Intellectual disability in children: Definition, diagnosis, and assessment of needs

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All topics are updated as new evidence becomes available and our peer review process is complete.

Literature review current through: Jul 2016. | This topic last updated: Apr 25, 2016.

INTRODUCTION — Intellectual disability (ID) is a neurodevelopmental disorder characterized by deficits in intellectual and adaptive functioning that present before 18 years of age [1]. ID is heterogeneous in etiology and encompasses a broad spectrum of functioning, disability, needs, and strengths.ID affects approximately 1 percent of the population. It is an important public health issue because of its prevalence and the need for extensive support services. Its management requires early diagnosis and intervention, coupled with access to health care and appropriate supports.

The definition and diagnosis of ID, including assessment of needs for support, are reviewed here. Other aspects of ID are discussed separately:

●(See "Intellectual disability in children: Evaluation for a cause".)

●(See "Intellectual disability in children: Management, outcomes, and prevention".)

 

DEFINITIONS

Intellectual disability — ID is a state of functioning that begins in childhood and is characterized by limitations in intelligence and adaptive skills. The term replaces and improves upon the older term of "mental retardation" [2]. A variety of other terms are used outside of the United States to describe ID (table 1).

Two definitions are commonly used in the United States. One is published by the American Psychological Association (APA), in the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5), and the other by the American Association on Intellectual and Developmental Disabilities (AAIDD). Although these definitions differed substantially in the past, current versions are similar [1,2]. Standardized intelligence quotient testing is no longer used to classify severity of impairment in ID.

ID is characterized by significant limitations in both of the following, with onset during the developmental period [1] (or before 18 years of age [2]):

●Adaptive behavior – Limitations in adaptive functioning are linked to underlying intellectual impairment and affect participation in multiple settings, such as home, community, and school. Adaptive deficits include limitations in at least one of three domains: conceptual, social, and practical (table 2A-B). Ongoing support is required as compared with others in the same age group. The severity of ID is defined according to the level of supports needed in each of these domains. (See 'Adaptive function' below.)

 

●Intellectual functioning – Limitations in general mental capacity (intelligence) including learning, reasoning, and problem solving, abstract thinking, and judgment. This limitation in intellectual ability typically corresponds to an intelligence quotient (IQ) that is 2 standard deviations or more below the mean; scores are typically less than 65 to 75. Although standardized IQ testing remains pertinent to the descriptive diagnostic profile, it is no longer used to classify the severity of impairment in ID. (See 'Intellectual function' below.)

 

The above definition of ID assumes that limitations in function must be assessed relative to the child's age, experience, and environment [1,2]. In addition, a valid assessment of adaptive and intellectual functioning considers differences in language and culture, as well as those in communication, motor, sensory, and behavioral factors. It is important to note that individuals with ID often have strengths as well as limitations. Defining ID severity by the level of support needed for adaptive functioning is valuable because appropriate individualized support usually will improve the life functioning of a person with ID. In addition, the combination of adaptive and intellectual function is more predictive of outcomes than intellectual function alone.

Syndromic versus nonsyndromic ID — ID may be further categorized as syndromic or nonsyndromic ID. The term syndromic ID is applied when a child presents with ID in addition to one or more clinical abnormalities or comorbidities of a known syndrome. The term nonsyndromic ID is usually applied when a child presents with ID alone [3]. The distinction between the two categories is not always clear cut if the associated clinical abnormalities are subtle.

Global developmental delay — Global developmental delay (GDD) is the preferred term to describe intellectual and adaptive impairment in children <5 years old, based on failure to meet expected developmental milestones in several areas of intellectual functioning [1]. Not all children with GDD will meet criteria for ID as they grow older. The term ID usually is applied to children ≥5 years old, when the clinical severity of impairment is more reliably assessed.

CLINICAL FEATURES

Presenting symptoms — Children with ID usually are brought to the attention of a pediatrician because of parental concerns of language delay, immature behavior and play, immature self-help skills, or difficulty in learning. Parents may first recognize delayed development when a younger sibling overtakes an older child in these skills. In other cases, the clinician may be alerted to the possibility of ID when a child fails to meet expected developmental milestones during developmental surveillance and screening.

Children with severe ID tend to come to medical attention earlier than those with mild ID. Most severely affected children are recognized before two years of age, whereas some children with mild ID may be undetected until school age. Individuals with a known genetic disorder and those presenting with obvious dysmorphic features and microcephaly may be diagnosed in infancy. (See "Microcephaly in infants and children: Etiology and evaluation".)

