Despite increased recognition of the prevalence and impact of psychiatric disorders among individuals with Intellectual Disability (ID), the early assessment and treatment of these concurrent comorbidities remains a significant societal challenge and affect the functional skills and intensity of daily support needed for those living with ID. Limitations in current assessment and diagnostic systems prevent the equitable access to services for people with ID in line with Article 25 of the United Nation's Convention on the Rights of Persons with Disabilities which outlines ‘the right to the enjoyment of the highest attainable standard of health without discrimination on the basis of disability’ (United Nations 2006; Stuart, 2012). Consequently it is critical to further develop and refine standardised diagnostic systems to achieve these aims. Furthermore, in order for intervention to be effective, it must address the multiple risk and resilience factors underlying ID and comorbid mental health problems. Therefore, from a public health perspective, rates of comorbid psychiatric disorders should be taken into account when planning for the equitable provision of services. Similarly, intervention research should consider the role of comorbidity in predicting outcomes and the nature of the barriers that prevent access to services, while basic science research would benefit from increasing understanding of these comorbidities by delineating possible heterogeneous aetiological and neurobiological determinants. This chapter will review and synthesise the literature on the prevalence, impact and determinants of the comorbidity of ID and psychiatric disorders, and discuss important considerations to improve accurate identification and effective management of these concerns.
Background
Intellectual Disability (ID) is characterised by deficits in both intellectual and adaptive functioning beginning during childhood or adolescence (APA, 2013). Adaptive behaviour includes 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 and occupation) (Maulik, Mascarenhas, Mathers, Dua, & Saxena, 2011).
A comprehensive meta-analysis found that the overall global prevalence of ID was approximately 1%, with higher prevalence rates found in low- and middle-income countries, and among child and adolescent populations (Maulik et al., 2011). Social gradients and ethnic disparities have been reported in previous research on the prevalence of ID. Lower socio-economic status (SES) has been found to be associated with a marked increase in the prevalence of ID, particularly mild ID (Emerson, 2012; Leonard et al., 2005; Leonard & Wen, 2002; Roeleveld & Zielhuis, 1997; Stromme & Magnus, 2000), and higher rates of ID have been found among African-American and Indigenous Australian children (Croen, Grether, & Selvin, 2001; Leonard, Petterson, Bower, & Sanders, 2003; Leonard et al., 2005; Yeargin-Allsopp, Drews, Decouflé, & Murphy, 1995).In a recent epidemiological study it was estimated 5.9% of the Indigenous population lives with ID compared to 2.5% of the non-Indigenous Australian population(Statistics, 2015).
It has been proposed that socioeconomic disadvantage can contribute to both causation and severity of ID. (Durkin & Yeargin-Allsopp, 2018)For instance the etiology of some causes of intellectual disability, such as Down syndrome have been associated with environmental factors and stress that are linked to low SES. Additionally low SES families may not have the resources to provide the early socio-cognitive stimulation that is associated with improved outcomes in ID. Of particular concern is the evidence that an “inverse care law” applies, in that the greater number of risk factors a child may have for ID, the less access they have to preventative and comprehensive health care (Woolfenden et al., 2013). This creates a vicious cycle whereby the risk factors that may predispose a child to ID are also the same risk factors that may mean they are likely to miss out on timely and effective identification and intervention.