ICD-116A00	DISORDERS OF INTELLECTUAL DEVELOPMENT Disorders of intellectual development
ICD-10F70–F79	Mental retardation
DSM-5-TRF70–F73	Intellectual Developmental Disorder (Intellectual Disability)

1. Definition and nosology

Intellectual developmental disorder (ICD-11: 6A00 Disorder of Intellectual Development; DSM-5-TR: F70–F73 Intellectual Developmental Disorder / formerly “Mental Retardation”) is a neurodevelopmental disorder that begins during the developmental period (as a rule, before the age of 22) and is characterized by significant limitations both in intellectual functioning (approximately 2 standard deviations below the population mean, with scores below ~70 on standardized tests) and in adaptive behaviour (in the conceptual, social, and practical domains).

The term “mental retardation” has been abandoned by the international scientific community since 2010 because of its stigma and imprecision (in the USA, Rosa's Law 2010 enacted the change at the federal legislative level).

2. History

• Esquirol (1838) introduced the term “idiotie”, distinguishing congenital intellectual limitation from dementia (an acquired impairment).
• Binet & Simon (1905) the first standardized intelligence scale — the origin of the IQ concept (Stern's 1912 IQ formula).
• AAMR/AAIDD (1959, 1973, 1992, 2002, 2010, 2021): evolution of the definition — a shift from IQ alone to a model of adaptive behaviour + support needs. The AAIDD 12th edition (2021): intellectual functioning + adaptive behaviour + onset during the developmental period + assessment of individual support needs.
• DSM-IV (1994) “Mental Retardation” → DSM-5 (2013) “Intellectual Disability (Intellectual Developmental Disorder)”. The basis for severity grading was shifted from IQ to adaptive functioning.
• ICD-11 (2019) “Disorders of Intellectual Development” — the ICD-10 “Mental Retardation” group (F70–F79) was renamed, with the clinical and adaptive dimensions emphasized jointly.

3. Epidemiology

• Prevalence: ~1% in the general population (Maulik P.K. et al. Res Dev Disabil 2011 meta-analysis, 52 studies); rates are higher in low- and middle-income countries.
• Severity distribution: ~85% mild, ~10% moderate, ~3.5% severe, ~1.5% profound.
• Sex: more frequent in males (1.2–1.9 : 1), associated with X-linked genetic causes (e.g., Fragile X).
• Comorbidity: epilepsy 20–30%, motor disorders 20–40%, visual/hearing impairments 10–25%; psychiatric comorbidity is 3–4 times higher than in the general population (ASD, ADHD, anxiety, conduct disorders).
• Mortality: in severe and profound forms life expectancy is shortened — mainly due to aspiration pneumonias, status epilepticus, and cardiovascular anomalies.

4. Aetiology and pathogenesis

4.1 Genetic causes (50–60% identified)

• Chromosomal anomalies: Down syndrome (Trisomy 21) — the most frequent genetic cause, ~1/700 births; Edwards (T18), Patau (T13), 22q11.2 deletion (DiGeorge), Williams (7q11.23), Prader-Willi/Angelman (15q11-q13), Cri-du-chat (5p-).
• X-linked: Fragile X syndrome (FMR1 gene, CGG repeats) — the most frequent inherited form, ~1/4000 boys; Rett syndrome (MECP2, almost always in girls).
• Single-gene disorders: tuberous sclerosis (TSC1/TSC2), neurofibromatosis (NF1), phenylketonuria (PAH — treatable), Wilson disease.
• CNV (copy number variations): detected by microarray analysis — 10–20% of cases of unknown aetiology.

4.2 Prenatal non-genetic causes

• Infectious: TORCH (toxoplasma, rubella, cytomegalovirus, herpes), Zika, syphilis.
• Toxic: fetal alcohol spectrum disorders (FASD — a preventable cause!), valproate in pregnancy, isotretinoin, cocaine, lead.
• Metabolic and endocrine: untreated maternal hypothyroidism, severe diabetes, phenylketonuria.
• Ischaemic/hypoxic: placental insufficiency, intrauterine growth restriction.

