Developmental delay

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 $\leq 70$, i.e. $\geq 2$ 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

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

Key Specific Causes

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

Validated Surveillance and Screening Tools

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

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).

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 ($\geq 2$ SD below mean) in $\geq 2$ 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:

1. IQ $\leq 70$ on standardised psychometric testing
2. Adaptive behaviour deficits in $\geq 1$ adaptive domain (conceptual, social, or practical)
3. Onset in the developmental period

Severity is determined primarily by adaptive functioning and support needs, not IQ alone:

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

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

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

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:

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

When to Refer

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.

Sources

• Royal Children's Hospital Melbourne, Clinical Practice Guidelines
• Therapeutic Guidelines (eTG)
• NHMRC clinical practice guidelines