Intellectual Disability

Overview

Treatment of psychiatric and behavioral disorders in individuals with an intellectual disability is both challenging and rewarding. Although it is unlikely that most psychiatrists will be called on to make a diagnosis of intellectual disability, knowledge about what defines intellectual disability, and about the clinical features of the most common syndromes related to the development of an intellectual disability, is crucial for the optimal treatment of individuals with this condition. Key questions to consider when evaluating a person with an intellectual disability include the following: How was the diagnosis made? Was the work-up complete? How severe is the cognitive impairment? What is the current developmental level of the patient? Are there any current or co-morbid medical issues that may be causing or contributing to aberrant behavior? Is there a functional aspect to accompanying problematic behavior? Which psychiatric disorders are prevalent in patients with an intellectual disability or commonly occur with identified syndromes?

Historically, those with an intellectual disability and those with severe psychiatric illness have shared the burden of a chronic illness, as well as the experience of stigmatization and alienation from society. However, individuals with an intellectual disability were initially thought to stand apart from others with brain disorders. The English Court of Wards and Liveries in the sixteenth century differentiated “idiots” from “lunatics.” Kraeplin, in his initial diagnostic schema, identified intellectual disability as a separate form of psychiatric illness. It was not until 1888 when the American Journal of Insanity used the phrase “imbecility with insanity” that intellectual disability and psychiatric illness were identified as potentially co-occurring conditions.

Epidemiology

Prevalence

The prevalence of intellectual disabilities is approximately 1%. Prevalence rates have varied between 1% and 3% depending on the populations sampled, the criteria used, and the sampling methods applied.

Currently, there are more than 750 known causes of intellectual disability. Categories include: pre-natal/genetic disorders, neurological malformations, external/pre-natal causes (such as prematurity and toxin exposure), peri-natal causes (such as hypoxia), and post-natal causes (such as infection and neglect). In up to 25% of cases no clear etiology is found. This can be disheartening for patients, parents, families, and caregivers, as they search for an understanding of a condition that will profoundly affect their lives. This aspect of a patient's history should be addressed at the start of treatment.

The three most common identified causes of intellectual disability are Down syndrome, fragile X syndrome, and fetal alcohol syndrome. Facial features of these conditions are provided in Figures 20-1 to 20-3 ; knowledge of the dysmorphic features associated with clinical syndromes aids in their identification. Down syndrome is the most common genetic cause of intellectual disability; it involves trisomy of chromosome 21. Fragile X syndrome is the most common inherited cause of intellectual disability with the FMR1 gene located on the X chromosome. Fetal alcohol syndrome, the most common “acquired” cause of intellectual disability, has no identified chromosomal abnormality, as it is a toxin-based insult. These three etiologies account for approximately one-third of cases of intellectual disability.

Co-Morbid Psychopathology

Individuals with intellectual disability experience the full range of psychopathology, in addition to some unique behavioral conditions. The rates of psychopathology in this population are roughly three to four times higher than in the general population ; exact determination is difficult as data collection in this area is confounded by methodological issues (including how to obtain an accurate assessment in the absence of self-report and determining how appropriate certain standardized measures might be in this population). In institutional settings up to 10% of individuals with intellectual disability also have some form of psychopathology or behavioral disorder.

Although it is generally accepted that the rates of psychopathology are higher in the intellectually disabled, there is less agreement as to why this is so. One theory posits that intellectual disability is a brain disorder with an as-yet unidentified damage to cortical and subcortical substrates. This damage confers heightened vulnerability to psychiatric disorders. Another theory holds that individuals with intellectual disability are chronically exposed to stressful and confusing environments, but they lack the cognitive capacity to successfully cope with this stress or to resolve affective conflicts. This eventually wears them down and makes them more vulnerable to psychiatric disorders. Still another theory points to the paucity of good psychological care they receive, which leads to inadequate preventive measures and to delays in diagnosis and treatment.

In the past there was unwillingness on the part of the psychiatric community to aggressively diagnose and to treat what historically has been a difficult-to-diagnose population. This was superimposed on a movement in the field of intellectual disability not to “over-pathologize” behavior. Related to this has been the problem of under-diagnosis, based on the concept of diagnostic overshadowing: the attribution of all behavioral disturbances to “being intellectually disabled.” In today's treatment climate, however, one must be on guard against over-treatment in the form of misguided polypharmacy. What is needed is a thoughtful approach to diagnosis with an understanding of functional behavior that leads to optimum treatment of both psychiatric and behavioral disorders.