4.3 Perinatal causes

• Severe prematurity (<32 weeks), low birth weight (<1500 g).
• Birth asphyxia, hypoxic-ischaemic encephalopathy.
• Neonatal kernicterus (treatable — phototherapy).
• Neonatal sepsis, meningitis.

4.4 Postnatal causes

• Bacterial meningitis (especially pneumococcal, meningococcal), viral encephalitis.
• Head trauma (TBI) — during childhood.
• Lead poisoning — a preventable cause (the CDC blood lead reference value has been 3.5 µg/dL since 2021, previously 5 µg/dL; no safe blood lead level in children has been identified).
• Severe iodine deficiency (endemic cretinism — largely eliminated through WHO prevention programmes).
• Socio-environmental deprivation — extremely severe psychological neglect can affect development (Romanian orphanage studies, Nelson C.A. et al. Science 2007).

5. Clinical features

Severity classification (DSM-5-TR / ICD-11 — based on adaptive functioning)

Mild — IQ ~50–70, mild adaptive deficit

Academic delay may not be noticeable before school age. Reading, mathematics, and abstract thinking are difficult. Social naivety, easily deceived. Practical skills (self-care, household tasks) are achieved. Independent living is possible with support; suitable for supported employment.

Moderate — IQ ~35–49

Delay is apparent in early childhood. Simple speech, concrete thinking. Self-care is acquired with training. Functions in structured social/occupational settings. Supervised living (group home, family) is typical.

Severe — IQ ~20–34

Very little speech, understands simple requests. Continuous help with self-care. Medical and behavioural comorbidity are frequent.

Profound — IQ < 20

Very little or no speech, sensorimotor-level communication. Requires total care. Motor, visual, and hearing impairments are very frequent.

Developmental trajectory: early motor and speech delays (sitting, standing, first words); academic limitation at school age; the difference becomes more evident in adulthood as adaptive demands increase.

6. Diagnosis

6.1 Unified diagnostic criteria (DSM-5-TR · ICD-11 · AAIDD 12th ed. consensus points)

A. Significant limitation in intellectual functioning — on a standardized, culturally contextualized test of intelligence, approximately 2 standard deviations below the mean (a score of ~70 or below, accounting for a measurement error of ±5 points).

B. Significant limitation in adaptive behaviour — a score 2 SD below in at least one of the following three domains:

• Conceptual (language, reading, writing, mathematics, memory, problem-solving);
• Social (interpersonal skills, social responsibility, matters of gullibility);
• Practical (self-care, work, health, safety).

C. Onset during the developmental period — symptoms are apparent in childhood and adolescence (before adulthood).

D. Exclusions — not fully explained by a sensory (visual, hearing) impairment, motor disorder, language origin, cultural-linguistic barrier, or a psychiatric disorder.

6.2 Source-specific clarifications

• DSM-5-TR (319): severity is determined by adaptive functioning, IQ is noted only as an additional indicator. Onset “during the developmental period” (before 18 years is intended, but AAIDD's definition of 22 years may be accepted).
• ICD-11 (6A00): mild, moderate, severe, profound (the same terminology); severity is assessed jointly by both IQ and adaptive functioning; additional qualifiers — aetiology (genetic, prenatal, perinatal, postnatal, unknown) and comorbidities.
• AAIDD 12th ed. (Schalock R.L. et al. 2021): formal assessment of support needs (SIS — Supports Intensity Scale) is incorporated into the clinical formulation; age of onset up to 22 (unlike DSM); intellectual functioning is directed not at exclusion but at supported processes.
• AAP (Moeschler J.B., Shevell M.S., Pediatrics 2014, reaffirmed 2020): after the diagnosis is made, the search for aetiology is stepwise — first line: history, physical examination (dysmorphic features), chromosomal microarray, Fragile X, treatable metabolic screening (PKU, biotinidase, thyroid, liver, kidney); selective: brain MRI, EEG, whole-exome sequencing (WES).

