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I. Overview
Modern investigations regarding the relationship between mental retardation and psychiatric illness.
1. Several hypothesizes have been formulated to explain why individuals with cognitive impairment seem to be at increased risk for psychiatric disorders.
2. With the occurrence of the "Decade of the Brain" and a shift in focus towards neurodevelopmental psychiatry, the relationship between mental retardation and psychiatric illness has generated more interest. Future investigation in this area is likely to shed light on complex issues, eg, behavioral phenotypes and the relationship between coping abilities and the vulnerability.
II. Epidemiology
A. Prevalence
1. The prevalence of mental retardation is between 1% and 3% depending on the criteria used, the methods used, and the populations sampled; most investigators believe the prevalence is probably closer to 1%.
2. Mental retardation is a condition with diverse etiologies, more than 350 causes of mental retardation are known.
3. However, approximately 40% of cases have no clear etiology.
4. The three most common causes of mental retardation account for about 30% of identified cases. They include:
a. Down Syndrome (chromosome 21), which is the most common genetic cause of mental retardation.
b. The Fragile X Syndrome (X linked gene FMR-1), which is the most common inherited cause of mental retardation.
c. The Fetal Alcohol Syndrome (with triad of growth retardation, developmental delay, and classic facial features), which is the third most common known cause of mental retardation.
B. Comorbid psychopathology
1. The mentally retarded population suffers from the full range of psychiatric illness, and likely is afflicted at a higher rate than the general population.
a. Some studies estimate that the prevalence of psychiatric disorders is 4 to 6 times that of the general population.
b. In institutional settings upwards of 10% of the mentally retarded population have some form of psychopathology; percentages are less clear in community samples. One must be cautious of methodological problems.
C. Etiology of psychopathology in the mentally retarded
1. Why there appears to be a higher prevalence of psychiatric illness in the mentally retarded population is unclear.
a. One theory centers on the idea that mental retardation is a manifestation of damage to cortical and subcortical substrate, regardless of whether this damage can be identified with available technology. This damage confers a special vulnerability.
b. Another theory holds that individuals with mental retardation are chronically exposed to a confusing and stressful world, based on their decreased ability to cope with the demands of a complex society and an inadequate cognitive capacity to resolve emotional conflicts.
c. A third theory highlights the lack of psychiatric care in this population. It questions the psychiatric profession's unwillingness to treat psychiatric illness in the mentally retarded due to a prevailing view that behavioral disturbance and psychopathology is somehow more acceptable in individuals with mental retardation.
III. Diagnostic Features
A. Definition
1. The definition of mental retardation comes from work done by the American Association on Mental Retardation; it has basically been adopted by DSM IV.
a. "Mental retardation refers to substantial limitations in present functioning. It is characterized by significantly subaverage intellectual functioning, existing concurrently with related limitations ...[in several adaptive areas and]...manifests before age 18.
b. In more objective terms, mental retardation is defined by having a standardized IQ score at least two standard deviations below the mean and impairment in at least two out of ten areas of adaptive functioning when compared to peers of the same age and culture. The areas identified include: communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health and safety. However, it should be kept in mind that these domains were not empirically selected and no single measure of adaptive function exists.
B. Classification of mental retardation
1. Mental retardation is classified into four levels of severity based on intellectual impairment as measured by IQ scores. These categories do not reflect functional capabilities; they have a standard error of measurement of approximately 5 points. The levels of severity, their IQ ranges, and their prevalence within the mentally retarded population are as follows:
a. Mild (IQ 55-70) 85%
b. Moderate (IQ 40-55) 10%
c. Severe (2540) 34%
d. Profound (<25) 1-2%
2. A substantial portion of individuals diagnosed with mild mental retardation as children lose this diagnosis in adulthood as their adaptive skills improve and with no psychologist around to test them. Those who retain this label into adulthood tend to be affected more severely.
IV. Evaluation and Differential Diagnosis
A. Evaluation
1. The evaluation for a diagnosis of mental retardation is relatively straight-forward; it is based on the previously mentioned diagnostic criteria.
2. History of adaptive functioning from ancillary sources like school and primary caregivers and neuropsychiatric and adaptive behavior testing are the cornerstones of evaluation.
3. A thorough medical and neurological examination is important to rule out correctable causes (including heating impairment, vision impairment, and seizures) of observed dysfunction.
4. These exams are also helpful to identify any physical features that may be associated with specific syndromes, as many syndromes have related psychiatric conditions.
5. There are no laboratory findings that are specifically associated with mental retardation.
6. However, some laboratory findings are associated with a variety of causes of mental retardation. (e.g. metabolic disturbances and chromosomal abnormalities) It is helpful to identify these if not done previously, as this information can again lead to diagnosis of syndromes that may have associated psychiatric pathology.
B. Differential Diagnosis
1. The differential diagnosis for mental retardation also includes physical disabilities.
2. In addition specific learning disorders, communication disorders, and borderline intellectual functioning must be considered.
3. Although pervasive developmental disorders eg, autism, are a separate diagnostic category, 75-80% of individuals with a pervasive developmental disorder also have co-morbid mental retardation.
V. Treatment Considerations
A. Overview
1. It is unlikely that a psychiatrist will be called upon to treat mental retardation per se, as specialized education and training in adaptive functioning is done in schools and vocational settings. It is more likely that a psychiatrist will be called upon to evaluate and treat a psychiatric or behavioral disorder.
2. The evaluation and differential diagnosis of psychiatric illness in this population is not as straight-forward as the diagnosis of mental retardation itself. The full range of psychiatric disorders are found in people with mental retardation, at rates that are probably higher.
3. There are also certain behavioral disorders and syndrome associated disorders that occur in people with mental retardation.
a. Syndrome-associated disorders are specific disorders that have a high probability of being exhibited by people with a given syndrome.
b. The terms pathobehavorial syndrome and behavioral phenotype are concepts.