Intellectual developmental disorder

• Intellectual developmental disorder (IDD) is a neurodevelopmental condition in which both intellectual functioning (reasoning, learning, problem-solving, judgement) and adaptive functioning (the everyday conceptual, social, and practical skills expected for age) are substantially limited, and these limits are evident from childhood. ^[3]
• DSM-5-TR calls the condition "Intellectual Developmental Disorder", ICD-11 calls it "Disorders of Intellectual Development", and the older, still common, clinical term is "intellectual disability"; severity (mild, moderate, severe, profound) is decided by adaptive functioning, not by an IQ score alone. ^[3]
• Its name changed from "mental retardation" to avoid stigma and to put the emphasis on function and support rather than on a single test score. ^[2]
• It is not rare: population estimates are around 1 to 3 percent of people, with higher rates in regions with limited healthcare access. ^[3][5]
• The work is mostly developmental support, education, behavioural strategies, and treating the (very common) additional physical and mental health conditions; medication is reserved for specific targets and is only an adjunct. ^[1]

By the end of this page you should be able to:

1. State how the DSM-5-TR and AAIDD definitions of IDD combine intellectual and adaptive limitations with onset in childhood.
2. Explain why severity is graded by adaptive functioning rather than IQ, and what the IQ range around 70 to 75 means.
3. List the main prenatal, perinatal, and postnatal causes and risk factors, and give examples of genetic and acquired causes.
4. Describe the clinical assessment: history, examination, standardised cognitive and adaptive testing, and the first-line investigations.
5. Use the diagnostic cut-off table to tell IDD from global developmental delay, borderline intellectual functioning, specific learning disorder, and dementia.
6. Plan management that covers lifelong support, behaviour that challenges, capacity, and mental health comorbidity, following NICE NG11 where relevant.
7. Conduct a basic risk and capacity assessment for an adult with IDD.
8. Explain how support and prognosis differ by severity and by age.

DSM-5-TR makes IDD one of the neurodevelopmental disorders and keeps the dual label "Intellectual Developmental Disorder (Intellectual Disability)". ^[3] AAIDD, whose definition is the best-known clinical standard outside North America, phrases it slightly differently but asks for the same three things: significant limitation in intellectual functioning, significant limitation in adaptive behaviour, and origin before age 22. ^[2] ICD-11 uses the term "Disorders of Intellectual Development" (6A00) and asks for performance about two standard deviations below the population mean on standardised testing, together with functional impairment; it also provides a provisional category for very young children or for those whose severe sensory or motor problems prevent standard testing. ^[3]

Severity is expressed as mild, moderate, severe, or profound. ^[3] In DSM-5-TR and ICD-11 this severity is based on adaptive functioning rather than on IQ. ^[3] The reason is simple: an IQ score is a snapshot of tested performance, whereas what matters for a person's life is whether they can manage money, use transport, plan a day, form relationships, and stay safe. ^[3] AAIDD gives the traditional IQ landmark of around 70, sometimes up to 75, as indicating a significant limitation in intellectual functioning, but a score in that range on its own does not make the diagnosis and does not decide the grade. ^[2]

• Global developmental delay (GDD) is a deliberately provisional label for a child under about 5 years who is at least two standard deviations below the mean in two or more developmental domains (motor, language, cognition, personal-social, daily living). ^[3][4] Many children with GDD later meet criteria for IDD, and many children who end up with IDD were first labelled with GDD. ^[4]
• Borderline intellectual functioning is low intellectual ability that falls short of the level seen in IDD but still causes practical difficulty at school or work; it is not a diagnostic category in DSM-5-TR, although it is coded as a V condition, and these patients still need support.
• Specific learning disorder affects one or a narrow set of academic skills (reading, written expression, mathematics) in a person whose overall intellectual ability is average or above.
• Communication disorders and language disorder can make a child look globally delayed; careful language assessment is part of every evaluation.
• Disorder of intellectual development, unspecified covers situations in which assessment is not possible yet (for example profound impairment early in life).

IDD is one of the commoner chronic conditions of childhood. Population estimates place it at roughly 1 to 3 percent of the general population, and a Global Burden of Disease analysis for 2019 put the figure at about 1.4 percent. ^[3] In children aged 3 to 17 years in the United States, diagnosed IDD has been stable at about 1.7 to 2.2 percent in recent surveys. ^[3] There is substantial regional variation, with more affected people in low- and middle-socio-demographic areas, estimated at five to six times the rate in high-socio-demographic areas, and higher rates also occur in places with limited healthcare access. ^[4][5] Prevalence varies between studies much more than biology does, because of differences in ascertainment, in the definition used, and in health-system infrastructure. ^[3]

There is a sex difference: males are affected more often than females, part of which is explained by X-linked conditions. The largest group is mild IDD (perhaps around two-thirds of cases), with severe and profound IDD far less common. ^[4]

Risk factors and causes are usually discussed by timing, a division that also helps a clinician build a differential diagnosis.

