Intellectual Disability

© Springer-Verlag Italia 2015
Angelo Sghirlanzoni, Giuseppe Lauria and Luisa Chiapparini (eds.)Prognosis of Neurological Diseases10.1007/978-88-470-5755-5_17

17. Intellectual Disability

Stefano D’Arrigo , Enrico Alfei1 and Chiara Pantaleoni1

Developmental Neurology Department, IRCCS Fondazione Istituto Neurologico “C. Besta”, Via Celoria 11, Milan, 20133, Italy



Stefano D’Arrigo

Intellectual disabilityDevelopmental delayGenetic syndromes


AS, Angelmann syndrome; CNS, central nervous system; DD, developmental delay; DS, Down syndrome; ELN, elastin; FAS, fetal alcohol syndrome; FMR1, Fragile X mental retardation 1; ID, intellectual disability; IQR, interquartile range; MR, mental retardation; SMR, standardized mortality ratios; VCFS, Velo-cardio-facial syndrome

Key Facts

  • Terminology and definitions – ID is a “significant limitation both in intellectual functioning and in adaptive behavior.” Its onset is before 18 years of age and IQ score is less than 70

  • Clinical features – Prevalence of ID is 1–2 %. ID may be mild (IQ = 50–70) (85 % of the mentally disabled), moderate (IQ = 30–49) (10 % of the ID), severe (IQ = 20–29) (3–4 % of the ID) or profound (IQ < 20) (1–2 % of the ID). ID may be associated dysmorphisms and behavioral disorders

  • Diagnostic markers

    • Clinical – Often complex clinical syndromes and constellations

    • Blood – Targeted screening (hypothyroidism, fetal alcohol syndrome, etc.)

    • CSF – Targeted screening

    • GeneticKaryotype studies is abnormal in 4 %, microarray analysis is abnormal 7.8 % of pts. Mutations of X-linked genes may diagnose up to 10 % of ID

    • Imaging

      • CT – Diagnostic in 30 % of patients

      • MRI – Diagnostic in 48.6–65.5 % of patients

    • NeurophysiologyEEG – For epileptic syndromes

  • Top differential diagnoses – Childhood illnesses or injuries, hereditary disorders, infection, inborn errors of metabolism, and poisoning

  • Prognosis

    • Principles of treatment – Treatment is most often supportive

    • Disability – Median age of death decreases with the decrease of IQ.

    • Mild ID – Patients can become self-sufficient and in some cases live independently, with family support (median age of death = 68 years). Moderate ID – Can work and perform self-care tasks with moderate supervision (median age of death = 64 years). Severe ID – May acquire very basic self-care and some communication skills (median age of death = 59 years). Profound ID needs a high level of assistance (median age of death = 46 years)

17.1 Terminology and Definitions

The condition currently defined as “mental retardation” (MR) comprises a cluster of disorders characterized by low intelligence and limitations in adaptive behavior [1].

The disorder “Iintellectual disability” (ID) is defined as “significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills.” This disability originates before age 18 years and has widely replaced MR for policy, administrative and legislative purposes.

In early infancy, and in the first 5 years of life in particular, the condition termed developmental delay (DD) affects 5–10 % of children, and defines a clinical condition coinciding with a performance of at least 2 standard deviations below the mean for chronological age in at least two of the following areas: (1) global and fine motor control, (2) language, (3) cognition, (4) personal/social relationships, and (5) activities of daily living [2].

The severity of ID can be classified on the basis of the intelligence quotient (IQ) measured by standardized tests. Scores below 70 (2 standard deviations below the mean) represent the cut-off in identifying intellectually disabled individuals. ID can be distinguished as mild (IQ 50–70) (85 % of the mentally disabled), moderate (IQ 30–49) (10 % of the mentally disabled), severe (IQ 20–29) (3–4 % of the mentally disabled) or profound (IQ <20) (1–2 % of the mentally disabled) [1, 3].

