Intellectual Disability — Viva

Q1: Take me through the DSM-5 definition of intellectual disability. How does it differ from global developmental delay and from autism spectrum disorder? (2 min)

Intellectual disability (intellectual developmental disorder) under DSM-5 (and the closely-aligned ICD-11 "disorder of intellectual development") requires all three core criteria:^[1]

1. Deficits in intellectual functions — reasoning, problem-solving, abstract thinking, judgement, planning, academic learning and learning from experience, confirmed by standardised IQ testing approximately 2 SD or more below the population mean (IQ under 70).
2. Deficits in adaptive functioning — failure to meet developmental and sociocultural standards for personal independence and social responsibility, in at least ONE adaptive domain: conceptual (language, reading, writing, maths, reasoning, knowledge, memory), social (empathy, social judgement, interpersonal communication, friendship, social problem-solving) or practical (personal care, money management, occupation, school, organisation, transport, safety). Without ongoing support, the deficits limit functioning in one or more activities of daily life.
3. Onset during the developmental period — before age 18 years.

Severity is assigned on the basis of adaptive functioning (the level of support required), not IQ alone: mild (IQ 50 to 69, ~85 percent of cases), moderate (35 to 49, ~10 percent), severe (20 to 34, ~3 to 4 percent), profound (under 20, ~1 to 2 percent). The AAIDD 12th-edition manual classifies alternatively by intensity of support — intermittent, limited, extensive, pervasive.

Global developmental delay (GDD) is the term used in children under 5 whose milestones are delayed in two or more domains, when reliable IQ testing is not possible. It is the SAME underlying phenomenon as ID; the diagnosis converts to formal ID once standardised IQ testing can be performed (typically after age 5).

Autism spectrum disorder (ASD) is distinguished by its core deficit in social communication and restricted/repetitive behaviour, not by IQ. Crucially, DSM-5 criterion E: ASD is ADDED to ID only when the social-communication deficit is MORE impaired than expected for the overall developmental level. ID and ASD co-occur in 30 to 40 percent of cases of severe ID; both diagnoses are made when criteria are met.

Borderline intellectual functioning (IQ 71 to 84) is, by definition, NOT intellectual disability — a frequent examination trap.

DSM-5 retired the term "mental retardation"; ICD-11 uses "disorder of intellectual development". Older terms (mental retardation, mental handicap, feeble-minded, idiot/imbecile) are stigmatising and must not be used.

Q2: Walk me through the pathophysiology. Why does Down syndrome cause early Alzheimer dementia, and why does Fragile X cause intellectual disability? (3 min)

ID is the clinical expression of altered brain development in the prenatal and early postnatal period. The unifying mechanism is disruption of cortical neurogenesis, neuronal migration, synaptogenesis, myelination and synaptic pruning, producing inefficient neural networks. The locus and timing of disruption determine the syndromic phenotype.^[9]

Down syndrome (trisomy 21) causes ID through gene-dosage effects of genes on chromosome 21. The most clinically important is APP (amyloid precursor protein): over-expression of APP drives the near-universal development of Alzheimer-type neuropathology — amyloid plaques and neurofibrillary tangles — by age 40 to 50, with overt clinical dementia in over 50 percent by age 60.^[7] DYRK1A over-expression contributes to cognitive and growth deficits and is a therapeutic target. Other genes contribute to the cardiac (AVSD), gut (duodenal atresia), haematological (leukaemia) and endocrine (hypothyroidism) phenotype.

Fragile X syndrome (FMR1) is the commonest inherited single-gene cause of ID. A CGG trinucleotide repeat expansion (over 200 repeats) in the 5' UTR of FMR1 on Xq27.3 becomes hypermethylated and silenced, producing deficiency of FMRP (Fragile X Mental Retardation Protein) — an RNA-binding protein that normally represses dendritic translation of target mRNAs at the synapse. Its absence causes excessive, dysregulated synaptic translation, abnormal dendritic-spine morphology (long, immature, tortuous spines), impaired synaptic plasticity and dysregulated mGluR5 signalling — the basis of the cognitive deficit and of targeted drug trials such as AFQ056 (mavoglurant).^[3] Inheritance is X-linked dominant: full penetrance in males, about 50 to 60 percent in females (milder due to skewed X-inactivation). The premutation (55 to 200 repeats) is unstable on maternal transmission (anticipation) and is associated with FXTAS in adult men and FXPOI in women.

