Paeds · genetics-dysmorphology-and-metabolism
Amino-acid disorders including phenylketonuria and MSUD
Also known as Aminoacidopathies · Phenylketonuria · PKU · Maple syrup urine disease · MSUD · Tyrosinaemia type I · Classical homocystinuria · BH4 deficiency
A fellowship approach to the inherited amino-acid disorders: recognise phenylketonuria, maple syrup urine disease, tyrosinaemia type I and homocystinuria as autosomal-recessive blocks in amino-acid catabolism that share a single dangerous mechanism — a blocked step, an accumulating neurotoxic or tissue-toxic metabolite, and preventable brain injury — distinguish them on newborn-screen, metabolite and clinical signatures, deliver the acute 'switch off catabolism, clear the toxin' protocol for an MSUD crisis, and lock in long-term diet-plus-cofactor-plus-transplant management knowing that outcome tracks time on the toxic metabolite.
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Target exams
Red flags
- Any neonate well at birth who deteriorates 48 to 120 hours into feeds with poor feeding, vomiting, dystonia or opisthotonus, a maple-syrup or earwax odour, and ketoacidosis — think maple syrup urine disease and send a leucine and quantitative amino acids
- An infant with progressive intellectual disability, fair skin and hair, eczema and a mousy or musty odour — untreated phenylketonuria, now usually caught on newborn screening
- An infant with liver failure, a bleeding tendency, renal Fanconi syndrome and rickets, or painful neurovisceral crises — tyrosinaemia type I until succinylacetone returns
- A tall, thin child with downward lens dislocation, a marfanoid habitus and a thrombotic event — classical homocystinuria (CBS), the great Marfan mimic
- Limb dystonia or an abnormal gait in an infant labelled 'cerebral palsy' with hyperphenylalaninaemia — a BH4 cofactor defect, not classical PKU
- A woman with phenylketonuria planning or already pregnant with poor phenylalanine control — maternal PKU causes microcephaly, congenital heart disease and intellectual disability in an unaffected fetus
- A consanguineous family or a sibling history of a neonatal or unexplained death — raises the pre-test probability of an autosomal-recessive aminoacidopathy
Life stages
Care settings
Clinical exam formats
Board mappings
- Newborn bloodspot screening for amino-acid disorders (PKU, MSUD, tyrosinaemia)
- Acute recognition of a neonatal metabolic decompensation (MSUD crisis)
- Principles of dietary management of an inherited amino-acid disorder
- Biochemistry and classification of PKU, BH4 defects, MSUD, tyrosinaemia type I and homocystinuria
- Cofactor and enzyme-substitution therapy: sapropterin, pegvaliase, nitisinone, betaine
- Maternal PKU and the management of pregnancy in amino-acid disorders
- Stabilising an acute MSUD crisis and interpreting quantitative amino acids
- Distinguishing classical PKU from a BH4 cofactor defect
- Counselling a consanguineous family on a 25 percent recurrence risk
- Acute case of a neonate with encephalopathy, ketoacidosis and a maple-syrup odour
- Long case of a child with PKU discussing diet, sapropterin or pegvaliase and transition
- Inherited metabolic disease in the neonate and infant
- Investigation of the developmentally delayed or acutely unwell child
- Genetic counselling and the family implications of autosomal-recessive disease
- Phenylalanine, tyrosine and branched-chain amino-acid biochemistry
- Newborn screening and the aminoacidopathies
- Cofactor therapy and the management of homocystinuria and tyrosinaemia
- Recognising a metabolic emergency in the neonate
- Communicating a new amino-acid-disorder diagnosis and the emergency regimen to a family
- Phenylketonuria: newborn screening, diet and BH4 responsiveness
- Maple syrup urine disease: acute crisis management
- Tyrosinaemia type I and the role of nitisinone
- Inherited amino-acid metabolism disorders
- Acute metabolic decompensation in the infant
- Newborn screening interpretation and follow-up
