Chapter 44 Intellectual Disability
PATHOPHYSIOLOGY
Intellectual disability (the term that is gaining widespread usage in replacing the term mental retardation) is foremost amongst the developmental disabilities in terms of its prevalence. The term intellectual disability refers to significant limitations in cognitive and adaptive functioning. This is a cognitive disability manifested during childhood (before age 18 years) that is characterized by below-normal intellectual functioning (intelligence quotient [IQ] is approximately 2 standard deviations below the norm, in the range of 70 to 75 or below) with other limitations in at least two adaptive areas of functioning: speech and language, self-care skills, home living, social skills, use of community resources, self-direction, health and safety, functional academics, leisure, and work. Newer definitions of intellectual disability adopt a functional or ecologic approach rather than applying the terminology formerly used to describe levels of mental retardation, such as mild, moderate, severe, and profound. Refer to Box 44-1 and Box 44-2 for diagnostic criteria for mental retardation (currently still in use by some organizations). Many advocates promote the use of newer designations—cognitive disability, intellectual disability, and learning disability—rather than the term mental retardation.
Box 44-1 Diagnostic Criteria for Mental Retardation
1. Significantly subaverage intellectual functioning: an intelligence quotient (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 and 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:
From American Psychiatric Association: Diagnostic and statistical manual of mental disorders, ed 4, text revision (DSM-IV-TR), Washington, DC, 2000, The Association.
Box 44-2 Operational Definition of Mental Retardation
Mental retardation is characterized both by a significantly below-average score on a test of mental ability or intelligence and by limitations in the ability to function in areas of daily life, such as communication, self-care, and getting along in social situations and school activities. Mental retardation is sometimes referred to as a cognitive or intellectual disability.
Children with mental retardation can and do learn new skills, but they develop more slowly than children with average intelligence and adaptive skills. There are different degrees of mental retardation, ranging from mild to profound. A person’s level of mental retardation can be defined by their intelligence quotient (IQ), or by the types and amount of support they need.
From Centers for Disease Control and Prevention: Mental Retardation, Developmental Disabilities (serial online): www.cdc.gov/ncbddd/dd/ddmr.htm. Accessed on June 10, 2007.
Causes of intellectual disability can be classified as prenatal, perinatal, and postnatal. Prenatal causes include chromosomal disorders (trisomy 21 [Down syndrome], fragile X syndrome), syndrome disorders (Duchenne’s muscular dystrophy, neurofibromatosis [type 1]), and inborn errors of metabolism (phenylketonuria [PKU]). Perinatal causes can be categorized as those related to intrauterine problems such as abruptio placentae, maternal diabetes, and premature labor, and those related to neonatal conditions, including meningitis and intracranial hemorrhage. Postnatal causes include conditions resulting from head injuries, infections, and demyelinating and degenerative disorders. Fragile X syndrome, Down syndrome, and fetal alcohol syndrome (FAS) account for one third of the cases of intellectual disability. The occurrence of associated problems such as cerebral palsy, sensory impairments, psychiatric disorders, attention deficit hyperactivity deficit (ADHD), and seizure disorders is more likely correlated with the more severe levels of intellectual disability. Diagnosis is established early in childhood. In a few instances, intellectual disability can be prevented as demonstrated with fetal alcohol syndrome by encouraging women not to injest alcohol during pregnancy. Children born with metabolic conditions such as PKU and congenital hypothyroidism can be medically treated to prevent the consequences of nontreatment resulting in intellectual disability. Long-term prognosis is determined ultimately by the extent to which the individual can function independently in the community (i.e., employment, independent living, social skills).
INCIDENCE
1. More than 85% of persons with intellectual disability have an IQ that classifies them in the mild level (IQ between 50 and 70).
2. Prevalence rate is 1% for children between the ages of 3 years to 10 years.
3. More than 600,000 children and youth ages 6 to 21 years are estimated to have intellectual disability.
4. One of every 10 children in special education has some variation of intellectual disability.
5. Intellectual disability is more common in older children (6 to 10 years) than in younger children (3 to 5 years).
6. Male/female ratio is 1.5:1. Incidence is higher in lower socioeconomic groups.
7. Prevalence rate is higher in African Americans than in whites.
8. Risk of recurrence in families is as follows:
9. Nearly 18% of infants with very low birth weight have severe disabilities.
10. Approximately 500,000 youths have an intellectual disability.
11. The high school dropout rate for students with disabilities is 25% to 30%.
12. The unemployment rate for persons with intellectual disabilities is estimated to be between 66% and 75%.
13. Lifetime costs for all individuals born with an intellectual disability in 2000 are anticipated to reach $51.2 billion, amounting to more than $1 million per person.
CLINICAL MANIFESTATIONS
COMPLICATIONS
3. Behavioral and/or psychiatric problems
5. Constipation (caused by decreased intestinal motility secondary to anticonvulsant medications, insufficient intake of fiber and fluids)
6. Associated congenital anomalies such as esophageal malformation, small bowel obstruction, and cardiac defects
LABORATORY AND DIAGNOSTIC TESTS
1. Cognitive and developmental assessment tests, including the following:
2. Measurements of adaptive behaviors
Table 44-1 Hypotheses and Strategies for Assessing Etiologic Risk Factors
| Hypothesis | Possible Evaluation Strategies |
|---|---|
| Prenatal Onset | |
| Chromosomal disorder | Extended physical examination |
| Referral to clinical geneticist | |
| Chromosomal and DNA analysis | |
| Syndrome disorder | Extended family history and examination of relatives |
| Extended physical examination | |
| Referral to clinical geneticist | |
| Inborn error of metabolism | Newborn screening using tandem mass spectrometry |
| Analysis of amino acids in blood, urine, and/or cerebrospinal fluid | |
| Analysis of organic acids in urine | |
| Measurement of blood levels of lactate, pyruvate, very long chain fatty acids, free and total carnitine, and acylcarnitines | |
| Measurement of arterial ammonia and gases | |
| Assays of specific enzymes in cultured skin fibroblasts | |
| Biopsies of specific tissue for light and electron microscopy and biochemical analysis | |
| Cerebral dysgenesis | Neuroimaging (computed tomography or magnetic resonance imaging) |
| Social, behavioral, and environmental risk factors | Intrauterine and postnatal growth assessment |
| Placental pathologic analysis |
