Intellectual Disability

Updated: Nov 16, 2021
  • Author: Ari S Zeldin, MD, FAAP, FAAN; Chief Editor: Stephen L Nelson, Jr, MD, PhD, FAACPDM, FAAN, FAAP, FANA  more...
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Practice Essentials

Intellectual disability (ID), is a descriptive term for subaverage intelligence and impaired adaptive functioning arising in the developmental period (< 18 years).

Signs and symptoms

Patients with ID and developmental delays may demonstrate the following:

  • Language delay: One of the first signs of ID may be language delays, including delays in expressive language (speech) and receptive language (understanding)

  • Fine motor/adaptive delay: Significant delays in activities such as self-feeding, toileting, and dressing are typically reported in children with ID

  • Cognitive delay: Difficulties with memory, problem-solving, and logical reasoning

  • Social delays: Lack of interest in age-appropriate toys and delays in imaginative play and reciprocal play with age-matched peers

  • Gross motor developmental delays: Infrequently accompany the cognitive, language, and fine motor/adaptive delays associated with ID unless the underlying condition results in both ID and cerebral palsy.

  • Behavioral disturbances: Infants and toddlers may be more likely to have difficult temperaments, hyperactivity, disordered sleep, and colic; associated behaviors may include aggression, self-injury, defiance, inattention, hyperactivity, sleep disturbances, and stereotypic behaviors.

  • Neurologic and physical abnormalities: Prevalence of ID is increased among children with seizure disorders, microcephaly, macrocephaly, history of intrauterine or postnatal growth retardation, prematurity, and congenital anomalies

Evaluation of patients for ID can include the following examinations:

  • Head circumference: Microcephaly correlates highly with cognitive deficits; macrocephaly may indicate hydrocephalus, is associated with some inborn errors of metabolism, and may be seen early on in some children later diagnosed with autism [1, 2]

  • Height: Short stature may suggest a genetic disorder, fetal alcohol syndrome, or hypothyroidism; tall stature may suggest fragile X syndrome (FraX), Soto syndrome, or some other overgrowth syndrome associated with ID

  • Neurologic: This examination should include assessments of head growth (for microcephaly/macrocephaly), muscle tone (for hypotonia or spasticity), strength and coordination, deep tendon reflexes, persistent primitive reflexes, ataxia, and other abnormal movements, such as dystonia or athetosis.

  • Sensory: Children with disabilities and ID are more likely than other children to have visual impairment and hearing deficits

  • Skin: Findings can include hyperpigmented and hypopigmented macules, such as café-au-lait macules (associated with neurofibromatosis type 1), as well as ash-leaf spots (associated with tuberous sclerosis), fibromas, and irregular pigmentation patterns

  • Extremities: Although ID with multiple congenital anomalies and major malformations accounts for only 5-10% of all cases, most of these affected individuals have 3-4 minor anomalies, especially involving the face and digits

See Clinical Presentation for more detail.


Laboratory studies

  • Array-based comparative genetic hybridization (CGH), or microarray

  • High-resolution karyotype

  • Fragile X testing

  • FISH probes

  • Next-generation sequencing through panels and/or whole exomes

Imaging studies

  • Brain magnetic resonance imaging (MRI): Should be conducted in any child with global developmental delays or ID [3]

  • Head computed tomography (CT) scanning: Preferred imaging study for calcifications that may be identified with TORCH infections (ie, toxoplasmosis, other infections, rubella, cytomegalovirus [CMV], herpes simplex) or when tuberous sclerosis is suspected or craniosynostosis is a concern

  • Skeletal films: Assist with phenotypic description, syndrome characterization, and assessment of growth

Additional tests

Detailed assessment by a licensed professional is necessary to confirm the diagnosis of ID. Some of the most commonly used tests in children include the following:

  • Bayley Scales of Infant Development

  • Stanford-Binet Intelligence Scale

  • Wechsler Preschool and Primary Scale of Intelligence-Revised (WPPSI-R)

  • Wechsler Intelligence Scale for Children–IV (WISC-IV)

  • Vineland Adaptive Behavior Scales-II

See Workup for more detail.


The mainstay of ID treatment is the development of a comprehensive management plan for the condition. The complex habilitation plan for the individual requires input from care providers from multiple disciplines, including special educators, language therapists, behavioral therapists, occupational therapists, and community services that provide social support and respite care for families affected by ID.

Neuropathic pain due to dysautonomia or motor spasms may create chronic disturbances. Treatment should be prompt and include the following:

  • Nonsteroidal anti-inflammatory drugs (NSAIDs) or acetaminophen for mild pain

  • Tramadol or equivalent for moderate pain

  • Opioids for severe pain as indicated

  • Management of sources of pain

No specific pharmacologic treatment is available for cognitive impairment in the developing child or adult with ID. [4] Medications, when prescribed, are targeted to specific comorbid psychiatric disease or behavioral disturbances.

