Research in Developmental Disabilities 35 (2014) 3156–3161

Contents lists available at ScienceDirect

Research in Developmental Disabilities

Co-occurring disorder clusters in adults with mild and

moderate intellectual disability in residential treatmentsettings

Nicole C. Turygin *, Johnny L. Matson, Hilary L. Adams, Lindsey W. Williams

Louisiana State University, United States

A R T I C L E I N F O

Article history:

Received 8 April 2014

Received in revised form 8 July 2014

Accepted 20 July 2014

Available online

Keywords:

Intellectual disability

Developmental disabilities

Psychopathology

Comorbidity

Adults

A B S T R A C T

In the typically developing population, co-occurring psychopathology is not uncommon

and is a topic of importance among psychologists. It is only recently that the

psychopathology in individuals with intellectual disability (ID) has become an area of

significant clinical and research interest. Individuals with ID are believed to be at a greater

risk for co-occurring disorders compared to the typical population. By definition, ID

involves deficits in adaptive behavior, which necessitates the use of community services,

or specialized services at residential facilities to manage severe challenging behaviors or

psychiatric disorders. The presence of co-occurring disorders in addition to ID can

complicate treatment, limit available services, and restrict opportunities for individuals

with ID. The present study examines the prevalence of co-occurring psychiatric disorders

and ID in a sample of 78 individuals with mild to moderate ID living in a long-term

residential treatment facility diagnosed with psychiatric disorders. Certain psychiatric

disorders were more likely to co-occur together in this population. Identifying and treating

individuals with multiple psychopathologies in addition to ID poses challenges unique to

the population.

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In addition to cognitive and adaptive skill deficits (American Psychiatric Association, 2000; Luckasson et al., 2002),individuals with intellectual disability (ID) frequently exhibit symptoms of comorbid psychopathology (Bhaumik, Tyrer,McGrother, & Ganghadaran, 2008; Matson & Shoemaker, 2011; Matson & Smiroldo, 1997; Matson & Williams, 2014; Tonge &Einfeld, 2003; Werner & Stawski, 2012; White, Chant, Edwards, Townsend, & Waghorn, 2005). Estimates of the prevalence ofco-occurring psychological disorders among individuals with ID vary widely (e.g., ranging from 7 to 97% across studies;Cooper, Smiley, Morrison, Williamson, & Allan, 2007; Matson, Smiroldo, Hamilton, & Baglio, 1997), partly due tomethodological differences across analyses (Morgan, Leonard, Bourke, & Jablensky, 2008; Vereenooghe & Langdon, 2013).There is not a consensus among researchers regarding whether individuals with ID have greater psychopathology prevalencerates than the general population overall (Costello & Bouras, 2006; Morgan et al., 2008; Whitaker & Read, 2006), althoughthere is evidence of this pattern for certain disorders (Deb, Thomas, & Bright, 2001; Dekker & Koot, 2003; Emerson, 2003).This pattern may also differ based on age of the individual and severity of cognitive deficit (Cherry, Matson, & Paclawskyj,1997; Paclawskyj, Matson, Bamburg, & Baglio, 1997; Whitaker & Read, 2006). Regardless, recent recognition of the common

* Corresponding author at: Department of Psychology, Louisiana State University, Baton Rouge, LA 70803, United States. Tel.: +1 225 578 7792.

E-mail address: nturyg1@tigers.lsu.edu (N.C. Turygin).

http://dx.doi.org/10.1016/j.ridd.2014.07.039

0891-4222/� 2014 Elsevier Ltd. All rights reserved.

N.C. Turygin et al. / Research in Developmental Disabilities 35 (2014) 3156–3161 3157

occurrence of psychiatric problems among this population has led to an increased interest in research on this important topic(Hove & Havik, 2008; Kozlowski, Matson, Sipes, Hattier, & Bamburg, 2011; Matson et al., 1997a), especially due to the criticalimplications of the presence of comorbid psychopathology among individuals with ID (Horovitz, Shear, Mancini, & Pellerito,2014; Turygin, Matson, & Adams, 2014).

