NADD Bulletin Volume XI Number 3 Article 3

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Assessing Mental Health Concerns in Individuals with Intellectual Disabilities

Karen L. Hobden, Ph.D.
Barbara W. LeRoy, Ph.D.


Historically, individuals with intellectual disabilities were assumed to be free from mental health concerns. More recently, however, research has suggested that people with intellectual disabilities may be at an increased risk for psychological problems (Deb, Thomas, & Bright, 2001; Stromme & Diseth, 2000). Estimates from prevalence studies indicate that between 15% and 50% of individuals with intellectual disabilities also have a co-occurring mental illness (Clay & Thomas, 2005; Deb et al., 2001; Gustafsson & Sonnander, 2004; Koskentausta & Almqvist, 2004; Prasher, 1995; Reiss, 1990; Salvador-Carulla, Rodríguez-Blázquez, Rodríguez de Molina, Pérez-Marín, & Velázquez, 2000). Findings vary as a result of the type of measure used and the population from which the sample is drawn. Screening instruments tend to find higher rates of psychopathology than clinical interviews (Aman, 1991). Clinic samples often have higher rates of psychopathology than community samples (Taylor, Dixon, & Douglas, 2004).

Researchers have found that individuals with intellectual disabilities experience the full range of psychiatric disorders. Clay and Thomas (2005) compared rates of specific types of psychopathology in their sample of 179 individuals with intellectual disabilities to those in the general population (National Institute of Mental Health, 2006). These authors found that the prevalence of major depression and bipolar disorder was three times higher in the individuals with intellectual disabilities than in the general population. In fact, only the prevalence of generalized anxiety disorder was higher in the general population. There is some evidence that certain types of disorders vary as a function of the severity of intellectual disability. Holden and Gitlesen (2004) found that individuals with mild or moderate intellectual disabilities are more likely to experience anxiety, depression, and psychosis than those with severe or profound intellectual disabilities.

The diagnosis of mental illness in individuals with intellectual disabilities can be challenging. Psychiatric disorders are often underdiagnosed in this population for a number of reasons (Gustafsson & Sonnander, 2004; Reiss, 1990). Some disorders may manifest differently (i.e., different symptoms may be evident) across a range of intellectual ability (Powell, 1999). Challenging or disruptive behaviors may be attributed to the intellectual disability instead of a potential mental illness (Moss, 2001). This is referred to as diagnostic overshadowing – an inability on the part of mental health professionals to see beyond the individual's intellectual disability (Reiss & Szyszko, 1983). Clinicians tend not to be trained in diagnosing psychiatric disorders in individuals with intellectual disabilities (Moss, Emerson, Bouras, & Holland, 1997). Further, many diagnostic tools rely on individuals’ ability to express their symptoms verbally (Moss, 2001). Certain symptoms cannot be displayed at all in individuals who are nonverbal (Moss, Emerson, et al., 1997).

Accurate diagnosis of mental illness in individuals with intellectual disabilities is important because the presence of a mental illness may effect functioning. Proper diagnosis can lead to better service planning. Further, having a formal diagnosis enables the person to be seen as "ill" rather than "bad." Steve Moss and his colleagues have recently developed the Mini Psychiatric Assessment Schedule for Adults with a Developmental Disability (Mini PAS-ADD: Moss, Ibbotson, Prosser, Goldberg, Patel, & Simpson, 1997). The Mini PAS-ADD was developed from the PAS-ADD (Moss, Ibbotson et al., 1997), which is a structured, clinical interview based on the ICD 10 classification of psychiatric disorders (World Health Organization, 1994). The administration of Mini PAS-ADD involves the collection of observational data by an informant who knows the respondent well. The Mini PAS-ADD produces 7 subscale scores related to the following psychiatric disorders: (a) depression, (b) anxiety, (c) expansive mood (bipolar disorder), (d) obsessive-compulsive disorder, (e) psychosis, (f) unspecified disorder (including dementia), and (g) autism. Individuals who exceed the threshold score on one or more of these subscales should have a subsequent psychiatric assessment (Prosser, Moss, Costello, Simpson, Patel, & Rowe, 1998).

The measure has been found to have reasonable internal and inter-rater reliability, but evidence of its construct validity has been relatively weak. Prosser et al. (1998) compared scores on the Mini PAS-ADD and subsequent psychiatric diagnoses and found a 91% agreement for psychiatric caseness (i.e., the presence or absence of a psychiatric disorder). However, the authors provide no information on the extent to which scores on the Mini PAS-ADD subscales match the specific diagnoses found by the study psychiatrists. The present study was designed to test the utility of the Mini PAS-ADD in identifying specific psychiatric disorders in individuals with intellectual disabilities. In conducting this research, we collaborated with the Macomb Oakland Regional Center (MORC) in southeast Michigan, a service provider for adults and children with developmental, physical, or psychiatric disabilities.



