- Intellectual disability (formerly called mental retardation) is characterized by a slow rate of learning or slow cognitive-processing abilities. By definition, there are significant cognitive and adaptive delays first evident in childhood. Significant cognitive delays are defined as 2 standard deviations below the population mean on a standard cognitive or IQ test.
- Usually indicates an IQ score of <70 to 75
- Adaptive skills are the functional skills of everyday life, including communication, social skills, daily living/self-care skills, and the ability to safely move about the home and community.
- Intellectual disability is typically subdivided into mild, moderate, severe, and profound categories, depending on the severity of the delays. A definition by the American Association on Intellectual and Developmental Disabilities (AAIDD) puts more emphasis on the level of functioning and the amount of support required by an individual.
- Children with behavioral problems may also be masking cognitive delays.
- Hearing impairment may present as a delay in development.
- Children with mild intellectual disability may not be diagnosed as having a problem until they are having difficulties keeping up in elementary school.
Found in both sexes and all racial and socioeconomic groups
- Prevalence of intellectual disability is generally listed as 2–3% of the population.
- Of the different subcategories of intellectual disability, the mild form is the most prevalent, at 85% of those with intellectual disability.
- Profound intellectual disability is least prevalent, at ~1% of this group.
- There is no specific prevention, but prevention of some underlying causes may be possible.
- Immunization programs, early detection of metabolic disorders, and education programs for head injury/asphyxia prevention may be useful in some cases.
- Avoidance of alcohol and some drugs during pregnancy may also decrease the likelihood of some specific brain insults.
The cause of the intellectual disability is usually an insult to the brain or abnormal development of the CNS but is not evident in many cases. The following represent potential causes.
- Fragile X syndrome
- Trisomy 21 (Down syndrome) and other chromosomal abnormalities
- Tuberous sclerosis
- Phenylketonuria (PKU)
- Other inborn errors of metabolism
- Nervous system anomalies
- Congenital hypothyroidism
- Prenatal cytomegalovirus, rubella, toxoplasmosis, HIV
- Postnatal bacterial meningitis, neonatal herpes simplex
- Environmental toxins
- Heavy metal poisoning such as lead
- In utero drug or alcohol exposure, including fetal alcohol syndrome
- Closed-head trauma
Commonly Associated Conditions
- Associated findings are more common in the more severe forms of intellectual disability.
- Intellectual disability has many associated findings, including seizures, autism, cerebral palsy, communication disorders, failure to thrive, sensory impairments, and psychiatric disorders.
- Behavioral disorders can be seen, including attention-deficit/hyperactivity disorder and self-injurious and self-stimulating behaviors.
- Families often face additional stressors when caring for a child with intellectual disability.
Complete information regarding the following:
- Pregnancy history
- Maternal age and parity
- Maternal complications (including infections and exposures)
- Medications/drugs used
- Tobacco or alcohol used, along with quantities
- Fetal activity
- Birth history
- Gestational age
- Birth weight
- Route of delivery
- Maternal or fetal complications/distress
- Apgar scores
- General health
- Significant illnesses, hospitalizations, or surgeries
- Accidents or injuries
- Hearing and vision status
- Medications used
- Known exposures to toxins
- Any new or unusual symptoms
- Developmental history
- Current developmental achievement in each stream of development
- Age when developmental milestones were achieved
- Any loss of skills
- Where parents think their child is functioning developmentally
- Educational history
- Type of schooling and services received, if any
- Any previous educational/developmental testing
- Behavioral history
- Any perseverative or stereotypical behaviors
- Interaction skills
- Attention and activity levels
- Family history
- Family members with developmental delays, neurologic disorders, syndromes, inherited disorders, or consanguinity
A complete physical exam including growth parameters is needed looking for etiology. Key features to include are the following:
- Observation of interactions and behavior
- Atypical behaviors and general impressions
- Head circumference
- Macro- or microcephaly
- Skin exam
- Neurocutaneous lesions
- Major or minor dysmorphic features
- Indication of a syndrome or anatomic malformation
- Neurologic exam
- Assess for cranial nerve deficits, neuromuscular status, reflexes, balance and coordination, and any soft signs.
The differential can include several other developmental diagnoses, including the following:
- Borderline cognitive abilities
- Developmental language disorder
- Autism spectrum disorder
- Specific learning disability
- Cerebral palsy
- Significant visual or hearing impairment
- Degenerative disorders
Diagnostic Tests and Interpretation
- There is no specific laboratory test battery for intellectual disability.
- Testing must be tailored to the individual situation based on history and physical exam. A high index of suspicion should be maintained for any associated findings and delays in the other streams of development. Listed below are some of the more common studies.
- Genetic testing
- For any dysmorphic features or a family history of delays or genetic disorder, a karyotype and fragile X DNA testing should be considered, particularly for significant cognitive delays, although the comparative genomic hybridization (CGH) microarray is now recommended as a 1st-line genetic test.
- Metabolic tests
- Quantitative plasma amino acid, quantitative urine organic acid, lactate, pyruvate, or ammonia levels should be considered if there is any loss of skills, indication of a metabolic disorder, or if no newborn metabolic screen was done.
- Additional metabolic tests may be indicated depending on symptoms.
- Thyroid function tests
- Most infants will have had screening for hypothyroidism shortly after birth. This should be rechecked if symptoms indicate.
- Head MRI: Consider for head abnormalities; significant neurologic findings; loss of skills; or for workup of a specific disorder, such as trauma or leukodystrophy.
- Genetic testing
- When developmental delays are present and intellectual disability is suspected, more formal developmental screening or testing should be done.