Other key mechanisms: Rett syndrome (MECP2 mutation in girls — loss of a methyl-CpG-binding protein that regulates BDNF and other activity-dependent genes, with regression at 6 to 18 months and stereotyped hand-wringing); PKU (PAH deficiency → phenylalanine accumulation → impaired myelination and neurotransmitter synthesis — completely preventable by newborn screening and diet); HIE (excitotoxicity, oxidative stress, apoptosis); lead poisoning (disrupts synaptic development and myelination).

Q3: How would you investigate a 4-year-old child with unexplained global developmental delay? (2 min)

The goal is to identify a treatable or syndromic cause (driving surveillance, prognosis, genetic counselling and increasingly specific therapy) and to detect comorbidity. I follow a tiered algorithm.^[1]

First-line (universal):

• Audiology (audiometry, auditory brainstem response) — mandatory in every case.
• Vision assessment — refraction, fundoscopy.
• Chromosomal microarray (CMA) — the first-line genetic test in unexplained ID, replacing karyotype; detects aneuploidies, deletions, duplications and copy-number variants; diagnostic yield 10 to 20 percent.
• Fragile X (FMR1) testing — second-line if CMA is negative.
• Thyroid function, FBC, U&E, LFT, fasting glucose, iron studies, vitamin B12/folate, calcium.
• Lead level if pica or environmental risk.

Second-line (guided by clinical picture):

• Metabolic workup (plasma amino acids, urinary organic acids, lactate, ammonia, urine MPS, VLCFA, transferrin IEF) — for regression, dysmorphism, organomegaly, parental consanguinity.
• MRI brain — NOT routine; reserved for focal neurology, abnormal head size, regression, neurocutaneous signs.
• EEG — if seizures, regression, paroxysmal events, or suspected Landau-Kleffner / electrical status epilepticus of slow sleep.
• Whole-exome / whole-genome sequencing — increasingly second- or third-tier for unexplained ID; per the ACMG 2021 guideline (Manickam et al.), recommended as a first- or second-tier test with a diagnostic yield of 30 to 50 percent.

The triad of preventable causes — PKU, congenital hypothyroidism, lead poisoning — is detected by newborn screening (PKU, hypothyroidism) and public-health programmes (lead).

Q4: A 30-year-old man with severe ID and minimal speech presents with a 3-day history of new agitation, self-injurious hand-biting and refusing food. How do you approach this? (3 min)

The cardinal principle is diagnostic overshadowing: a new behavioural change in a person with ID is a physical-health problem until proven otherwise. I follow a structured ABCDE-anchored approach.^[6][5]

1. Safety of the patient and others; reduce sensory triggers (quiet room, dim lights, remove unnecessary staff; allow trusted caregiver and familiar objects).
2. De-escalation (first-line) — calm predictable language, visual supports, reduce demands.
3. Systematically exclude organic precipitant FIRST:
   • Pain — dental abscess, otitis, headache, fracture, joint dislocation (consider atlantoaxial instability if Down syndrome), gastro-oesophageal reflux, corneal abrasion, constipation.
   • Infection — UTI, chest, skin, ear, dental, septicaemia.
   • Constipation — extremely common and often overlooked.
   • Medication side-effect — anticholinergic load, antipsychotic EPSE/akathisia, benzodiazepine, SSRI activation.
   • Seizure — post-ictal, subclinical.
   • Metabolic/endocrine — hypoglycaemia, hyponatraemia, hypothyroidism, dehydration.
   • Sleep deprivation, change in routine, bereavement, abuse.
4. Pharmacological intervention only as last resort when imminent risk: lorazepam 0.05 mg/kg PO/IM (max 2 mg); or low-dose antipsychotic (risperidone 0.5 to 1 mg, olanzapine 5 mg orodispersible, or haloperidol 2 to 5 mg IM with promethazine). Cautions: airway, QTc, EPSE, paradoxical disinhibition; review within 1 hour.
5. Multidisciplinary review (psychiatry, psychology, OT, SALT, adult ID, primary care) and a Positive Behaviour Support (PBS) plan based on functional analysis (ABC) to address the FUNCTION of the behaviour (communication, escape, attention, sensory, pain) rather than suppress it.

Q5 (examiner's probe): The same man has Down syndrome. What surveillance does he need over his adult life?