6.3 Diagnostic algorithm

1. Developmental history (congenital, perinatal, postnatal) + family history (3 generations).
2. Physical examination — height, head circumference, dysmorphic features, neurological, skin (café-au-lait, hypopigmentation).
3. Standardized intellectual test (WISC-V 6–16 years; Stanford-Binet 5 2+ years; Bayley-III <3.5 years).
4. Adaptive behaviour assessment (Vineland-3, ABAS-3).
5. Vision and hearing tests (exclusion of sensory impairment).
6. First-line aetiological laboratory tests — chromosomal microarray analysis (CMA), Fragile X (FMR1), TSH, lead level, liver/kidney, creatine and amino acid profile.
7. Selective: brain MRI (microcephaly, focal neurological signs), EEG (suspected epilepsy), trio exome sequencing (when CMA is negative).
8. Comorbid screening: ASD (M-CHAT-R/F, ADOS-2), ADHD (Conners), anxiety, depression, conduct disorder.

6.4 Differential diagnosis

7. Examination and assessment

7.1 Standardized tests

• Intelligence: WISC-V (6–16 years), Stanford-Binet 5 (2+), WAIS-IV (16+), Bayley Scales of Infant Development 4 (<3.5 years), Kaufman ABC-II.
• Adaptive behaviour: Vineland-3, ABAS-3, Scales of Independent Behavior-Revised.
• Support needs: SIS-A (adults), SIS-C (children) — AAIDD.

7.2 Laboratory investigations (AAP 2014 / 2020 protocol)

First line (all cases): complete blood count, liver and kidney function, TSH, creatinine, ammonia, lactate, creatine and amino acid profile (urine and blood), lead level.

First-line genetic: chromosomal microarray analysis (CMA — yield 15–20%), Fragile X (FMR1) in all boys (yield ~2%, in females ~1%).

Selective: trio exome sequencing (WES — yield 25–40% when CMA is negative), MECP2 (suspected Rett in girls), specific syndromic genes.

7.3 Instrumental investigations

• Brain MRI — in focal neurological signs, micro/macrocephaly, seizures, regression.
• EEG — suspected seizures, regression.
• Vision and hearing — baseline indicators in all cases.
• Cardiac echocardiography and EKG — in syndromes such as Down and 22q11.2 deletion.

8. Treatment

8.1 General principles (AAIDD 12th ed. · AAP 2014 · NICE NG54 · WHO mhGAP consensus)

1. There is no “cure” for intellectual developmental disorder — intellectual functioning itself is not restored pharmacologically. Intervention is directed at individualized support, education, behaviour, and management of comorbid symptoms.
2. Identification of aetiology: early detection of treatable causes (PKU, congenital hypothyroidism, lead, biotinidase deficiency) is critical — the basis of newborn screening programmes.
3. Early intervention (0–3 years): the period with the strongest evidence base (IDEA Part C in the USA; similar programmes in all developed countries). Improves social and cognitive outcomes.
4. Individualized education programme (IEP): academic and adaptive goals at school; AAIDD support-needs-based approach.
5. Behavioural interventions: Applied Behavior Analysis (ABA) and positive behaviour support (PBS) — effective in reducing challenging behaviours and developing new skills.
6. Family support: psychoeducation, respite care, peer support groups, social service coordination.
7. Treatment of comorbidity: epilepsy (standard antiepileptics), ADHD (stimulants; the Cortese 2018 network meta-analysis makes methylphenidate the first choice in children, but it excluded trials with comorbid disorders — the evidence base in IDD is separate and thinner), anxiety/depression (start SSRI at a low dose), aggression (behavioural intervention is first line; risperidone or aripiprazole only in severe cases — FDA approved for irritability in ASD: risperidone ages 5–16, aripiprazole ages 6–17; no official approval for IDD, evidence base limited).
8. Medical monitoring: in Down syndrome — thyroid, heart, hearing, atlanto-axial instability (AAP Health Supervision for Children with Down Syndrome 2022).
9. Transition to adulthood: supported employment (IPS model), independent living options, coordination of social and medical services.

