Overview
- Developmental delay refers to a child performing significantly below age-expected norms in one or more developmental domains: gross motor, fine motor, language/communication, personal-social, and cognitive/adaptive functioning.
- When delay affects multiple domains simultaneously, it is termed global developmental delay (GDD).
- By convention, GDD is the preferred term for children under 5 years, reflecting the relative variability of developmental assessments in young children and acknowledging diagnostic uncertainty (though ability patterns are relatively stable from age 2 years).
- When delays persist, are confirmed on formal psychometric testing, and affect adaptive functioning, the term intellectual disability (ID) is applied.
Intellectual disability is defined by:
- Significant limitations in intellectual functioning (IQ approximately ≤70, i.e. ≥2 SD below the mean)
- Significant limitations in adaptive behaviour (conceptual, social, and practical domains)
- Onset during the developmental period
- The distinction between delay and disability is clinically important: delay implies potential for catch-up; disability implies permanence requiring long-term support planning.
- Many parents assume that any delay will eventually resolve, use of the word "disability" when appropriate avoids false reassurance.
Epidemiology
Prevalence of ID
| Severity | IQ Range | Approximate Prevalence |
|---|---|---|
| Borderline | 70-79 | ~14% of population (1-2 SD below mean) |
| Mild ID | 55-69 | ~2% of population (2-3 SD below mean) |
| Moderate-Severe ID | <55 | ~0.13-0.4% (>3 SD below mean) |
- Overall prevalence of GDD/ID: 0.67-3% of the paediatric population (variation due to methodology and case definition).
- Approximately 68% of the population have IQ within the average range.
Aetiology
- The cause of ID can be identified in 25-57% of mild ID cases and 70-90% of moderate-to-severe ID cases.
- Prenatal factors predominate; chromosomal abnormalities (Down syndrome most common) form the largest single aetiological group.
Aetiology by Timing
| Timing | Causes |
|---|---|
| Prenatal | Chromosomal abnormalities (Down syndrome/trisomy 21 most common), single-gene disorders (fragile X, Rett syndrome), copy number variants (CNVs), congenital infections (TORCH), teratogen exposure (alcohol → FASD), neural tube defects, cortical malformations, inborn errors of metabolism (PKU, hypothyroidism if untreated), iodine deficiency (greatest cause worldwide) |
| Perinatal | Hypoxic-ischaemic encephalopathy, preterm birth (IVH, periventricular leukomalacia), neonatal hypoglycaemia, neonatal meningitis/encephalitis |
| Postnatal | CNS infections, traumatic brain injury (including non-accidental), lead encephalopathy, near-drowning, severe psychosocial deprivation |
| Unknown | Remains significant, especially in mild ID |
Key Specific Causes
| Cause | Notes |
|---|---|
| Down syndrome (trisomy 21) | Most common chromosomal cause |
| Fragile X (FMR1 expansion) | Most common inherited cause; X-linked; predominantly males; dysmorphic features |
| FASD | Most common preventable cause |
| Copy number variants (CNVs) | Detected by CMA; submicroscopic deletions/duplications |
| Iodine deficiency | Greatest cause of ID globally |
| Rett syndrome (MECP2) | Females; developmental regression ~12-18 months |
| Tuberous sclerosis complex, NF1 | Neurocutaneous syndromes with autistic features |
| Inborn errors of metabolism | PKU, organic acidaemias, mucopolysaccharidoses, creatine deficiency, mitochondrial disorders |
| Intracranial tumours, storage disorders, demyelinating disorders | Progressive/regressive course, must be differentiated from static causes |
| Landau-Kleffner syndrome / electrical status epilepticus during slow-wave sleep | Epilepsy-associated regression; subclinical seizures; important treatable cause |
An alternative analytical framework considers risk factors: poor nutrition, low social class, and low maternal education are all associated with ID and have population-level management implications.
