This is the first report of the prevalence of autism spectrum disorders in a Caribbean country. In the Aruba birth years 1990–1999 we found a prevalence for autistic disorder of 1.9 per 1,000 (95% CI 1.2–2.8) and for autism spectrum disorders of 5.3 per 1,000 (95% CI 4.1–6.7). These prevalence estimates should be considered minimum prevalence. Centralized psychiatric services, with excellent coverage and penetration notwithstanding, it is possible that ASD cases have escaped detection within the study period. Children who have left the frame of observation may have been missed cases, but they still were included in the denominator. Higher functioning cases and young children (i.e. year of birth in the late nineties) may not have been referred.
The prevalence found in the present study is in the mid range of estimates reported for similar birth years in studies conducted in the US and Europe, using diverse methodologies [4
]. In Table we present selected autism prevalence studies published from 2000 reporting on children born during the same time period, but diagnosed using varied methodologies.
A comparison of selected descriptive epidemiology studies of autism
The Aruban findings are consistent with these reports in two additional respects: the proportion of autism spectrum cases with AD (25/69
36.2%), and the proportion of cases with co-morbid mental retardation (41% ASDs, 64% AD). The sex ratio (87.0% males) is also within the range of previous reports [1
The validation study showed a rate of agreement between study diagnosis and ADOS that was higher than expected at 95.8%. For example, Gray et al. [32
] showed that the overall agreement between ADOS and clinical diagnosis was .87 (95% CI .81–.91) in a group of young children (aged 20–55 months). A possible explanation for the higher agreement found in the present study could be the preponderance of AD (16 of 24 subjects), and a higher age distribution; both conditions contribute to a better performance of the ADOS [32
A recent epidemiological study investigating children aged 3–9 years within different health services in Venezuela [34
] yielded a treated prevalence of 1.7 per 1,000 (95% CI 0.1–2.0) for all ASDs, and 1.1 per 1,000 (95% CI 1.0–1.4) for autism. There could be various explanations for the markedly lower prevalence estimates in Venezuela, especially for all ASDs, compared to our study. Aruba is distinctly different from Venezuela, not only in terms of availability and access to health care, but also with respect to socioeconomic and population characteristics. One explanation is a lower degree of service coverage and penetration in the population compared to the Aruba study, where 6% of all children born in Aruba between 1990–1999 were evaluated at the clinic. As mentioned by the authors, a possible lack of awareness of autism and treatment options in the general population could have resulted in underrecognition and lower referral levels for higher functioning ASD cases. Of course, lower prevalence estimates can also reflect lower prevalence in the underlying population.
Because epidemiologic findings with respect to autism spectrum disorders are particularly sensitive to study methodology, the comparison analysis was undertaken in which we drew a direct comparison of cumulative incidence to age five of ASDs in Aruba to that reported by a study conducted in two areas of the West Midlands, UK [26
]. In this UK study case ascertainment methods are similar to the present study: children were identified through Child Developmental Centres’ treatment records, and similar diagnostic criteria were in use during the period of case detection. Powell reported a cumulative incidence (to age five) of 1.6 per 1,000 for AD, and 3.4 per 1,000 for ASDs. We estimated the cumulative incidence to age five of AD and ASDs to be 2.4 per 1,000, 4.5 per 1,000 respectively, at the high end of the confidence intervals of the UK study. This difference may be one of methodology. In the UK study onset was defined as the age at which a definite or probable diagnosis of ASD was first communicated to the child’s family, whereas in the present study we used date of first contact. However, it is uncertain whether this slightly higher cumulative incidence could be accounted for by this difference.
The ideal study upon which to base comparisons in prevalence and cumulative incidence across cultures would use the same rigorous methods at all research sites. However, in reality the development of comparable prevalence estimates is hampered by methodological issues, and differences across countries and services.
