Prevalence of intellectual disability: A meta-analysis of population-based studies

Abstract

Intellectual disability is an extremely stigmatizing condition and involves utilization of large public health resources, but most data about its burden is based on studies conducted in developed countries. The aim of this meta-analysis was to collate data from published literature and estimate the prevalence of intellectual disability across all such studies. The review includes studies published between 1980 and 2009, and includes data from populations that provided an overall estimate of the prevalence of intellectual disability. Meta-analysis was done using random effects to account for heterogeneity. Sub-group analyses were also done. The prevalence of intellectual disability across all 52 studies included in the meta-analysis was 10.37/1000 population. The estimates varied according to income group of the country of origin, the age-group of the study population, and study design. The highest rates were seen in countries from low- and middle income countries. Studies based on identification of cases by using psychological assessments or scales showed higher prevalence compared to those using standard diagnostic systems and disability instruments. Prevalence was higher among studies based on children/adolescents, compared to those on adults. Higher prevalence in low and middle income group countries is of concern given the limitations in available resources in such countries to manage intellectual disability. The importance of using standardized diagnostic systems to correctly estimate the burden is underlined. The public health and research implications of this meta-analysis have been discussed.

Introduction

Intellectual disability (ID) or mental retardation is defined as “a condition of arrested or incomplete development of the mind, which is especially characterized by impairment of skills manifested during the developmental period, which contribute to the overall level of intelligence, i.e., cognitive, language, motor, and social abilities” (World Health Organization, 1992). The International Classification of Functioning, Disability and Health (ICF) (World Health Organization, 2001) complements the definition of the International Classification of Diseases, Version 10 (ICD10) (World Health Organization, 1992) and incorporates the concept of disability and functional adaptation to disability. The American Association on Intellectual and Developmental Disabilities (AAIDD) defines it as “limitations both in intellectual functioning and adaptive behavior” (American Association on Intellectual and Developmental Disabilities, 2010). Adaptive behavior assesses conceptual skills (e.g., language, money and time concepts), social skills (e.g., interpersonal skills and social problem solving) and practical skills (e.g., activities of daily living, occupation). Throughout this paper, the term intellectual disability has been used in most areas. Mental retardation has been used in some places only where it is related to other sources, but has been used with the same meaning as intellectual disability.

Harris (2006) reported the prevalence of ID to vary between 1% and 3%, globally. Among those with ID, mild, moderate, severe, and profound mental retardation affects about 85%, 10%, 4%, and 2% of the population, respectively (King, Toth, Hodapp, & Dykens, 2009). There are few incidence studies. Katusic et al. (1996) and Heikura et al. (2003) reported incidence of 9.1 and 12.6 per 1000 population, in their studies from the U.S. and Finland, respectively. While the initial study reported a cumulative incidence of a 5-year birth cohort, the latter was based on two birth cohorts. Similarly large population-based mortality studies on people with intellectual disabilities are also few, and a 35-year cohort study (Patja, Iivanainen, Vesala, Oksanen, & Ruoppila, 2000) reported no differences in mortality rates between people with intellectual disability and the general population. However, mortality rates are much higher among those with more severe forms of intellectual disability. Given the paucity of research in areas of incidence and mortality, studies reporting on prevalence assume greater significance as sources of critical data for estimating the burden of intellectual disability. However, to the best of our knowledge there are no studies that report any global prevalence estimates, or have reported on data collated through systematic reviews of different studies. Most currently reported estimates are based on extrapolated figures from studies conducted in the U.S., or some other developed country.

This meta-analysis is part of a larger study that reviewed research in the area of incidence, prevalence, mortality and etiology of intellectual disability in community-based population, as part of the Global Burden of Disease estimates. The current paper uses a sub-group of those studies. The primary aim of this paper is to report on the findings of the meta-analysis of population-based studies reporting prevalence estimates for intellectual disability. The focus was not on any particular disorder associated with intellectual disability or intellectual disability among a sub-population. For example, studies that reported on prevalence of comorbid mental disorders among people with intellectual disability were not included. Similarly, intellectual disabilities examined only among populations with Down's syndrome, autism, lead toxicity, or population within correctional facilities were excluded.