The psychostimulant class of drugs is commonly prescribed in individuals with ID, because of the diagnosis of attention deficit with or without hyperactivity disorder (ADHD/ADD) in 6-80% of these patients. However, few studies on stimulants in people with ID are available. The studies that do exist indicate that benefits vary, and significant adverse events, such as severe social withdrawal, increased crying, drowsiness, and irritability, have been noted, especially at higher doses of methylphenidate (0.6 mg/kg). [5]

The neuroleptic drugs are the most frequently prescribed agents for targeting behaviors such as aggression, self-injury, and hyperactivity in people with ID. These indications are generally off-label for ID and caution is advised.

See Treatment and Medication for more detail.



Intellectual disability (ID) is a descriptive term for subaverage intelligence and impaired adaptive functioning arising in the developmental period (< 18 y). ID and other neurodevelopmental disabilities are seen often in a general pediatric practice.

The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, (DSM-5), characterizes ID by deficits in general mental abilities, such as reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience. The deficits result in impairments to adaptive functioning, such that the individual fails to meet standards of personal independence and social responsibility in one or more aspects of daily life, including communication, social participation, academic or occupational functioning and personal independence at home or in community settings. [6]

Approximately 10% of children have some learning impairment, while as many as 3% manifest some degree of ID. The population prevalence of these combined disorders of learning rivals that of the common childhood disorder asthma.

ID originates during the developmental period (i.e., conception through age 18 years). ID is categorized as a neurodevelopmental disorder and is distinct from the neurocognitive disorders, which are characterized by a loss in cognitive functioning. Major neurocognitive disorder may occur with ID such as when an individual with Down syndrome develops Alzheimer's disease, for example. In a case like this, both ID and neurocognitive disorder would be diagnosed.

ID also can be categorized as syndromic, if associated with dysmorphic features, or nonsyndromic, if not associated with dysmorphisms or malformations. The understanding of specific ID syndromes is expanding with recent molecular genetic advances. More than 800 recognized syndromes listed in the Online Mendelian Inheritance in Man (OMIM) database are associated with ID, reflecting clinical diagnostic advances in the field. The most common associated chromosomal abnormality is trisomy 21, or Down syndrome. The most common X-linked abnormality associated with ID is fragile X syndrome. However, for most cases of ID, no specific genetic abnormalities are found.

Some forms of ID are due to nongenetic factors and may be identifiable by their associated dysmorphisms and clinical presentation. Examples include prenatal exposure to teratogens (e.g., anticonvulsants, warfarin, alcohol) or prenatal thyroid dysfunction. Prenatal and postnatal exposure to lead and the associated decrement in IQ may increase an individual's chance of functioning in the ID range. ID may result from an acquired infection or injury during the developmental period from, for example, a severe head injury or meningitis or encephalitis infection

Diagnostic criteria (DSM-5)

Intellectual disability (intellectual developmental disorder) is a disorder with onset during the developmental period that includes both intellectual and adaptive functioning deficits in conceptual, social and practical domains. According to the DSM-5, the following three criteria must be met: [6]

  • Deficits in intellectual functions, such as reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience, confirmed by both clinical assessment and individualized, standardized intelligence testing.

  • Deficits in adaptive function that results in failure to meet developmental and sociocultural standards for personal independence and social responsibility. Without ongoing support, the adaptive deficits limit functioning in one or more activities of daily life, such as communication, social participation, and independent living, across multiple environments, such as home, school, work, and community.

  • Onset of intellectual and adaptive deficits during the developmental period.

Severity is specified as mild, moderate, severe, or profound based on the level of impairment in adaptive functioning, and not IQ scores, because it is adaptive functioning that determines the level of support required. The three domains of adaptive functioning are conceptual, social, and practical.

In addition to severity, the specifier “associated with a known medical or genetic condition or environmental factor” may be given. Examples include genetic disorders, such as fragile X syndrome, tuberous sclerosis, and Rett syndrome; and environmental factors including fetal alcohol exposure (even in the absence of fetal alcohol syndrome).



Intellectual disability (ID) is the end result of many disorders of CNS function. Most individuals with significant intellectual impairment have no discernible structural abnormalities of the brain. CNS malformations, a visual correlate of the disorders, are diagnosed in 10-15% of cases; dysfunction is localized primarily to the cortical structures, including the hippocampus and the medial temporal cortex. The most common malformations consist of neural tube defects, hydranencephaly, and microcephaly. Less commonly, CNS malformations include disorders of migration (the lissencephalies) and agenesis of the corpus callosum.

