We identified 1410 persons with a first recorded diagnosis of PDD during the study period: 1097 were categorised as autism and 313 as other PDD, for 294 (94%) of whom the diagnosis was Asperger's syndrome (appendix).
Table shows the crude rates of first diagnosis of PDD and age standardised incidence ratios by year of diagnosis and gender.
First diagnoses of PDD by year of diagnosis and gender
The person-years at risk varied from year to year because of changes in the number of contributing practices, which increased until the mid 1990s and then fell. During the study period incidence rates increased progressively for both males and females. The patterns of crude rates and of standardised incidence ratios were very similar. The overall rate increased seven-fold from 0.40/10,000 person-years (95% CI 0.30 to 0.54) in 1991 to 2.98/10,000 person-years (95% CI 2.56 to 3.47) in 2001. A similar change occurred in the age standardised incidence ratios, from 35 (95% CI: 26–47) in 1991 to 365 (95% CI: 314–425) in 2001. The overall male to female ratio was 4.8 and during 1991 to 2001 ranged from 2.7 to 8.4 (chi-squared test for heterogeneity, p = 0.21), with no clear temporal trend.
Table shows the number of cases, person-years of follow-up and rate of first recorded diagnosis by date of diagnosis and by date of birth, both in two year intervals. The diagonally linked numbers identify rates among children of the same age in different years. The temporal increase observed in rates of diagnosis is not limited to children born during specific years or to children diagnosed in a specific time period. The rates of diagnosis for children at the same age, born in successive birth cohorts, increased in each successive cohort, including for relatively old ages at diagnosis. For example, for diagnoses at the ages of two to four years, the rates of new diagnosis per 10 000 person-years from 1992–93 to 2000–01 were 1.41, 2.70, 3.07, 5.56 and 7.74. Corresponding rates at ages eight to ten years were 0.36, 0.40, 0.67, 2.46 and 4.46.
Table 2 Number of cases, person-years follow-up and rates of first recorded diagnosis of PDD (per 10,000 person-years) by year of diagnosis and year of birth (in two year categories). The diagonally linked numbers identify rates among children of the same age (more ...)
Table shows the rates of first recorded diagnosis and age standardised incidence ratios of autism and other PDDs by year of diagnosis.
Crude rates of first diagnosis (per 10 000 person-years) and age standardised incidence ratios for autism and other PDDs by year of diagnosis
There was a striking rise in the rate of diagnoses of other PDDs: from zero for the period 1988 to 1992 rising to 1.06/10,000 person-years by 2001. Over the study period the rate of diagnosis of autism also increased substantially. The patterns were broadly similar for males and females (data not shown). Few diagnoses of other PDDs were made before 1997, but by 2001 over one third of all diagnoses were PDDs other than autism.
Table shows the rates of first recorded autism and other PDDs by geographical area.
Crude incidence rates (per 10 000 person-years) and age standardised incidence ratios (SIR) for the period 1988 to 2001 by geographical area of the United Kingdom (listed in order of SIR)
There was marked geographical variation in rates of first diagnosis, with the lowest rate in Wales being less than half the rate in the South East of England (chi-squared test for heterogeneity in SIRs by region, p < 0.001). In general, the regional variations were less marked for autism than for other PDDs and the ratio of the diagnoses of autism compared with other PDDs varied greatly between areas (chi-squared test for heterogeneity, p = 0.0006).