Developmental Surveillance Schedule
- Developmental surveillance is an ongoing longitudinal process integrated into routine child health visits.
- It differs from screening (a one-time test) and formal assessment (a structured multidisciplinary evaluation).
Key Surveillance Timepoints
| Age | Key Milestones to Assess |
|---|---|
| 6-8 weeks | Social smile, fixes and follows, axial tone, feeding |
| 4 months | Head control, reaching, cooing, social responsiveness |
| 6 months | Sits with support, rolls, babbles, reaches for objects |
| 9 months | Sits unsupported, developing pincer, stranger anxiety, "mama/dada" non-specifically |
| 12 months | Pulls to stand/cruises, pincer grasp, 1-2 words, social referencing, waves bye-bye |
| 18 months | Walks independently, ≥10 words, points, symbolic play, follows 2-step commands |
| 2 years | 2-word phrases, runs, ≥50 words, parallel play; refer if <20 words or no 2-word combinations by 2.5 years |
| 3 years | 3-word sentences, rides tricycle, toilet training, imaginative play |
| 4 years | Complex sentences, dresses independently, cooperative play, draws a person |
| 5 years (school entry) | Formal psychometric assessment if delay suspected |
Validated Surveillance and Screening Tools
| Tool | Type | Age Range |
|---|---|---|
| Ages and Stages Questionnaire-3 (ASQ-3) | Parent-completed; 5 domains | 1-66 months |
| Parents' Evaluation of Developmental Status (PEDS) | Parent-report; detects concerns | 0-8 years |
| Child Development Inventories (CDI) | Parent-completed developmental survey | Infancy-6 years |
| BRIGANCE Screens | Criterion-referenced; multiple domains | 0-7 years |
| Modified Checklist for Autism in Toddlers (M-CHAT) | ASD-specific screening | 16-30 months |
| Griffiths Mental Developmental Scales | Formal assessment; yields DQ/GQ | 0-6 years |
| Wechsler scales (WPPSI-IV, WISC-V) | IQ psychometric testing | From 2.5 years |
| Differential Ability Scales (DAS) | Cognitive assessment; used at school entry | 2.5-17 years |
| Stanford-Binet | IQ psychometric testing | From 2 years |
Note: The Denver II (DDST-II) is widely known but regarded as insensitive and lacking specificity for ASD; the R-DPSDQ is not recommended for primary-care developmental surveillance.
Developmental Quotient (DQ/GQ) (from Griffiths/Gesell scales) includes self-care and motor development, a broader concept than Intelligence Quotient (IQ), which relates more specifically to cognitive capacity.
Red Flags Requiring Urgent Referral at Any Age
- Loss of previously acquired skills (regression), always urgent
- No social smile by 8 weeks
- No babbling by 12 months
- No single words by 18 months
- No 2-word phrases by 24 months
- Fewer words than 6 months previously (regression in language)
- Limited or absent imaginative play by 3 years
- Parental concern at any stage, take seriously
Investigations and Diagnostic Algorithm
Principles
- Investigations are directed by clinical findings, not ordered universally.
- History (including family history, psychosocial environment, pregnancy/birth/neonatal details, developmental milestones, specific illnesses), examination (growth including head circumference, dysmorphic features, neurocutaneous signs, neurological examination, vision and hearing), and developmental profile guide the strategy.
Offer investigations proactively: not all families are focused on aetiology, but identifying the cause provides information on natural history, likely complications, recurrence risk, and helps parents process grief.
