Overview of Intellectual/Developmental Disabilities

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.

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.

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.

The number of genetic syndromes is growing in conjunction with the development of new genetic techniques. 30 years ago genetic testing usually meant chromosome testing and linkage studies. Nowadays we can obtain genome wide scans, florescence in situ hybridization, and microarrays that hone in on single nucleotide polymorphisms, copy number variants and the structure of specific genes (18). Recent breakthroughs in Fragile X research have opened another doorway with the discovery that mglut5 antagonists appear to reverse both molecular and cognitive/behavioral features of Fragile X (18, 19). There is growing research in epigenetics that point to changes in genes regulation due to trauma or faulty parenting. These findings challenge genetic determinism and force us to rethink nature-nurture arguments in terms of a newer understanding of gene–environment interactions.

Behavioral phenotypes are patterns of cognition and behavior associated with specific genetic disorders. Examples include: gaze aversion and anxiety in Fragile X (20); hyperphagia and binge eating in Prader Willi syndrome (21); personality traits linked to Trisomy 21 (22); increased risk for schizophrenia in 22q deletion (Velocardiofacial syndrome) (23); and epilepsy, dysmorphologies, hypotonia, and autism in 15q11.2-13 duplications (24). Many behavioral phenotypes bear some resemblance to temperamental features of affected individuals. Others seem more like prodromal primary psychiatric disorders (18). Unfortunately, there are too many genetic and neuroanatomical “smoking guns” for polygenic disorders like psychosis, bipolar disorder/schizophrenia, autism, and epilepsy in patients with IDD. This observation suggests that we are dealing with complex disorders and reinforces the idea that many syndromes are related to disruptions in pathways rather than specific lesions or related to single neurotransmitter abnormalities.

The most commonly neglected component of the biopsychosocial model is the role played by socio-cultural forces in dual diagnosis. Over the past three centuries, attitudes and values about individuals with IDD changed dramatically. These changes impact the social risk factors on the epidemiology, availability and types of habilitation and treatment services for those with IDD and mental disorders. In industrialized societies, urbanization and changing modes of living and working placed new occupational and educational demands that changed the niche occupied by individuals with ID. For example, many individuals with IDD now live in a society where there are fewer less technologically demanding jobs (requiring basic skill sets) and these jobs are often subject to increasing competition. These social changes are complicated by smaller families, greater geographic mobility and the problem of isolation from traditional sources of family support. Today families rely upon “professional services” to fill this void (8).

These social and cultural changes are redefining our sense of community. Historically, attitudes such as relative pessimism by medical providers about a lack of effective interventions and prognosis; marginalization and socio-economic segregation of individuals and their families; and lack of community-based resources contributed to out-of-home residential placement. Governmental actions focused mainly on developing programs that eventually became large custodial institutions (25). As an outgrowth of the Civil and Human Rights movement and numerous lawsuits, there is a shift to community care, inclusion, freedom of choice, active treatment and protection from various forms of restraint. But these changes are quite vulnerable to economic downturns; legislative battles ensue over cost-effectiveness instead of treatment efficacy. Unfortunately, there has been a resurgence of ambivalence about placing group homes in residential neighborhoods as well as availability and adequate resources for staff and educators. Finally, limitations on funding and access to an integrated system of comprehensive health and mental health care (26) are present in many states. The care of high service users such as those with dual diagnosis is especially vulnerable to these socio-political forces (8, 27–29). This unfortunate scenario may underlie the recent trends toward increasing use of psychotropic drugs (30, 31).