8.2 Source-specific clarifications

• AAIDD 12th ed. (2021): “supports model” — planning tailored to the individual's support needs rather than to a given severity level; intermittent, limited, extensive, pervasive support categories.
• AAP 2014/2020: screening intervals and treatment algorithms for comorbidity (autism, ADHD, conduct); a separate AAP 2022 protocol for Down syndrome.
• NICE NG54 (2016) — Mental health problems in people with learning disabilities: diagnosis and treatment of psychiatric comorbidity; behavioural intervention is first line; nonspecific use of psychotropic drugs is not recommended — this is a popular “safe” practice, but the burden of side effects and the risk of interactions are high.
• WHO mhGAP Intervention Guide v2.0: assessment and basic intervention at the primary care level in low- and middle-income countries; family training and community rehabilitation are emphasized.

Treatment methods

1. Applied Behavior Analysis (ABA) — Shaping and reducing specific target behaviours based on a functional analysis of behaviour; the antecedent-behaviour-consequence (ABC) framework, discrete trial training (DTT), naturalistic behavioural interventions. Effect: moderate-to-high effect size in acquiring adaptive skills and reducing challenging behaviour, especially in cases comorbid with autism and IDD (Eldevik S. et al. J Clin Child Adolesc Psychol 2009 meta-analysis). Can be adapted to home, school, and community settings at all stages.
2. Early Intervention (0–3 years) — Multidisciplinary (developmental, physical, occupational, speech therapies + family training) home- or centre-based programmes. Evidence: the Carolina Abecedarian Project (Ramey C.T., Campbell F.A. — 30+ years of follow-up) improved IQ, academic, and employment outcomes in low-resource children. IDEA Part C.
3. Individualized Education Program (IEP) — A legal document that documents academic and adaptive goals, support measures, and assessment intervals. Mandatory in the USA under IDEA, with similar frameworks in other countries (United Kingdom — EHCP).
4. Positive Behaviour Support (PBS) — Functional analysis of challenging behaviours and replacement of behaviour (teaching an appropriate behaviour with the same function). Not punishment, but changing the environment and new skills. NICE NG11 (2015) makes it first line in managing challenging behaviour.
5. Vineland Adaptive Behavior Scales-3 (Vineland-3) — The “gold standard” assessment of adaptive behaviour — communication, daily living, social, and motor domains. Interview (parent/caregiver) and teacher forms.
6. WISC-V / Stanford-Binet 5 — Standardized intelligence tests; WISC-V — 6–16 years, 5 main indices (verbal, visuo-spatial, abstract reasoning, working memory, processing speed); Stanford-Binet 5 — from 2 years to adulthood.
7. Supports Intensity Scale (SIS, AAIDD) — A scale that measures the intensity of support needs; SIS-A for adults, SIS-C for ages 5–16. For clinical planning and allocation of services.

9. Prognosis

Good prognostic factors

• Mild severity (~85% of all IDD); intellectual and adaptive functioning at a close level.
• Treatable aetiology (PKU, congenital hypothyroidism, lead) — early detection.
• Early intervention (<3 years).
• Stable and supportive family environment.
• Active treatment of comorbid psychiatric and medical conditions.
• Accessibility of community and school resources.

Poor prognostic factors

• Profound or severe severity.
• Genetic syndrome with a complex phenotype (epilepsy, cardiac anomalies, motor).
• Severe behavioural disorders, self-harm.
• Absence of family support and community resources.
• Unidentified and untreated comorbidity (epilepsy, ASD, ADHD).

Follow-up targets

• Annual reassessment of adaptive development.
• Medical monitoring — according to syndrome-specific protocols (e.g., AAP Down 2022).
• Screening for comorbid psychiatric conditions (ASD, ADHD, anxiety, depression).
• Functional analysis and intervention for behavioural disorders.
• Transition planning for supported employment and living options in adulthood.
• Offering the family respite care and peer support.

