The ADDM Network monitored CP among 8-year-old children living in four areas in the United States, including central Alabama; metropolitan Atlanta, Georgia; metropolitan St. Louis, Missouri; and southeastern Wisconsin in 2006, 2008, and 2010. The prevalence of CP for the surveillance years 2006 and 2008 was published previously for a broader study area than the area under surveillance in 2010.^12,13 In one site, the size of the surveillance area changed over time. To allow comparisons over time, this report is restricted to counties in the four states that were under surveillance in all 3 years, 2006, 2008, and 2010 (Table 1).

The methodology for CP surveillance was based on that developed by the CDC’s Metropolitan Atlanta Developmental Disabilities Surveillance Program, an ongoing, population-based, multisource surveillance programme that monitors the occurrence of developmental disabilities among 8-year-old children in metropolitan, Atlanta, and that has been described in detail previously.^12–16 For surveillance purposes, we defined CP as a group of permanent disorders of movement and posture that are attributed to nonprogressive disturbances in the developing brain. CP is more than a motor impairment: it is a disorder often accompanied by disturbances of sensation, perception, cognition, communication, and behaviour; by epilepsy; and by secondary musculo-skeletal problems.¹⁷ This surveillance case definition includes cases with a documented postneonatal (>28 days after birth to age 8 years) cause. For analyses monitoring trends in the percent LBW, we restricted the CP cases to those with no documented postneonatal aetiology, which included approximately 93% of the CP cases each year.

Case inclusion criteria were as follows: residence in one of the respective surveillance areas during years 2006, 2008, or 2010 at any time during the age of 8 years; and documentation of a CP diagnosis or physical findings consistent with CP in an evaluation by a qualified professional at or after 2 years of age. Children suspected of having CP were identified by screening comprehensive evaluations at multiple sources, including hospitals, clinics, diagnostic centres, health care providers, and state public health and rehabilitation agencies. In Georgia, potential cases also were identified through public school special education programmes. Records containing one or more evaluations with a confirmed or suspected CP diagnosis or descriptions of physical findings consistent with CP were abstracted.

Demographic data, diagnostic summaries, descriptions of physical findings and motor functioning, and information about co-occurring disabilities were collected. Race and ethnicity of each child was determined from information in clinical or education records or from birth certificate information and classified as white non-Hispanic, black non-Hispanic, Hispanic, other, or undetermined. Data were combined into one composite record per child and subsequently reviewed by clinicians using a specified protocol to determine case status. Clinician reviewers included developmental paediatricians, a paediatric neurologist, physical therapists, and occupational therapists. In the absence of excludable conditions, such as progressive disorders and neuromuscular diseases, children were classified as confirmed CP cases based on diagnostic information and physical finding descriptions consistent with CP in the abstracted records.

Two methods for classifying gross motor function were incorporated into clinician review. When sufficient information on gross motor function at or after 4 years of age was abstracted, clinician reviewers used the Gross Motor Function Classification System (GMFCS)¹⁸ to assign a functional level. GMFCS is based on the gross motor skills needed for self-initiated movement, including sitting, transferring, and mobility. Due to reliance on the documented findings in the source files, we were unable to assign a GMFCS level for all cases.¹⁹

Motor function was also classified based on walking ability as ‘unaided walking’, ‘walking with aids’, or ‘unable to walk’.²⁰ In determining walking ability, children classified as GMFCS Levels I or II were categorised as ‘walks independently’; Level III as ‘walks with handheld mobility device’; and Levels IV or V as ‘limited or no walking ability’. For children with insufficient information to GMFCS level, walking ability was independently assigned.²¹

The prevalence of CP per 1000 8-year-old children in the combined surveillance area declined from 3.5 (95% CI 3.2, 3.9) in 2006 to 2.9 (95% CI 2.6, 3.2) in 2010 (Table 2, test for trend P = 0.036). Prevalence was lower in 2010 than in 2006 for both boys and girls, in each of the four surveillance sites, and among white non-Hispanic and Hispanic children, though the trends within most subgroups were not statistically significant. In contrast to other subgroups, no decline in the prevalence of CP was evident among black non-Hispanic children (Table 2).

