A Young Adult with Episodic Ataxia and Vomiting: When to Escalate Testing for a Late-Onset Metabolic Disorder
A 24-year-old man presents with a 3-year history of recurrent episodes of vomiting, confusion, and unsteadiness, occurring every few months, sometimes after protein-rich meals or illness. Between episodes he is well. During an episode: ammonia 95 micromol/L (mildly elevated), glucose 4.2 mmol/L, lactate 2.0 mmol/L, anion gap normal, liver enzymes normal, urine organic acids show increased glutamine and a mildly elevated orotic acid. Plasma amino acids: citrulline 12 micromol/L, arginine 18 micromol/L, glutamine elevated, alanine elevated. He has a brother with similar episodes and a maternal uncle who had 'seizures' as a young adult. He is worried about his own children and asks whether 'this is genetic.'
1. What diagnosis best fits this presentation, and how do the family history and biochemical pattern support it?
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Diagnosis: a late-onset urea cycle disorder, most likely OTC deficiency (X-linked).
Supporting features:
- Episodic encephalopathy/ataxia/vomiting with mild hyperammonemia and elevated glutamine/alanine, low-normal citrulline, and mildly elevated orotic acid - the biochemical signature of partial OTC deficiency.
- Triggers: protein-rich meals and illness (catabolism) precipitate hyperammonemia.
- Family history: a brother with similar episodes and a maternal uncle with 'seizures' fits X-linked inheritance through the maternal line (males affected; females variable). This is a key clue.
Caveats: partial/late-onset defects can show only mild or intermittent abnormalities; ammonia may be normal between episodes. The combination of episodic encephalopathy, low citrulline, elevated glutamine, and elevated orotic acid in a male with a maternal family history is highly suggestive of OTC deficiency. Other considerations (organic acidemias, fatty acid oxidation defects) are less consistent with this pattern.
2. How would you confirm the diagnosis, and what is the role of molecular testing versus biochemical testing in this late-onset presentation?
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Confirmation strategy:
- Molecular testing of OTC is the preferred confirmatory test: sequencing plus deletion/duplication analysis (large deletions occur). A pathogenic variant confirms the diagnosis and enables family testing.
- Biochemical testing supports the diagnosis but is less definitive in partial defects: repeat plasma amino acids and ammonia during and between episodes, quantitative urine orotic acid, and consider a controlled protein challenge in a monitored setting if baseline testing is nondiagnostic. Liver enzyme assay is invasive and rarely needed when molecular testing is available.
- Test the mother for the same variant - she may be an asymptomatic or mildly affected carrier with skewed X-inactivation, which has implications for her own health and for counseling.
Why molecular first here: late-onset/partial defects can have normal or near-normal biochemistry between episodes, so a normal biochemical panel does not exclude the diagnosis; a molecular result is durable and informs cascade testing. If molecular testing is uninformative and suspicion remains high, pursue provocative biochemical testing and possibly enzyme assay.
3. The patient asks about risks to his children and whether his brother and uncle should be tested. What is your counseling approach?
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Inheritance: OTC deficiency is X-linked. A male with a pathogenic OTC variant transmits the variant to all of his daughters (who become carriers, with variable expression depending on X-inactivation) and to none of his sons (sons receive his Y).
Counseling points:
- Daughters: obligate carriers; they may be asymptomatic or have variable symptoms; they should be offered testing and education about avoiding triggers (protein loading, illness, valproate) and about monitoring during pregnancy.
- Sons: not at risk from him (they inherit his Y).
- Brother: if the brother has the same variant, his risks to children follow the same X-linked pattern; if he is unaffected, he may not carry it. Testing the brother is appropriate.
- Maternal uncle: his 'seizures' may represent undiagnosed OTC deficiency; he should be evaluated, and his daughters would be at risk.
- Mother: test her; if she carries the variant, her relatives (including the uncle's branch) are at risk - cascade testing.
- Reproductive options: carrier daughters can consider prenatal diagnosis or preimplantation genetic testing once the familial variant is known.
Emphasize that a normal newborn screen does not exclude OTC deficiency, so at-risk infants need biochemical monitoring regardless of NBS results.