10. Myths and misconceptions

10.1 Aetiology myths

Myth 1: “Intellectual disability is the parent's fault / punishment”

Why it is widespread: a cultural and religious-moral explanatory framework; the image of a “child atoning for sin”; a mechanism of blaming the parent (especially the mother) in poorly educated communities.

Biological logic and evidence: The aetiology of IDD is broad — genetic (Down, Fragile X), prenatal infections, fetal alcohol spectrum (the only one completely preventable through parental intervention), perinatal asphyxia, postnatal trauma or meningitis. The position of AAIDD, WHO, and APA — IDD is a biomedical and/or developmental aetiological disorder, not a matter of moral judgement.

Real clinical step: the search for aetiology (genetic counselling, syndromic and metabolic screening) — turns blame into a precise risk calculation for a future pregnancy.

Myth 2: “Vaccines (especially MMR) cause IDD”

Why it is widespread: Wakefield A.J. Lancet 1998 — a study later retracted for fraud, which attempted to link autism with MMR (not IDD, but in popular belief both conditions are conflated).

Evidence: Taylor L.E., Swerdfeger A.L. et al. Vaccine 2014 meta-analysis n>1.2 million children — there is no association between vaccines and autism or IDD. The position of the CDC, WHO, and NICE is the same. Down syndrome arises from nondisjunction of chromosome 21 in meiosis (most often maternal meiosis I) — mechanistically it cannot be related to vaccines.

Myth 3: “‘Stress’ or ‘bad thoughts’ during pregnancy cause IDD”

Evidence: psychological stress may be a modest risk factor for neurodevelopment in the prenatal period, but it is not identified as a direct cause of IDD. Established prenatal risks — alcohol (FAS — the most preventable cause), valproate, infections (TORCH), nutritional deficiency (iodine, folate). Reducing stress is beneficial for general health, but it is defined medical protective measures (vaccination, abstaining from alcohol, perinatal care) that are effective, not “correct thoughts”.

10.2 Harmful “treatment” methods

Myth 4: “Chelation therapy cures IDD by removing heavy metals”

Why it is widespread: the expansion of the “heavy metals” concept in mass media; the marketing of alternative medicine centres.

Evidence and risk: AAP 2010 and FDA warning: in 2005 a 5-year-old child during a chelation procedure died (cardiac arrest due to calcium deficiency). Chelation is applied only for confirmed heavy metal poisoning (high lead, mercury) on medical indication. There is no evidence of effect for IDD.

Myth 5: “Stem cell injections (in unofficial clinics) cure IDD”

Evidence and risk: The position of the ISSCR (International Society for Stem Cell Research) and the FDA: for IDD (and neurodevelopmental disorders in general) there is no approved stem cell treatment. Illegal “clinics” have caused serious adverse effects accompanied by infections, brain tumours, and deaths (Berkowitz A.L. et al. NEJM 2016).

Myth 6: “The hyperbaric oxygen chamber (HBOT) is a treatment for IDD and autism”

Evidence: Cochrane (Xiong T. et al. 2016) — there is no evidence of effect for autism; there is also no evidence base for IDD. Risks: pneumothorax, ear barotrauma, fire (oxygen + static electricity).

Myth 7: “Miracle Mineral Solution (MMS) / chlorine dioxide”

Evidence: FDA 2019, EMA warning — caustic (chlorine dioxide), liver and kidney damage, and deaths have been recorded. All clinical use is prohibited.

10.3 Ineffective methods or those that delay essential support

Myth 8: “Mega-doses of vitamin B complex / niacin restore cognitive functioning in IDD”

Evidence: the “orthomolecular” psychiatry approach (Hoffer A., Osmond H. 1950–70) has not shown efficacy in IDD or autism; numerous RCTs are negative (Pfeiffer S.I. et al. J Autism Dev Disord 1995 meta-analysis). Mega-doses of niacin cause liver damage and hyperuricaemia. AAP and AAIDD do not recommend it.

Myth 9: “Patterning” (Doman-Delacato method)

Why it is widespread: it gained mass popularity in the 1960s–80s; the claim of “reprogramming the brain”.

