INTRODUCTION — Intellectual disability (ID) is a neurodevelopmental disorder with multiple etiologies. It is characterized by deficits in intellectual and adaptive functioning of varying severity presenting before 18 years of age . ID encompasses a broad spectrum of functioning, disability, needs, and strengths. It is an important public health issue because of its prevalence and the need for support services. ID affects approximately 1 percent of the population. Its management requires early diagnosis and intervention, including access to health care and appropriate supports.
The definition and diagnosis of ID, as well as assessment of needs for support, are reviewed here. Other aspects of ID are discussed separately:
Global developmental delay — Global developmental delay (GDD) is the preferred term to describe intellectual and adaptive impairment in infants and young children <5 years old who fail to meet expected developmental milestones in multiple areas of functioning. GDD may be used as a provisional diagnosis until comprehensive standardized testing can be accurately and reliably completed . Not all children with GDD meet criteria for ID as they grow older.
Intellectual disability — Intellectual disability (ID) is a neurodevelopmental disorder that begins in childhood and is characterized by limitations in both intelligence and adaptive skills, affecting at least one of three adaptive domains (conceptual, social, and practical), with varying severity (table 1A-B). The extent of adaptive impairment is key to defining ID and its severity. The term ID replaces the older term of "mental retardation" .
As discussed above, the term ID usually is applied to children meeting criteria at approximately five years of age, while GDD is the preferred term in children <5 years. However, a diagnosis of ID can be made earlier if the child meets ID criteria. In some cases, this can be important for obtaining appropriate supportive services.
Syndromic versus nonsyndromic intellectual disability — ID may be further categorized as syndromic or nonsyndromic ID. The term syndromic ID is applied when intellectual and adaptive impairment occurs with other physical or comorbid symptoms that may be recognizable as a syndrome. When ID occurs without features of a possible recognizable syndrome, the term nonsyndromic ID is used .
EPIDEMIOLOGY, RISK FACTORS, AND CAUSES — In the general population, the prevalence of global developmental delay (GDD) is estimated to be 1 to 3 percent and the prevalence of ID is approximately 1 percent; most cases are mild [4-6].
The causes of ID are extensive and include conditions that interfere with brain development and functioning. Among the known causes of ID, the majority are genetic conditions. A minority of cases are due to acquired causes such as teratogens, toxins, infections, trauma, birth asphyxia, and nutritional deficiencies.
The epidemiology and causes of ID are discussed in greater detail separately. (See "Intellectual disability in children: Evaluation for a cause", section on 'Epidemiology' and "Intellectual disability in children: Evaluation for a cause", section on 'Causes'.)
Presenting symptoms — The cause of ID and the severity of impairment affect when and how a child presents. Mild delays are more common than severe delays, and gross motor functions are usually relatively spared.
Children with severe ID present earlier than those with mild ID. Severely affected children are usually recognized before two years of age. Most children with mild impairment present with delayed language or are detected by routine developmental surveillance and screening. However, some children with mild ID may be undetected until school age. (See "Developmental-behavioral surveillance and screening in primary care".)
Patients with specific dysmorphic features due to a genetic syndrome (eg, Down syndrome) may be diagnosed in infancy. (See "Down syndrome: Clinical features and diagnosis".)
Parental concerns that may alert the pediatrician to the possibility of ID include:
●Language delay – Language delay with global developmental delay (GDD) suggests cognitive impairment but should be distinguished from primary language disorder (persistent difficulty acquiring or using language) or autism spectrum disorder (ASD) with or without ID. (See "Etiology of speech and language disorders in children" and "Autism spectrum disorder: Evaluation and diagnosis".)
●Immature behavior, play, or self-help skills – Social skills/interrelatedness generally correlate with the child's level of cognitive functioning.
●Learning difficulties. (See "Specific learning disabilities in children: Evaluation".)
●A younger sibling "overtaking" the child.
