Skip to main content
WoMWeb of Medicine
SearchGraphHistory
Medical Genetics›Condition·Updated Jul 11, 2026·v1

Urea Cycle Disorders

Urea cycle disorders are a group of inherited metabolic diseases causing hyperammonemia due to defects in six enzymes or two transporters. Acute management requires immediate IV sodium benzoate and dialysis for severe crises. Chronic therapy uses nitrogen scavengers (preferably glycerol phenylbutyrate), dietary protein restriction, and for arginase 1 deficiency, pegzilarginase. Despite advances, neonatal-onset disease still carries high mortality and neurocognitive sequelae in one-third of survivors. Early diagnosis through newborn screening and genetic counseling is critical for improving outcomes.

High Evidence90 references·9,657 words·39 min read·v1
urea cycle disordershyperammonemiagenetic metabolic disordersornithine transcarbamylase deficiencyarginase deficiencycitrullinemianewborn screeningsodium benzoateglycerol phenylbutyratepegzilarginase

Quick Reference

RxDrug of choiceFor acute hyperammonemic crisis: IV sodium benzoate (loading 250 mg/kg over 2 h, then maintenance 250 mg/kg/day). For chronic management: glycerol phenylbutyrate (GPB) 1.5-31.8 g/day in adults, titrated to maintain fasting ammonia <60 µmol/L. For arginase 1 deficiency: pegzilarginase weekly.
AltAlternativesSodium phenylbutyrate (NaPB) 7.15 g·m⁻²·day⁻¹; sodium benzoate 5.5 g·m⁻²·day⁻¹; combination of half-dose NaPB + half-dose NaBz for cost reduction.
⊘AvoidValproate (precipitates hyperammonemia); non-dihydropyridine CCBs (no specific contraindication but avoid catabolic triggers); prolonged fasting; high-protein meals without scavenger coverage.
DxTest of choiceStat plasma ammonia (must be drawn on ice, processed within 15 min). If >100-150 µmol/L with encephalopathy, proceed with plasma amino acids, urine orotic acid, and molecular genetic testing of the causal gene (OTC, CPS1, ASS1, ASL, ARG1, etc.).
ScKey scorePlasma ammonia level >500 µmol/L with encephalopathy (GCS <15) is the threshold for immediate dialysis. Fasting ammonia ≥1.0 × ULN predicts a 4.5-fold higher risk of hyperammonemic crisis.
→When to referRefer for liver transplantation evaluation if hyperammonemia is uncontrolled despite maximal medical therapy, if progressive liver fibrosis/cirrhosis develops, or if neurocognitive function declines. Also refer for genetic counseling and reproductive options (prenatal diagnosis, PGT-M) for at-risk families.
UCDs cause hyperammonema leading to brain injury; early diagnosis and aggressive treatment with nitrogen scavengers and dialysis improve outcomes. Long-term management requires a multidisciplinary approach with metabolic specialists, dietitians, and genetic counselors.
Urea cycle disorders (UCDs) are inherited metabolic diseases caused by defects in one of six enzymes or two transporters of the hepatic urea cycle, leading to impaired ammonia detoxification and life-threatening hyperammonemia. With a combined prevalence of approximately 1 in 35,000 live births, UCDs represent a significant cause of intoxication-type metabolic disease. The key to preventing irreversible neurologic injury is early recognition, suspect UCD in any neonate with unexplained encephalopathy, respiratory alkalosis, and hypertension, and immediate initiation of ammonia-lowering therapy. Modern management combines nitrogen-scavenger medications (e.g., glycerol phenylbutyrate, sodium benzoate), dietary protein restriction, and, for arginase 1 deficiency, enzyme replacement with pegzilarginase. Despite advances, neonatal-onset disease still carries a mortality of 25-30%, and neurocognitive deficits affect one-third of survivors.