• Prenatal (before birth). Chromosomal abnormalities such as trisomy 21; copy number variants (for example deletions at 22q11.2); single-gene disorders such as fragile X syndrome (FMR1), Rett syndrome (MECP2), tuberous sclerosis (TSC1/TSC2), and Angelman and Prader-Willi syndromes. ^[3] Maternal infections (CMV, rubella, toxoplasmosis, syphilis, Zika), maternal alcohol (fetal alcohol spectrum disorder, which is common and preventable), other teratogens, and prematurity-related risks that begin before delivery. ^[3] Congenital CMV alone occurs in about 1 in 200 births in high-income regions and is the most common congenital infection. ^[3]
• Perinatal (around birth). Extreme prematurity, hypoxic-ischaemic encephalopathy, severe hyperbilirubinaemia, neonatal infection, and brain injury around delivery. ^[3]
• Postnatal (in infancy and childhood). Severe traumatic brain injury, meningitis or encephalitis, lead and other toxin exposure (the blood lead reference value for intervention is 3.5 microg/dL), child maltreatment and severe neglect, and iodine deficiency. ^[3]
• Acquired and environmental factors overall underlie roughly 11 to 55 percent of global developmental delay or intellectual disability, while genetic causes account for up to about 47 percent in cohorts that have been worked up. ^[4]

Across the whole life course, the causes are heterogeneous even within one person: a genetic vulnerability (say a de novo variant) often acts together with an environmental insult (a premature delivery complicated by hypoxia) rather than as a single switch. ^[4]

IDD is a final common pathway rather than a single disease. Different causes disrupt a small set of processes that the developing brain depends on, and this convergence is why seemingly different conditions look so similar.

• Synaptic signalling. Mutations and copy number variants frequently affect proteins involved in synapse formation and plasticity: neurexins and neuroligins, SHANK scaffolding proteins, NMDA and AMPA glutamate receptor subunits, SYNGAP1, and STXBP1. ^[4] When these fail, the brain's ability to strengthen and weaken connections in response to experience is impaired from the start.
• Protein homeostasis. The ubiquitin-proteasome system and growth-factor kinase pathways (for example RPS6KA3) regulate how many proteins a neuron holds and for how long. Disruption here leads to abnormal neuronal shape and connectivity. ^[4]
• Epigenetic regulation of transcription. MECP2 and CHD8 control whether whole sets of genes are turned on or off during neuronal maturation. Defects give characteristically uneven profiles of impairment. ^[4]
• Network and circuit level. Structural and functional abnormalities have been described in cerebral cortex, basal ganglia, thalamus, cerebellum, and long white-matter tracts. ^[4]
• Secondary injury. Hypoxia, infections, toxins, and trauma in the perinatal or postnatal period damage developing tissue directly, and severe epilepsy (for example in young children) can compound whatever the primary cause already inflicted.

The key point for a clinician is that by the time a person presents, the brain's architecture is already set; management cannot "cure" the underlying biology. What can change the trajectory is enriched environment, education tailored to the person, sensory correction, treatment of seizures and infections, and avoidance of further injury.

The history is the diagnosis in most cases; the brain has been not meeting expectations since early childhood.

• Developmental milestones. Sitting, walking, first words, phrases, toilet training, self-feeding, dressing and money use, all later than expected or never achieved. Ask the parents directly; do not rely on the patient's own account.
• Schooling. Need for special education, failure to learn to read or count, needing the curriculum broken down, repeated years.
• Adaptive skills. Managing money, travelling alone, cooking, personal care, telling time, safety, social judgement, friendships, employment.
• Regression or plateau. A child who was making progress and then stops is a red flag for a metabolic, genetic, or neurodegenerative condition (for example Rett syndrome, or a treatable metabolic disorder) and needs urgent investigation. ^[3][4]
• Sensory function. Unrecognised hearing loss or visual impairment can masquerade as or worsen cognitive delay.
• Seizures, movement disorder, sleep, feeding, and behaviour.
• Family history. Consanguinity, affected siblings, miscarriages, similarly affected relatives, and a known syndrome.
• Pregnancy, birth, and neonatal course. Perinatal infection, prematurity, jaundice, hypoxia.
• Social context. Housing, carer support, schooling available locally, abuse or neglect. ^[1]

IDD is a clinical syndrome; there is no single psychiatric sign. The examination looks for the level of function and for clues to cause.