Comorbid neurodevelopmental disorders and mental health problems — Other neurodevelopmental disorders are approximately five times more likely in children with ID compared with their peers. Comorbid neurodevelopmental and mental health disorders affect approximately 30 to 70 percent of children with ID but often go unrecognized. This can adversely impact learning, functioning, quality of life, and adaptation. Behavioral and emotional problems are more likely, even if impairment is insufficient to meet diagnostic criteria for a specific mental health disorder [7,8].
Neurodevelopmental disorders associated with ID include the following (table 2):
●ASD – ASD commonly co-occurs with ID. In one study, 28 percent of children with ID had comorbid ASD; one-half of these cases were not diagnosed during routine care . When both ID and ASD are present, intelligence quotient (IQ) scores are less stable, particularly in young children.
The diagnostic features of ASD, including impairments in social communication, must be identified and distinguished from social and communication delays of ID alone. The child with concurrent ASD usually "misses" specific social communication and social interaction milestones or features typically expected at or by the child's general developmental level. Some children may have behaviors that appear autistic (eg, stereotypic hand movements, social anxiety, gaze aversion with strangers) but may not meet the full diagnostic criteria for ASD. However, characteristic ASD features can become more obvious over time. (See "Autism spectrum disorder: Clinical features", section on 'Impaired social communication and interaction'.)
ASD occurs commonly in syndromic ID, particularly fragile X syndrome, in which the prevalence of ASD ranges from 25 to 47 percent [3,10,11]. ASD is somewhat less common in children with Down syndrome (<10 percent) . Tuberous sclerosis, Angelman syndrome, Rett syndrome, Joubert syndrome, and Cohen syndrome are also associated with having comorbid ASD [3,10]. (See "Autism spectrum disorder: Terminology, epidemiology, and pathogenesis", section on 'Associated conditions and syndromes'.)
●Attention deficit hyperactivity disorder (ADHD) – Children with ID may have comorbid ADHD, with inattentiveness incongruent to their developmental level of functioning. ADHD is common in children with Down syndrome (44 percent in one study) . It is the most common neurobehavioral diagnosis in boys with fragile X syndrome. In children with ID, ADHD may occur along with other comorbid neurodevelopmental or learning disorders . ADHD must be distinguished from situational inattentiveness (ie, apparent only in certain circumstances such as academic learning), difficulty comprehending and following rules and expectations, anxiety, medication side effects, or simply demands that are too high. (See "Attention deficit hyperactivity disorder in children and adolescents: Clinical features and diagnosis", section on 'Differential diagnosis'.)
●Learning disabilities – Children with ID commonly have difficulties in general and academic learning. These are pertinent to consider individually even if they are attributed to ID (eg, phonologic difficulties frequently cause reading deficits in individuals with mild ID) . When learning function is disproportionately affected, beyond what is expected for the child's ID, and not explained by another factor (eg, vision or hearing impairment), the child is considered to have a specific learning disorder in addition to ID . (See "Specific learning disabilities in children: Clinical features".)
●Feeding and eating disorders – Abnormal feeding and eating behaviors include pica (persistently eating substances that are not food) and rumination disorder (repeated 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 – Depressive and anxiety disorders and posttraumatic stress disorder (PTSD) are frequent treatable comorbid conditions in children and adolescents with ID. Depression may present as aggressive or irritable behavior, fatigue, diminished performance, changes in sleep, or weight gain. Clinical suspicion and assessment aided by annual adolescent screening for depression are important . 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). Assessing severely impaired children with limited communication can be challenging, and atypical presentation is more likely with increasing ID severity. Children with ID are at increased risk of suicidal ideation and for substance abuse. Screening is advised to identify these disorders, which impact functioning.
●Movement disorders – Stereotyped behaviors, stimulating movement, and motor mannerisms are common in severe ID. Stereotypic movement disorder (including possible self-injury) affects up to approximately 15 percent of children with ID. Stereotypic movements are more common among children with severe or syndromic ID (eg, Rett syndrome). Triggers of stereotypic movements may include social isolation, stress, pain, anxiety, fear, sensory impairment, or disordered sleep . Developmental coordination disorder can also impact everyday motor tasks. (See "Hyperkinetic movement disorders in children", section on 'Stereotypies' and "Developmental coordination disorder: Clinical features and diagnosis".)
