.8 | 10 | 24.4 | .044 |
Normal/mild moderate Functioning | 4 | 36.4 | 9 | 60.0 | 30 | 75.0 | 45 | 71.4 | 31 | 86.1 | .003 |
Age of parental recognition <24 months | 8 | 80.0 | 9 | 81.8 | 30 | 85.7 | 47 | 88.7 | 34 | 91.9 | .198 |
Regression | 3 | 27.3 | 3 | 20.0 | 12 | 27.9 | 12 | 17.6 | 13 | 31.7 | .789 |
Probable/definite Language delay3
| 9 | 81.8 | 11 | 91.7 | 41 | 100.0 | 62 | 96.9 | 38 | 97.4 | .069 |
DSM-IV Symptoms: | | | | | | | | | | | |
–Mean | 6.91 | | 6.00 | | 8.16 | | 7.62 | | 8.29 | | .127 |
–SD | 4.13 | | 3.70 | | 4.48 | | 3.19 | | 4.24 | | |
Interrater reliability
Interrater reliability was examined on the subset of 50 randomly selected children. Agreement between the two raters was good for the presence/absence of a PDD in the child (Kappa = .73), and there were only 2 cases where raters originally disagreed. The agreement on the number of DSM-IV symptoms was excellent (ICC = .92). PDD symptom scores for each of the three domains separately showed high intra-class correlations as well, with ICC values of .87 for the social domain, .75 for the communication/language domain, and of .91 for repetitive behaviours. The agreement was also good to excellent on the presence/absence of language delay (Kappa = 1.0), of regression or loss of skills in the course of development (Kappa = .58), of epilepsy (Kappa = .84), on overall language level (Kappa = .62), on estimate of intellectual functioning coded on 3 levels (normal range, mild retardation, moderate to profound retardation) (Kappa = .72), and on the presence/absence of any developmental disorder amongst first degree relatives (Kappa = .74). Reliability was lower for regression due to the difficulty in differentiating loss of skills from developmental stagnation and to establish language level before the reported loss occurred.