Most children with severe intellectual impairment will present with language delay. Language development is considered a reasonably good indicator of future intelligence in a child without hearing impairment. Thus, language delay associated with global developmental delay (GDD) suggests cognitive impairment. This should be distinguished from isolated familial expressive language delay (without other deficits), which has a more favorable prognosis. In contrast, gross motor skills in children with ID are often less significantly delayed than intellectual and adaptive function, except when neuromuscular abnormalities result in delay. (See "Etiology of speech and language disorders in children" and "Overview of expressive language delay ("late talking") in young children".)

Associated conditions — As the severity of ID increases, other conditions are more likely to be associated. Common problems that occur in children with severe or profound ID include medical conditions, neurodevelopmental disorders including autism and other behavioral disorders, and obesity.

Medical conditions — Medical conditions commonly associated with ID include seizure disorders, motor impairment affecting gross, fine, and speech motor functions, structural abnormalities, dysmorphism, and vision, hearing, and other sensory impairments. In some cases, these other morbidities are the presenting features, while in others they may be unrecognized. Comorbid endocrine abnormalities may also be present, such as abnormal thyroid function, short stature, and growth hormone deficiency [3-5].

Individuals with ID have greater rates of overweight and obesity as compared with typically developing peers. Obesity in ID poses significant health risk which increases with age. Risk factors that predispose to obesity include altered or inappropriate eating habits, underlying genetic syndromes, less physical activity, chronic health conditions, and psychotropic medication use. (See "Intellectual disability in children: Management, outcomes, and prevention", section on 'Monitoring for associated disorders'.)

When a genetic cause is identified, the clinician should evaluate the patient for known associated conditions (eg, hypothyroidism in patients with Down syndrome). Such comorbidities may require specific management and also may alter the approach to diagnostic testing for ID (if they affect optimal participation or require test modification).

Other neurodevelopmental disorders and mental health problems — Other neurodevelopmental disorders in addition to ID affect approximately 30 to 70 percent of children with ID, occurring five times more often than in children without ID [6]. However, these associated disorders may be underdiagnosed in ID because developmental delay (especially poor language skills), medical problems, andpersonal/family circumstances make it challenging to apply the diagnostic behavioral criteria described in the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM–5) [7]. These neurodevelopmental disorders adversely affect functioning, quality of life, and adaptation. Individuals with cognitive and adaptive impairments are more likely to have behavioral and emotional problems; however, some of these may not necessarily meet diagnostic criteria for a specific mental health disorder [8].

Neurodevelopmental disorders associated with ID are reviewed in the practice parameter of the American Academy of Child and Adolescent Psychiatry (AACAP) [7] and include the following:

●Autism – Autism and autism spectrum disorder (ASD) may occur with ID. The disordered social and communication skills that characterize ASD must be distinguished from the developmentally delayed social and communication skills that typically affect children with ID. (See "Autism spectrum disorder: Clinical features", section on 'Impaired social communication and interaction'.)

 

In addition, intelligence quotient (IQ) scores may be less stable where both ID and autism are present, particularly in young children. In one study, 28 percent of children with ID had comorbid autistic disorder, and half of these cases were not diagnosed during routine care [9]. Autistic disorder occurs more commonly in syndromic ID (eg, in 25 to 47 percent of those with fragile X syndrome, 5 to 10 percent of those with Down syndrome); individuals with tuberous sclerosis, Angelman syndrome, Rett syndrome, Joubert syndrome, and Cohen syndrome are also more likely to have comorbid autism [3,10].

 

●ADHD – Some children with ID also have attention deficit hyperactivity disorder (ADHD). ADHD must be distinguished from situational inattentiveness at school due to demands that may be too high, inability to comprehend and follow rules and expectations, or the effect of medication. (See "Attention deficit hyperactivity disorder in children and adolescents: Clinical features and diagnosis", section on 'Differential diagnosis'.)

 

●Learning disabilities – Varied difficulties in general and academic learning commonly occur with ID; these are pertinent to consider in each individual even if they are attributed to the underlying cause of the ID. For example, phonological difficulties frequently cause reading deficits in individuals with mild ID [11]. A specific learning disorder may be identified in addition to ID when a specific learning function is disproportionately affected, beyond expected for the child's ID profile, and not considered due to the underlying ID [1]. (See "Specific learning disabilities in children: Clinical features".)