Evidence: AAP 1968 and 2010 separate Statements: no evidence of effect; excessive burden and emotional strain on the family; delays children from other evidence-based interventions. AAP — strongly does not recommend it.

Myth 10: “Auditory Integration Training (AIT)”

Evidence: ASHA (American Speech-Language-Hearing Association) Position Statement 2004 — there is no evidence of effect of AIT in IDD and autism; it is considered an experimental intervention.

Myth 11: “IDD will resolve on its own if the child is stimulated”

Reality: to be honest, early intervention improves adaptation, but IDD is not a state but a neurodevelopmental disorder — when the aetiology is genetic or structural, intellectual ability does not change significantly. Some specific causes (PKU, congenital hypothyroidism) can be completely prevented through early detection — this is not “resolving”, but timely prevention.

Real clinical step: the AAIDD support model — the goal is not elimination of the disorder, but ensuring a meaningful and independent life; supporting the family with realistic expectations.

Myth 12: “IDD will improve if the parent applies a special diet (gluten-free, casein-free)”

Evidence: Hyman S.L. et al. (AAP 2020 Clinical Report on ASD): in children without gastrointestinal problems, the effect of special diets on cognitive functioning has not been proven. If coeliac disease is proven — a gluten-free diet, but not for IDD.

11. Sources

1. Schalock R.L., Luckasson R., Tassé M.J. Intellectual Disability: Definition, Diagnosis, Classification, and Systems of Supports, 12th ed. AAIDD; 2021.
2. American Psychiatric Association. DSM-5-TR. Washington DC: APA Publishing; 2022.
3. WHO. ICD-11 for Mortality and Morbidity Statistics. 6A00 Disorders of Intellectual Development. 2024.
4. Moeschler J.B., Shevell M., Committee on Genetics. Comprehensive evaluation of the child with intellectual disability or global developmental delays. Pediatrics 2014;134(3):e903–e918 (reaffirmed 2020).
5. NICE Guideline NG54. Mental health problems in people with learning disabilities. 2016.
6. NICE Guideline NG11. Challenging behaviour and learning disabilities: prevention and interventions for people with learning disabilities whose behaviour challenges. 2015.
7. WHO mhGAP Intervention Guide for mental, neurological and substance use disorders, version 2.0. Geneva; 2016.
8. Maulik P.K., Mascarenhas M.N., Mathers C.D. et al. Prevalence of intellectual disability: a meta-analysis of population-based studies. Res Dev Disabil 2011;32(2):419–436.
9. Bull M.J. et al. AAP Health Supervision for Children and Adolescents With Down Syndrome. Pediatrics 2022;149(5):e2022057010.
10. Eldevik S., Hastings R.P., Hughes J.C. et al. Meta-analysis of Early Intensive Behavioral Intervention for children with autism. J Clin Child Adolesc Psychol 2009;38(3):439–450.
11. Ramey C.T., Campbell F.A. Carolina Abecedarian Project — long-term outcomes. Applied Developmental Science 2002;6(1):42–57.
12. Nelson C.A., Zeanah C.H., Fox N.A. et al. Cognitive recovery in socially deprived young children: the Bucharest Early Intervention Project. Science 2007;318(5858):1937–1940.
13. Taylor L.E., Swerdfeger A.L., Eslick G.D. Vaccines are not associated with autism: an evidence-based meta-analysis. Vaccine 2014;32(29):3623–3629.
14. Berkowitz A.L., Miller M.B., Mir S.A. et al. Glioproliferative lesion of the spinal cord as a complication of “stem-cell tourism”. NEJM 2016;375(2):196–198.
15. Xiong T., Chen H., Luo R., Mu D. Hyperbaric oxygen therapy for people with autism spectrum disorder. Cochrane Database Syst Rev 2016;(10):CD010922.
16. Hyman S.L., Levy S.E., Myers S.M.; AAP Council on Children with Disabilities. Identification, Evaluation, and Management of Children With Autism Spectrum Disorder. Pediatrics 2020;145(1):e20193447.