Cerebral creatine deficiency syndromes (GAMT, GATM, SLC6A8)
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A 5-year-old boy has intellectual disability with expressive speech far more impaired than comprehension, autistic features, and seizures. Metabolic and genetic first-tier testing is unrevealing. Brain MR spectroscopy shows an absent creatine peak.
Three disorders, one shared endpoint of no creatine in the brain:
| Disorder | Gene | Inheritance | Mechanism |
|---|---|---|---|
| GAMT deficiency | GAMT | Autosomal recessive | Cannot complete creatine synthesis; neurotoxic guanidinoacetate builds up |
| AGAT deficiency | GATM | Autosomal recessive | Cannot start creatine synthesis |
| Creatine transporter defect | SLC6A8 | X-linked | Creatine is made but cannot enter the brain |
SLC6A8 is by far the most common, and being X-linked it is a recognized cause of X-linked intellectual disability in males. Carrier females can have milder learning and behavioral difficulties.
- Intellectual disability with disproportionate expressive language impairment
- Autistic features and behavioral difficulty
- Seizures, often difficult to control, most severe in GAMT deficiency
- Movement disorder, particularly a dystonic or extrapyramidal picture in GAMT deficiency
- Growth is usually normal, and there is no dysmorphism, so nothing on examination points here
- Brain MR spectroscopy showing a reduced or absent creatine peak is what unifies all three and is often the finding that opens the case
- Guanidinoacetate (GAA) separates them:
| Plasma/urine GAA | Urine creatine:creatinine | |
|---|---|---|
| GAMT deficiency | High | Normal or low |
| AGAT deficiency | Low | Normal or low |
| SLC6A8 transporter defect | Normal | High in males |
- Confirm with sequencing. In a male with a raised urine creatine:creatinine ratio, go straight to SLC6A8
- GAMT deficiency is on some newborn screening panels via elevated GAA
- The synthesis defects respond to treatment; the transporter defect does not. This is the single most important distinction to make
- AGAT deficiency: oral creatine monohydrate, with good response, particularly if started early
- GAMT deficiency: creatine monohydrate plus ornithine supplementation and arginine restriction to lower guanidinoacetate, which is itself neurotoxic and drives the seizures. Treating with creatine alone is inadequate
- SLC6A8: creatine supplementation does not work, because the defect is getting creatine into the brain rather than making it. Care is supportive; arginine and glycine have been tried with limited benefit
- Screen at-risk relatives, including carrier females in SLC6A8 families
GAMT: GAA Goes up. AGAT is the other one, so its GAA is low. That single letter hook carries the whole biochemical distinction, and the direction of GAA is what the diagnosis turns on.
For treatment: you can supply what is not made, but you cannot supply your way past a broken door. Synthesis defects (GAMT, AGAT) respond to creatine; the transporter defect (SLC6A8) does not.
- Consider this in any unexplained intellectual disability with speech disproportionately affected, especially in a boy with a suggestive family history. It is one of the few metabolic causes of nonsyndromic intellectual disability with a specific treatment.
- Urine GAA and creatine:creatinine are cheap and non-invasive. They are a reasonable add-on when first-tier testing is negative, before or alongside spectroscopy.