- The pediatrician can do some in office developmental screening, but the diagnosis needs to be made based on standardized tests, usually done by a clinical psychologist. Such standardized testing might involve the Stanford-Binet Intelligence Scale, the Wechsler Adult Intelligence Scale, and the Vineland Adaptive Behavior Scale.
- Audiologic testing
- For any child with speech and language and/or cognitive delays
- An EEG should be considered if there is any concern about seizures.
- There is no specific cure for intellectual disability. The ultimate goal of all therapies is to help the child reach his or her full potential.
- Therapy should consist of appropriate treatment for any underlying or associated medical condition.
- Early intervention and special education programs are available for an individualized education program based on the child’s needs and abilities.
- Behavior management programs or selected use of medications is available for patients with severe behavioral problems.
Issue for Referral
- A referral is made to a clinical psychologist for the formal diagnosis.
- Referral to other medical specialists may also be indicated.
- These specialists may include developmental pediatrics, neurology, genetics, or ophthalmology.
- Children with intellectual disability will need regular pediatric preventive care in addition to management of any underlying medical conditions.
- Ongoing monitoring of the educational programs, to ensure that it is still meeting the child’s needs, is important.
- The family will also need ongoing counseling and support in dealing with a child having special needs.
- The prognosis for longevity varies with the associated findings and overall health, but individuals with intellectual disability can live to adulthood and old age.
- An individual’s level of functioning is variable depending on the level of retardation, special individual skills, and family or community supports. In general, the following applies:
- Mild intellectual disability (IQ 55 to 70): formerly called educable. May be in school with extra help and may achieve roughly a 4th to 6th grade level in reading and math. May be employed in an unskilled to semiskilled job; may live in a group home or independently
- Moderate intellectual disability (IQ 40 to 54): may learn to recognize basic words and learn basic skills. May work in a sheltered workshop or with supported employment in an unskilled job; may live with family or in a group home doing much of their own care
- Severe intellectual disability (IQ 25 to 39): may live with family or in a group home or institution. Some may be in a sheltered workshop; may be able to do some daily self-care or chores with supervision
- Profound intellectual disability (IQ <25): live with family, in a group home, or in institution; usually require full-time care
- Battaglia A. Neuroimaging studies in the evaluation of developmental delay/mental retardation. Am J Med Genet C Semin Med Genet. 2003;117C(1):25–30. [PMID:12561055]
- Battaglia A, Carey JC. Diagnostic evaluation of developmental delay/mental retardation: an overview. Am J Med Genet C Semin Med Genet. 2003;117C(1):3–14. [PMID:12561053]
- Gropman AL, Batshaw ML. Epigenetics, copy number variation, and other molecular mechanisms underlying neurodevelopmental disabilities: new insights and diagnostic approaches. J Dev Behav Pediatr. 2010;31(7):582–591. [PMID:20814257]
- Jeste SS. Neurodevelopmental behavioral and cognitive disorders. Continuum (Minneap Minn). 2015;21(3):690–714. [PMID:26039849]
- Moeschler JB, Shevell M; for American Academy of Pediatrics Committee on Genetics. Clinical genetic evaluation of the child with mental retardation or developmental delays. Pediatrics. 2006;117(6):2304–2316. [PMID:16740881]
- Shea SE. Intellectual disability (mental retardation). Pediatr Rev. 2012;33(3):110–121. [PMID:22383514]
- Shevell M. Global developmental delay and mental retardation or intellectual disability: conceptualization, evaluation, and etiology. Pediatr Clin North Am. 2008;55(5):1071–1084. [PMID:18929052]
- Stankiewicz P, Beaudet AL. Use of array CGH in the evaluation of dysmorphology, malformations, developmental delay, and idiopathic mental retardation. Curr Opin Genet Dev. 2007;17(3):182–192. [PMID:17467974]
- Walker WO Jr, Johnson CP. Mental retardation: overview and diagnosis. Pediatr Rev. 2006;27(6):204–212. [PMID:16740804]
- 319 Unspecified intellectual disabilities
- 317 Mild intellectual disabilities
- 318.0 Moderate mental retardation
- 318.1 Severe intellectual disabilities
- 318.2 Profound intellectual disabilities
- F79 Unspecified intellectual disabilities
- F70 Mild intellectual disabilities
- F71 Moderate intellectual disabilities
- F72 Severe intellectual disabilities
- F73 Profound intellectual disabilities
- 91138005 Mental retardation (disorder)
- 86765009 Mild mental retardation (I.Q. 50-70) (disorder)
- 61152003 Moderate mental retardation (I.Q. 35-49) (disorder)
- 40700009 Severe mental retardation (I.Q. 20-34) (disorder)
- 31216003 Profound mental retardation (I.Q. below 20) (disorder)
- Q: Will my child be “normal” by adulthood?
- A: Generally, intellectual disability is considered a lifelong condition. Some individuals, usually with the milder form of intellectual disability, can function well in the community, especially when given added supports.
- Q: Can my child learn?
- A: Except for the most severe forms of intellectual disability, children do learn. This learning may not be as rapid or as extensive as that of a typically developing child.
- Q: But my child looks fine and has had appropriate motor development. How can he or she be mentally retarded?
- A: Mental retardation or intellectual disability is a slowed rate of cognitive development. Many children with intellectual disability do not have obvious dysmorphic features. Other streams of development, such as gross motor skills, may be reached on time or nearly so; yet, the cognitive developmental streams can be significantly delayed.
Rita Panoscha, MD
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