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

Currently, there are more than 750 known causes of mental retardation; however, in up to 30% of cases no clear etiology is found.²^–⁶ 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 mental retardation are Down syndrome, fragile X syndrome, and fetal alcohol syndrome. Facial features of these conditions are provided in Figures 20-1 through 20-3; knowledge of the dysmorphic features associated with clinical syndromes aids in their identification. Down syndrome is the most common genetic cause of mental retardation; it involves trisomy of chromosome 21. Fragile X syndrome is the most common inherited cause of mental retardation with the FMR1 gene located on the X chromosome. Fetal alcohol syndrome, the most common “acquired” cause of mental retardation, has no identified chromosomal abnormality, as it is a toxin-based insult. These three etiologies account for nearly one-third of cases of mental retardation.

Figure 20-1 Image of an individual with Down syndrome.

Figure 20-2 Image of an individual with Fragile X syndrome.

Figure 20-3 Image of an individual with fetal alcohol syndrome.

CO-MORBID PSYCHOPATHOLOGY

Individuals with mental retardation experience the full range of psychopathology, in addition to some unique behavioral conditions.⁷^–⁹ The rates of psychopathology in this population are roughly four times higher than in the general population ^2,⁶; 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 mental retardation also have some form of psychopathology or behavioral disorder.

Although it is generally accepted that the rates of psychopathology are higher in the mentally retarded, there is less agreement as to why this is so. One theory posits that mental retardation is a brain disorder with as yet unidentified damage to cortical and subcortical substrate. This damage confers heightened vulnerability to psychiatric disorders. Another theory holds that individuals with mental retardation 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.

For many years there was an unwillingness on the part of psychiatrists to aggressively treat a difficult-to-diagnose population. This was superimposed on a movement in the field of mental retardation not to “over-pathologize” behavior. Related to this has been the problem of under-diagnosis, based on the concept of diagnostic overshadowing, that is, the attribution of all behavioral disturbances to “being mentally retarded.”¹⁰^–¹² This stance further marginalizes an already vulnerable population. However, in today’s treatment climate one must also be on guard against over-treatment in the form of misguided polypharmacy. What is needed is a thoughtful approach to diagnosis and treatment of both psychiatric and behavioral disorders in a challenging population.

CLINICAL FEATURES AND DIAGNOSIS

Clinical Features

Familiarity with the diagnostic criteria of mental retardation and its clinical manifestations will aid in the assessment of the functional strengths and weaknesses of a patient with mental retardation who presents for diagnosis and treatment. Table 20-1 presents the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) criteria for mental retardation.¹ These criteria have been adopted from work done by the American Association on Mental Retardation (AAMR), now called the American Association on Intellectual and Developmental Disabilities (AAIDD). Key criteria include the following: below-average intellectual function (as defined by standard intelligence quotient [IQ] testing results that are at least 2 standard deviations below the mean [i.e., an IQ ≤ 70]), in conjunction with impairment in 2 out of 11 areas of adaptive function (compared to peers of the same age and culture), and the symptoms must have been present before age 18 (to denote the congenital nature and life-long course of the diagnosis and to differentiate mental retardation from acquired brain injury and other impairments in cognition that can occur later in life).

Table 20-1 DSM-IV Criteria for Mental Retardation

A. Subaverage intellectual function (i.e., an IQ of 70 or below)

B. Concurrent deficits in adaptive function (relative to the person’s age) in at least two areas (e.g., communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety)

C. Onset before age 18 years

Table 20-1 also lists adaptive areas of function that are assessed when one considers a diagnosis of mental retardation. These domains were not empirically tested and no “gold standard” exists in terms of assessment of level of function. Multiple instruments are available to assist in the assessment; their use varies widely from state to state and from agency to agency. In addition, one can see individuals who may have been diagnosed as mentally retarded, but who do not meet strict IQ criteria. IQ testing can have a standard error of measurement of approximately 15 points. Conversely, there are individuals who may have had a diagnosis of mental retardation, but whose functional level and family support system have allowed them to live and work in society without need for services or supervision. Psychiatrists should be cognizant of the general cognitive level of their patients and assess how cognitive ability may affect their clinical presentation, as well as their compliance and response to treatment.

Table 20-2 presents the DSM-IV-TR classification system (based on severity) for mental retardation along with estimates of their approximate prevalence.¹ These categories do not reflect functional capabilities per se but should be used by the psychiatrist to get a better understanding of how an individual with such an IQ might be expected to present. The overwhelming majority of individuals with mental retardation fall in the mild to moderate category and are the ones most likely to present for community treatment. Individuals with severe to profound mental retardation are more commonly seen in institutional settings, but this can vary greatly from state to state and from region to region.

Table 20-2 Classification of Severity and Approximate Percentages of Mental Retardation in the Population