Tiered Investigation Algorithm
| Clinical Scenario | Investigations |
|---|---|
| All children with unexplained GDD/ID | CMA (first-line), fragile X DNA testing, TFTs, FBC, formal hearing assessment, vision assessment |
| Dysmorphic features / multiple congenital anomalies | CMA (first-line), genetics consultation before targeted single-gene testing |
| Boys with DD + hypotonia or motor concerns | Creatine kinase (CK), exclude Duchenne muscular dystrophy |
| Macrocephaly + CNS signs / neurocutaneous features | MRI brain, ophthalmology; CT if calcification suspected; check family history of macrocephaly |
| Microcephaly | TORCH serology, MRI brain, consider mild maternal PKU, Zika serology if relevant exposure |
| Regression / loss of skills | Overnight/sleep EEG (for Landau-Kleffner, ESES), metabolic screen (lactate, pyruvate, plasma amino acids, urine organic acids, ammonia), MRI, mucopolysaccharide screen |
| Severe ID | Lactate/pyruvate, urine amino acids, urine organic acids, mucopolysaccharide screen |
| Pica / old housing / paint exposure | Blood lead level |
| Seizures + DD | EEG, MRI, CMA; consider mitochondrial disorders |
| Suspected metabolic disorder | Plasma amino acids, urine organic acids, acylcarnitine profile, lysosomal enzymes |
| ASD with ID or dysmorphic features | CMA, fragile X; consider WES if CMA non-diagnostic |
| Low-average/borderline delay alone, no dysmorphism | Consider variation from family norms before extensive investigation |
Chromosomal Microarray (CMA), Key Investigation
CMA has replaced standard karyotype as the first-line genetic test for unexplained GDD/ID, per current Australian and international guidelines (RACP, ACMG).
| Feature | Standard Karyotype | CMA |
|---|---|---|
| Resolution | Detects abnormalities ≥5 Mb | Detects submicroscopic CNVs (deletions/duplications, few kb-Mb) |
| Diagnostic yield (GDD/ID + dysmorphism) | ~3-5% | ~15-20% |
| Detects balanced translocations | Yes | No |
| Detects trisomies | Yes | Yes |
CMA indications:
- Unexplained GDD or ID, with or without dysmorphic features
- Multiple congenital anomalies
- ASD with associated ID or dysmorphic features
- When single-gene disorder is not strongly suspected clinically
Limitations of CMA:
- Cannot detect single nucleotide variants (point mutations), requires WES or targeted gene panels
- May detect variants of uncertain significance (VUS), genetic counselling required
- Cannot detect balanced translocations, standard karyotype still required if clinically suspected
- Cannot detect trinucleotide repeat expansions, fragile X requires separate PCR/Southern blot
- Cannot detect epigenetic disorders (e.g. Prader-Willi, Angelman), genome-wide methylation analysis used for these
When standard karyotype is still indicated:
- Clinical suspicion of Down syndrome or other trisomy (pending rapid FISH/QF-PCR)
- Family history of chromosomal translocation
- Recurrent miscarriage in the family
Whole Exome Sequencing (WES):
- Used when CMA and fragile X testing are non-diagnostic, particularly in severe/unexplained ID, consanguineous families, or suspected monogenic cause.
- Trio WES (proband + both parents) maximises diagnostic yield.
- Over 9% of females and >7% of males with ASD have abnormalities on CMA; a similar proportion have abnormalities on WES.
- EEG: Indicated if epileptic events suspected, developmental slowing, or regression.
- Overnight/sleep EEG critical for Landau-Kleffner syndrome and subclinical electrical status during slow-wave sleep, conditions where epilepsy may not be clinically apparent yet causes developmental regression.
Diagnosis and Severity Classification
Global Developmental Delay (GDD)
- Significant delay (≥2 SD below mean) in ≥2 developmental domains
- Applied in children under 5 years when formal IQ testing is not yet feasible
- Repeat assessment prior to school entry determines whether delay has resolved or represents permanent disability
Intellectual Disability (ID), DSM-5 Framework
Diagnosed when all three criteria are met:
- IQ ≤70 on standardised psychometric testing
- Adaptive behaviour deficits in ≥1 adaptive domain (conceptual, social, or practical)
- Onset in the developmental period
Severity is determined primarily by adaptive functioning and support needs, not IQ alone:
| Severity | Approximate IQ | Adaptive Functioning | Support Needs |
|---|---|---|---|
| Mild | 50-69 | ~6th grade academic level; semi-independent living possible | Intermittent |
| Moderate | 35-49 | Limited academics; supervised work and living | Limited-extensive |
| Severe | 20-34 | Limited communication; significant daily support required | Extensive |
| Profound | <20 | Minimal self-care; constant support required | Pervasive |
Behaviour, socialisation, and activities of daily living should all be considered in gauging severity before predicting impact on academic performance.
