The term Intellectual Disability (ID), commonly used in Europe, is now becoming preferred to the previous one Mental Retardation (MR), although both are still used interchangeably. Developmental disability is a legal term defined by the Developmental Disabilities Assistance and Bill of Rights Amendments of 1996, and is used in statutes referring to entitlements. Its requirements are: The condition is due to mental and/or physical impairment, is manifested prior to age 22, is likely to be lifelong, results in substantial functional limitations in three or more major life activities and in need for lifelong services or supports.

ID does not denote an illness or a single disorder entity but a behavioral syndrome of variable etiology characterized by intellectual and adaptive functioning below level expected for the person’s age, education and socio-cultural context. In fact, strictly speaking, it is not a mental disorder like a mood or psychotic disorder. Formal definitions of ID have evolved over time. The modern ones are tri-factorial, requiring impairment in (1) intellectual functioning, (2) adaptive functioning, and (3) onset before age 18. In the US, the standard definition is of the Diagnostic and Statistical Manual of Mental Disorders, fourth edition, Text Revision (DSM-IV-TR) (American Psychiatric Association 2000).

DSM-IV-TR Diagnostic Criteria

Mental Retardation

1. 
   

   Significantly subaverage intellectual functioning: An IQ of approximately 70 or below on an individually administered IQ test (for infants, a clinical judgment of significantly subaverage intellectual functioning).

   
   
2. 
   

   Concurrent deficits or impairments in present adaptive functioning (i.e., the person’s effectiveness in meeting the standards expected for his or her age by his or her cultural group) in at least two of the following areas: Communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.

   
   
3. 
   

   The onset is before age 18 years

Code based on degree of severity reflecting level of intellectual impairment:

Mental Retardation has the following levels of severity and codes:

(Reprinted, with permission, from the Diagnostic and Statistical Manual of Mental Disorders, 4th edn. Text Revision. Copyright 2000 American Psychiatric Association.)

The second most common definition is of the American Association on MR. It also requires onset prior to age 18 and presence of significant limitations in (A) intellectual functioning (defined as an IQ score of 2 SD or more below the mean of an individually administered assessment instrument) and (B) adaptive behavior (in conceptual, social, and practical skills). For the purpose of either definition the adaptive behavior is commonly assessed clinically, but use of testing instruments such as Vineland Adaptive Behavior Scales is recommended. The American Association on Mental Retardation (AAMR) definition specifies scores on such scales of 2 SD or more below the instrument’s mean.

Thus, if a person has an IQ close to the cutoff point (within the error range of the instrument, usually ±5 points), the diagnosis will depend on the level of adaptive functioning. Therefore, such person may qualify for the diagnosis of ID, for example, in school age when learning is impaired by poor academic skills but may lose this diagnosis later, with the acquisition of work skills important for an adult.

1. 
   

   ID is not a single disorder.

   
   
2. 
   

   ID a behavioral syndrome of a triad of significant limitation in intellectual and adaptive functioning, and onset prior to age 18. It is NOT diagnosed by subaverage IQ alone.

   
   
3. 
   

   The diagnosis must be made considering person’s socio-cultural and linguistic diversity and associated disabilities.

   
   
4. 
   

   ID does not have to be lifelong.

   
   
5. 
   

   All people with ID have an ability to learn and progress and all possess strengths which must be supported in any habilitation program.

Epidemiology

Estimates of the prevalence of ID have changed over time depending on its definition. With current definition it is thought to be about 0.7–1% of the population and in about two-thirds of cases are within the mild range of severity, although there are regional differences. Persons with ID are also at increased risk for comorbid mental disorders, with the prevalence ranging between 16% and 71%, and virtually all diagnostic categories are represented.

Etiology

Etiologic consideration of intellectual and other developmental disabilities requires a conceptual framework that takes into account the timing of insult or risk to the developing brain and its impact on developmental functioning. There are multiple factors that may contribute to ID, with occurrence in the prenatal, perinatal, and postnatal periods. ID has many different causes, and sometimes biologic, social, behavioral, and educational factors interact to affect how a person functions (Table 31–1). There may be similarities in type of insults in all three of these time frames, such as infection, toxins, and deprivation of nutritional, environmental, and chemical substances. The timing of these insults however, often determines the degree of ID. In addition, the more significant the etiologic insult, the more likely that other types of disabilities will result, such as in motor function, communication, social, and self-help skills.