Although co-occurring psychopathologies among typically developing individuals is a frequent area of research, lessresearch has been conducted regarding ID and multiple comorbid disorders, including the analysis of symptom clusters thatmay present among individuals in this population (Matson & Rivet, 2008). In contrast, co-occurring psychopathologiesamong typically developing individuals is a frequent topic of study. Researchers have observed a tendency for severaldisorders to frequently manifest concurrently in those without ID, including anxiety and mood disorders (Brown, Campbell,Lehman, Grisham, & Mancill, 2001; Cerda, Sagdeo, & Galea, 2008; Erwin, Heimberg, Juster, & Mindlin, 2002; Kessler,Avenevoli, & Ries Merikangas, 2001), attention-deficit/hyperactivity disorder (ADHD) and conduct disorder or oppositionaldefiant disorder (Dunn & Kronenberger, 2012; Kessler et al., 2006), depression and substance abuse (Cerda et al., 2008; Grantet al., 2004), conduct disorder and substance use (Cerda et al., 2008), conduct disorder and schizophrenia (Hodgins, Tiihonen,& Ross, 2005; Regier et al., 1990; Swofford, Scheller-Gilkey, Miller, Woolwine, & Mance, 2000), autism and obsessive-compulsive disorder (Matson & Dempsey, 2009). Personality disorders are often comorbid with a wide variety of Axis Idisorders (Lenzenweger, Lane, Loranger, & Kessler, 2007). As compared to typically developing individuals with a singlepsychiatric disorder, individuals with multiple psychiatric diagnoses often have more severe symptoms and worseprognoses (Angold, Costello, & Erkanli, 1999). Those with ID and autism have an especially high rate of comorbidpsychopathology (Matson & LoVullo, 2009; Matson, LoVullo, Rivet, & Boisjoli, 2009).

Similarly, evidence suggests that having multiple comorbid disorders in addition to ID appears to exacerbate existingproblems with everyday functioning across multiple domains (Dekker & Koot, 2003; Matson et al., 1999; Smith & Matson,2010). Furthermore, some researchers suggest that psychopathology is more impairing for children with ID than childrenwithout ID (Dekker & Koot, 2003). Thus, for individuals with ID, having multiple comorbid disorders may have even greaternegative implications, including across wide ranging skills such as adaptive behavior (Matson, Dempsey, & Fodstad, 2009;Matson, Rivet, Fodstad, Dempsey, & Boisjoli, 2009). Although less common than research about single disorders comorbidwith ID, a few studies have been conducted regarding symptom clusters exhibited by this population. For instance,Kozlowski and colleagues (2011) found significant correlations among pairs of disorders within a population with ID:organic and mood, mood and mania, and PDD/autism and stereotypies, although some overlapping items used to determinediagnoses in each pair likely played a role in the correlations. Additionally, Dekker and Koot (2003) found that 14.2% ofchildren with ID in their sample had multiple disorders. Of the children in their sample who met criteria for ADHD, 44% alsomet criteria for ODD.

The presence of co-occurring disorders in addition to ID can complicate treatment, limit available services, and restrictopportunities for individuals with ID. As of now, the similarities between patterns of multiple co-occurring disorders amongtypically developing individuals and persons with ID have yet to be determined. This study is an extension of a previousstudy of the prevalence of psychopathologies in adults with mild to moderate ID in a residential setting (Turygin et al., 2014).The purpose of the current study was to determine symptom clusters exhibited by a sample of adults with ID, to examinepatterns of co-occurring psychopathologies in this sample. Some symptoms of multiple psychopathologies tend to occurtogether, with important clinical implications for assessment and treatment in this population. This issue will be explored inthe present study.

1. Method

1.1. Participants

The present study initially included 102 adults with an initial diagnosis of borderline, mild, or moderate ID, many with co-occurring medical and psychiatric disorders. Due to inability to ascertain the level of ID, three participants were excludedfrom the analyses, thus leaving a sample of 99 participants. Of these, 78 had a co-occurring diagnosis of a psychiatric disorderand were included in the present study. At the time of data collection, all participants were residing in either of two state-rundevelopmental centers designed to treat individuals with a wide variety of intellectual and developmental disabilities. As ina preceding study (Turygin et al., 2014), diagnoses were recorded from the individual’s file and not diagnosed for thepurposes of the present study. Intellectual disability status according to the DSM-IV-TR (APA, 2000) was determined atintake by an on-site licensed psychologist. Diagnoses incorporated information from a standardized measure of adaptivefunctioning (most frequently the Vineland Adaptive Behavior Scales, Second Edition; Sparrow, Cicchetti, & Balla, 2005) andadministration of a standardized, individually administered IQ test (e.g., Stanford Binet). Reassessment of cognitivefunctioning occurred yearly thereafter. The licensed psychologist assigned diagnoses according to a consensus model aftermeeting with the individual, reviewing the individual’s file, and consulting with members of the individual’sinterdisciplinary team. DSM-IV-TR criteria were used by licensed psychologists and psychiatrists working at the centersto assess co-occurring psychiatric disorders. These assessments were supplemented by administration of other measures asdeemed appropriate by the psychologist. Inclusion criteria included a diagnosis of borderline, mild, or moderate ID.