 

●Eating disorders – Associated eating disorders include pica (eating substances that are not food) and rumination (regurgitation of undigested food into the mouth, during or shortly after eating). (See "Eating disorders: Overview of epidemiology, clinical features, and diagnosis", section on 'Pica' and "Eating disorders: Overview of epidemiology, clinical features, and diagnosis", section on 'Rumination disorder'.)

 

●Depression and anxiety – Depression, anxiety, and posttraumatic stress disorder (PTSD) are common comorbid conditions. Depression may be manifested as aggressive or irritable externalizing behaviors. Depression and/or anxiety can be triggered by relocation, caregiver changes, and effects of medication (eg, beta blockers, neuroleptic drugs) or associated conditions (eg, hypothyroidism). It may be difficult to assess children with ID for these problems due to limited communication skills. Children with ID are at increased risk of suicidal ideation and for substance abuse. Screening for these problems is advised.

 

●Physical and sexual abuse – Children with ID are at increased risk of being maltreated, victimized, manipulated, neglected, and abused, including physical, sexual, and emotional abuse. They may be vulnerable to abuse because of cognitive, learning, and communication deficits, gullibility, social naiveté, dependence on others, and the desire to please. Up to 14 percent of patients with ID admitted to hospitals have reported abuse [12]. Estimates of the prevalence of sexual abuse among children with ID range from 15 to 30 percent; however, sexual abuse may be under-identified in this population [13]. Female adolescents and children with mild ID (compared with more severely affected children) are at greatest risk of being sexually abused [13].

 

Individuals with ID are more likely to experience frequent and persistent abuse, multiple abusers, and greater involvement of unfamiliar or non-family perpetrators compared with those without ID [14]. However, perpetrators are most commonly a male member of the immediate or extended family or a peer residing in the same living facility [13]. Children with ID who have suffered sexual abuse are also at greater risk of becoming a perpetrator [13].

 

Abuse may cause other behavioral disorders, such as conduct disorder, PTSD, and depression. Sensitivity and a high index of suspicion for abuse are needed. Despite communication deficits, adolescents with severe ID often are able to disclose their victimization and abuse [12].

 

Physical and sexual abuse and maltreatment in children and adolescents are discussed in greater detail in separate topic reviews. (See "Physical abuse in children: Epidemiology and clinical manifestations" and "Evaluation of sexual abuse in children and adolescents" and "Child neglect and emotional maltreatment".)

 

●Movement disorders – Stereotyped behaviors, stimulating movement and motor mannerisms, including tic disorders, are common in severe ID. (See "Hyperkinetic movement disorders in children", section on 'Stereotypies'.)

 

●Self-injurious behaviors – Self-injurious behavior frequently occurs and may suggest syndromes such as Lesch-Nyhan. Self-injurious and aggressive behaviors can also occur in individuals who have limited communication who are experiencing stress, depression, or anxiety, or may result from side effects of sedative-hypnotic and neuroleptic medications.

 

CLINICAL EVALUATION — Developmental surveillance should be performed at well-child visits with targeted screening at selected visits (9, 18, and 24 or 30 months), and whenever a parent or provider raises developmental concerns [15]. The goal is early identification of children with possible developmental delay or ID and prompt referral for further evaluation and early intervention services. (See "Developmental-behavioral surveillance and screening in primary care", section on 'When to perform developmental-behavioral screening'.)

Screening — A number of standardized screening tools are available (table 3). These vary in their sensitivity and specificity. The following are some of the screening tools in common use. Details about these tests, including their accuracy in identifying infants or children with developmental delays, are discussed in a separate topic review. (See "Developmental and behavioral screening tests in primary care".)

●Ages and Stages Questionnaires (ASQ) – The ASQ-3 provides age-specific questions about the child that are completed by the child's parent in 10 to 15 minutes in English or Spanish; these can be used to screen development in children from 1 month to 5.5 years of age. These questionnaires are used in clinical and research settings and applied in some community screening and intervention programs [15].

 

●Bayley Infant Neurodevelopmental Screener (BINS) – The BINS can be used to screen development in children 3 to 24 months of age; this tool takes 10 to 15 minutes to administer. It applies directly elicited items per three- to six-month age range to screen neurologic processes (reflexes and tone) and receptive and expressive language, cognitive, and motor functions.