Perinatal Insults

Perinatal insults associated with birth asphyxia has been noted to be the etiology for children with a range of intellectual disabilities and cerebral palsy. Events associated with hypoxia and ischemia can be causative, both in preterm and full term infants). Other perinatal events, such as infections and their consequences, may also contribute to developmental outcome.

Postnatal Causes

Postnatal causes are those that are acquired. Mild MR has been associated with postnatal environmental deprivation, with lack of adequate nutritional and social support. In addition, infections, anoxia, trauma, cerebral vascular events, and malignancy and the effects of its treatment may all influence the level of cognitive function. Risk factors for mild ID have been associated with multiple births, nutritional and social deprivation, low maternal education, and teen pregnancy. While male gender has been noted to be a risk factor, this high male-to-female ratio difference is less apparent as the severity of ID increases. Other risk factors that have been associated with ID include low birth weight, and older maternal age.

Genetics

Most diagnoses of ID are felt to have prenatal causes. Genetic and metabolic disorders, central nervous system malformations, congenital infections, toxic exposures, deprivation of essential elements in utero, and many idiopathic etiologies are all considered to be potential prenatal etiologic factors. Severe to profound ID is more likely associated with genetic etiologies, although genetics certainly may play a role in those with moderate and mild ID. A larger percentage of children with mild ID are also felt to have idiopathic causes.

Signs & Symptoms

By definition, the onset of ID is before 18 years of age. The diagnosis, however, is usually made much earlier. Language delays often become apparent during the second year of life, with motor delay being the more common developmental concern during the first year. Children with ID often present with delay in receptive and expressive language skills. Language and cognition are closely associated; children with severe ID are more apt to have more significant language delays. Children with more significant cognitive delay also usually present earlier than those with milder cognitive deficits. Table 31–2 provides information regarding behavioral phenotypes for specific genetic and developmental disorders.

Tests of developmental functioning in the very young child are not felt to be predictive of later cognitive skills, unless there are profound delays. The diagnosis of MR or ID is generally made after infancy, usually before the child is enrolled in school. Sometimes the diagnosis is made later, if a thorough assessment had not been performed earlier. To make the diagnosis, cognitive and adaptive abilities are assessed. Consideration also needs to be given to the environment in which a person lives, ad the manner in which he or she interacts with others on a daily basis.

There are times when the exact level of functioning cannot be determined with certainty. Sometimes a child has not yet received the needed intensive educational support and it is unclear how he or she will respond to these services. Other times, testing cannot be completed or is not felt to reflect a child’s potential. In those instances, further testing is required at a future date to more accurately determine the level of functioning.

In addition to language delays, children may present with delays in play, social, and adaptive skills. Gross motor skills generally develop well, except in certain disorders associated with significant hypotonia. Fine and oral motor skills may also be affected. Developmental milestone acquisition is generally slow but steady. A child may also display developmental plateaus or regression of developmental skills. In those instances, assessment may be required to rule out the presence of metabolic, neurodegenerative, or neurophysiologic disorders.

Children with intellectual disabilities continue to learn, albeit at a rate slower than typically developing peers.

It is important to obtain a thorough medical history, which often provides etiologic information. Parental concern has been shown an important indicator of the presence of developmental problems. The history should include information about pregnancy, delivery, and the perinatal period. Information needs to be obtained regarding past and present medical issues, including illnesses and injuries. A child’s response and recovery from illnesses also need to be explored. Developmental history should be obtained, including developmental milestones in the various developmental domains, as well as overall developmental trajectory. The social environment, with potential stressors, needs to be explored. Family medical history, going back three generations, is needed in the evaluation potential genetic factors. In addition to psychological and psychiatric assessment, the child requires a physical examination that includes a thorough neurological assessment. This examination should include assessment of growth parameters, including head circumference. Dysmorphology should be identified, as it may indicate to prenatal contributing factors. Other physical findings are important, such evaluation of integument for neurocutaneous lesions, coarse facies and/or enlarged liver and spleen for certain storage diseases, muscle distribution, bulk, or asymmetries in certain neurological disorders.

Psychological Testing

Indications for Psychological Assessment

Psychological assessment measures intelligence and adaptive functioning, as well as behavior and academic achievement. It aids in diagnosis, determining abilities and disabilities, treatment plan formulation, eligibility for habilitative and/or rehabilitative services and planning for other interventions. The selection of specific tests is tailored to the age and ability of the individual. Obtaining collateral information from parents, caregivers, teachers, and employers is essential for accurate assessment, since children, nonverbal individuals and persons with significant ID are unable to construct an adequate developmental and behavioral history.