In this sample, 60.25% of participants (n = 47) were males and 39.74% (n = 31) were females, with no significantdifferences observed between sites, x2 (1, N = 78) = 0.50, p = .48,. Ages of participants ranged from 18 to 96 years (M = 38.39;

N.C. Turygin et al. / Research in Developmental Disabilities 35 (2014) 3156–31613158

SD = 20.28); significant differences [F(1, 71) = 12.09; p = .00] were found between sites with site 1 (N = 36) having greatervariation in age (M = 34.13, SD = 19.57) than site 2 (N = 37; M = 31.91, SD = 11.68). Of this sample, 69.20% (n = 54) had mild ID,26.90% (n = 21) had moderate ID, and 3.80% (n = 3) had borderline ID. Out of these, 60.30% (n = 47) were Caucasian, 38.5%(n = 30) were African American, and 1.30% (n = 1) was of another ethnic background. With regard to communication, 97.40%(n = 76) of individuals in our sample were able to communicate verbally, which was defined as the ability to at least verballyexpress one’s basic needs. Two participants (2.60%) were not able to communicate verbally. Twenty-three (29.48%)participants had a history of seizures, and 51 (65.38%) had no history of seizures, and seizure history information was notavailable for four participants. Demographic information is presented in Table 1.

1.2. Procedure

Approval from the Institutional Review Board was obtained prior to onset of data collection for the present study and wasconducted after approval by and in accordance with the policies of the Human Rights Committee at the residential facilities.Informed assent was obtained and witnessed by a staff member of the participant’s choice. Data used in the present studywas gathered from pre-existing information in the individual’s treatment file and entered into a database, excluding anypersonally identifiable information.

Types of disorders were then coded into categories, with psychiatric disorders divided into autism spectrum disorders(ASD); bipolar disorder; other mood disorders; schizophrenia and other psychotic disorders; impulse control disorder,oppositional defiant disorder (ODD), and conduct disorder (CD); schizoaffective disorder; substance use disorders; rule outs(undifferentiated); and other disorders. Combination of three disorders into the ‘‘impulse control disorder’’ category and anumber of other disorders into the ‘‘other’’ group for coding purposes were based on relatively low frequency of thesediagnoses in the sample. The ASD category included all DSM-IV-TR disorders in the ASD category (i.e., autism spectrumdisorder, Asperger disorder, and Pervasive Developmental Disorder-Not Otherwise Specified [PDD-NOS]). Bipolar disorderincluded both Bipolar I and Bipolar II. Major depressive disorder, dysthymic disorder, and mood disorder NOS comprised theother mood disorder group. Schizophrenia and other psychotic disorders included schizophrenia, delusional disorder, andpsychotic disorder NOS. The ‘‘other’’ category included eating disorders, narcolepsy, obsessive-compulsive disorder (OCD),paraphilias, sleep disorders, and other DSM-IV-TR Axis 1 disorders, each of which occurred at rates of no more than n = 4 inthe present sample.

1.3. Statistical analyses

Out of the entire sample, there were 78 participants with a psychiatric diagnosis in addition to ID. Twelve participants hadan Axis II disorder but were not diagnosed with any Axis I disorder, and 22 had no diagnosis in addition to ID. Of the 78participants with ID and at least one comorbid psychiatric diagnosis, 49 participants had a second psychiatric diagnosis. Datafor Axis I diagnoses were reported in narrative form and coded into one of eight categories: ASD; bipolar disorder; othermood disorder; schizophrenia and psychotic disorders; impulse control, ODD and CD; schizoaffective disorder; and otherdisorders (which included eating disorders, OCD, anxiety disorders). Turygin et al. (2014) provide greater detail on thespecific distribution of all ‘‘other’’ diagnoses in this sample.

