Urea cycle disorders and hyperammonaemia — branching viva

Branching viva on the urea cycle disorders: recognising the neonatal and late-onset presentations of hyperammonaemia, localising the defect with plasma amino acids and urinary orotic acid, delivering the 'treat on suspicion' emergency protocol, and locking in long-term medical and transplant-based management with family counselling.

branching clinical structured oral3 min readVerification in progress

Prompt

Neonatal unit: a term male infant, well at birth, now 60 hours old with lethargy, poor feeding, vomiting, hyperventilation and seizures. Blood gas shows a respiratory alkalosis. The ammonia returns at 580 micromoles per litre. The examiner asks: why is this a urea cycle disorder until proven otherwise, what is the 'treat on suspicion' emergency protocol, how do plasma amino acids and urinary orotic acid localise the defect, what is the ammonia-glutamine mechanism of cerebral oedema, and what are the dialysis thresholds — then branches to the mother, found to be an OTC heterozygote, and asks you to explain the X-linked counselling, carrier testing of siblings, and the role of liver transplantation.

Opening framework

My framework has four layers. First, the recognition — a well neonate deteriorating 24 to 72 hours into feeds with a respiratory alkalosis and encephalopathy is a urea cycle disorder until proven otherwise, and the ammonia is the test that decides it. Second, the emergency protocol — treat on suspicion: stop protein, calorie-load, scavenge nitrogen, give arginine, and dialyse at threshold. Third, the biochemistry — the cycle spans the mitochondrion and cytosol, and each block produces a recognisable metabolite pattern. Fourth, the family — an X-linked OTC diagnosis obliges carrier testing of the mother and sisters and reproductive counselling. [1]

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References5Show ledger

[1]Häberle J, Burlina A, Chakrapani A, Dixon M, et al. Suggested guidelines for the diagnosis and management of urea cycle disorders: First revision. J Inherit Metab Dis, 2019.PMID 30982989
[3]Brusilow SW, Maestri NE. Urea cycle disorders: diagnosis, pathophysiology, and therapy. Adv Pediatr, 1996.PMID 8794176
[5]Raina R, Bedoyan JK, Lichter-Konecki U, Jouvet P, et al. Consensus guidelines for management of hyperammonaemia in paediatric patients receiving continuous kidney replacement therapy. Nat Rev Nephrol, 2020.PMID 32269302
[10]García Vega M, Andrade JD, Morais A, et al. Urea cycle disorders and indications for liver transplantation. Front Pediatr, 2023.PMID 36937980
[12]Lo RS, Cromie GA, Tang M, et al. The functional impact of 1,570 individual amino acid substitutions in human OTC. Am J Hum Genet, 2023.PMID 37146589