Epidemiology and Natural History
Current lifetime prevalence data suggest that between 20%–70% of individuals with IDD experience significant behavioral problems or meet current diagnostic criteria for a mental disorder (
1,
3–
5). These high prevalence rates are linked to the severity of intellectual disability, global impairments in adaptability and resilience, coexisting neurological/medical conditions, histories of major psychosocial and familial dysfunction, and abuse and neglect (
5–
7). In addition, life transitions such as physiological changes associated with puberty, the end of educational programs, transitioning to out-of-home living arrangements, declining vigor of aging parents and the physiological effects of aging create major challenges for individuals with IDD. Each of these transitions require major psychological and social adaptation and frequently trigger the onset of both challenging behaviors and the onset of many mental disorders. Unfortunately our clinical understanding of age related changes in brain function, dementias, and other neuropsychiatric conditions are still in their infancy. Much remains to be explored (
2,
7).
Each life transition also influences the timing of referrals for mental health services. Most mental health referrals result from increases in troublesome or challenging behaviors (e.g. self-injury or aggression). Because the severity of these behaviors garners immediate attention, the clinician may overlook or not recognize associated primary psychiatric disorders unless a high index of suspicion for their presence is maintained. The reverse can also be true: clinicians can assume that a psychiatric disorder is present even though there is insufficient evidence to make an appropriate diagnosis. For example hallucinations prompt the diagnosis of schizophrenia regardless of contradictory findings or other core features of this severe mental illness.
Why might such misdiagnoses occur? Many patients with IDD are frequently unable to adequately describe mood or cognitive states because of limited expressive language, difficulty describing such internal states, or are subject to regression and cognitive/behavioral disorganization in response to environmental stressors. In addition, some are unable to provide useful information or fully understand the process of the psychiatric examination. As a result the clinician must rely on secondary sources or direct observation, inference and extrapolation of complex behaviors to fit the diagnostic criteria of psychiatric disorders. In addition, a failure to consider the contribution of medical or neurological illness or medication side effects on challenging behaviors can lead to misdiagnosis of serious organic mental disorders (e.g. delirium) as primary mental illness. Such misdiagnoses can also lead to further complications secondary to inappropriate treatment strategies.
To accommodate these complex features of neurodevelopmental disorders we will expand the usual definition of diagnostic overshadowing (misattributing symptoms of mental illness to IDD) to include behavioral changes associated with neurological and medical disorders. Thus the misdiagnosis of medical/neurological disorders as primary psychiatric conditions and misdiagnosis of primary psychiatric conditions as medical/neurological disorders are added diagnostic overshadowing. Resolving these issues requires a high index of suspicion and a reasonable working knowledge of neuropsychiatry. Such awareness can improve our understanding of the complex nature of differential diagnosis and help us empathize with the problems faced by colleagues working in the field of dual diagnosis (IDD and MH) (
2).
A second set of challenges may be as complicated as those outlined above. Understanding and treating individuals with dual diagnosis also forces the clinician to integrate multiple clinical perspectives. For example:
1.
Behavioral therapy and learning theory: This system is based on target behaviors rather than syndromes (e.g. aggression in response to changes in key environmental factors or escape from task demands) (
8). The challenge to mental health professionals is to determine when appropriate target symptoms are identified with specific psychiatric syndromes, e.g. dysphoric mood in major depressive disorder.
2.
Nomenclature issues: Clinicians need to adapt our current nomenclature to better fit adult patients with IDD. The Diagnostic Manual-Intellectual Disability (DM-ID) and the Diagnostic Criteria-Learning Disorders respectively represent attempts to adapt the DSM-IV-TR and ICD-9 uniquely to these differences in nosology.
3.
Adapting to multidisciplinary teams. Best practices in the field of dual diagnosis include mental health professionals as part of a team that includes behavioral psychologists, occupational therapists, physical therapists, pediatric neurology, medicine, psychiatry, educators, family members, and direct support professionals and others. The goal is to integrate different points of view and treatment models into a coherent treatment plan (
5–
7).
A third challenge arises from an information explosion in the clinical neurosciences of IDD and mental illness. At a practical level we need to translate new information into clinical practice. At a theoretical level we need to update the biopsychosocial model to include:
1.
Genetics: Genes serve as the blueprint for brain development. Our older models were deterministic, hence a nature-nurture debate; current models are probabilistic and interactional. Clinical researchers moved beyond Mendelian (autosomal dominant, recessive inheritance) analysis and now emphasize the sequence and integration of gene activation and de-activation during critical periods of development. As a result the focus instead is on epigenetics and gene-environmental interactions. In this world, the nature-nurture argument is an anachronism (
9–
11).
2.