One hundred individuals participated in this research (42 females and 58 males). The sample was predominantly white (94%). Forty-three percent of participants had a mild intellectual disability, 53% a moderate intellectual disability. Sixty-seven percent had a pre-existing psychiatric disorder (according to their case file). The mean age of our sample was 49.5 years.


Potential study participants were contacted initially by mail. They received a letter describing the study, a consent form, and a return-addressed, stamped envelope. For each study participant for whom we received signed consent, an informant was identified who knew the individual in question well. The informant was typically a family member, care worker, or manager of the group home where the participant lived. The mean length of acquaintance of informants with study participants was eight years. The Mini PAS-ADD was administered by a trained interviewer, typically in the informant's home or place of employment. The interviewers were employees of MORC and were either psychologists or social workers.


The Mini PAS-ADD is a semi-structured interview that collects information about an individual’s symptoms by asking questions of an informant who knows the individual well. It takes about 20 minutes to administer and yields subscale scores related to seven psychiatric disorders: (a) depression, (b) anxiety, (c) expansive mood (bipolar disorder), (d) psychosis, (e) obsessive-compulsive disorder, (f) unspecified disorder (including dementia), and (g) autism.


Pre-existing psychiatric diagnoses were compared to scores on the Mini PAS-ADD. As stated previously, 67% of participants had one or more psychiatric diagnosis on file, but only 32% of participants exceeded the threshold score on one or more of the subscales of the Mini PAS-ADD. This difference is significant ?2(1) = 24.50, p <.001. When we compared rates of specific types of diagnoses, we found that 12% of participants had a previous diagnosis of depression, whereas only 4% reached the threshold for depression on the Mini PAS-ADD, ?2(1) = 4.35, p<.05.  In 42% of participants, there was a diagnostic match between scores on the Mini PAS-ADD and pre-existing diagnoses.  However, in 39% of respondents, those with a pre-existing diagnosis failed to meet the threshold on any of the Mini PAS-ADD subscales (see Table 1). Because a large percentage of our participants, 63%, were taking psychotropic medication, which may have reduced or eliminated their symptoms, we analyzed the frequency of diagnostic matching based on whether participants were or were not taking psychotropic medication. Chi-square analysis found that individuals who were taking psychotropic medication were less likely to have a diagnostic match than those who were not (33% versus 57%, respectively, ?2(4) = 20.35, p < .001).

Table 1 – Diagnostic Matched by Psychotropic Medication



Any Psychotropic Meds?Total   


Different diagnoses givenN





Mini PAS-ADD, but no pre-existingN





Pre-existing diagnosis, but no Mini PAS-ADDN





Exact diagnostic matchN





Pre-existing diagnosis not covered by Mini PAS-ADDN












The Mini PAS-ADD is a quick, cost effective, and easy-to-use screen for psychiatric disorders in individuals with intellectual disabilities. It was developed specifically for use with this population. Because it asks behaviorally-based questions of informants, individuals with communication problems can be assessed. Our findings suggest that the measure may be most effective in individuals not currently on psychotropic medication. There was a higher degree of agreement between scores on the Mini PAS-ADD and the diagnosis on file for individuals who were not on psychotropic medication at the time of assessment than for those who were. More research is needed to demonstrate the construct validity of this measure.

Even for our participants who were free from psychotropic medications, the Mini PAS-ADD only agreed with pre-existing diagnoses in 57% of the cases. There are a number of reasons for the lack of agreement between the Mini PAS-ADD and clinical observations. In some cases, the diagnoses on file had been given many years ago. It is possible that some of our participants experienced improvement in their symptoms over time. Also, MORC's current diagnostic system is based on either behavioral plans or individuals’ responses to medication. Potentially, some of our participants may have been misdiagnosed on the basis of their response to medication.

One possible direction for future research would involve the use of the Mini PAS-ADD in a prospective study of diagnoses and outcomes in individuals with intellectual disabilities who are just coming into contact with service providers. Specific diagnoses on Mini PAS-ADD could then be compared to subsequent psychiatric diagnoses to determine the degree of diagnostic matching. Furthermore, these two groups of individuals could be followed longitudinally and compared to determine whether one or the other form of diagnoses leads to better service planning and/or better outcomes.


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