The psychologist has to understand the individual’s ability to sustain attention and comprehend tasks required by a particular test, in order to make an informed decision which tests yield the greatest information about cognition and behavior. Specific instruments have been developed to measure intelligence and associated cognitive abilities including attention, memory, motor function, receptive and expressive language and executive functions (planning, sequencing, organization and task management). Tests administered during childhood may be restricted to a single domain of function. With maturity it is possible to measure additional areas of cognition and behavior with greater reliability and validity. In a comprehensive assessment, a battery of tests is employed to measure multiple domains of cognition and behavior.

While personality testing is within the scope of practice for psychologists, it is less useful for individuals with significant ID who usually lack language and cognitive skills necessary for exploring intrapersonal dynamics through tests such as the Rorschach, personality inventories (Minnesota Multiphasic Personality Inventory-2 [MMPI-2]) or projective measures like picture apperception tests (Thematic Apperception Test).

Measurement of Intelligence

The diagnosis of ID requires that intelligence, as measured on a standardized test, is two or more standard deviations below the mean, with an IQ below 70 (the other two diagnostic criteria are: Deficits in adaptive functioning and onset prior to age 18). To gain an accurate appraisal of the person’s function, individual rather than group testing is required. Testing should occur in a quiet and comfortable place free from distraction. This generally precludes bedside testing except for screening tests designed for that purpose. Sufficient time should be allotted to pace the testing to the comfort level and ability of the individual. It may be necessary to provide for frequent breaks. Testing may often require several sessions over different days. The person being tested should not be fatigued. If they become hyper-aroused or extremely anxious, test data will not be valid and testing should be discontinued and administered at a later time. Tests are used for diverse populations stratified by socioeconomic status, culture, language, geographic location and gender. Individuals of low socioeconomic status may test less well than more affluent and educated persons. The low scores in such situations should not be automatically attributed to mild intellectual impairment. This is one reason why in individuals with suspected ID, measures of intelligence and adaptive behavior are critical in order not to confound lack of education, poverty or inexperience with ID. If necessary, an interpreter should be used for persons not fluent in English. The validity of test results can be significantly affected by translation and the interpretation of results should be viewed with the caveat that tests have been normed on native English speakers. Several IQ tests have been translated into Spanish with appropriate norms for native speakers.

Psychological tests that are standardized are based on a normal distribution of scores with a mean of 100 and a standard deviation of 15 points. Normal scores range from 85–115, representing one standard deviation from the mean. Abnormal scores on standardized tests fall within 1.5–2.0 standard deviations below the mean. Subtests on intelligence tests have scores measured in scaled points ranging from 1–18 with a mean of 10 and a standard deviation of 3. Standardized tests are constructed to enable valid comparisons between scores. For example, an IQ of 85 corresponds to a scaled score of 7 on a subtest that might measure a variable like fund of knowledge (Information on the Wechsler scales) and a Z score of −1.0 standard deviations. All of these scores are at the 16th percentile and equivalently scaled for comparison.

Interpretation of Test Results

The report, prepared by the psychologist following completion of testing, lists all tests administered, raw scores and standard scores for test comparisons. It is important to stress that the summary scores, such as a Full Scale IQ, do not adequately capture individual differences enumerated by testing. The clinician needs to be aware that summary scores combine data and may lead to a false impression since they represent a mean of all tests administered. The psychologist should look at all scores, and their ranges, to gain an accurate impression of an individual’s profile defining patterns of strengths and weakness across all domains evaluated. For IQ tests this means examining index or cluster scores where subtests are divided into similar constructs. Different clusters measure verbal comprehension, perceptual organization, working memory and processing speed.

Following are the other areas of importance to consider:

1. 
   

   Gross and fine motor skills and coordination.

   
   
2. 
   

   Language including vocabulary, syntax, pragmatics, fluency and articulation.

   
   
3. 
   

   Memory including visual and verbal domains for encoding and retrieval.

   
   
4. 
   

   Motor strength.

   
   
5. 
   

   Visual-motor and perceptual abilities.

   
   
6. 
   

   Executive skills involving planning, organization, task management, and cognitive flexibility.

   
   
7. 
   

   Achievement testing including reading, arithmetic, and writing.

Commonly used psychological tests are listed in Tables 31–3, 31–4, and 31–5.