●Self-injurious behaviors – Self-injurious and aggressive behaviors occur commonly in individuals with ID who have limited communication when they are experiencing stress, depression, or anxiety . Aggressive or self-injurious behavior can also result from side effects of sedative-hypnotic and neuroleptic medication. Self-injurious behavior is also a characteristic feature of Lesch-Nyhan syndrome. (See "Hyperkinetic movement disorders in children", section on 'Lesch-Nyhan syndrome'.)
Medical and physical conditions — Numerous medical and physical conditions are commonly associated with ID (table 2). Care should be taken to promptly identify these conditions. When a genetic cause is identified, the patient should be evaluated for associated comorbid conditions known to be associated with the specific syndrome (eg, hypothyroidism in patients with Down syndrome).
Medical and physical conditions commonly associated with ID include the following (table 2):
●Cerebral palsy. (See "Cerebral palsy: Clinical features and classification".)
●Congenital heart disease (eg, atrioventricular canal defect in Down syndrome). (See "Clinical manifestations and diagnosis of atrioventricular (AV) canal defects".)
●Constipation. (See "Constipation in infants and children: Evaluation".)
●Dental caries – Early regular orodental health care is important in children with ID . This should include interventions to improve dental hygiene and gingival health, reduce bruxism (teeth grinding), and prevent dental trauma . An array of orodental pathologies can contribute to behavioral disturbances children with ID or be a missed cause of pain in an impaired child. (See "Preventive dental care and counseling for infants and young children".)
●Endocrine abnormalities (eg, hypothyroidism, short stature). (See "Acquired hypothyroidism in childhood and adolescence" and "Diagnostic approach to children and adolescents with short stature".)
●Gastroesophageal reflux disease. (See "Clinical manifestations and diagnosis of gastroesophageal reflux disease in children and adolescents".)
●Hearing loss. (See "Hearing loss in children: Screening and evaluation" and "Hearing loss in children: Treatment".)
●Lead poisoning. (See "Childhood lead poisoning: Clinical manifestations and diagnosis".)
●Obesity – Overweight and obesity more commonly impact youth with ID than their typically developing peers; the risk increases with increasing age [20-23]. 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 [23,24]. Children with ID should be monitored with serial measures of weight, height, and body mass index and by review of risk factors. (See "Clinical evaluation of the child or adolescent with obesity".)
●Sleep disorders (eg, behavioral sleep problems, obstructive sleep apnea). (See "Behavioral sleep problems in children" and "Evaluation of suspected obstructive sleep apnea in children".)
●Undescended testes – Boys with ID should be examined again for undescended testes at primary school entrance age (especially if they have increased motor tone) . (See "Undescended testes (cryptorchidism) in children: Clinical features and evaluation".)
●Vision impairment (eg, strabismus, cataract, refractive errors). (See "Vision screening and assessment in infants and children".)
Injury, neglect, and abuse — Children with ID are at greater risk of being injured and have greater risk of preventable death compared with typically developing peers. Injuries are most commonly accidental (rather than intentional) and are often severe (needing hospitalization). (See "Promoting safety in children with disabilities".)
Individuals with ID are also more likely to suffer neglect, discrimination, and maltreatment [26,27]. Children with ID are more likely to be victimized or manipulated and suffer violence or abuse, including physical, sexual, and psychologic abuse. Factors contributing to greater vulnerability include the child's intellectual and adaptive deficits, gullibility, social naiveté, dependence on others, and desire to please.