• Level of alertness, cooperation, and rapport; the overall developmental level.
• Speech and language: expressive and receptive language, non-verbal communication.
• Attention, memory, and problem solving at a practical level (can the person follow a two-step instruction, tell a story, make change?).
• Social interaction: eye contact, reciprocity, ability to read social cues.
• Behaviour that challenges: triggers, form, and function.
• A brief structured cognitive screen only as a triage tool; in IDD the full assessment is formal and must be carried out by a trained assessor.
• Motor and neurological examination. Hypotonia, spasticity, ataxia, focal deficits, head circumference and growth, dysmorphic features (epicanthus, upslanting palpebral fissures, macroorchidism in fragile X), skin findings (hypopigmented macules in tuberous sclerosis), and hepatosplenomegaly.
• Sensory examination and ophthalmology and audiology assessment.

• Developmental regression at any age.
• Seizures, focal neurological signs, or a rapidly increasing head circumference.
• Dysmorphism, organomegaly, or multisystem involvement.
• A very young child with profound impairment.
• Consent or safeguarding concerns around the person or their carers.

Diagnosis is clinical and functional, then etiological. A useful sequence, usable from primary care through to specialist services, is:

1. Routine developmental surveillance and, in children, standardised screening at the recommended ages (for example 9, 18, and 30 months, with autism-specific screening at 18 and 24 months in the United States). ^[3]
2. Detailed history and examination covering milestones, schooling, adaptive skills, prenatal and perinatal factors, sensory function, family history, and social context. ^[3]
3. Audiology and vision assessment before or alongside cognitive testing, because unrecognised sensory loss can mimic or worsen apparent cognitive delay. ^[3]
4. Standardised cognitive assessment appropriate to the person's age, language, sensory, and motor state. ^[3]
5. Standardised adaptive behaviour assessment completed by multiple informants (parents, carers, teachers) rather than by one rater. ^[3]
6. Genomic investigation: chromosomal microarray first in established practice, with exome or genome sequencing increasingly recommended as a first- or second-tier test because of higher yield and cost-effectiveness; fragile X testing in males and where the history suggests it; methylation studies for syndromes suggested by the phenotype. ^[3][4]
7. Metabolic testing when there are red flags (regression, metabolic crises, multisystem involvement, consanguinity, or no newborn screening history). ^[3][4]
8. Neuroimaging and EEG when there are neurological signs, seizures, abnormal head growth, or regression, used as targeted rather than blanket investigations. ^[3]
9. Environmental and teratogenic review, including lead exposure and history of congenital infections. ^[3]

The table is a working summary for bedside use. IQ thresholds alone do not diagnose IDD, and severity is graded by adaptive functioning, not by IQ. ^[2][3]

The conditions most often confused with IDD are set out below. The distinguishing feature is in the third column rather than the label.

IDD is a lifelong condition, and the goal of treatment is support and participation, not cure. The same person needs different things at 5, 25, 45, and 75, so the management plan should be reviewed at each major transition.

• Support and independence are the end points: teach skills, adapt the environment, and use assistive technology rather than doing things for the person.
• Every physical and mental health problem that coexists should be identified and treated on its own merits, in line with the relevant guideline, with the format and delivery adjusted for the person's cognitive level. ^[1]
• Care should happen in the least restrictive setting possible, close to home, with services built around the person. ^[1]
• Shared understanding of the person (their communication, sensory profile, likes and triggers) comes before behaviour management. ^[1]
• Information must be given in accessible form (Easy Read, pictures, speech, different fonts) and decisions must be made with the person, their family, and carers. ^[1]

• Early intervention from as soon as delay is recognised.
• Individualised education plans with a functional curriculum (literacy and numeracy at a usable level, plus daily living, money, time, travel, communication, and community skills).
• Applied behaviour analysis and skills teaching for specific targets; always worth trying even when progress is slow.
• Speech and language therapy, including augmentative and alternative communication (sign, picture exchange, devices) for those with limited speech.
• Occupational and physiotherapy for daily living and motor skills.
• Transition planning well before school leaving age (education, employment, housing, health, and legal arrangements).

