Individuals with intellectual disability and mental illness: a literature review.

People with intellectual disability may be limited not only in their cognitive and adaptive behaviour skills, but also by emotional and behavioural disorders that further limit their ability to learn new skills, adapt to changing environments and develop appropriate social interaction skills. When these disorders are of a sufficient severity and intensity, they may constitute a diagnosable psychiatric disorder (Einfeld & Tonge, 1996). Hence when intellectual disability is complicated by mental illness, the common clinical term "dual diagnosis" is used to describe these individuals (Bongiorno, 1996; Matson & Sevin, 1994; Lovell & Reiss, 1993). The term "dual diagnosis" indicating a co-existence of intellectual disability and mental illness is relatively new and has only recently been acknowledged in the field (Borthwick-Duffy, 1994; Fuller & Sabatino, 1998; Parmenter, 2001). Individuals with intellectual disability and mental illness present several challenges to both community and health services in terms of the difficult behaviours they present, the complexities of diagnoses and treatment, and the complex service needs they require (Dudley, Ahlgrim-Delzell & Calhoun, 1999; King, DeAntonio, McCracken, Forness, & Ackerland, 1994; Moss, Emerson, Bouras & Holland, 1997). In order to appreciate the challenge people with intellectual disability and mental illness present, it is pertinent to understand the prevalence of the dual diagnosis in this population. The prevalence of mental illness is higher among people with intellectual disability than in the general population than previously reported (Borthwick-Duffy, 1994; Fuller & Sabatino, 1998; Linaker & Nitter, 1990). However, the prevalence rates reported in the research literature vary from 10% (Borthwick-Duffy & Eyman, 1990) to 91% (Linaker & Nitter, 1990). It is also important to distinguish prevalence rates in individuals with intellectual disability in institutional care (Linaker & Nitter, 1990) and those living in the community (Bouras & Drummond, 1992; Torrey, 1993). There are several factors that might account for the wide range of reported prevalence rates (Borthwick-Duffy, 1994). Briefly, Borthwick-Duffy (1994) identified issues relating to definition and identification of intellectual disability and mental illness, and the problems due to sampling of subjects. Clinical presentations of mental illness in people with intellectual disability may also be masked by poor language skills and life circumstances, where often reports of mental illness may be conveyed via a support worker rather than the individual himself or herself (Szymanski & King, 1999). Therefore Szymanski and King (1999) argue that diagnostic assessment might hinge heavily on observable behavioural symptoms. Fuller and Sabatino (1998) also reported that mental health practitioners might find it difficult to shift diagnostic practices to a dual focus rather than differential diagnoses. Whatever the discrepancy, there is growing evidence that mental illness is prevalent across all age groups in individuals with intellectual disability. Prevalence in Children and Adolescents with Intellectual Disability Children and adolescents with intellectual disability and mental illness may experience a range of psychiatric disorders (Hurley, 1996; Masi, 1998; Walters, Barrett, Knapp & Borden, 1995). Examples of psychiatric disorders include mood disorders, psychotic disorders, personality disorders, anxiety disorders, depression and suicide ideation (Hurley, 1996; Masi, 1998; Walters, Barrett, Knapp & Borden, 1995). Both Hurley (1996) and Masi (1998) argue that often the symptoms presented by these children and adolescents are often mistaken for aberrant behaviours associated with intellectual or developmental disability. As a result, many of these individuals do not receive the appropriate psychiatric care that they require. …