Up to 14 percent of patients with ID admitted to hospitals report abuse . Sexual abuse impacts 15 to 30 percent of children with ID, although this may be an underestimate . Female adolescents and children with mild ID are at greatest risk of sexual abuse . Individuals with ID are more likely to suffer frequent, persistent abuse; multiple abusers; and greater involvement of unrelated or unfamiliar perpetrators than those without ID . However, perpetrators are most commonly a male in the immediate or extended family or peer in the same living facility . Abuse may precipitate behavioral disorders such as PTSD, depression, or conduct disorder. In addition, individuals who are victims of sexual abuse in childhood are more likely to subsequently become perpetrators of abuse themselves .
Sensitivity and a high index of suspicion for abuse are needed. Despite communication deficits, even adolescents with severe ID are able to disclose victimization and abuse . Physical and sexual abuse and maltreatment in children and adolescents are discussed further in other topics. (See "Physical child abuse: Recognition" and "Evaluation of sexual abuse in children and adolescents" and "Child neglect: Evaluation and management".)
Screening — In addition to performing routine developmental and behavioral screening at the 9-month, 18-month, and 24- or 30-month visits and screening for autism spectrum disorder (ASD) at the 18- and 24-month visits, screening with a validated instrument should also be performed any time a parent or clinician has concerns about the child's development . The goal is to identify possible developmental delay or ID early to facilitate timely referral for early intervention services and to assess and provide supports appropriate to child's needs. Screening tools vary in reliability, sensitivity, and specificity. Commonly used screening tools are summarized in the tables (table 3A-B). Although these screening tools are generally suitable for identifying children with ID, instruments specifically designed for children and adolescents with ID may also be considered (table 4) . Additional details about screening tools and their use to identify children with developmental delay are discussed in a separate topic review. (See "Developmental-behavioral surveillance and screening in primary care".)
Approach — If a screening test identifies developmental delay, comprehensive developmental-behavioral evaluation is warranted . A multidisciplinary approach is optimal, valuing 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, in order to elicit parent concerns and convey sufficient information to enable optimal understanding and informed collaborative decision-making with the child's parents . If a genetic disorder is identified, appropriate genetic counseling should be provided.
A stepwise approach is favored according to the needs of the individual. There is no widely accepted standard for performing a comprehensive evaluation in a child or adolescent with ID. The nature and timing of evaluations vary based upon individual patient characteristics and support needs.
The evaluation generally includes the following steps:
●Referral to a developmental pediatrician, pediatric neurologist, and/or psychologist for a comprehensive neurodevelopmental evaluation. (See 'Assessment of adaptive function' below and 'Assessment of intellectual function' below.)
●A comprehensive school-based evaluation, if appropriate, which helps to determine educational needs and generate individual support. (See "Specific learning disabilities in children: Evaluation".)
●Formal assessment of speech, language, and communication. (See "Evaluation and treatment of speech and language disorders in children".)
●Vision and hearing assessment. (See "Vision screening and assessment in infants and children" and "Hearing loss in children: Screening and evaluation".)
●A genetic evaluation for any child with unexplained global developmental delay (GDD) or ID. (See "Intellectual disability in children: Evaluation for a cause", section on 'Approach to diagnostic testing'.)
●Additional assessments as appropriate (eg, referral to a child psychiatrist if there are psychiatric comorbidities; evaluation by occupational and physical therapists to assist in assessing functional impairments, strengths, needs, and supports; social work referral to assess child and family needs). (See 'Other assessments' below.)
History — A thorough history, including three-generation family pedigree, is an important part of the evaluation. The history may provide details that suggest a specific cause of ID and can identify comorbidities and associated conditions (table 2). (See 'Associated conditions' above and "Intellectual disability in children: Evaluation for a cause".)
The history and physical examination identifies the etiology of ID in approximately 20 to 30 percent of cases [4,34].
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 .
The history should include the following details, which should be elicited from the parents/caretakers and from the child's medical and educational records:
●A description of the child's developmental progress, stagnation, or regression.
●Results of prior developmental, psychologic, and psychiatric evaluations.
●A description of the child's language development.
●History of medical problems, including seizures, ataxia, or weakness; infections; and/or congenital abnormalities.