 

●Brigance Early Childhood Screens-III – The Brigance Early Childhood Screens-III can be used to screen development in children from zero to five years of age and takes 10 to 15 minutes to administer. This tool screens language, motor, self-help, social-emotional, cognitive, or academic skills.

 

●Infant-Toddler Checklist (Communication and Symbolic Scale Developmental Profile) – This tool can be used to screen communication and symbolic behavior in children 6 to 24 months of age [16,17]. It includes early screening of gestures, eye gaze, and object use and can generate concern for atypical social communication development.

 

●Parents' Evaluation of Developmental Status (PEDS) – The PEDS can be used as a brief developmental screen in children from zero to eight years of age. This tool includes eliciting of parent concerns, and it can be swiftly completed in various languages. In addition, it provides longitudinal surveillance, helping to determine when referrals are necessary and when patient education, in-office counseling, watchful waiting, or additional screening is needed.

 

Detailed evaluation — When a screening test suggests developmental delay, further evaluation is needed. This includes a focused history and physical examination that elicits characteristics that suggest a specific cause of the ID, and examines for possible co-morbidities or associated conditions. The history should incorporate a three-generation family pedigree. The evaluation also includes a comprehensive developmental assessment, including standardized testing of intellectual and adaptive functioning and assessment of behaviors. Evaluations should apply both assessment tools and clinical judgment to establish the diagnosis of ID and to evaluate the child's needs for support and services. (See "Intellectual disability in children: Evaluation for a cause" and 'Diagnosis of ID and needs assessment' below.)

A multidisciplinary approach is recommended, while still stressing the importance of the medical home and collaboration with the family. Throughout the evaluation, it is important to use skilled professional communication and allow sufficient time for the interview, to elicit parent concerns, provide genetic counseling, and convey sufficient information to enable optimal understanding and informed collaborative decision-making with the child's parents [18].

●Referral to a developmental pediatrician, pediatric neurologist, and/or psychologist is usually needed, for a comprehensive neurodevelopmental evaluation. This includes cognitive-adaptive and behavioral assessments to determine the need for intellectual, adaptive and behavioral supports. A comprehensive school-based evaluation can also help determine educational needs and generate individual support.

 

●Speech, language, communication, and adaptive functioning should be formally assessed. Adaptive functions are assessed in three domains of conceptual, social, and practical functioning, so as to identify what ongoing supports are needed individually. (See "Evaluation and treatment of speech and language disorders in children".)

 

●A child psychiatrist can help evaluate the child's psychopharmacologic needs in more complex comorbidities.

 

●Occupational and physical therapists can assist in assessing functional impairments, strengths, and needs.

 

●A social worker can assist in assessment of the family and family needs, provide counseling, and generate social supports and contribute to long-term planning.

 

●A geneticist can evaluate for suspected genetic disorders and provide genetic counseling.

 

Various other specialists may be necessary to help evaluate and manage other associated conditions. Examples include:

●A speech-language pathologist for children with swallowing dysfunction and meal-related behavior problems. (See "Aspiration due to swallowing dysfunction in infants and children".)

 

●A gastroenterologist for children with gastroesophageal reflux disease or constipation. (See "Clinical manifestations and diagnosis of gastroesophageal reflux disease in children and adolescents" and"Constipation in infants and children: Evaluation".)

 

●An ophthalmologist for children with vision impairment. (See "Visual development and vision assessment in infants and children".)

 

●An audiologist and otolaryngologist for children with hearing impairment. (See "Hearing impairment in children: Evaluation".)

 

A stepwise approach is favored according to the needs of the individual. There is no widely accepted, standardized comprehensive approach to the clinical, adaptive, intellectual, and behavioral evaluation of a child or adolescent with ID, so as to ascertain their needs for support. The nature and timing of evaluations vary based upon individual patient characteristics.

History — The first step of the evaluation is a comprehensive history and physical examination. In three series, the history and physical examination identified the etiology of ID or developmental delay in 17 to 34 percent of cases [19-22].

Ample time should be allocated for the evaluation. This should include an opportunity for the parents to describe the perceived needs of their child, and communicate their concerns, perceptions, and misconceptions [18].