All analyses were conducted using SPSS version 18. The cluster analysis was conducted with all 78 participants includedin the analysis. Dichotomous variables representing each diagnostic category were created and entered in the analysis. Theeight diagnostic categories created by coding DSM-IV-TR diagnoses as stated above were determined by their total in thesample, such that each category had at least four occurrences in the sample. These categories were then chosen as thepotential clusters for the analysis to investigate trends of co-occurring psychiatric diagnoses. The furthest neighbor linkagemethod of cluster analysis was used. In this method of analysis, each variable is first considered to be an independent cluster;next these items are sequentially combined into larger clusters by combining the most similar two clusters. The furthest

Table 1

Demographic information for sample (N = 78).

Mild Moderate Borderline

Sex (percent)

Male 33 (42.31) 12 (15.38) 2 (2.56)

Female 21 (26.92) 9 (11.55) 1 (1.28)

Race (percent)

Caucasian 31 (39.74) 13 (16.67) 3 (3.84)

African American 22 (28.20) 8 (10.27) 0 (0.00)

Other 1 (1.28) 0 (0.00) 0 (0.00)

Verbal (percent)

Yes 53 (67.94) 21 (26.92) 2 (2.56)

No 1 (1.28) 0 (0) 1 (1.28)

Seizures (percent)

Yes 14 (17.94) 7 (8.97) 2 (2.56)

No 36 (46.18) 14 (17.94) 1 (1.28)

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neighbor linkage method tends to produce compact clusters which contain data points that are very similar (Kaufman &Rousseeuw, 2009). Russell and Rao distance parameter was used as a similarity measure; this parameter can be used withasymmetric dichotomous variables and marks the proportion of cases in which both observations share the trait of interest.The furthest neighbor linkage rule and the Russell and Rao distance parameter were chosen as we are searching for thoseclusters that correspond to participants who are most similar to one another with respect to the dichotomous variable ofdiagnosis. These particular parameters were chosen in order to increase the likelihood that those with the most similardiagnosis profile would be placed into the same cluster.

2. Results

Eight clusters were evident within the remaining data based on a review of the resulting dendrogram. Follow up analysisrevealed that Cluster 1 consisted of 13 participants with a personality disorder. Six of the participants in this cluster had onlya personality disorder, and 7 had at least one co-occurring disorder. See Table 2 for all co-occurring disorders present withinthe cluster. Cluster 2 consisted of all participants with ASD (n = 8). Co-occurring disorders within this category were mostcommonly observed to be from the ‘‘other’’ category, while one had a co-occurring personality and ‘‘other’’ disorder. Cluster3 consisted of participants who all had ‘‘other mood’’ disorders (n = 12). Within this group, 5 participants had an ‘‘other’’disorder, 1 had a psychotic disorder, and 1 had a substance use disorder. Cluster 4 consisted solely of participants withbipolar disorder (n = 8). Co-occurring disorders in Cluster 4 included impulse, schizoaffective and ‘‘other.’’ All participants inCluster 5 had schizophrenia or a psychotic disorder (n = 10), with a co-occurring ‘‘other’’ disorder, or co-occurring ‘‘other’’,impulse and substance use disorders. Cluster 6 consisted of 10 participants with schizoaffective disorders, 2 of whom had co-occurring other and substance use disorders, 2 with co-occurring substance use disorder, and 1 with impulse disorder.Cluster 7 consisted of participants with an ‘‘other’’ diagnosis (n = 10) with co-occurring substance and personality disorder(n = 2), or substance use disorder (n = 1), and schizophrenia or a psychotic disorder (n = 1). Cluster 8 consisted of participants(n = 7) diagnosed with substance use disorder with one with another from the other category.

3. Discussion

The present study illustrates the disorders that commonly co-occur among adults with mild or moderate intellectualdisability who reside in a residential treatment setting. These individuals represent those who are at present unable to residein less-restrictive settings as a result of behavioral or medical problems. The successful management of co-occurringpsychiatric disorders in this population may result in their ability to transition to less-restrictive settings and obtain a widervariety of personal and occupational opportunity. The analysis provided information on those co-occurring disorders thatare likely to occur together in this population. For example, those with ASD only carried co-occurring disorders of apersonality disorder, or diagnoses from the ‘‘other’’ category (enuresis/encopresis, PTSD, rule-out dementia, and Tourette’s).Those with other mood disorders were diagnosed with schizophrenia, ‘‘other’’ disorders, or substance use disorder.Schizophrenia or other psychotic disorders tended to co-occur with substance use, reflecting tendencies found in the generalpopulation (Hodgins et al., 2005; Regier et al., 1990; Swofford et al., 2000). The present study represents an initial step inunderstanding the relationships between psychiatric disorders as they occur in individuals with intellectual disability.