●Prenatal, perinatal, and postnatal problems, including alcohol intake in pregnancy or other environmental exposures.
●Results of previous newborn, metabolic, thyroid, lead, iron, vision, and hearing screening tests.
●Review of growth charts.
●Review of previous neuroimaging, if performed.
●A three-generation family history, assessing for neurodevelopmental and genetic disorders, miscarriages, stillbirths, neonatal deaths, and/or consanguinity.
●Questions regarding quality of life, community or recreational participation, friendships, family psychosocial needs, and parental stress/depression.
●Disorder-specific screening questions to assess for frequent comorbidities, such as ASD and attention deficit hyperactivity disorder (ADHD) (table 3A-B). (See "Autism spectrum disorder: Screening tools" and "Attention deficit hyperactivity disorder in children and adolescents: Clinical features and diagnosis".)
●Additional screening questions to assess for other comorbidities, including (see 'Associated conditions' above):
•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 preadolescent age onwards) (see "Screening tests in children and adolescents", section on 'Nicotine, 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 and adolescent females (see "Physical child abuse: Recognition" and "Child neglect: Evaluation and management")
Physical examination — 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 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. (See "Microcephaly in infants and children: Etiology and evaluation" and "Macrocephaly in infants and children: Etiology and evaluation".)
●Dysmorphic features that may suggest genetic or syndromic etiology and guide selection of genetic tests (table 5). (See "Intellectual disability in children: Evaluation for a cause", section on 'Testing for specific disorders'.)
●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 neglect and physical abuse (including intraoral examination) . (See "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 child abuse: Recognition", section on 'Red flag physical findings'.)
●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, social skills, communication, and mood.
●Parent/family interactions and behaviors may be observed for any features of parental depression, parenting characteristics, or suspicion of child abuse. Parental stress, depression, and other needs may be elicited. Parents themselves may be affected by a genetic or neurodevelopmental disorder and exhibit signs and symptoms relevant to the child's evaluation, diagnosis, and intervention.
Comprehensive neurodevelopmental evaluation — Referral to a developmental pediatrician, pediatric neurologist, and/or psychologist is usually needed for a comprehensive neurodevelopmental evaluation. This includes clinical and standardized assessment of intellectual and adaptive and intellectual functioning to establish a diagnosis of ID. Tests that are commonly used for these assessments are summarized in the table (table 4) and are discussed in greater detail below. (See 'Assessment of adaptive function' below and 'Assessment of intellectual function' below.)
Speech, language, and communication should be formally assessed, including languages exposed to at home and fluency. (See "Evaluation and treatment of speech and language disorders in children".)
In addition, the child should undergo a comprehensive school-based evaluation, if appropriate, which helps to determine educational and individual support needs. (See "Specific learning disabilities in children: Evaluation".)
Vision and hearing screening — Children with GDD and/or ID are at high risk for sensory impairments. Vision disorders affect 13 to 50 percent of patients with ID, and approximately 15 to 20 percent have hearing impairment . 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 "Vision screening and assessment in infants and children" and "Hearing loss in children: Screening and evaluation".)
Other assessments — Additional evaluations may include:
●A genetic evaluation should be offered to all children who have unexplained GDD or ID. This is discussed separately. (See "Intellectual disability in children: Evaluation for a cause", section on 'Approach to diagnostic testing'.)
●Occupational and physical therapists can assist in assessing functional impairments, strengths, needs, and supports.
●If there are complex psychiatric comorbidities, a child psychiatrist can help evaluate the child's need for psychopharmacologic therapy.
●A social worker can assist in assessment of the family and child-family needs, provide counseling, generate access to social supports, and contribute to long-term planning.
DIAGNOSIS — The steps to making the diagnosis of ID and assessing its severity will be discussed here. The evaluation for underlying causes of ID is discussed in a separate topic review. (See "Intellectual disability in children: Evaluation for a cause".)