Details of the child's medical, developmental, behavioral, social, and educational history should be elicited from the parents or caretakers, as well as a record of medications, treatments, and the extent and nature of the support services. Developmental progress, stagnation, or regression should be elicited. Medical problems should be ascertained including history of seizures, ataxia, weakness, obesity, and birth defects. Results of any previous newborn metabolic and thyroid testing, neuroimaging, lead and iron screening, growth records, and vision and hearing surveillance should be reviewed. Features that are characteristic of a specific cause of ID or behaviors of other neurodevelopmental disorders should be noted. (See 'Other neurodevelopmental disorders and mental health problems' above.)

The evaluation also should include a detailed family history inquiring about any close family member with a neurodevelopmental or genetic disorder (including three generations), whether the mother has a history of miscarriages or stillbirth/neonatal death, and whether there is any known consanguinity. The prenatal, perinatal, and neonatal course should be reviewed, including an inquiry about alcohol intake during pregnancy, medications, infections, or further environmental exposures (their timing and extent), as well as a complete review of systems and immunization needs. (See "Intellectual disability in children: Evaluation for a cause".)

Questions regarding quality of life are important and a history of community or recreational participation and friendship can provide further insight into the needs of a person with ID. Questions or measures of family psychosocial factors, or parent depression may further delineate needs. Results of prior developmental, psychological, and psychiatric evaluations should be obtained [7].

Disorder-specific questions or screens can help assess for more frequent comorbidities, such as autism spectrum disorder and attention deficit hyperactivity disorder, where identification and interventions improve everyday functioning (table 3). (See "Autism spectrum disorder: Screening tools" and "Attention deficit hyperactivity disorder in children and adolescents: Clinical features and diagnosis".)

Additional screening questions that are important in the assessment of children with ID include the following:

●Questions about sleep quality and sleep problems (which may include daytime sleepiness or features of obstructive sleep apnea) (see "Assessment of sleep disorders in children")

 

●Targeted substance abuse questions (particularly for mildly impaired school attendees, from pre-adolescent age onwards) (see "Screening tests in children and adolescents", section on 'Alcohol and substance use')

 

●Targeted questions to assess for depressive symptoms and suicidality (see "Screening tests in children and adolescents", section on 'Depression screening')

 

●A high index of suspicion for abuse and neglect is warranted, particularly in verbally inexpressive children (see "Physical abuse in children: Epidemiology and clinical manifestations" and "Child neglect and emotional maltreatment")

 

Physical examination — A physical examination may help establish the etiology of ID and identify associated conditions. The examination should be comprehensive, with specific attention to the following (see "The pediatric physical examination: General principles and standard measurements"):

●Measurements of height, weight, and head circumference, including growth velocity and the use of syndrome-specific growth charts. Micro- or macrocephaly and short or tall stature are important growth parameters that are useful for evaluation of syndromic forms of ID.

 

●Dysmorphic features that may suggest genetic or syndromic etiologies to guide selection of genetic tests. (See "Intellectual disability in children: Evaluation for a cause", section on 'Testing for specific disorders'.)

 

●Examination of the ears, nose, and throat, including hearing assessment. (See "The pediatric physical examination: HEENT".)

 

●Eye examination, including, visual acuity, visual fields, extra-ocular movements, strabismus, and evaluation of the fundi. (See "The pediatric physical examination: HEENT".)

 

●Assessment of the heart, lungs, abdomen, genitourinary system, back, extremities, and skin. Cutaneous findings that are important to note include café-au-lait macules (suggestive of neurofibromatosis) (picture 1), ash-leaf spots (suggestive of tuberous sclerosis) (picture 2), and bruises or other signs suggestive of physical abuse. (See "The pediatric physical examination: Back, extremities, nervous system, skin, and lymph nodes" and "The pediatric physical examination: Chest and abdomen" and "The pediatric physical examination: The perineum" and "Neurofibromatosis type 1 (NF1): Pathogenesis, clinical features, and diagnosis", section on 'Clinical manifestations' and "Tuberous sclerosis complex: Genetics, clinical features, and diagnosis", section on 'Clinical features' and "Physical abuse in children: Epidemiology and clinical manifestations", section on 'Clinical manifestations'.)

 

●Complete neurologic and neurodevelopmental assessment. (See "Detailed neurologic assessment of infants and children".)

 

●Detailed observation of the child's behavior, including attention, impulsivity, activity, affect, motor mannerisms, disordered social communication, internalizing (eg, anxiety, depression) and externalizing behaviors (eg, oppositional and aggressive behavior), and behavioral phenotype.

