Why this matters
Intellectual disability (ID) and global developmental delay affect 1-3% of children, and most diagnostic effort rightly goes to defining the cause. But the single most actionable question at the bedside is different: is this one of the treatable inborn errors of metabolism (IEM)? A systematic review identified more than 80 IEMs that cause ID and have a disease-modifying treatment — dietary modification, vitamin/cofactor supplementation, substrate reduction, or specific drugs — that can prevent further neurological deterioration and, if started early, preserve or improve cognition. Missing them is missing the one chance to change the trajectory. This monograph emphasises screening for treatable causes, not a comprehensive ID etiology workup.
The core concept: screen treatable-first
The Treatable Intellectual Disability Endeavor (TIDE) reframes the workup so that treatable conditions are excluded before (or in parallel with) extensive, slower genomic evaluation. It is a two-tier protocol:
- Tier 1 — non-targeted metabolic screening: a small, inexpensive, widely available panel of blood and urine tests applied to every child with unexplained ID/developmental delay. This first tier alone can capture the majority (~60%) of the known treatable IEMs (on the order of ~50 conditions). Typical tier-1 investigations include plasma amino acids, urine organic acids, homocysteine, plasma/urine creatine metabolites and guanidinoacetate, copper/ceruloplasmin, and a urine purine/pyrimidine and oligosaccharide/glycosaminoglycan screen, among others.
- Tier 2 — targeted testing: single-metabolite assays, enzymology, or specific molecular tests pursued when the phenotype points to a particular disorder not covered by tier 1.
See the full criteria: Treatable Intellectual Disability Endeavor (TIDE).
Clinical features that raise the index of suspicion (red flags)
- Developmental regression or loss of previously acquired skills.
- ID/delay with additional features: seizures (especially treatment-resistant or vitamin-responsive), movement disorder, behavioural/psychiatric symptoms, or autism.
- Multisystem involvement — hepatosplenomegaly, coarse facies, cardiomyopathy, cataracts, dysmorphism, or unusual hair/skin.
- Consanguinity or a family history of a similarly affected sibling.
- Episodic decompensation with intercurrent illness, or biochemical clues (metabolic acidosis, hyperammonemia, hypoglycemia, lactic acidosis).
Importantly, many treatable IEMs can present as isolated, non-syndromic ID with a normal examination — which is exactly why a non-targeted tier-1 screen is justified rather than testing only “metabolic-looking” children.
Diagnostic approach
Apply the tier-1 screen at first assessment of any child with unexplained ID/developmental delay, in parallel with first-line genetic testing (chromosomal microarray, fragile X, and increasingly exome sequencing). Treat positive metabolic findings as urgent. The protocol demonstrably shortens time-to-diagnosis and identifies a treatable IEM in a clinically meaningful minority (>5%) of children screened.
Management overview
Management is disorder-specific and is the entire point of screening: examples of treatment strategy classes include dietary protein/substrate restriction (e.g., aminoacidopathies, urea-cycle defects), cofactor/vitamin supplementation (e.g., biotin, pyridoxine/B6, B12/folate, riboflavin, creatine), substrate-reduction or scavenging therapy, and metal-chelation or copper-replacement strategies. The unifying principle: confirm the specific diagnosis, then start the targeted, evidence-based therapy promptly under metabolic specialist guidance — early initiation is the strongest determinant of neurocognitive outcome. Specific agents and dosing are individualized by the treating metabolic team.
References
- van Karnebeek CDM, Stockler S. Mol Genet Metab. 2012.
- van Karnebeek CDM, et al. Paediatr Child Health. 2014.
- TIDE-BC protocol resources, treatable-id.org.
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.