In the birth cohort serving as the comparison group for the CP cases, the percent born LBW increased slightly, from 7.8 (95% CI 7.6, 7.9) in birth year 1998 to 8.1 (95% CI 8.0, 8.3) in 2002, while the percentage born VLBW was constant during the study period at 1.3% (Table 3).

In contrast, among children with CP not attributed to postneonatal causes and with available birthweight information, the percentage born LBW declined by 9.6% during the study period, from 54.3% (95% CI 48.4, 60.1) in 2006 to 49.1% (95% CI 42.9, 55.2) in 2010. This trend was not statistically significant (test for trend P = 0.213, Table 3). The percentage of these cases born VLBW declined by >20%, from 35.4% (95% CI 30.0, 41.2) in 2006 to 28.1% (95% CI 22.9, 34.0) in 2010 (P = 0.066, Table 3). The direction of these trends was fairly consistent across demographic subgroups (Table 3). Analogous results are provided in Appendix 3 for the categories of preterm and very preterm birth.

Comparing CP cases not attributed to postneonatal causes to the birth cohort overall, and using births ≥2500 g as the reference group, the relative risks indicating associations between CP and both LBW and VLBW appeared to decline during the study period, from 13.9 (95% CI 11.0, 17.4) in 2006 to 10.8 (95% CI 8.5, 13.7) in 2010 for LBW and from 39.1 (95% CI 30.9, 49.5) to 28.6 (95% CI 22.0, 37.2) for VLBW (Table 4). The differences between the relative risks in 2006 and 2010, however, were not significantly different from zero for either LBW (P = 0.136) or VLBW (P = 0.082).

The population attributable risks for LBW were 50.2% (95% CI 41.2, 56.1) in 2006 and 44.3% (95% CI 37.9, 50.7) in 2010. For VLBW, they were 33.1% (95% CI 16.3, 38.7) in 2006 and 26.4% (95% CI 21.5, 32.0) in 2010.

This is the first population-based study in the United States to show a significant decline in the population prevalence of CP. It also showed preliminary evidence, though not statistically significant, of declining trends in the association between CP and both LBW and VLBW. The findings are encouraging given concern raised in earlier decades that increases in the survival of infants at greatest risk of CP (i.e. those born preterm and VLBW) may have given rise to increases in the population prevalence of CP.^5–7 This study was only able to monitor CP prevalence in a common surveillance area for the years 2006, 2008, and 2010. Although the results are encouraging, it is not possible to know from the data available whether the significant decline in CP prevalence observed during the study period is due to advances in health care for infants born VLBW. Nor is it possible to predict from the initial results reported here whether declines in CP prevalence and its association with LBW will continue after 2010.

Less hopeful were our findings of no decline in CP prevalence for black non-Hispanic children during the study period, and evidence that the racial disparity in CP prevalence appeared to increase over time. In 2010, the prevalence of CP was 68% higher in black children relative to white children (Table 2). These findings are difficult to reconcile with our observation that the contribution of LBW and VLBW to the risk of CP appeared to decline for both black and white children, and with previous research showing that the excess prevalence of CP among black children in the United States is largely explained by the excess frequency of LBW among black infants.^31,32 Further research is needed to identify other risk factors contributing to the excess prevalence of CP among black children and to identify and monitor strategies to reduce the risk of CP and to reduce racial disparities in CP risk in the United States.