- Amino-acid disorders including PKU and MSUD
- Newborn screening and the metabolic emergency
- Long-term management and maternal PKU
The fellowship mark goes to the candidate who reads these diseases through one shared mechanism rather than as four separate lists. The child in front of you may be a neonate in ketoacidotic coma, an infant with liver failure, or a tall thin child with a dislocated lens. The question is never "which rare disease is this" in isolation; it is "what is accumulating, where is the block, and how fast can I remove the substrate and clear the toxin". The biochemistry is the treatment plan, because the accumulating compound — phenylalanine, leucine, succinylacetone, homocysteine — is itself the disease. [1] [12]
Overview & Definition
The inherited amino-acid disorders are a family of autosomal-recessive diseases in which a single enzyme that breaks down a dietary amino acid is missing or sluggish, so the amino acid — and its downstream metabolites — accumulate in blood and tissue. The accumulation is not a marker of disease; it is the disease. Phenylalanine is neurotoxic to the developing brain, leucine causes cerebral oedema, succinylacetone destroys the liver and kidney, and homocysteine injures the lens, bone and blood vessels. This shared logic is why the management of all four disorders looks the same: restrict the precursor amino acid, supply the missing chemistry with a cofactor or enzyme when possible, and prevent the catabolic states that flood the blocked pathway. [1] [12]
Clinically these conditions sit among the intoxication-type inborn errors of metabolism, alongside the organic acidaemias and the urea cycle disorders. The unifying idea is a blocked catabolic pathway that generates a circulating toxin. That framing matters at the bedside because it tells you the priority: remove the toxin, do not wait for the enzyme name. Phenylketonuria is the prototypical and the most common, and it is the disorder that proved the entire concept — a toxic metabolite, a diet that removes it, and a normal intellect rescued by newborn screening. [1] [5]
References12ShowHide
- [1]Blau N, van Spronsen FJ, Levy HL. Phenylketonuria. Lancet, 2010.PMID 20971365
- [2]van Wegberg AMJ, MacDonald A, Ahring K, et al. European guidelines on diagnosis and treatment of phenylketonuria: First revision. Mol Genet Metab, 2025.PMID 40378670
- [3]Singh RH, Cunningham AC, Mofidi S, et al. Updated, web-based nutrition management guideline for PKU: An evidence and consensus based approach. Mol Genet Metab, 2016.PMID 27211276
- [4]Cunningham A, Rohr F, Splett P, et al. Nutrition management of PKU with pegvaliase therapy: update of the web-based PKU nutrition management guideline recommendations. Orphanet J Rare Dis, 2023.PMID 37349772
- [5]Camp KM, Parisi MA, Acosta PB, et al. Phenylketonuria Scientific Review Conference: state of the science and future research needs. Mol Genet Metab, 2014.PMID 24667081
- [6]Zori R, Thomas JA, Shur N, et al. Induction, titration, and maintenance dosing regimen in a phase 2 study of pegvaliase for control of blood phenylalanine in adults with phenylketonuria. Mol Genet Metab, 2018.PMID 30146451
- [7]Strauss KA, Puffenberger EG, Carson VJ Maple Syrup Urine Disease. GeneReviews, 1993.PMID 20301495
- [8]Frazier DM, Allgeier C, Homer C, Marriage BJ, Ogata B, Rohr F, et al. Nutrition management guideline for maple syrup urine disease: an evidence- and consensus-based approach. Mol Genet Metab, 2014.PMID 24881969
- [9]Shellmer DA, DeVito Dabbs A, Dew MA, et al. Cognitive and adaptive functioning after liver transplantation for maple syrup urine disease: a case series. Pediatr Transplant, 2011.PMID 20946191
- [10]Ficicioglu C Tyrosinemia Type I. GeneReviews, 1993.PMID 20301688
- [11]Sacharow SJ, Levy HL Homocystinuria due to Cystathionine Beta-Synthase Deficiency. GeneReviews, 1993.PMID 20301697
- [12]Arnold G, Vockley J Phenylalanine Hydroxylase Deficiency. GeneReviews, 1993.PMID 20301677