Overview and Recommendations

Background

  • •Urea cycle disorders (UCDs) are a group of inherited metabolic diseases caused by defects in one of six enzymes or two transporters in the hepatic urea cycle, resulting in impaired ammonia detoxification and hyperammonemia. The combined prevalence is approximately 1 in 35,000 live births, making them a significant cause of intoxication-type metabolic disease in children and adults worldwide.
  • •The cycle comprises five enzymes (CPS1, OTC, ASS1, ASL, ARG1) and two transporters (ORNT1, citrin). Ornithine transcarbamylase (OTC) deficiency is the most common UCD, accounting for ~53% of cases, and is X-linked; all other UCDs are autosomal recessive. Null variants produce neonatal-onset disease, while hypomorphic alleles cause late-onset forms.
  • •Ammonia is detoxified in astrocytes by glutamine synthetase, which converts ammonia and glutamate to glutamine. In hyperammonemia, glutamine accumulates, causing osmotic swelling, astrocyte dysfunction, cerebral edema, and ultimately herniation. Beyond osmotic stress, ammonia impairs neurotransmitter systems, inhibits α-ketoglutarate dehydrogenase, and induces mitochondrial dysfunction and oxidative stress.
  • •The four pillars of acute management, IV nitrogen scavengers, dialysis, caloric support, and protein restriction, have reduced mortality but not eliminated neurologic injury. A landmark shift occurred with the introduction of glycerol phenylbutyrate (GPB), which provides superior overnight ammonia control compared to sodium phenylbutyrate, and for arginase 1 deficiency, pegzilarginase enzyme replacement therapy has shown sustained motor and biochemical improvements.

Evaluation

  • •Suspect a UCD in any neonate with progressive lethargy, poor feeding, tachypnea, vomiting, hypothermia, or seizures within the first 24-72 hours of life. In older children and adults, consider UCD when recurrent vomiting, headache, ataxia, confusion, or behavioral changes are precipitated by catabolic stressors (infection, surgery, high-protein intake, or valproate use).
  • •Ask about protein aversion, cyclic vomiting, family history of unexplained neonatal death, and exposure to medications that can trigger hyperammonemia (e.g., valproate, corticosteroids). In females, inquire about neuropsychiatric symptoms such as mood disorders and sleep disturbances, which may precede overt hyperammonemia.
  • •Examine for encephalopathy: irritability progressing to obtundation, asterixis, hyperreflexia, clonus, and extensor plantar responses. In chronic UCDs, especially arginase 1 deficiency, look for spastic diplegia, hypertonia, cognitive deficits, and gait impairment. Fundoscopy may reveal papilledema from cerebral edema.
  • •Order a stat plasma ammonia level, the sentinel test. A level >150 µmol/L (or >100 µmol/L in neonates after day 1) with a normal anion gap and hypoglycemia excluded points strongly to a UCD. The sample must be drawn without tourniquet stasis, placed on ice, and processed within 15 minutes to avoid spuriously elevated values.
  • •Obtain plasma amino acid analysis to localize the enzyme block: low citrulline in OTC and CPS1 deficiency; markedly elevated citrulline (>200 µmol/L) in citrullinemia and argininosuccinic aciduria; elevated arginine in arginase 1 deficiency; elevated glutamine as a surrogate for ammonia burden.
  • •Order urine organic acids and orotic acid to differentiate proximal disorders: elevated orotic acid and uracil occur in OTC deficiency, whereas both are absent in CPS1 deficiency. In argininosuccinic aciduria, argininosuccinic acid is detectable in urine.
  • •Confirm the diagnosis with molecular genetic testing (targeted gene panel or whole-exome sequencing). This identifies the causative variant, enables carrier testing, and informs reproductive counseling. In males with OTC deficiency and a negative panel, sequence the OTC promoter and enhancer, as regulatory variants account for up to 24% of cases.
  • •Diagnostic criteria for hyperammonemic encephalopathy include: plasma ammonia >100-150 µmol/L, encephalopathy (Glasgow Coma Scale <15 or abnormal neurologic exam), and exclusion of other causes (sepsis, hepatic failure, organic acidemias, fatty acid oxidation defects). In neonates, blood pressure above the 95th percentile is a red flag distinguishing UCD from sepsis, present in 81% of cases.
  • •Also consider: transient hyperammonemia of the newborn, organic acidemias (e.g., propionic acidemia, methylmalonic acidemia), fatty acid oxidation disorders, and liver failure. A normal ammonia level does not exclude late-onset UCD; in females with suspected OTC deficiency, direct genetic sequencing is more sensitive than biochemical testing alone.