 

●Parent/family interactions and behaviors may be observed for any features suggesting parental depression, parenting characteristics, or suspicion of child abuse. Parental stress, depression, and other needs may be elicited by parent-report measures. In some cases, parents themselves are affected by a genetic or neurodevelopmental disorder and exhibit signs and symptoms relevant to the child's evaluation, diagnosis, and intervention.

 

Sensory screening — Children with global developmental delay and/or ID are at high risk for sensory impairments. Vision disorders affect 13 to 50 percent of patients with ID, and approximately 18 percent have hearing impairment [23]. Thus, assessment of vision and hearing is an essential part of the initial evaluation. This may include a complete ophthalmologic examination and audiometry, preferably using brainstem auditory evoked response. (See "Visual development and vision assessment in infants and children" and "Hearing impairment in children: Evaluation".)

DIAGNOSIS OF ID AND NEEDS ASSESSMENT — ID is diagnosed by documenting significant limitations in both adaptive and intellectual function with onset during the developmental period [1,2]. The steps to making the diagnosis of ID and assessing its severity will be discussed here. The evaluation of a child for underlying causes of ID is discussed in a separate topic review. (See "Intellectual disability in children: Evaluation for a cause".)

Adaptive function — The diagnosis of ID requires the presence of deficits in both intellectual and adaptive functioning; the adaptive impairment must be directly related to the intellectual impairment [1]. The severity of ID is also defined by the degree of impairment in adaptive functioning, rather than by an intelligence quotient (IQ) score. Although both intellectual and adaptive impairment measures are pertinent in describing ID, impaired adaptive functions are more likely to be the presenting feature of ID than low IQ; impaired adaptive skills that affect activities of daily living and the child's ability to respond to common situations are more readily perceived than impaired intellectual functioning.

The diagnosis of ID requires impaired functioning in at least one of the following three domains, affecting participation in multiple settings (eg, home, community and/or school) and requiring ongoing support (table 2A) [1,2]:

●Conceptual domain – These skills include language, reading, and writing (literacy); money, time, and number concepts (mathematics); reasoning; memory; self-direction; and judgment in novel situations.

 

●Social domain – These skills include interpersonal social communication, empathy, ability to relate to peers as friends, and social problem-solving. Social responsibility, self-esteem, gullibility, and the ability to follow rules and avoid being victimized may also be included.

 

●Practical domain – These skills include activities of personal care or daily living, such as eating, dressing, mobility, and toileting. Further skills may include following a schedule or routine, using a telephone, managing money, preparing meals, occupational skills, and abilities in transportation/travel, health care, and safety.

 

Both clinical assessment and individualized standardized testing are used to delineate adaptive functioning [1]. Impaired function is defined by a score or performance at least 2 standard deviations below the mean on a standardized test assessing these domains [2]. The symptoms of adaptive impairment need to have begun during child development. The severity of the ID is defined according to the supports that are needed in each of these domains of adaptive functioning. The "support intensity" may vary from intermittent to pervasive [2].

Standardized assessment tools using established developmental criteria are available to measure adaptive function and provide a general or total adaptive composite score, as well as adaptive domain scores or subscale measures. Tools vary in the adaptive domains, functions, and skills that are measured and in the reliability of scores within adaptive domains. The most commonly used tool is the parent-reported Vineland Adaptive Behavior Scale (the Vineland-III is the most recent version, although the Vineland-II is also widely used), which can be administered in English or Spanish.

Other tools include the American Association on Intellectual and Developmental Disabilities (AAIDD)'s Diagnostic Adaptive Behavior Scale (DABS), the Woodcock-Johnson Scales of Independent Behavior-Revised (SIB-R), and the Adaptive Behavior Assessment System (ABAS-3rd Edition, or ABAS-III).

The instrument that is used to measure adaptive functioning must be appropriate for the child's age, gender, socioeconomic, experience, culture, and disability profile, and the results must be interpreted accordingly. As an example, normative values on an adaptive scale may be invalidated by a child's disability or medical condition. The assessment must also be appropriate for chronological and developmental age, as not all adaptive skills are applicable to young children. The instrument needs to be able to identify adaptive impairment as defined by the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) criteria in order for an ID diagnosis to be made. The ability to detect the impairment of a single adaptive domain (ie, conceptual, social, or practical) is necessary for applying the DSM-5 criteria to children whose adaptive impairment is limited to one domain. Adaptive impairments should impact multiple settings, limiting the child's participation. Clinical assessment and interpretation are vital to the adaptive assessment of an individual with ID.