The results of this study suggest that initial progress was made by 2010 towards the HP2020 objective of reducing by 10% between 2006 and 2020 the percentage of children with CP who were born LBW. By 2010, this percentage among CP cases with no documented postneonatal aetiology had declined by 9.6%. Even more impressive was the 20% decline during the study period in the percentage of CP cases born VLBW. Although encouraging, these trends are limited to three surveillance years and were not statistically significant due to the limited size of the population covered by the ADDM Network CP surveillance system. Nor are the results based on a probability sample selected to be representative of the nation. We reiterate that ongoing monitoring is necessary to evaluate future trends and progress towards the HP2020 CP objective. If improvements in neurodevelopmental outcomes of VLBW have occurred due to advances in perinatal and neonatal medicine in the 1990s and early 2000s, it is unclear that the pace of those advances will continue. Future trends in the percentage of CP cases born LBW might also be affected by overall trends in the frequency of LBW and preterm birth among all livebirths. After decades of gradually increasing, the percentages of births in the United States that were LBW or preterm peaked in 2006 and have declined slightly over time since then.³³

Additional limitations of this study are that birth-weight information was available only for the approximately 75% of CP cases who were born in the state in which they resided at age 8 years and that the occurrence of CP could not be ascertained for an unknown number of birth cohort members no longer residing in the surveillance area at that age. Because of these limitations, we reported the prevalence of CP in the population of 8-year-olds using census counts for the denominator, rather than the prevalence or cumulative incidence in the birth cohort. For evaluating birth-weight trends, it was necessary to rely on the birth cohort rather than census counts and the subset of CP cases with available birth certificate information. A limitation of this approach is that the relative risks could be affected by selection bias if the birthweight distributions of CP cases residing in the surveillance area at age 8 were different from those of an unknown number of CP cases from the birth cohort who could not be identified by the surveillance system, because they were no longer living in the surveillance area at age 8 years. However, we have no evidence that the birthweight distribution of CP cases from the birth cohort who could not be ascertained by the surveillance system because they did not reside in the surveillance area at age 8 differed from the distribution of CP cases from the birth cohort who were represented in the surveillance system. Moreover, even if these distributions did differ, we have no reason to think that the differences would have changed during the surveillance period to bias the relative risks or trends in the percentage of CP cases that were born LBW.

Another limitation of this study is that detailed information on prenatal, perinatal, and neonatal care was not available for the case group or cohort, precluding our ability to examine potential impacts at the population level of specific health care interventions that have been shown to improve neurodevelopmental outcomes of preterm birth and VLBW. Our observation that the frequency of LBW increased and the frequency of VLBW did not change over the study period in the overall cohort, while the relative risk for CP and both LBW and VLBW declined, though not significantly, is consistent with the possibility that advances in the perinatal and neonatal care of preterm infants might have contributed to declines in the risk of CP associated with LBW and the prevalence of CP in the population. Further research is needed to confirm this possibility and monitor future progress.

This study has a number of implications for public health monitoring related to CP. One is that to establish whether progress has been made towards the HP2020 objective of reducing by 10% the percentage of CP cases born at LBW, it is necessary to monitor this percentage in a population large enough to be able to find a 10% reduction to be statistically significant. Based on our baseline of 54.3%, and the CP prevalence estimates and percent with available birthweight information estimated from this study, it would be necessary to perform ongoing surveillance of CP in a population of at least 512 000 8-year-olds in order to have 80% power to detect a 10% reduction as statistically significantly different from no change. For the study year 2010, the size of the ADDM CP Network surveillance population was limited to 131 352, and the power to detect a 10% reduction in the percentage of CP cases born LBW in this population was only 28%. With the sample size available, the minimum reduction in the HP2020 CP indicator that would be detectable given a power of 80% would be about 20%. If the initial trend from 2006 to 2010 were to continue beyond 2010, >a 20% reduction in the indicator could be achieved by the year 2020. Given the fact that VLBW is a much stronger risk factor for CP than the broader category of LBW, future public health objectives for CP would benefit from including reductions in the percentage of children with CP who were born VLBW.

In summary, our preliminary findings of progress towards the HP2020 objective for CP and the significant decline during the study period in the population prevalence of CP are encouraging trends. Future monitoring of progress will require ongoing surveillance in relatively large surveillance areas that are consistent over time. These results also point to the need for further research to investigate the long-term developmental impacts at the population level of improvements in perinatal and neonatal care and to identify strategies to eliminate racial disparities in the United States in the risk of both LBW and CP.