It was therefore surprising that in spite of stated differences we found, using similar methods, that the prevalence estimate for ASDs previously reported in a narrow range of countries, also pertains in a place as distinctive as Aruba.
Strengths and limitations of the study
The strength of evidence derives from the coverage and penetration of the health care system, the quality of diagnostic services, and the ability to accurately enumerate the population at risk. During the 1990’s, health insurance was nearly universal for legal residents, but access to specialty services was limited. From 2001 forward, access to child psychiatry services was effectively universal; over the study period, six percent of all children born in Aruba from 1990 to 1999 were assessed at the clinic.
The consistency and reliability of diagnosis upon which the prevalence estimates are based also contribute to the strength of the study findings. All children were fully assessed by the clinic psychiatrist and detailed clinical notes were systematically collected and included in charts. Study inclusion was based on standardized chart abstractions; a study diagnosis was assigned in accordance with standardized diagnostic criteria. In an effort to validate study classification of ASD the ADOS was used to examine 35% (24/69) of ASD subjects included in the study. This showed confirmation of study classification in all cases, but one.
Finally, the population of Aruban births 1990–1999 was enumerated based on population registry data. Because virtually all births in Aruba are attended [15
], registry data should include the births of all legal Aruban-born children. All children identified with ASDs reported here are legal Aruban residents.
The limitations of this study are those common to record-based prevalence studies, and fall into two principal categories: factors affecting case ascertainment and factors affecting diagnosis. With respect to case ascertainment, a fundamental limitation of record review methodology is that prevalence will only include children who presented for clinical assessment, and who elicited clinical suspicion of falling within the autism spectrum. In past studies, reliance on a single source for identifying cases has yielded low estimates [31
]. In this Aruban context, diagnostic assessment and treatment is centralized in the first and only child psychiatry service on the island; competing diagnostic services do not exist.
Other ascertainment effects are specific to segments of the cohort, and are only relevant in the context of the main prevalence analysis. Some cases among children born in the earliest birth years and in the latest birth years may not have been referred to the child psychiatric service. Because the clinic opened in 1997, it is possible that cases born in the early 1990s emigrated in search of services prior to the clinic’s opening and thereby escaped detection. It is also possible that higher functioning cases born in the late 1990s have not attained a sufficient age for referral and are therefore underrepresented in this study.
Another ascertainment issue may be relevant to both the main prevalence analysis and the comparison analysis, that is that ascertainment of children with prominent co-morbidities, especially mental retardation, may also have been limited. Interviews with directors of Aruban schools and day-care programs for the disabled concerning the diagnostic distribution in their institutions, however, lead us to believe that few lower functioning children were overlooked.
With respect to diagnosis, as in any record-based study, the findings are limited by the absence of in-person standardized research interviews and assessments of every study classified case. Due to restraints in time and finances only 24 of the 69 children (almost 35%) with a study classification of ASD were assessed with the ADOS. One study classified ASD case was not confirmed by the ADOS rating.
A potential for misdiagnosis specific to this population arises from the multilingual environment. It is possible that some children with late language development and behavioral disturbances (e.g., ADHD) present with ASD-like profiles at some point in their development.
In conclusion, it is clear that interest in the distribution of ASDs is intensifying, as concern over possible environmental contributions to the occurrence of these disorders continues to grow [3
]. Standardizing future research methodology would permit geographic cross-cultural comparisons. Finding areas of high and low contrast will also motivate additional international epidemiologic investigations. In addition, each epidemiologic study will contribute to local appreciation of the magnitude of ASDs impact on local public health resources and services.
This study shows that the prevalence of ASDs in Aruba is similar to previous prevalence reports from a narrow range of developed countries. We hesitate to draw inferences about the causes of autism based on findings from a single setting. However, as studies in diverse settings accumulate, we believe that the emerging picture will provide important clues to some causes and help to rule out others. The significance of these findings for Aruba is clear. Aruba joins the developed world in needing to respond with services and care for a significant number of seriously disabled individuals.