Intellectual function — In addition to impaired adaptive functioning as described above, the diagnosis of ID requires the presence of impaired intellectual functioning, which includes learning, reasoning, and problem solving, abstract thinking, and judgment. Both clinical assessment and individualized standardized testing are required to confirm deficits in intellectual functioning [1]. The degree of impairment in intellectual function that is consistent with a diagnosis of ID typically corresponds to an IQ less than 65 to 75.

An IQ score provides a useful description of a child's ID. However, the IQ score is no longer applied as the diagnostic measure of severity, and is not used alone to drive clinical or legal decision-making. Moreover, IQ determination alone is an imprecise measure of a child's true intellectual abilities. The intellectual profile obtained by comprehensive developmental assessment is frequently more helpful than a composite or global IQ score, and generates a better description of individual strengths, and impaired functions that need intervention.

Because the diagnosis of ID requires both intellectual and adaptive deficits, a child with an IQ less than 70 unaccompanied by an adaptive deficit would not be considered to have ID, while one with an IQ between 70 and 75 that is accompanied by significant adaptive deficit would be diagnosed with ID. Although most children with an IQ less than 70 have adaptive deficits, their adaptive skills may improve with appropriate interventions; over time, they may no longer meet the severity of adaptive impairment criteria needed for an ID diagnosis.

Standardized intelligence tests — Intellectual function is typically measured by the administration of standardized tests to compare measured performance to that expected for age. The most commonly used tests for children are the Wechsler Scales. The instrument used for testing intellectual function must be appropriate for the child's level of adaptive function. In order to be valid, the test must take into account the child's age, culture, language, socioeconomic status, and profile of motor, sensory, and communication functioning [1]. Tests must use currently applicable norms, as out-of-date norms falsely inflate scores ("Flynn effect"). (See 'Adaptive function' above.)

The following are some of the commonly used tests to assess children over specific age ranges:

●Wechsler Preschool and Primary Scale of Intelligence, 4th edition (WPPSI-IV) – Ages 2 years 6 months to 7 years 7 months [24]

 

●Wechsler Intelligence Scales for Children (WISC-V) – Ages 6 to 16 years 11 months [24]

 

●Wechsler Adult Intelligence Scale (WAIS-IV) – Ages 16 to 90 years [24]

 

●Stanford-Binet Intelligence Scales (SB-5) – Ages 2 to 85 years [25]

 

●Kaufman Assessment Battery for Children (KABC-II) – Ages 3 to 18 years [24]

 

●Differential Abilities Scales (DAS-II) – Ages 2 years 6 months to 17 years [24]

 

●Leiter International Performance Scale, 3rd Edition (Leiter-3) – Ages 3 to 75 years; this evaluates nonverbal intelligence [26]

 

●Test of Nonverbal Intelligence-4th Ed (TONI 4) – Ages 6 to 89 years [24]

 

Evaluation of individuals with suspected ID during infancy and early childhood is valuable to detect and define discrepancies needing early intervention. However, early assessment is not considered accurate in predicting long-term future intelligence [23,27]. With appropriate supportive services, some mildly affected children progress into typical range functioning by five years of age and do not meet criteria for ID.

Tests used to evaluate infants or children under two years of age include the Bayley Scales of Infant Development and the Griffiths Mental Development Scales. The Bayley Scales are widely used as a standardized test of infant mental and psychomotor development from 1 to 42 months of age [24]. The Griffiths scales, revised in 1996, are widely used internationally for the standardized comprehensive evaluation of children 0 to 24 months of age [28]. Griffith's criteria have been restandardized, extended, and revised for children from zero to eight years old; the Griffiths-III (restandardized in 2015) will enable continuous use in children from 1 to 71 months of age (5 years 11 months) [28,29]. The Griffiths criteria are applicable for low functioning infants or children with global developmental delay or ID, including those with severe or profound impairment.

Interpretation and limitations of IQ testing — Appropriate interpretation of the results is essential. An intelligence quotient (IQ) score that is at least 2 standard deviations below the mean indicates significant intellectual impairment. The standard error of measurement is approximately five points, although it varies with the instrument used. This means that for a child whose Wechsler-derived IQ score is 70, the true IQ may range from 65 to 75.