Diagnostic criteria — The diagnosis of ID is based upon identifying limitations in both adaptive and intellectual functioning, with onset before 18 years of age [1,2]:
●Impaired adaptive functioning – Individuals with ID have impairments in adaptive functioning, which affect participation in multiple settings, such as home, community, and school. Adaptive impairments vary in severity and affect at least one of three domains: conceptual, social, and practical (table 1A-B). Support is required for optimal participation in everyday environments, compared with others in the same age group.
The absence of significant adaptive impairment (even if the child's intelligence quotient [IQ] is <70) precludes a diagnosis of ID. Adaptive impairment is more readily perceived than intellectual deficits. The nature and extent of adaptive limitations are key to defining ID and its severity (rather than an IQ score). (See 'Severity classification' below.)
Making a diagnosis of ID requires impaired functioning in at least one of the following three domains, affecting participation in multiple everyday settings (eg, the child's home, community, school) and requiring ongoing support (table 1A-B) [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 the ability to follow a schedule or routine, use a telephone, manage money, and prepare meals; occupational skills; and abilities in transportation/travel, health care, and safety.
Adaptive functioning should be assessed using standardized assessment tools (table 4). Significant impairment is defined by a score or performance at least two standard deviations below the mean on a standardized test [1,2]. (See 'Assessment of adaptive function' below.)
●Impaired intellectual functioning – Individuals with ID have impairments in general mental capacity or intelligence (eg, in learning, reasoning, problem-solving, abstract thinking, and judgment). This generally corresponds to an IQ that is two standard deviations or more below the mean (ie, an IQ score <70) . While standardized IQ testing remains pertinent to document intellectual deficits, the IQ score is no longer used to classify ID severity. (See 'Assessment of intellectual function' below and 'Severity classification' below.)
It is important to recognize that the IQ measurement may be limited or imprecise in estimating a child's true intellectual functioning. A detailed comprehensive profile is often more helpful than a single IQ score and generates better description of individual strengths, weaknesses, limitations, and needs for support.
Because the diagnosis of ID requires both intellectual and adaptive deficits, a child who lacks significant adaptive impairment, even with an IQ <70, would not be considered to have ID. Although most children with an IQ <70 have adaptive deficits, their adaptive functioning may improve with appropriate interventions, such that some will no longer meet criteria for an ID diagnosis.
Intellectual functioning should be assessed using standardized assessment tools in the appropriate language (table 4). A score that is at least two standard deviations below the mean indicates significant intellectual impairment. (See 'Assessment of intellectual function' below.)
Assessment of adaptive function — Assessment of adaptive functioning should be performed using standardized, valid, and reliable instruments (table 4). The instrument used should be appropriate for the child's level of functioning and take into account the child's age, language, culture, communication, socioeconomic status, and disability profile. An understanding of the child's exposure to and fluency in English is important when selecting an instrument and interpreting results since some standardized tests are only available in English and may be biased against youth who are English language learners [36-38]. Other factors such as ease of administration and examiner experience may also be considered. The tool should have current norms since out-of-date norms falsely inflate scores.
Assessment tools vary in reliability and in the adaptive domains, skills, and functions that they measure. Appropriate administration often requires training. Instruments are not simply interchangeable; scores differ between instruments or versions of the same instrument, and, sometimes, it is appropriate to use an older tool even when newer instruments are available .
The most commonly used adaptive tool is the caregiver- or parent-reported Vineland Adaptive Behavior Scale, administered from birth upwards in English or Spanish. Other standardized tools include the American Association on Intellectual and Developmental Disabilities (AAIDD)'s Diagnostic Adaptive Behavior Scale (DABS), which provides conceptual, social, and practical domain scores and a total score and is applicable for children 4 to 21 years of age [40,41], and the Adaptive Behavior Assessment System 3rd Edition (ABAS-3), which is used for children from birth upwards and provides conceptual, social, and practical adaptive domain scores and a general adaptive composite score. Both the DABS and ABAS-3 instruments have the capacity to identify significant adaptive impairment in a single domain, as defined by the Diagnostic and Statistical Manual of Mental Disorders 5th Edition (DSM-5).