Management

  • •In acute hyperammonemic crisis, initiate IV sodium benzoate immediately: loading dose 250 mg/kg over 2 hours, followed by maintenance infusion 250 mg/kg/day. This regimen reduces median plasma ammonia from 245.5 µmol/L to 40.0 µmol/L in survivors, with 92.8% achieving ≤100 µmol/L. Continue until ammonia is consistently <100 µmol/L and the patient is neurologically stable.
  • •If ammonia exceeds 500 µmol/L with encephalopathy, or if it rises despite medical therapy, refer for hemodialysis or continuous veno-venous hemodiafiltration without delay. Earlier dialysis (at ≤500 µmol/L) improves neurologic survival compared to waiting for higher levels; median ammonia at dialysis start in one series was 1199 µmol/L, far above the threshold for irreversible injury.
  • •Monitor plasma ammonia every 1-2 hours during the acute phase and after each dialysis session. Fasting ammonia correlates with daily exposure and predicts crisis risk: patients with fasting NH₄⁺ ≥1.0 × ULN have a 4.5-fold higher relative risk of hyperammonemic crises compared to those with <0.5 × ULN.
  • •Once ammonia falls below 100 µmol/L and the patient tolerates enteral feeds, transition to oral long-term nitrogen scavenger therapy. Glycerol phenylbutyrate (GPB) is preferred over sodium phenylbutyrate (NaPB) due to its slow-release liquid formulation, superior overnight ammonia control, and improved executive function in children. GPB dose: 1.5-31.8 g/day for adults; titrate to maintain fasting ammonia <60 µmol/L.
  • •If GPB is not available or tolerated, use sodium phenylbutyrate (NaPB) at 7.15 g·m⁻²·day⁻¹. Pre-prandial administration (30 minutes before breakfast) maximizes systemic exposure and glutamine consumption, reducing total daily drug burden. Monitor branched-chain amino acids (BCAA) as NaPB therapy depletes valine, leucine, and isoleucine; supplement when needed.
  • •Alternative and combination therapy: sodium benzoate (NaBz) at 5.5 g·m⁻²·day⁻¹ can be used alone or in half-dose combination with NaPB. Combination therapy lowers cost without sacrificing efficacy, as nitrogen disposal is similar between the two arms. However, conjugation efficiency plateaus at 65% for both drugs, limiting the benefit of further dose escalation.
  • •For arginase 1 deficiency (ARG1-D), start pegzilarginase weekly at the approved dose plus standard of care. In open-label studies, this improved 6-minute walk test by 68.2 m (+19%), resolved spasticity (Modified Ashworth Scale 0) in 12 of 25 patients, and maintained plasma arginine below 115 µmol/L. Early intervention is critical for optimal motor outcomes.
  • •Dietary protein restriction is the cornerstone of supportive therapy: target approximately 13-15% of total energy from protein, with essential amino acid supplementation to maintain anabolic balance. Avoid prolonged fasting; provide high-carbohydrate, low-protein snacks between meals. During intercurrent illness, implement an emergency sick-day protocol: stop protein, provide high-calorie fluids, and contact the metabolic team.
  • •What NOT to do: Do not delay dialysis until ammonia reaches 1199 µmol/L, by that point, neurologic injury is often irreversible. Do not rely on respiratory alkalosis or hypertension alone to exclude sepsis; these findings are common in UCD crisis but do not rule out concomitant infection. Do not use bicarbonate for metabolic acidosis correction unless pH is critically low (<7.2) and hyperammonemia is being simultaneously treated.
  • •When to refer: Refer for liver transplantation evaluation if hyperammonemia is uncontrolled despite maximal medical therapy, if progressive liver disease develops (fibrosis, cirrhosis, hepatocellular carcinoma), or if neurocognitive function declines. Transplant offers definitive metabolic correction but carries operative mortality and lifelong immunosuppression risks.
  • •Discharge criteria after acute crisis: plasma ammonia <100 µmol/L, patient neurologically stable (awake, tolerating enteral feeds, GCS 15), and transition to oral long-term therapy established. Provide a written emergency protocol and a follow-up appointment within 1-2 weeks with the metabolic team. Annual monitoring includes liver ultrasound with elastography, neuropsychological screening, and nutritional assessment.

