Travis A. Cos, M.S., Arthur M. Nezu, Ph.D., Paul M. Spangler, Ph.D. & Christine Maguth Nezu, Ph.D.
The extant research has indicated that individuals with intellectual disability exhibit rates of psychiatric and behavioral co-morbidity that are two to three times greater in this population compared to the general population (Rutter, Tizard, Yule, Graham, & Whitmore, 1976; Nezu, Nezu, & Gil-Weiss, 1992). Estimates of the point-prevalence of mental illness in this population have repeatedly produced findings in the range of 30 to 40 percent with diagnosed psychiatric disorders (e.g., Reiss, 1997). However, specific attention to the presence of psychopathology in this population is a relatively recent phenomenon: fifty years ago it was widely believed by field experts that persons with intellectual disability were incapable of exhibiting mental illness (Sevin & Matson, 1994).
As a field we have come a long way, demonstrating that individuals in this population can exhibit the full range of psychiatric and behavioral disorders. Pioneering researchers have established the presence, and prevalence, of affective disorders, anxiety disorders, psychotic disorders, and personality disorders, in addition to various others disorder clusters (Corbett, 1979; Eaton & Menolascino, 1982; Jacobson, 1982; Sovner & Hurley, 1983). Furthermore, it has been estimated that 10-20% of individuals with intellectual disability have a variety of behavioral disorders including aggressive, self-injurious, self-stimulatory/stereotyped, and sexually problematic behaviors (McClintock, Hall, & Oliver, 2003).
The specific level of mental retardation appears differentially associated with the rates of psychiatric disorders and behavioral problems that are diagnosed in individuals with intellectual disability. Specifically, psychopathology in individuals diagnosed with mild mental retardation has been associated with psychiatric disorder, while those with a profound level of mental retardation are associated less with psychiatric disorders and more with behavioral problems (e.g., Jacobson, 1982, Bouras, Cowley, Holt, Newton, & Sturmey, 2003; O'Brien, 2003). However, behavioral and psychiatric disorders have been demonstrated to exist at similar rates within the domains of moderate and severe mental retardation. It is not unlikely to observe frequent co-morbid behavioral and psychiatric disorders in intellectually-disabled individuals, particularly in the moderate and severe levels (Dudley, Ahlgrim-Delzell, & Calhoun, 1999): the most commonly demonstrated relationship has been significant, high correlations between aggressive behavior and major depressive disorder (Reiss & Rojahn, 1993; Holden & Gitlesen, 2003)
The exact association between specific psychiatric disorders and behavioral problems remains unclear among individuals with intellectual disability. Several studies have been conducted that focus on examining the relationship between behavioral and psychiatric problems in this population, utilizing factor analytic techniques (Leudar, Fraser, & Jeeves, 1984; Benson & Reiss, 1984; Dudley et al., 1999; Rojahn, Matson, Naglieri, & Mayville, 2004). While results have been mixed in demonstrating significant relationships, comparison across these studies have been limited by the following factors: samples with widely different compositions of the levels of mental retardation (e.g. some with mostly mild intellectual disability, others with mostly severe or profound intellectual disability); the problem of sample-specificity inherent in utilizing factor analytic techniques (Kazdin, 2003); and questions regarding the psychometric qualities of some the measures utilized to assess behavioral and psychiatric problems.
An important question to ask when assessing psychopathology in individuals with intellectual disability is what diagnostic criteria can and should be utilized in making clinical and research determinations? Concerns have been expressed with regard to the current DSM or ICD criteria, since these diagnostic systems have not been normed on this population, and generally do not include behavioral disorders. There is, also, a paucity of empirical evidence validating the use of the aforementioned diagnostic criteria with this population (Sturmey, Reed, & Corbett, 1991). In recent years, there has been a movement to create specialized diagnostic criteria (Deb, Matthews, Holt, & Bouras, 2001; Royal College of Psychiatrists, 2001; Fletcher, Stavrakaki, First, & Loschen, 2005) specifically for individuals with intellectual disability. In addition, it is not widely accepted that separate diagnostic criteria are needed for individuals with intellectual disability.
The purpose of this research study was to examine the relationship between behavioral and psychiatric disorders in light of the limitations expressed about previous studies. Therefore, the aim of this research was to solicit a representative participant sample of individuals across the various levels of mental retardation. In addition, a correlation matrix was utilized instead of factor analytic techniques to obtain a more conservative estimate of the relationship between behavioral and psychiatric disorders. Finally, assessment tools for behavioral problems and psychiatric disorders were chosen based on the best available measures in terms of psychometric properties and development specifically for a developmentally disabled population, at the time of this study (1997-2004). Consequently, the Psychopathology Instrument for Mentally Retarded Adults (PIMRA) (Matson, 1996) and the Reiss Screen for Maladaptive Behavior (Reiss Screen) (Reiss, 1997) were selected to measure psychiatric disturbance and behavioral problems, respectively.
We administered the PIMRA and the Reiss Screen for Maladaptive Behavior to 116 individuals with intellectual disability at an outpatient clinic specializing in developmental disabilities. This retrospective, archival analysis examined individual testing records as part of a comprehensive psychological and adaptive evaluation, required for continuation of services from the state-sponsored funding source. All participants had been diagnosed with mental retardation based on the administration of intellectual testing and an assessment of adaptive functioning. All participants were older than 18 years old at the time of assessment. A psychiatric interview was conducted during their assessment, as well as prior completion of both the PIMRA and Reiss Screen by another person, such as a family member, staff personnel, or other, who had frequent contact with the individual and could accurately rate their daily behavior.
The Psychopathology Instrument for Mentally Retarded Adults (PIMRA) (Matson, 1996) is a measure that assesses for the presence of psychiatric disorders in individuals with mental retardation and will be used as an indicator of psychiatric disorder in this study. It was originally designed based on DSM-III disorder criteria, and since has been validated on DSM-IV.