• Employment: supported or sheltered employment, with job coaching and task breakdown.
• Housing: supported living or residential care, chosen to maximise independence and safety.
• Day activities and structured daytime occupation, which both improve quality of life and reduce behaviour that challenges. ^[1]
• An named key worker or case manager who coordinates health, social care, education, and housing.

People with IDD have higher rates of epilepsy, sensory impairment, obesity, constipation, gastro-oesophageal reflux, dental disease, congenital heart disease, and hypothyroidism, and communication difficulties mean these problems are often missed. An annual physical health check using a standardised template, and carried out with someone who knows the person, is the single most practical preventive step. ^[1] The check should review behaviour, physical health, all current medicines and their side effects or interactions, and result in a shared care plan that includes pain management. ^[1]

People with IDD experience more illness than the general population and are less able to report it. The list below is the minimum that a clinician should keep in mind when the person seems "not themselves".

Intimate relationships and sexuality are part of normal life for people with IDD, including those with severe impairment. The clinical tasks are to support the person's autonomy, to provide relationship and sex education in accessible form, and to make sure that any relationship is genuinely consensual and free from exploitation. Consent to a sexual relationship is a capacity issue and must be assessed specifically for that decision. Safeguarding also means recognising that people with IDD are disproportionately exposed to abuse, financial exploitation, neglect, and institutional mistreatment, and that signs may be non-verbal or be attributed to "the disability". Any suspicion of abuse should trigger the local safeguarding pathway rather than an informal discussion.

• Carers should be involved in planning and given information in accessible form. ^[1]
• Advise carers of their right to a carer assessment, respite care, and other support. ^[1]
• Offer skills training, emotional support, and family support or information groups, especially when behaviour that challenges is emerging. ^[1]
• If a carer has a mental health problem, offer interventions or referral in line with the relevant guideline. ^[1]

Behaviour that challenges (aggression, self-injury, property destruction, withdrawal, paradoxical or unusual behaviours) is common but not inevitable, and it usually signals an unmet need, pain, communication difficulty, or environmental mismatch rather than "the diagnosis". The NICE NG11 approach is the standard taken here. ^[1]

• Understand the function of the behaviour: what triggers it, what it achieves (escape, attention, sensory input, pain relief), and what individual and environmental factors maintain it. ^[1]
• Carry out a functional assessment first, varying its depth with the complexity of the behaviour, and develop a written behaviour support plan based on a shared formulation. ^[1]
• Proactive strategies before reactive ones: adjust the environment (noise, predictability, task demands), build predictable routines, develop a positive daily programme, and teach replacement skills such as improved communication and emotional regulation. ^[1]
• Use biological and medical review: untreated pain, constipation, reflux, infection, seizures, and psychiatric illness are common maintainers. ^[1]
• Medication:
  • Medication should be considered or optimised for any coexisting mental or physical health problem that contributes to the behaviour, and psychological or other interventions should be offered alongside any drug treatment. ^[1]
  • Antipsychotics should be used only after psychological or other interventions have failed or been judged unlikely to work in time, or when treatment of a coexisting problem has not reduced the behaviour, or when the risk to the person or others is very severe, and only in combination with psychological or other interventions. ^[1]
  • The antipsychotic should be started by a specialist (psychiatrist or neurodevelopmental paediatrician) at a low dose, with the minimum effective dose, a single drug, a defined target behaviour, monitoring after 3 to 4 weeks, cessation at 6 weeks if there is no response, a 3-month then 6-monthly review if there is a response, and a plan to stop. ^[1]
  • Rapid tranquillisation follows the NICE violence and aggression guideline. ^[1]
• Reactive strategies, including restraint, are last-resort measures within the Human Rights Act 1998, the Mental Health Act 1983, and the Mental Capacity Act 2005; they must be proportionate, regularly reviewed, and paired with a reduction programme. ^[1]
• For sleep problems, behavioural interventions (functional analysis of the sleep behaviour, structured bedtime routine) come first; melatonin, which is off-label in people aged 55 years and under, is considered only after behavioural approaches, alongside non-drug measures. ^[1]

Mental health problems are very common in IDD and are under-recognised because of diagnostic overshadowing, communication barriers, and atypical presentation. ^[1][6] Depression, anxiety, psychosis, attention-deficit hyperactivity disorder, and autistic conditions occur more often than in the general population, and challenging behaviour is often the only visible expression of an underlying mental illness. ^[1][6] A recent systematic review of parents with intellectual disability found an overall mental disorder prevalence of 45 percent, and screening studies report rates up to 63 percent for depression, 30 percent for anxiety, and 17 percent for psychosis. ^[7] Because the person may not be able to report low mood or worry in words, the carer's observation of change in sleep, appetite, energy, pleasure, or self-care matters more than in other patients. ^[1]