Board Review — High Yield

  • •OTC deficiency, Most common UCD, X-linked; presents with hyperammonemia and elevated orotic acid; neonatal males have severe disease, females may be asymptomatic or have late-onset crises.
  • •Arginase 1 deficiency, Presents with spastic diplegia, hyperargininemia, and minimal hyperammonemia; pegzilarginase is first-line disease-modifying therapy.
  • •Acute management, IV sodium benzoate 250 mg/kg loading; dialysis if ammonia >500 µmol/L and encephalopathy.
  • •Glycerol phenylbutyrate, Preferred chronic therapy due to slow-release, better overnight ammonia control, and improved executive function in children.
  • •Newborn screening, Detects citrullinemia and argininosuccinic aciduria but not OTC deficiency; second-tier molecular testing improves detection.
  • •Prognosis, Neonatal-onset mortality 25-30%; 34% of survivors have intellectual disability; earlier diagnosis and treatment have improved outcomes since 2000.
  • •Complications, Liver fibrosis in 46-52% of UCD patients; annual ultrasound with elastography recommended.
  • •Pregnancy, Catabolic stress can unmask UCD; check ammonia in any pregnant woman with vomiting, coma, or unexplained encephalopathy.

Deep Dive — Evidence Details

References

  1. [1]

    Rudebeck M, Braverman N, Chang R et al.. “Clinical Characteristics of Arginase 1 Deficiency: Natural History Insights From International Clinical Trials.” Journal of inherited metabolic disease (2026). PMID: 41651652 ↗

    L4TRIAL_NONRANDOMCited in: Definition, Classification & Genetic Nomenclature, Clinical Presentation & Phenotypic Spectrum, History and Evolution of Treatment, Prognosis & Natural History
  2. [2]

    Slenter DN, Hemel IMGM, Evelo CT et al.. “Extending inherited metabolic disorder diagnostics with biomarker interaction visualizations.” Orphanet journal of rare diseases (2023). PMID: 37101200 ↗

    L5OTHERCited in: Definition, Classification & Genetic Nomenclature, Variant Interpretation & ACMG/AMP Classification, Genotype-Phenotype Correlation, Severity & Risk Stratification
  3. [3]

    Sin YY, Baron G, Schulze A et al.. “Arginase-1 deficiency.” Journal of molecular medicine (Berlin, Germany) (2015). PMID: 26467175 ↗

    L5REVIEW_NARRATIVECited in: Definition, Classification & Genetic Nomenclature, Pathophysiology & Molecular Mechanism
  4. [4]

    Posset R, Zielonka M, Gleich F et al.. “The challenge of understanding and predicting phenotypic diversity in urea cycle disorders.” Journal of inherited metabolic disease (2023). PMID: 37702610 ↗

    L5REVIEW_NARRATIVECited in: Definition, Classification & Genetic Nomenclature
  5. [5]

    Marini JC, Lee B, Garlick PJ. “In vivo urea kinetic studies in conscious mice.” The Journal of nutrition (2006). PMID: 16365083 ↗

    L5OTHERCited in: Definition, Classification & Genetic Nomenclature
  6. [6]

    Yuan G, Liu Z, Chen Z et al.. “Clinical characteristics and molecular genetic analysis of ten cases of ornithine carbamoyltransferase deficiency in southeastern China.” Italian journal of pediatrics (2024). PMID: 39256843 ↗

    L4OTHERCited in: Definition, Classification & Genetic Nomenclature, Diagnosis & Genetic Testing Cascade
  7. [7]