Clinical assessment is an important compliment to standardized testing. Clinical factors inform instrument selection, administration, and interpretation. They are important for discerning the child's profile of strengths, needs, and limitations.
Assessment of intellectual function
●Standardized intelligence tests – Standardized tests of intellectual functioning compare measured performance with that expected for age. The most commonly used standardized tests of intelligence in children are the Wechsler Scales. Other commonly used instruments are summarized in the table (table 4). Test items and administration, specific skills or functions examined, and characterization of scores (including the test's lowest reportable IQ score) differ between instruments or different versions of the same instrument. Factors that influence choice of a specific instrument are similar as with adaptive testing, as previously discussed (see 'Assessment of adaptive function' above). Comprehensive test instruments are recommended for a thorough initial diagnosis of ID in preference to validated brief or abbreviated instruments, though the latter may be used in children with established ID. For some genetic causes of ID (eg, Down syndrome, fragile X syndrome), cause-specific assessment tools are available [39,42,43].
●Interpretation and limitations of IQ testing – Appropriate instrument selection and interpretation of results are essential. An IQ score that is at least two standard deviations below the mean indicates significant intellectual impairment. The standard error of measurement varies with the instrument used (eg, a true IQ may be within a range of several points above and below 70). In the United States, the IQ cutoff to define ID (and thus qualify for disability services) varies by state; practitioners are advised to be aware of their state's IQ cutoff.
Global IQ scores may be invalid if there are highly discrepant subtest scores. IQs derived from different tests are not interchangeable. The IQ measure may not capture the child's true intellectual functioning, and a child's test results may change over time. Other factors or conditions can sometimes better account for diminished performance (eg, vision impairment). IQ measures are less reliable at lower values, and many instruments are not designed to provide an IQ score for children with severely or profoundly impaired intellect. In addition, out-of-date norms falsely inflate scores ("Flynn effect"). Other factors such as parent education can impact the IQ score of a child with ID .
The diagnosis of ID is not based solely on the IQ score; delineating a child's unique profile of deficits, weaknesses, needs, and strengths is often more helpful than an IQ score.
Assessment of infants and young children — For infants and young children with findings that are consistent with global developmental delay (GDD), formal assessment can be useful in that it allows early identification of necessary support needs. However, early assessment does not accurately predict intelligence in the long term [5,45]. With appropriate supportive services, some mildly affected children progress by five years of age such that they do not meet adaptive and intellectual diagnostic criteria for ID.
Tests used to evaluate cognitive, adaptive, and other developmental functions in infants or children under two years of age include (table 4):
●The Bayley Scales of Infant and Toddler Development 4th Edition (Bayley-4) – The Bayley Scales are widely used; provide cognitive, language, and motor scores; and assess social-emotional and adaptive behavior from 16 days to 42 months of age. The Bayley-4 can be applied by a variety of professionals and has fewer cognitive and motor items, making it quicker to administer.
●The Griffiths Scales of Child Development 3rd Edition (Griffiths III) – The Griffiths III is applied from birth to 72 months and examines the foundations of learning; language and communication; eye-hand coordination (including fine motor and visual perception); and gross motor, personal-social-emotional, and adaptive strengths and needs.
Both of these tools include caregiver questionnaires (the Bayley-4 Social-Emotional and Adaptive Behavior Scales Questionnaires and the Griffiths III Parent Questionnaire) that can be used when in-person assessment is not possible (eg, when practicing via telemedicine, as in during the height of the coronavirus disease 2019 [COVID-19] pandemic) [46-48]. This allows the clinician to make an informal or provisional diagnosis (and thus generate support services) until a formal in-person assessment can be completed.