Assessment therefore needs:

• A low threshold to look for mental illness whenever there is a change in behaviour, function, or self-care. ^[1]
• Use of adapted tools where possible (for example the Psychiatric Assessment Schedule for Adults with a Developmental Disability, the Diagnostic Assessment Schedule for the Severely Handicapped, or the Psychopathology Instrument for Mentally Retarded Adults) and expert opinion when a mental health problem may underlie behaviour that challenges. ^[1]
• Assessment of suicide and self-harm risk, harm to others, self-neglect, carer breakdown, exploitation, and rapid escalation, all reviewed regularly. ^[1]
• Adaptation of both psychological and pharmacological treatments: the diagnosis is still made against the standard criteria, but the content, pace, format, and setting should be adjusted for the person's developmental level. ^[1]
• A stepped, multidisciplinary pathway, with early referral to specialist intellectual disability or learning disability mental health services, which is where most of this care belongs. ^[1]

Capacity in IDD is decision-specific and time-specific, not a single global status. The principles that follow are those used in UK law, and other jurisdictions have equivalents.

• Presume capacity unless there is evidence to the contrary. ^[1]
• Support the person to make the decision (accessible information, pictures, extra time, a trusted supporter, the best time of day).
• An unwise decision is not proof of incapacity.
• Decide for the person (in their best interests, by the least restrictive means) only when they truly cannot decide even with support.
• Formal capacity assessment covers understanding, retaining, using or weighing, and communicating a decision, and it must be carried out for the specific decision (consent to a procedure, managing money, where to live).
• Best-interests decisions should involve the person as far as possible, family and carers, and independent advocates, and should consider the person's past and present wishes, beliefs, and values.
• Where a person with a severe learning disability lacks capacity to consent to care or treatment, and the care would amount to a deprivation of liberty, the safeguards of the Mental Capacity Act (or the relevant local law) apply, as noted in NICE NG11. ^[1]
• Capacity and consent also govern day-to-day issues that dominate this person's life: financial affairs, sexual relationships, marriage, voting where the law allows, and research participation. Specialist advice is often needed.

• Epilepsy, which occurs in a substantial minority of people with IDD, especially with severe impairment or specific syndromes, and which itself contributes to further cognitive and behavioural disturbance.
• Sensory impairment: vision and hearing loss are over-represented and often missed.
• Dysphagia, aspiration, and under-nutrition or over-nutrition.
• Constipation and gastro-oesophageal reflux.
• Dental disease.
• Skeletal and respiratory complications of severe motor impairment.
• Diagnostic overshadowing, where treatable coexisting illness is attributed to the disability itself.
• Social exclusion, unemployment, bullying, and abuse, which are more common and compounded by communication difficulty.
• Premature mortality, much of which is preventable and is driven by respiratory, cardiac, and undetected illness.

Prognosis depends on severity and on the quality of support.

• Mild IDD (the largest group) is compatible with independence in daily living, employment in supportive settings, relationships, and a normal lifespan, although the person may always need help with complex or unfamiliar tasks and with major decisions. ^[3][4]
• Moderate IDD usually needs structured support in daily living, and many adults live in supported accommodation and work in supported settings.
• Severe and profound IDD needs extensive, often lifelong support with self-care, communication, mobility, and health.
• Prognosis in IDD is not fixed: enriched early environments, timely sensory correction, education, management of epilepsy, and safe housing move the actual outcome well above the predicted one. ^[3]
• People with severe IDD and complex health needs have a shorter life expectancy than the general population, and much of this gap is preventable through better medical surveillance. ^[1]

• Primary prevention. Iodine sufficiency in pregnancy; folate before and early in pregnancy; maternal rubella vaccination; prevention of fetal alcohol exposure; antenatal and newborn screening for infection and metabolic disorders; safe delivery and neonatal care; lead abatement and environmental toxin control.
• Early detection. Standardised developmental screening at recommended ages and systematic developmental surveillance at every well-child visit; hearing and vision testing; prompt access to early intervention after a delay is noticed, without waiting for a diagnostic label. ^[3]
• Newborn and carrier screening for conditions that are treatable or that change management (for example, metabolic disorders and congenital CMV where screening is available).
• Genetic counselling for families after a diagnosed cause, which is especially important for X-linked and autosomal recessive conditions and for recurrence-risk planning.
• Prevention of secondary complications through annual health checks and the proactive medical management described above.