Psychometric distribution:
-
2 SD below mean (IQ <70): affects ~2% of population
- 1-2 SD below mean (IQ 70-85, borderline): affects ~14% of population
- ~68% of population have IQ within average range
Clinical Presentations by Age
| Age Group | Typical Presentation |
|---|---|
| Neonatal/Early Infancy | Hypotonia, poor feeding, absent social smile, failure to fix and follow, recognisable dysmorphic syndrome (e.g. Down syndrome) |
| Infant (3-12 months) | Delayed motor milestones, poor social responsiveness, absent babbling, asymmetric tone |
| Toddler (1-3 years) | Absent or very limited words, not walking by 18 months, limited social/symbolic play; children with more severe ID typically present in first 2-3 years |
| Preschool (3-5 years) | Speech and language delay most common concern; may identify GDD on school readiness assessment; children with milder ID frequently present here or later |
| School age (5+ years) | Academic underperformance, learning difficulties, behavioural problems; late identification of mild ID |
| Adolescent | Emotional/behavioural difficulties, difficulties with abstract reasoning, transition and vocational concerns |
At-risk groups warranting active follow-up: preterm infants, small for gestational age, past history of meningitis/encephalitis, neonatal HIE, known genetic syndrome.
Management
Multidisciplinary Team
- Paediatrician, psychologist, speech pathologist, occupational therapist, physiotherapist, social worker, educator, and clinical geneticist as indicated.
- Assessment sessions often span several hours; each professional works with the child and clarifies history from parents.
Breaking the News
- State the team's findings clearly; allow time for parents to react
- Provide written summaries, much of what is said may be forgotten, especially when news is shocking; the manner in which it is imparted is remembered for life
- Copies of all reports to family and (with consent) to all involved clinicians
- Acknowledge grief as a normal reaction; unresolved grief can cause ongoing anger, depression, marital conflict, and delay remedial action for the child
- Arrange follow-up to review investigations and establish intervention services
Therapeutic Interventions
| Domain | Intervention |
|---|---|
| Communication | Speech-language pathology; augmentative and alternative communication (AAC) |
| Motor skills | Physiotherapy, occupational therapy |
| Adaptive skills | Occupational therapy, behaviour support |
| Cognitive/educational | Early intervention, individualised education plan (IEP) |
| Behavioural | Positive behaviour support; treat co-occurring ADHD, anxiety, ASD |
| Medical | Treat underlying cause (e.g. thyroid replacement); epilepsy management; lead chelation if indicated |
Early Intervention
- Early intervention (ideally before age 5) capitalises on neuroplasticity and is associated with better long-term outcomes.
- In Australia, early childhood early intervention (ECEI) is primarily funded through the NDIS.
Educational Planning
- IEP developed collaboratively with school, family, and allied health team
- School entry assessment (~age 5) using formal tools (WPPSI-IV, DAS, Griffiths) guides placement
- Options include integration aides, specialist support classes, or special schools depending on severity
- Communication with teachers about specific learning needs is essential
NDIS Pathways
The National Disability Insurance Scheme (NDIS) is the primary funding mechanism for disability supports in Australia for eligible individuals aged 0-65 years.