The Reiss Screen for Maladaptive Behavior (Reiss Screen) (Reiss, 1997) is a 58-item measure that assesses the likelihood that an individual with mental retardation is exhibiting psychopathology. While this measure provides multiple estimates of psychopathology, this study will focus on the scores for the individual behavioral problems.
Both measures have been demonstrated to display strong correlations for test-retest reliability, internal consistency, and validity related to psychiatric diagnoses. Adequate to strong correlations for internal consistency, test-retest reliability, and validity have been observed (Linaker, 1991; Sturmey, Burcham, & Perkins, 1995; Matson, 1996; Reiss, 1997).
Rates of psychiatric disorders were determined via a psychiatric interview with the given participant and significant individuals in their life (e.g., residential staff, family, etc), collateral reports, study measures, and DSM-IV criteria. The study sample demonstrated psychopathology prevalence rates similar to that found in previously published studies concerning dual diagnosis (mental retardation and co-morbid psychiatric or behavioral disorders). Specifically, 47% of the sample received a dual-diagnosis of at least one psychiatric disorder. This rate was increased to 54% if pervasive developmental disorders and autism were included as diagnoses. Moreover, the sample was composed of individuals with a diverse range of intellectual disability (See Table 1). The self-identified ethnicity closely approximated the regional population demographics.
Relationship between Behavior Problems & Psychiatric Disorders
A correlation matrix was conducted to determine the relationship between behavioral problems, as measured by the individual problem behaviors from the Reiss Screen, and psychiatric disorder, as defined by the psychiatric scales from the PIMRA. Table 2 & Table 3 demonstrates the correlations that were greater than or equal to .30, and were significant to the .01 level. It is important to note that the "Psychosexual Disorder" scale from the PIMRA was excluded, due to no significant correlations with the Reiss Screen problem behaviors.
There were many interesting findings as a consequence of this research. First, the observed descriptive statistics and rates of psychopathology, which both closely approximates previously published population parameters, provide some confidence in the generalizability of the results of this study.
Additionally, based on the correlation matrix analysis, a number of significant relationships were observed between behavioral problems and psychiatric disorders, suggesting considerable overlap and comorbidity between these two forms of psychopathology. More important, distinct profiles emerged, with specific behavioral items tending to be highly correlated to certain psychiatric disorders. These correlational profiles also seem to be conceptually consistent, with each psychiatric category being directly correlated with related behavioral problems. For example, in terms of "Affective Disorder" on the PIMRA, the behaviors that demonstrated the highest correlations ("Tiredness", "Sleep Problems", and "Sadness") are components of the DSM-IV-TR diagnostic criteria for major depressive disorder. Similar results were also observed in terms of all the PIMRA psychiatric subscales, but particularly for "Anxiety Disorder", "Somatoform Disorder", and "Personality Disorder", in which behavioral problems were analogous with defining criteria for these disorders. These results support the argument that that psychiatric disorders and behavioral problems may be related forms of psychopathology.
In addition to these overall patterns, there were some interesting trends within the correlation analyses. First, "Aggressive" behavior on the Reiss Screen was highly correlated with the PIMRA scale of "Adjustment Disorder" but not "Affective Disorder", which is contrary to the observed literature demonstrating a correlation between depression and aggression. This observed result may be potentially due to the combined influence of an under-diagnosis of adjustment disorder in this population and aggression being more common in adjustment disorder and at the early onset of depression, but as the disorder becomes more chronic, symptomatology may change. One possible example may be represented by the individual who is at first aggressive, but later more passive and resigned to a sense of helplessness or failure. Two other interesting trends were observed in the PIMRA category of "Somatoform Disorder." First, higher correlations were observed for the problem behavior of "Suicidal Tendencies" with the scale for "Somatoform Disorder" (r=.43, p<.01) than for the "Affective Disorder" scale (r=.31, p<.01). A similar trend has been observed in the general population, regarding high rates of suicide in health anxiety (hypochondrias) patients. This research, coupled with intellectual and communication deficits that are often common in individuals with developmental disabilities, may yield suicide gestures as an attempt to escape a pain they can neither understand nor alleviate. It was also observed that the behavioral problem of "Hallucinations" was more highly correlated with "Somatoform Disorder" (r=.40, p<.01), than its observed relationship with "Schizophrenia" (r=.29, p<.01). One possible explanation for this difference could be that unfounded somatic complaints are more recognizable hallucinations than what might be observed in individuals with intellectual disability and schizophrenia, given potential communication difficulties. Another possible explanation is somatic complaints are erroneously being called hallucinations in this population without enough medical testing being conducted to rule out the legitimacy of these complaints. Further research is recommended to explore this relationship.
Given the demonstrated associations between behavioral and psychiatric disorder categories in this study, it may be likely that specific common behavioral problems may be directly related to certain psychiatric disorders. With further research elucidating these potential relationships, commonly concurring behavioral problems may be used to inform the diagnostic criteria of certain psychiatric disorders in the future. Additionally, the results of this study suggest the applicability of utilizing the DSM-IV-TR with individuals with intellectual disability. These authors recommend that instead of "reinventing the wheel" and developing brand new diagnostic criteria for this population, existing psychiatric criteria should be further researched with ID populations and relevant adaptation be integrated into existing criteria.
There are several limitations that should be mentioned. First, these researchers were not able to assess genetic disorders in this sample. Given the emerging understanding on genetic disorders and their common expression of behavioral phenotypes, future research should consider the role of genetics when conducting similar studies. Secondly, the authors of the measures utilized have reservations about the applicability to individuals with profound intellectual disability: given these were among the most highly rated measures at the beginning of this study, they were utilized throughout.
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