Neurosciences: Rapid advances in the neurosciences are deepening our understanding of central coherence, executive functions and adaptive behaviors, theory of mind, social communication, memory, attention, impulse control and affect regulation (
11). Our challenge is to understand how errors in brain development (errors in neuronal migration, maturation, neuroplasticity, myelinization and interactive specialization and regional interconnections) relate to cognitive, social and emotional development; and how these errors translate into real life problems for adults with dual diagnoses.
3.
Translational research needs: Clinicians concentrate on challenging behaviors and mental disorders and often eschew molecules as well as policy. But we cannot lose sight of the interactions between macrocosmic forces such as socio-cultural, governmental and economic forces and microcosmic ones related to genetics and neurobiology.
As a result, we need to redefine our understanding of the biopsychosocial model to incorporate this new knowledge (
12). The challenge for the 21st century is to adapt the biopsychosocial model in order to make sure that our ideas about the multidirectional and systemic forces underlying this concept are kept up to date with scientific and clinical reality. Translating the innovations in the neurosciences into best practices, positive practice models, and evidence-based and outcome-based treatment for busy clinicians is one of our greatest challenges.
Deficits in measured intellectual/cognitive abilities are not enough
IDD are most often considered in terms of performance on individualized measures of intelligence. Historically, the development and introduction of intelligence testing represented a major breakthrough in our ability to measure and classify IDD. But it wasn’t long before researchers and clinicians realized that normalized measures of intelligence provided only a portion of what was needed. Yet in many programs, service providers still use IQ scores to group patients into broad categories (mild, moderate, severe, and profound levels of ID). As we shall see, this view is incomplete (
6,
13)
In educational settings, this IQ score shorthand is used in decision-making about classroom placement, setting performance expectations as well as designing educational programs to match academic and daily life skills training needs. Yet the validity of these predictions is often derailed by positive effects of early intervention and better management of neurodevelopmental disorders such as seizures and the negative effects of severe behavioral problems, neuropsychiatric and medical disorders (e.g., cerebral palsy or epilepsy). In addition, disruptive behaviors and the presence of unrecognized or inadequately treated mental disorders have a deleterious effect on educational performance (
14). These persist into adulthood as behavioral problems that adversely affect structured employment, community independence, social activities, and residential living and produce levels of psychosocial stress on caregivers and alter the quality of life (
7).
Psychodynamic-oriented clinicians use IQ scores to formulate hypotheses about their role in psychosexual, psychosocial and cognitive development. The relationship between mental age and developmental stages is a permeable one but these data do provide a system of staging strengths and vulnerabilities and clarify developmental levels of functioning. Within a psychodynamic framework, arrests and derailments of many lines of development underlie working hypotheses about the sources of challenging behaviors and how these may represent manifestations of various mental disorders. This system is less often applied and often less useful in the presence of severe IDD, cerebral palsy, epilepsy and other neurological, genetic disorders (including recognizable behavioral phenotypes) (
7,
15).
In the final analysis, it is a combination of these models with behavioral and neuropsychological data that help us integrate biology, past life experiences and psychological development into our understanding of aggression, self-injurious behavior (SIB), hyperactivity and stereotypies. The increased use of attachment and separation-individuation models are providing additional insights into how an individual with IDD adapts and responds to his psychosocial environment (
16). In addition, ethology, neuro-ethology and behavioral pharmacology are becoming useful tools in understanding complex social behaviors such as aggression or avoidance behaviors. This information may be as valuable as measured intellectual abilities in explaining what initiates and maintains these behaviors.
Deficits in adaptive skills are more than meets the eye
Adaptive skills represent an amalgam of abilities associated with 1) behavioral flexibility, 2) experiential learning (“plastic” intelligence), 3) problem solving, 4) coping with novel settings 5) task demands (procedural learning) and 6) mastery of skills of daily living. Social intelligence is another subset of adaptive skills that do not seem directly tied to other forms of intelligence but play a key role in all forms of skill learning (
14). This relative disconnect reinforces the importance of adaptive skills in the definition of IDD.
For example, individuals with mild IDD may have sufficient adaptive skills to “disappear” into the workday world after completing school. In general, their cognitive limitations are camouflaged by good communication and reasonable adaptive skills. Yet abuse/neglect, coexisting developmental disorders (e.g., autism spectrum disorder or cerebral palsy) and mental disorders can derail this more favorable developmental trajectory (
15). From a neuropsychological perspective, early environmental insults can adversely affect behavior regulation, cognitive flexibility, stress management as well as lower the threshold for major mental disorders. For many supportive caregivers, positive learning experiences and the availability and utilization of individually-directed and designed programs can compensate for these psychosocial insults and improve outcome by enhancing resilience and coping skills.