Eligibility Criteria
- Australian citizen or permanent resident
- Age under 65 at time of first access
- Has a permanent disability that significantly affects daily functioning
GDD/ID and NDIS Access
| Pathway | Description |
|---|---|
| Early Childhood Early Intervention (ECEI) | For children under 9 with developmental delay or disability; accessed via NDIS Early Childhood partner organisations; does not require a formal disability diagnosis |
| NDIS Plan | For confirmed ID/GDD; provides individualised funding for therapy, supports, equipment, and community access |
| Evidence Required | Diagnosis from paediatrician/psychologist confirming permanence and functional impact; allied health functional reports |
Paediatrician's Role in NDIS
- Providing diagnostic confirmation letters specifying permanence and functional impact
- Completing NDIS access request documentation
- Linking families with NDIS Local Area Coordinators (LACs) or ECEI partners
- Coordinating allied health reports to support access and plan reviews
Co-occurring Conditions
Children with GDD/ID have high rates of co-occurring conditions requiring active surveillance:
| Condition | Prevalence in ID | Key Consideration |
|---|---|---|
| Epilepsy | 20-30% | EEG; antiseizure medication; exclude epileptic regression |
| Autism spectrum disorder | 15-40% | ASD-specific supports; chromosomal abnormalities in 15-25% of ASD |
| ADHD | 30-40% | Behavioural strategies; medication |
| Anxiety/mood disorders | Elevated | Adapted psychological therapies |
| Sensory impairment (vision/hearing) | Elevated | Regular screening at every assessment |
| Sleep disorders | Common | Sleep hygiene; melatonin consideration |
| Feeding difficulties | Common in severe ID | Dietitian, SLT, gastroenterology |
| Behavioural challenges | Common | Positive behaviour support; medication in severe cases |
Prognosis and Follow-up
Prognosis depends on aetiology (static vs. progressive), severity of ID, early access to intervention, co-occurring conditions, and family/social supports.
- Mild ID: Many achieve semi-independent living with appropriate support; supported employment achievable
- Moderate-to-profound ID: Lifelong support needs; supervised living; quality-of-life focus
Developmental regression at any point requires urgent reassessment to exclude progressive or treatable neurological conditions: intracranial tumours, storage disorders, demyelinating disorders, epileptic encephalopathy, Landau-Kleffner syndrome, subclinical electrical status during slow-wave sleep.
Follow-up Schedule
| Period | Review Focus |
|---|---|
| Initial diagnosis | Aetiology investigations, sensitive disclosure, initial referrals, grief support, written summary |
| 6-monthly (early childhood) | Developmental progress, therapy response, behaviour, family wellbeing |
| School entry | Formal psychometric assessment, IEP planning, NDIS plan review |
| Annual | Co-occurring condition surveillance, growth, vision, hearing, puberty |
| Adolescence/young adulthood | Transition to adult services, NDIS plan review, vocational and sexual health planning |
When to Refer
| Situation | Action |
|---|---|
| Parental or clinician concern about development | Refer to developmental paediatrician; do not delay with false reassurance |
| Developmental regression | Urgent referral; same-day evaluation if acute |
| Dysmorphic features + DD | Clinical genetics consultation before targeted genetic testing |
| Elevated CK in boy with DD | Paediatric neurology + genetics; Duchenne MD excluded |
| Seizures + DD | Paediatric neurology; EEG, MRI, CMA |
| Language delay, no clear cause | Audiology first, then speech pathology and paediatrician |
| Suspected metabolic disorder | Metabolic/genetics team; may require inpatient stabilisation |
| NDIS access | ECEI partner for children under 9; LAC for older children |
| Severe behavioural disturbance | Developmental/behavioural paediatrician; inpatient if safety concern |
- Key clinical principle: GDD/ID should prompt a structured, staged investigation algorithm with CMA and fragile X testing as cornerstones, guided by history and examination findings.
- Always ensure families receive sensitive disclosure, genetic counselling regarding recurrence risk, and timely connection to early intervention and NDIS services.
- Identifying the aetiology matters, it defines natural history, anticipatory guidance for complications, recurrence risk, and informs family planning.