    Silvera-Ruiz SM, Gemperle C, Peano N et al.. “Immune Alterations in a Patient With Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome: A Case Report.” Frontiers in immunology (2022). PMID: 35711415 ↗

    L4CASE_REPORTCited in: Pathophysiology & Molecular Mechanism, Acute & Crisis Management, Complications & Multisystem Surveillance
  8. [8]

    Ivanovski I, Ješić M, Ivanovski A et al.. “Metabolically based liver damage pathophysiology in patients with urea cycle disorders - A new hypothesis.” World journal of gastroenterology (2017). PMID: 29209134 ↗

    L5CASE_REPORTCited in: Pathophysiology & Molecular Mechanism
  9. [9]

    Ribas GS, Lopes FF, Deon M et al.. “Hyperammonemia in Inherited Metabolic Diseases.” Cellular and molecular neurobiology (2021). PMID: 34665389 ↗

    L5REVIEW_NARRATIVECited in: Pathophysiology & Molecular Mechanism
  10. [10]

    Duff C, Baruteau J. “Modelling urea cycle disorders using iPSCs.” NPJ Regenerative medicine (2022). PMID: 36163209 ↗

    L5REVIEW_NARRATIVECited in: Pathophysiology & Molecular Mechanism
  11. [11]

    Lacabanne D, Sowton AP, Jose B et al.. “Current Understanding of Pathogenic Mechanisms and Disease Models of Citrin Deficiency.” Journal of inherited metabolic disease (2025). PMID: 40145619 ↗

    L5REVIEW_NARRATIVECited in: Pathophysiology & Molecular Mechanism
  12. [12]

    Bigot A, Tchan MC, Thoreau B et al.. “Liver involvement in urea cycle disorders: a review of the literature.” Journal of inherited metabolic disease (2017). PMID: 28900784 ↗

    L5REVIEW_NARRATIVECited in: Pathophysiology & Molecular Mechanism
  13. [13]

    Diaz GA, Bechter M, Cederbaum SD. “The role and control of arginine levels in arginase 1 deficiency.” Journal of inherited metabolic disease (2022). PMID: 36175366 ↗

    L5REVIEW_NARRATIVECited in: Pathophysiology & Molecular Mechanism
  14. [14]

    Nagamani SC, Erez A, Lee B. “Argininosuccinate lyase deficiency.” Genetics in medicine : official journal of the American College of Medical Genetics (2012). PMID: 22241104 ↗

    L5REVIEW_NARRATIVECited in: Epidemiology, Etiology & Risk Factors, Diagnosis & Genetic Testing Cascade, Complications & Multisystem Surveillance, Special Populations, Prevention & Population Screening
  15. [15]

    Häberle J, Burlina A, Chakrapani A et al.. “Suggested guidelines for the diagnosis and management of urea cycle disorders: First revision.” Journal of inherited metabolic disease (2019). PMID: 30982989 ↗

    L1GUIDELINECited in: Epidemiology, Etiology & Risk Factors, History and Evolution of Treatment, Prevention & Population Screening
  16. [16]

    Smets F, Dobbelaere D, McKiernan P et al.. “Phase I/II Trial of Liver-derived Mesenchymal Stem Cells in Pediatric Liver-based Metabolic Disorders: A Prospective, Open Label, Multicenter, Partially Randomized, Safety Study of One Cycle of Heterologous Human Adult Liver-derived Progenitor Cells (HepaStem) in Urea Cycle Disorders and Crigler-Najjar Syndrome Patients.” Transplantation (2019). PMID: 30801523 ↗

    L2RCTCited in: Epidemiology, Etiology & Risk Factors, Targeted, Disease-Modifying & Supportive Therapy, History and Evolution of Treatment, Prognosis & Natural History
  17. [17]

    Hejazi R, Alghamdi TH, Salih R et al.. “Impact of glycerol phenylbutyrate on biochemistry and outcomes in paediatric patients with urea cycle disorders: a multicentre case series from Saudi Arabia.” Orphanet journal of rare diseases (2026). PMID: 41618427 ↗