Children. Most diagnoses are first suspected in early childhood, and the label should be applied only when the full picture is present; for younger children, the provisional diagnosis of global developmental delay is appropriate. ^[3][4] Early intervention should not wait for the diagnosis. ^[3] Children with IDD have high rates of co-occurring autism (around 40 percent of eight-year-old children with autism meet IQ-based criteria for IDD) and ADHD, and a structured, multidisciplinary assessment is needed to separate these. ^[3]

Adolescents and transition. Puberty, sexual development, and the move from paediatric to adult services are high-risk periods; education and housing transitions should be planned early, and mental health surveillance stepped up.

Adults. The focus moves from diagnosis to housing, employment, finances, legal capacity, relationships, and age-related health. People with IDD sometimes lose supports in adulthood and regress functionally without a change in the underlying brain.

Older adults. Dementia and decline of vision and hearing must be distinguished from IDD itself; cognitive testing in a person with IDD is only meaningful against their own baseline. Carer breakdown is common. Safeguarding, honest conversation about goals of care, and continuity of familiar carers are the central issues.

Pregnancy. Women with IDD may have higher risks of perinatal complications and are entitled to the same antenatal care as anyone else, with information adapted to their understanding, extra time, and a dedicated support person. Some will be parents themselves and need tailored, non-judgemental parenting support rather than assumptions about capacity.

Low-resource settings. The diagnostic work-up can be scaled: most diagnoses are made on history and careful examination; a hearing test and, where available, a vision test and a screening tool for adaptive skills are the highest-yield investigations; basic blood, thyroid, and iron studies are cheap and useful; microarray and exome sequencing will not be locally available and can be requested through referral or research pathways. Drug treatment for comorbid conditions and behaviour should follow local formularies; non-drug behavioural and environmental approaches remain the foundation everywhere.

• IDD is a neurodevelopmental condition requiring impairment in both intellectual and adaptive functioning with onset in childhood; the label is double-barrelled by design. ^[2][3]
• AAIDD frames the onset as before age 22 and gives an IQ around 70 to 75 as indicating a significant limitation, but adaptive functioning, not IQ, decides both diagnosis and severity. ^[2][3]
• Population prevalence is roughly 1 to 3 percent, higher in deprived regions, with a male excess and the largest group being mild IDD. ^[3][4][5]
• Causes are heterogeneous: genetic in up to about 47 percent in well-worked-up cohorts, environmental and acquired factors in 11 to 55 percent, and an interacting combination in most real patients. ^[4]
• The common pathophysiology runs through synaptic signalling, protein homeostasis, epigenetic regulation, and brain development, which is why different causes produce a similar phenotype. ^[4]
• Global developmental delay is the provisional label under age 5, and standardised IQ testing becomes reliable after about 7 years. ^[3][4]
• Assessment must include hearing and vision, standardised cognitive testing, adaptive behaviour from more than one informant, and tiered genetic and metabolic investigation guided by red flags. ^[3][4]
• Common co-occurring conditions include autism, ADHD, epilepsy, and sensory impairment; about 40 percent of eight-year-old children with autism also meet IQ-based criteria for IDD. ^[3]
• Treatment is lifelong support: early intervention, adapted education and skills training, accessible information, carer support, annual physical health checks, and treatment of coexisting medical and psychiatric illness. ^[1]
• Behaviour that challenges usually reflects unmet need, pain, or environmental mismatch; NICE NG11 puts proactive behavioural assessment and environmental change before medication. ^[1]
• Antipsychotics, when used at all, are a last-resort adjunct, started low and reviewed early by a specialist, alongside non-drug interventions. ^[1]
• Mental health comorbidity is common and under-diagnosed; adapt assessment and treatment to the person's developmental level. ^[1][6]
• Capacity is decision-specific; the Mental Capacity Act principles (presume capacity, support, unwise is not incapacity, best interests, least restrictive) apply, and restrictive interventions must sit within the legal framework set out in NICE NG11. ^[1]
• Prevention centres on antenatal health, newborn and developmental screening, and supportive environments; much of the excess mortality in severe IDD is preventable. ^[1][3]

Plain-text related topics: autism spectrum disorder; attention-deficit hyperactivity disorder; global developmental delay; specific learning disorder; dementia and major neurocognitive disorders; cerebral palsy; epilepsy; communication disorders; fragile X syndrome; Down syndrome; tuberous sclerosis; fetal alcohol spectrum disorder; spina bifida.