IDs have a major impact on functioning and disability throughout the life course, and high comorbidity with other cognitive disorders. Frequently misdiagnosed, ID is associated with poor access to health care services and involves very high costs for the health care system and for society as a whole [4].

17.2 Epidemiology

ID is the most common developmental disorder. Its prevalence is around 1 % in high income countries and 2 % in low and middle income countries. On average, about 1 % of children aged from 3 to 10 years have ID. ID is more frequent in older children (ages 6–10 years) than in younger children (ages 3–5 years). ID is also more common in boys than in girls and in black than in white children. Mild ID accounts for 85 % of the mentally disabled, moderate ID for 10 %, severe ID for 3–4 %, and profound ID for 1–2 % of the mentally disabled [5, 6].

17.3 Clinical Features

Infants and children who have ID generally do not reach developmental milestones within the expected age range (e.g., sitting up, crawling or walking, talking or using coherent language). They also have other symptoms of cognitive impairment such as deficits in short-term memory, concept formation, understanding social rules, problems solving, and understanding cause and effect relationships [1].

17.4 Diagnosis

Genetic syndromes

(alias dysmorphic-genetic syndromes) are complex pathological constellations in which genetic abnormalities are often associated with mild to severe dysmorphisms, psychomotor delay, mental retardation, and behavioral disorders. They can be secondary to chromosomal abnormalities, numerical or structural, or to single gene mutations. Often, the underlying abnormality is unknown and diagnosis may be based exclusively on clinical data remembering that


The presence of a single minor abnormality cannot be deemed to have clinical significance



At least three typical features are needed for a correct diagnosis



The collection of photographic evidence is useful for reassessing a patient over time



Each diagnostic hypothesis can be assessed with the help of computerized systems (Possum database, London Dysmorphology) [1, 2]


Common causes of ID are


Genetic conditions (fragile X syndrome, Down syndrome, inborn errors of metabolism)



Pathogenic noxae during pregnancy or birth (e.g., congenital cytomegalovirus, fetal alcohol syndrome [FAS], etc.)



Anomalies affecting CNS at birth or soon after (e.g., hydrocephalus, asphyxia, etc.)



CNS syndromes (e.g., stroke, infections, traumas, etc.) during infancy, childhood, and adolescence


Many causes of ID can be prevented or appropriately treated after birth.

Thirty to forty percent of mental retardation is associated with dysmorphic-genetic syndromes that can be secondary to chromosomal abnormalities or gene mutations.

17.5 Clinical Markers

  • Blood – Targeted screening

  • Neuropsychological tests – Severity ID/DD is ascertained by means of neuropsychological tests: Griffiths Mental Developmental 0–2, and 2–8 years, Wechsler Scales (WPPSI, WISC III or WISC IV), Leiter-R test, and others.

  • Genetic – Karyotype studies are abnormal in at least 4 % of subjects with ID and in 18.6 % of patients with syndromic features [7].

Chromosomal microarray analysis (CMA) or microarray-based comparative genomic hybridization according to clinical phenotypes. Microarray testing is abnormal in 7.8 % of subjects with ID and in 10.6 % of patients with syndromic features [8, 9].

Mutations of X-linked genes may be diagnosed in up to 10 % of ID. Testing of XLID genes has a yield of 42 % in males from definitely X-linked families and of 17 % in males from possibly X-linked families. FMR1 testing has a combined yield of at least 2 % in male and female subjects with mild ID. MeCP2 mutations are found in 1.5 % of girls with moderate/severe ID and in less than 0.5 % of males with ID.

  • CT – Contributes to the etiological diagnosis of intellectual disability in approximately 30 % of patients (Class III studies).

  • MRI – Demonstrates abnormalities in 48.6–65.5 % of patients with global delay. The chance of positive results increases if clinical deficits are also present.

  • EEG – Suggests the presence of epilepsy and specific epileptic syndrome.

  • Top differential diagnoses – Childhood illnesses or injuries, hereditary disorders, infection, inborn errors of metabolism, and poisoning, etc.