Individuals with severe-profound IDD (SPIDD) present a different picture. These individuals may lack the intellectual or adaptive capacities to deal with many routine life experiences. The increased prevalence of genetic, neurological, medical and other developmental disorders limit the pace of learning and can produce less reversible ceiling effects on adaptive skills (
15,
17). Yet individuals with SPIDD are more likely to respond to physiological and psychological distress by increases in the frequency or severity of either pre-existing or new patterns of challenging behaviors (baseline exaggeration). For example, changes in expected routine or caregiver can be highly stressful events. Because of limited communication skills, challenging behaviors become the primary mode of expressing distress or pain. Understanding and interpreting these behavioral responses can require a combination of provider familiarity and clinical savvy.
Problems arise when clinicians misattribute these transient or environmentally driven changes in behavior (baseline exaggerations, behavioral regression and cognitive disorganization) as a primary neuropsychiatric disorder. The reverse can also be true: the emergence of a primary psychiatric disorder may increase the individual’s vulnerability to that stress or be the cause of the reaction. Impaired adaptive skills can become a setting event, contributing factor, or an impediment to the natural resolution or the underlying factor in the diagnostic misattribution.
Onset during the developmental period
The developmental period is most often defined as between birth and age 18. Although useful for legal and guardianship purposes, this legalistic cutoff is inconsistent with current models of neurodevelopment. Research suggests that brain maturation and neuroplasticity continue well past age 18. For individuals with IDD, ongoing brain development suggests a lifelong capacity for skill learning, provided the individual is free of neurodegenerative, neuropsychiatric or medical illnesses. But there is a paradox inherent to this observation. The timing of any insult on brain development is complex and incompletely understood. There are fundamental neurobiological differences between acquired lesions in adulthood and those occurring during critical periods in brain development. Even though some insults during infancy are more amenable to interventions, there may still be potential limits imposed on later developing neurocognitive skills. The critical piece is the timing of the intervention: early and intensive intervention during periods of rapid brain development is most effective for many but not all areas of development.
Many genetic syndromes present another twist: the age of onset is often inversely related to the severity, clinical course and prognosis. Milder forms of autosomal disorders present with minimal findings during childhood but then emerge and slowly progress during adolescence or early adulthood. Classification based on a cutoff age of 18 becomes confusing when there is a very slow cognitive decline (drop in IQ score and decreasing adaptive skills) that is finally recognized in late adolescence. This scenario is more consistent with neurodegenerative or dementing illness that was masked by learning disability or low normal intelligence until clear cut neurological or metabolic symptoms emerged. Both traumatic brain injury and the onset of neurodegenerative disorders during late adolescence may contribute to behavioral or mental health problems prior to obvious cognitive decline. This gap can make it more difficult to determine the age of onset issue.
Treatment and Outcomes
The diagnosis and treatment of mental disorders in adults with IDD requires that the clinician is able to negotiate two worlds. As a team member, the psychiatrist often plays the role of translator between the world of the neurosciences and biomedicine and the team. As we have seen, the differential diagnosis can present some of our greatest challenges. For example, schizophrenia is frequently diagnosed and treated based on the presence of hallucinations or delusions without a comprehensive exploration of psychosis; psychotic forms of major depressive, bipolar disorder, PTSD, delirium, dissociative states and intoxication can occur (
3). A parallel problem arises when clinicians prescribe psychopharmacological treatment for aggression, self-injury, agitation and disruptive/destructive behaviors without thorough reviews of environmental factors (social ecology), review of neurobiological setting events and functional behavioral analyses. In either of these scenarios the rush to treat can result in the long-term exposure but often ineffective treatment regimens that all too often result in unnecessary polypharmacy (
30,
44).
Traditionally, most mental health clinicians accept the view that mental disorders arise from the interaction between psychosocial stresses and biological/genetic risks for a particular mental disorder (“two hit theories”). Some colleagues take a different approach, viewing challenging behaviors as learned phenomena. These behavioral specialists tend to focus less on accurate diagnosis and biogenetic etiology (
8). Yet in light of our current knowledge, neither approach provides the ultimate solution. As clinicians we need to consider both challenging behaviors and mental disorders as lying on a continuum. From this perspective, challenging behaviors are attributed to environmental stressors and patterns of reinforcement or consequences; mental disorders, consequences of complex gene-environmental interactions. On closer analysis, gene-environmental interactions also bias the individual toward specific behavioral reactions to specific circumstances and the risk for psychiatric disorders. Sorting out these complex relationships and matching them with current evidence-based treatment or best practices are some of our greatest challenges.
Mental health professionals entering the world of IDD and dual diagnosis must be open to learning from other disciplines while thinking through problems both systemically and systematically. The time-consuming, multidisciplinary nature of assessment, diagnosis and treatment planning requires understanding of many bodies of knowledge. For psychiatrists, the challenge is to view psychopharmacological interventions as but one component of a complex toolkit (
Table 1).