Global IQ scores may be invalid if there are highly discrepant scores on the subtests. IQs derived from different tests are not interchangeable because of differences in the measurement error and the particular skills that are tested. Another limitation is that a child's test results may change over time [30]. As a result, while they provide information about abilities measured at the time of the test, they are not necessarily an accurate measurement of a child's long-term potential. IQ measures are less valid at lower values, and out-of-date norms falsely inflate scores ("Flynn effect").

Classification of severity — Although ID is highly heterogeneous and comprises a diverse spectrum of functioning, strengths, weaknesses, impairments, disabilities, and needs, it is helpful to recognize categories of severity when assessing needs for supports. The severity of ID is currently defined according to the level of support needed to address impaired adaptive functioning in one or more settings (eg, school, home, work). The DSM-5 describes characteristic impairments in ID that affect one or more adaptive domains (conceptual, social, or practical), and categorizes levels of severity within a range of mild to profound (table 2B) [1]. The AAIDD uses a scheme that is similar except that it classifies severity by describing the supports needed as intermittent, limited, extensive, and pervasive [2].

In the past, ID severity was categorized based on intellectual functioning alone, using the following ranges [31]:

●Mild – IQ between 50 to 55 and 70

●Moderate – IQ between 35 to 40 and 50 to 55

●Severe – IQ between 20 to 25 and 35 to 40

●Profound – IQ less than 20 to 25

 

SUMMARY AND RECOMMENDATIONS

●Intellectual disability (ID) is characterized by significant limitations both in intellectual functioning and in adaptive functioning that affect everyday social, conceptual, and practical functioning, with onset in childhood (before 18 years of age). The term is preferred over the former term, mental retardation. (See 'Intellectual disability' above.)

 

●Global developmental delay (GDD) is the term applied to children under five years of age who fail to meet expected developmental milestones and have significant impairments in several areas of intellectual functioning. Not all children with GDD will meet criteria for ID as they grow older. (See 'Global developmental delay' above.)

 

●The cause of ID and the severity of impairment affect when and how a child presents with ID. Children with severe ID present earlier than those with mild ID. Most children with intellectual impairment present with language delay and disorder or delay in other developmental domains. Gross motor skills in affected children are often relatively preserved. (See 'Clinical features' above.)

 

●Problems that are commonly associated with most severely affected children with ID include seizure disorders, motor impairments, and vision, hearing, and other sensory impairments, as well as neurodevelopmental disorders including autism, attention deficit hyperactivity disorder, depression and anxiety, and self-stimulating or self-injurious behaviors. Children with ID are also at increased risk of being victimized or abused. (See 'Associated conditions' above.)

 

●A number of standardized tests are available to screen development and behavior for features of GDD or ID and their frequently associated conditions (table 3). When a screening test suggests developmental delay, further evaluation is needed. Evaluation for ID includes a comprehensive developmental assessment, including standardized testing of intellectual and adaptive functioning and assessment of behaviors. These assessments are used to establish the diagnosis of ID and to evaluate the child's needs for supports and services. (See 'Screening' above and 'Detailed evaluation'above.)

 

●ID is diagnosed when there are significant deficits in both intellectual and adaptive function, which are measured using clinical assessment and standardized tests. The adaptive deficits are directly associated with the underlying intellectual impairment. Selection of the appropriate testing instruments depends on the child's age, gender, culture, language, socioeconomic status, and neurodevelopmental profile including motor, sensory, and communication functioning. (See 'Intellectual disability' above.)

 

•Tests of adaptive function are used to assess for deficits in conceptual skills, social skills, and practical daily living skills (table 2A). A diagnosis of ID requires impaired functioning in at least one of these domains, across multiple environments, requiring ongoing support. (See 'Adaptive function' above.)

 

•The severity of ID is defined by the degree of impairment in adaptive functioning rather than by intellectual function or IQ score (table 2B). (See 'Classification of severity' above.)

 

•For tests of intellectual function, significantly impaired intellect is considered when IQ scores are 2 standard deviations below the mean or more, with values typically less than 70. (See 'Intellectual function' above.)

 

●The evaluation of a child for underlying causes of ID is discussed in a separate topic review. (See "Intellectual disability in children: Evaluation for a cause".)

 

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