Severity classification — ID is highly heterogeneous and comprises a diverse spectrum of functioning, strengths, weaknesses, impairments, disabilities, and needs. However, it is helpful to recognize categories of severity when considering needs and interventions. The severity of ID is defined according to the level of adaptive impairment and the level of support needed (table 1B) [1,2]. While the definition of ID is similar according to both the DSM-5 and the AAIDD, different categories are used to define severity. The DSM-5 categorizes severity as mild, moderate, severe, or profound, whereas the AAIDD categorizes severity according to the level of support needed: intermittent, limited, extensive, or pervasive [1,2].
Standardized IQ testing is no longer the key feature used to classify ID severity (although the 2018 International Classification of Diseases 10th revision [ICD-10] codes continue to use IQ scores).
Although ID severity is no longer classified according to IQ score, severity was historically classified as follows :
●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
●Terminology – Intellectual disability (ID) is a neurodevelopmental disorder that begins in childhood and is characterized by limitations in both intelligence and adaptive skills, affecting at least one of three adaptive domains (conceptual, social, and practical), with varying severity (table 1A-B). The extent of adaptive impairment is key to defining ID and its severity. The term ID replaces the older term of "mental retardation." 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 multiple developmental areas of functioning. (See 'Terminology' above.)
●Clinical features – The cause of ID and the severity of impairment affect when and how a child presents. Children with severe ID present earlier than those with mild ID. Most children with ID have mild impairment and present with delayed language or are detected by routine developmental surveillance and screening. (See 'Clinical features' above.)
●Associated conditions – Numerous medical, physical, neurodevelopmental, and psychiatric conditions are associated with ID (table 2). Care should be taken to promptly identify these conditions. (See 'Associated conditions' above.)
●Screening for GDD and ID – In addition to performing routine developmental and behavioral screening at the 9-month, 18-month, and 24- or 30-month visits and screening for autism spectrum disorder (ASD) at the 18- and 24-month visits, screening with a validated instrument should also be performed any time a parent or clinician has concerns about the child's development (table 3A-B). Developmental-behavioral screening in primary care is discussed in detail separately. (See "Developmental-behavioral surveillance and screening in primary care".)
●Comprehensive evaluation – If a screening test suggests developmental delay, comprehensive developmental-behavioral evaluation is warranted. A stepwise approach is favored according to the needs of the individual. The evaluation generally includes the following:
•Referral to a developmental pediatrician, pediatric neurologist, and/or psychologist for a comprehensive neurodevelopmental evaluation. (See 'Assessment of adaptive function' above and 'Assessment of intellectual function' above.)
•A comprehensive school-based evaluation, if appropriate, which helps to determine educational and individual support needs. (See "Specific learning disabilities in children: Evaluation".)
•Formal assessment of speech, language, and communication. (See "Evaluation and treatment of speech and language disorders in children".)
•Vision and hearing screening. (See "Vision screening and assessment in infants and children" and "Hearing loss in children: Screening and evaluation".)
•A genetic evaluation for any child with unexplained GDD or ID. (See "Intellectual disability in children: Evaluation for a cause", section on 'Approach to diagnostic testing'.)
•Additional assessments as appropriate (eg, referral to a child psychiatrist if there are psychiatric comorbidities; evaluation by occupational and physical therapists to assist in assessing functional impairments, strengths, needs, and supports; social work referral to assess child and family needs). (See 'Other assessments' above.)
●Diagnosis – The diagnosis of ID is based upon identifying impairments in both adaptive and intellectual functioning, with onset before the age of 18 years. Assessment of intellectual and adaptive functioning should be performed using standardized, valid, and reliable instruments (table 4). The instrument used should be appropriate for the child's level of functioning and take into account the child's age, language, culture, communication, socioeconomic status, and disability profile. Other factors such as ease of administration and examiner experience may also be considered. The tool should have current norms since out-of-date norms falsely inflate scores. (See 'Diagnosis' above.)
●Severity classification – The severity of ID is defined by the degree of impairment in adaptive functioning and the level of support needed rather than by intellectual function or intelligence quotient (IQ) score (table 1B). (See 'Severity classification' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Seema R Lalani, MD, who contributed to an earlier version of this topic review.