Down syndrome (trisomy 21) carries a defined surveillance programme because of the multisystem comorbidity:^[7]

• Cardiac — baseline echocardiogram in infancy (AVSD, VSD, PDA); monitor valvular disease in adulthood.
• Thyroid — annual TFTs (hypothyroidism common).
• Audiology and vision — annual; cataracts, refractive errors, keratoconus, conductive hearing loss.
• Atlantoaxial instability — radiological screening before anaesthesia, surgery and contact sports.
• Obstructive sleep apnoea — sleep study if symptoms; CPAP.
• Haematological — awareness of leukaemia risk (transient abnormal myelopoiesis in neonate; ALL in childhood).
• Gastrointestinal — coeliac disease screening, constipation, gastro-oesophageal reflux.
• Skin — dry skin, folliculitis.
• Mental health — depression, anxiety, OCD; depression presents atypically (somatic/behavioural equivalents).
• Cognitive decline / dementia — Alzheimer-type neuropathology is near-universal by age 50 because APP on chromosome 21 is over-expressed; overt dementia in over 50 percent by age 60. Baseline cognitive assessment in early adult life is essential so that later decline can be detected; watch for withdrawal, apathy, loss of skills, sleep disturbance, new urinary incontinence, seizures (new-onset seizures in a 40+ adult with Down syndrome strongly suggest Alzheimer disease).
• Annual health check in primary care with a health action plan.

Q6 (final probe): A 14-year-old girl with profound intellectual disability, multiple physical disabilities and treatment-resistant epilepsy is admitted with recurrent aspiration pneumonia. Her parents ask what the future holds. How do you counsel them?

This is a profound and multiple intellectual disability (PMID) scenario. The counselling is honest, compassionate, family-centred and framed around palliative-principles care (not necessarily end-of-life, but a comfort-focused, dignity-focused model integrated with active medical care):^[2]

• Prognosis is guarded: profound ID with multiple physical disabilities, treatment-resistant epilepsy and recurrent aspiration has a substantially reduced life expectancy; aspiration pneumonia and SUDEP are leading causes of death.
• Goals of care discussion: balance active treatment (epilepsy optimisation, anti-reflux, nutritional support — which may include gastrostomy) with comfort, dignity and avoidance of burdensome interventions that do not add quality. The principle is shared decision-making in the person's best interests (Mental Capacity Act 2005 / RPwD Act 2016).
• Epilepsy optimisation — review with the epilepsy service; consider dietary therapy (ketogenic), vagus-nerve stimulation, epilepsy surgery if a resectable focus; rescue medication for clusters (buccal midazolam); SUDEP counselling.
• Aspiration prevention — speech and language therapy swallow assessment, texture modification, positioning, anti-reflux medication, consider gastrostomy/jejunostomy; treat reflux and constipation.
• Pain and symptom management — proactive, using the DisDAT tool for non-verbal distress; access to specialist palliative care.
• Family support — respite, sibling support, psychological support for parents, advance care planning, signposting to parent-led organisations; financial/benefits advice; respecting the family's central role.
• Capacity and ethics — decisions made in the best interests of the person, with the least restrictive option, and with the family as partners. Advance care planning and agreed escalation status (ceiling of care) are appropriate and should be revisited regularly.

The over-arching principle is that every life has equal value, and care is shaped around dignity, comfort and quality — not the absence of active treatment, but the right balance for this individual at this time.

References8ShowHide

1. [1]Manickam K, McClain MR, Demmer LA, et al. Exome and genome sequencing for pediatric patients with congenital anomalies or intellectual disability: an evidence-based clinical guideline of the American College of Medical Genetics and Genomics (ACMG). Genetics in Medicine, 2021.PMID 34211152
2. [2]Scheffer IE, Zuberi S, Mefford HC, et al. Developmental and epileptic encephalopathies. Nature Reviews Disease Primers, 2024.PMID 39237642
3. [3]Kaufmann WE, et al. Drug Treatments for Neurodevelopmental Disorders: Targeting Signaling Pathways and Homeostasis. Current Neurology and Neuroscience Reports, 2024.PMID 39641900
4. [5]Robertson J, Hatton C, Emerson E, et al. The impact of health checks for people with intellectual disabilities: an updated systematic review. Research in Developmental Disabilities, 2014.PMID 24984052
5. [6]Cooper SA, McLean G, Guthrie B, et al. Multiple physical and mental health comorbidity in adults with intellectual disabilities: population-based cross-sectional analysis. BMC Family Practice, 2015.PMID 26310664
6. [7]Rafii MS, et al. Down syndrome and Alzheimer's disease: insights into biomarkers, clinical symptoms, and disease mechanisms. Lancet Neurology, 2025.PMID 40818475
7. [8]Leblay Y, et al. From Gene to Hope: Rett Syndrome and the Rise of Molecular Therapies. Molecular Diagnosis and Therapy, 2026.PMID 41975033
8. [9]Glasson EJ, et al. Systematic Review and Meta-analysis: Mental Health in Children With Neurogenetic Syndromes. Journal of the American Academy of Child and Adolescent Psychiatry, 2020.PMID 31945412