17.6 Prognosis

17.6.1 Principles of Treatment

Treatment is most often supportive. With improved understanding of genetic and cellular mechanisms, novel treatment options are beginning to appear for a number of specific conditions. Fragile X and tuberous sclerosis offer paradigms for the development of targeted therapeutics, but advances in understanding of other disorders, such as Down and Rett syndromes, are also offering encouraging treatment directions.

In addition, better understanding of the underlying neurobiology is leading to novel developments in enzyme replacement for storage disorders and adjunctive therapies for metabolic disorders.

17.6.2 Disability and Mortality

Patients with developmental disabilities are at increased risk for behavioral and psychopathological disorders that impact on individuals, family functioning, and possibly on the wider community. For individuals, it is associated with reduced vocational, social and occupational opportunities, restricted participation in educational and recreational programs, and problematic placement in factories for disabled workers or even in residential communities. Child behavioral problems are associated with high ratings of parent stress and depressive symptoms. Moreover, the higher the ratings of behavioral and emotional disturbances in the child, the greater are the direct and indirect financial costs of care to parents.

Left untreated, behavioral problems in children with developmental disabilities are likely to persist, with evidence of only a small decline in the levels of emotional and conduct problems as children move into young adulthood [10].

People with ID experience a variety of health inequalities compared with the general population. They are more likely to die younger. Many studies have observed substantially higher standardized mortality ratios (SMR) in people with ID than in the general population, but the magnitude of these differences differed considerably. Much of the excess mortality in the ID population occurs in younger people (under the age of 40 years). People with more severe ID have particular disadvantage; an SMR of 8.4 has been reported in children and adolescents with moderate to profound ID compared with 1.4 for those with mild ID. The risk of dying at an early age was greatest for people with more severe intellectual disabilities, but the median age at death of people with mild intellectual disabilities (68 years) was still substantially younger than in the general population. Factors found to increase premature mortality in this population are a greater number of disabilities, reduced mobility, presence of epilepsy, presence of cerebral palsy, or presence of Down syndrome [11, 12]

17.6.3 Mild Mental Retardation (IQ 50–70)

Individuals with mild ID may have problems in reaching developmental milestones and acquiring language. With adequate training, most of these people will achieve full independence in most domains of function but may have some problems with adaptive integrative domain.

Most problems occur when they are matriculating through the educational system and developing interpersonal relationship skills with peers. Job coaches or aides may be necessary to help them in work routines and to develop solid employment skills. Most individuals (87 %) with mild ID will exhibit barely noticeable problems of learning; however, as the demands of academic work become more complex, differences will appear more pronounced.

Median age of death is inversely proportional to the severity of intellectual impairment and decreases when the intellectual disability is more pronounced. In mild ID, the median age of death is 68 years (IQR 58–77) [10].

17.6.4 Moderate Mental Retardation (IQ 30–50)

In addition to the deficits and needs of the above people, patients with moderate mental retardation may have additional insufficiencies in language expression and comprehension. They will need guidance and support throughout their lives. Their academic skills will always be limited and may not develop beyond a basic level.

Semi-independent living conditions are usually best and safest. They will need close supervision in employment endeavors but can be very dependable and loyal workers if given the appropriate structured tasks, training, and support. They can develop simple friendships and engage in appropriately supervised and developed physical activities. In moderate retardation, the median age of death is 64 years (IQR 52–75) [10].

17.6.5 Severe Mental Retardation (IQ 20–29)

As the degree of functioning decreases, the amount of needed support and supervision increases. Limitations in expected levels of achievement also increase. Problems with marked degrees of motor impairment or other associated deficits are prevalent. Clinically significant damage to or maldevelopment of the CNS is also a negative factor. These patients will need care and supervision to perform most activities of daily living. In severe retardation, the median age of death is 59 years (IQR 31–72) [10].

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Nov 10, 2016 | Posted by in NEUROLOGY | Comments Off on Intellectual Disability

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