The widest variety of co-occurring disorders occurred in those who were diagnosed with a personality disorder. Co-occurring disorders included bipolar disorder, impulse disorder, other mood disorders, schizoaffective disorder, and otherdisorders. This finding is unsurprising given the pervasive nature and complexity of personality disorders; furthermore, thisresult follows the trend found in the general population wherein personality disorders tend to co-occur with a wide varietyof other psychiatric disorders (Lenzenweger et al., 2007). Bipolar disorder also tended to occur with impulse disorder, and‘‘other’’ disorders. Impulse disorder occurred on its own more often than any other disorder, and also frequently co-occurredwith a variety of other disorders, particularly bipolar disorder, schizoaffective, and personality disorders, indicating moreheterogeneity of co-occurring disorders in individuals with impulse disorder.

Table 2

Co-occurring disorders within each cluster.

Cluster 1 Cluster 2 Cluster 3 Cluster 4

Personality (n = 13) ASD (n = 8) Other mood (n = 12) Bipolar (n = 8)

Bipolar (n = 1) Other (n = 4) Schizophrenia/Psychotic (n = 1) Impulse (n = 3)

Other mood (n = 4) Personality (n = 1) Other (n = 5) Schizoaffective (n = 1)

Impulse (n = 2) Substance (n = 1) Other (n = 3)

Schizoaffective (n = 1)

Other (n = 1)

Cluster 5 Cluster 6 Cluster 7 Cluster 8

Schizophrenia/Psychotic (n = 10) Schizoaffective (n = 10) Other (n = 10) Impulse (n = 7)

Impulse (n = 1) Substance (n = 4) Personality (n = 2) Other (n = 1)

Substance (n = 4) Other (n = 2) Schizophrenia/Psychotic (n = 1)

Other (n = 2) Impulse (n = 1) Substance (n = 3)

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One major limitation of the present study resulted from the low frequency of certain disorders within the sample, whichwere placed within the ‘‘other’’ category and included disorders such as anxiety disorders, obsessive-compulsive disorder,and pica. This limited the results of the present study to the focus of the relationships between those disorders which madeup a large enough portion of the sample to be included within the study. The absence of discrete groups of these disorderswas a reflection of the population at the two centers from which we obtained data, in which the disorders were observed atlow rates. This information would be useful, and further studies should continue to explore the relationship between thedisorders observed in the present study along with the disorders omitted from the present study. Future studies may alsowish to cluster with respect to age groups, race, or gender, particularly as more is known about gender differences indisorders in ID. Gender information would be particularly useful when examining co-occurring disorders such as ASD thatare known to be present more frequently in males (Rivet & Matson, 2011).

The relative low frequency of ‘‘other’’ disorders in the present study poses additional questions. Given the fact that asample of individuals residing at a residential treatment facility is inherently a sample with greater needs than a communitysample, individuals with ID in these residential settings may be more likely than a community sample to be diagnosed withdisorders perceived to be more ‘‘severe,’’ such as psychotic disorder and schizoaffective disorders. It is unsurprising that aresidential facility would represent a setting in which higher numbers of these ‘‘severe’’ disorders are observed. Diagnosticovershadowing is an alternate explanation, as symptoms of co-occurring disorders from the ‘‘other’’ category (e.g., anxiety,OCD) may be assumed to relate to the intellectual disability or ‘‘more severe’’ psychiatric condition and thus not warrant aseparate diagnosis. Mental health and training needs of individuals residing in these settings are complicated by co-occurring medical, psychosocial, and economic conditions, all of which pose further diagnostic challenges.

Overall, however, it appears that individuals dually diagnosed with ID and psychopathology have a high likelihood of notonly one, but multiple psychiatric diagnoses. This co-occurrence is unsurprising given the frequency of concomitantpsychiatric disorders in the general population. These results highlight the need for thorough assessment of thepsychological needs and individualized treatment plans for dually diagnosed individuals. Given the high use of psychotropicdrug use and the potential side effects that occur, this data should result in more precise and thus safer treatment (Advokat,Mayville, & Matson, 2000; Matson et al., 1998; Matson & Wilkins, 2008; Singh, Matson, Cooper, Dixon, & Sturmey, 2005).Additionally, a host of psychological methods are also applicable for treatment (Matson, Mahan, & LoVullo, 2009). Betterunderstanding of the nature of these overlapping disorders, then, should result in more precise and complimentarytreatment approaches.

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