Multiple congenital anomaly syndromes with malformations confined to nonneurologic organ systems may be present in 5% of all patients with ID. Between 3% and 7% of cases may be associated with a wide array of inborn errors of metabolism complicated by multiorgan system disease. Alcohol exposure in utero may account for as many as 8% of those with mild ID.

Most individuals with mild ID and other learning disorders do not have other neurologic complications, CNS malformations, or dysmorphisms. They are more likely, however, to be born into families of low socioeconomic status, low IQ, and little education. The etiologic contribution of poverty to their poor cognitive function remains unclear. Clearly, however, poor cognitive functioning and ID are correlated positively with a life of poverty.



United States

The frequency of intellectual disability (ID) of all degrees ranges from 1% to 3% of the population. ID has an overall general population prevalence of approximately 1% and prevalence rates vary by age. Prevalence for severe intellectual disability is approximately 6 per 1,000. [6]


A study with excellent ascertainment conducted in Aberdeen, Scotland, yielded a prevalence of 1 in 300 for severe ID and 1 in 77 for mild ID. Among those with severe ID were more boys than girls (male-to-female ratio 1.2:1), and among those with IQ > 70, in the mild range of deficiency, boys exceeded girls by a ratio of 2.2:1. [7]

Although prevalence rates vary from country to country, the variance in prevalence may be attributed to ascertainment bias, the standardization methods employed from study to study, and a generalized upward drift in IQ scores over time. Even so, the greatest variance in statistics of prevalence is most likely to fall within the category of mild ID, a group for which the ascertainment bias is large.

Race-, sex-, and age-related differences in incidence

Consistent racial differences in prevalence of ID and associated mortality rates are not known to exist.

The gender ratios for mortality and morbidity do not differ from the gender ratio noted in the severe/profound ranges of intellectual disability (ie, male-to-female ratio, 1.2:1).

ID refers to intellectual impairment that develops prior to the age of 18 years.

Certain syndromes associated with ID, such as Down syndrome, are associated with shorter life expectancy than the general population. In a comparison of those with ID with and without Down syndrome from the California Department of Developmental Services cohort, excess mortality in the Down syndrome group tended to decrease with advancing age up to 35-39 years but increased thereafter. The increase in death rate from age 40 years was steeper in patients with Down syndrome than in those without Down syndrome. [8]



Intellectual disability (ID) itself is not necessarily associated with an increased premature death rate. However, individuals with severe to profound ID experience a decreased life expectancy related to the underlying etiology or additional complicating neurologic disorders, such as epilepsy. Neurologic dysfunction resulting in immobility, significant oral motor incoordination, dysphagia, and aspiration confers a greater risk of premature death than ID itself. When significant neurologic dysfunction is associated with other organ system anomalies, an individual's life expectancy is shortened further.

Respiratory disease is the most prevalent cause of death among individuals with profound ID. In particular, respiratory infections were the leading cause of death among a Finnish cohort of children with ID. [9] For those affected by mild ID, life expectancy does not differ from that of the general population.

Comorbid psychiatric conditions are diagnosed more frequently in those with intellectual disabilities than in the general population. Even so, psychiatric disorders probably are underappreciated in this population.

  • Attention deficit/hyperactivity disorder (ADHD) is diagnosed in 8-15% of children and 17-52% of adults with ID.

  • Self-injurious behaviors require treatment in 3-15%, particularly in the severe range of ID.

  • Major depression, bipolar disorders, anxiety disorders, and other mood disorders are the most common psychiatric diagnoses in adults with ID. Obsessive-compulsive disorder, conduct disorder, tic disorders, and other stereotypic behaviors are also diagnosed more commonly in those with ID. Schizophrenia may have a prevalence of 3% in individuals with ID, compared to 0.8% in the general population.

  • In the 1970 Isle of Wight study, as many as 30% of children with ID exhibited an emotional or behavioral disorder, compared to 6% of children in the general population. ID compounded by epilepsy conferred a 56% risk of comorbid psychiatric disease in this study. [10]

  • Occult visual and auditory deficits occur in 50% of those with ID, particularly when refractive errors are considered.

  • The rates of transmittable diseases, including sexually transmitted diseases (STDs), hepatitis B, and Helicobacter pylori infection, are increased significantly among individuals with ID.

  • One in 5 individuals with ID also has cerebral palsy (CP).

  • As many as 20% of individuals with ID have seizures.

  • GI complications with ID include feeding dysfunction, excess drooling, reflux esophagitis, and constipation.

  • GU complications with ID include urinary incontinence and poor menstrual hygiene.

  • A profound social morbidity affects individuals with ID and their families. This morbidity can be measured in lost wages, dependence on social services, impaired long-term relationships, and emotional suffering.

  • Individuals with a diagnosis of intellectual disability with co-occurring mental disorders are at risk for suicide. Screening for suicidal thoughts is essential in the assessment process.