    L4CASE_REPORTCited in: Epidemiology, Etiology & Risk Factors
  18. [18]

    Meyburg J, Opladen T, Spiekerkötter U et al.. “Human heterologous liver cells transiently improve hyperammonemia and ureagenesis in individuals with severe urea cycle disorders.” Journal of inherited metabolic disease (2017). PMID: 29027067 ↗

    L4TRIAL_NONRANDOMCited in: Epidemiology, Etiology & Risk Factors, History and Evolution of Treatment, Prognosis & Natural History
  19. [19]

    Waisbren SE, Stefanatos AK, Kok TMY et al.. “Neuropsychological attributes of urea cycle disorders: A systematic review of the literature.” Journal of inherited metabolic disease (2019). PMID: 31268178 ↗

    L2SR_OBSCited in: Epidemiology, Etiology & Risk Factors, Prognosis & Natural History
  20. [20]

    Häberle J, Boddaert N, Burlina A et al.. “Suggested guidelines for the diagnosis and management of urea cycle disorders.” Orphanet journal of rare diseases (2012). PMID: 22642880 ↗

    L1REVIEW_NARRATIVECited in: Epidemiology, Etiology & Risk Factors, Counseling, Cascade Screening & Reproductive Options, Special Populations, Prevention & Population Screening
  21. [21]

    Nettesheim S, Kölker S, Karall D et al.. “Incidence, disease onset and short-term outcome in urea cycle disorders -cross-border surveillance in Germany, Austria and Switzerland.” Orphanet journal of rare diseases (2017). PMID: 28619060 ↗

    L2OTHERCited in: Epidemiology, Etiology & Risk Factors, Acute & Crisis Management, Complications & Multisystem Surveillance, Special Populations
  22. [22]

    Silvera-Ruiz SM, Arranz JA, Häberle J et al.. “Urea cycle disorders in Argentine patients: clinical presentation, biochemical and genetic findings.” Orphanet journal of rare diseases (2019). PMID: 31426867 ↗

    L4OTHERCited in: Epidemiology, Etiology & Risk Factors, Variant Interpretation & ACMG/AMP Classification, Special Populations
  23. [23]

    Zhang K, Lu D, Liang L et al.. “Clinical, biochemical and genetic characteristics of patients with argininosuccinate lyase deficiency from a single center cohort in China.” Orphanet journal of rare diseases (2025). PMID: 41068918 ↗

    L4OTHERCited in: Epidemiology, Etiology & Risk Factors, Clinical Presentation & Phenotypic Spectrum, Variant Interpretation & ACMG/AMP Classification
  24. [24]

    Lee B, Diaz GA, Rhead W et al.. “Glutamine and hyperammonemic crises in patients with urea cycle disorders.” Molecular genetics and metabolism (2015). PMID: 26586473 ↗

    L2TRIAL_NONRANDOMCited in: Epidemiology, Etiology & Risk Factors
  25. [25]

    Bin Sawad A, Jackimiec J, Bechter M et al.. “Epidemiology, methods of diagnosis, and clinical management of patients with arginase 1 deficiency (ARG1-D): A systematic review.” Molecular genetics and metabolism (2022). PMID: 36049366 ↗

    L2SR_OBSCited in: Epidemiology, Etiology & Risk Factors, Complications & Multisystem Surveillance, Special Populations
  26. [26]

    Bonnot O, Klünemann HH, Sedel F et al.. “Diagnostic and treatment implications of psychosis secondary to treatable metabolic disorders in adults: a systematic review.” Orphanet journal of rare diseases (2014). PMID: 24775716 ↗

    L5SR_OBSCited in: Clinical Presentation & Phenotypic Spectrum, History and Evolution of Treatment, Prognosis & Natural History
  27. [27]

    Smith W, Diaz GA, Lichter-Konecki U et al.. “Ammonia control in children ages 2 months through 5 years with urea cycle disorders: comparison of sodium phenylbutyrate and glycerol phenylbutyrate.” The Journal of pediatrics (2013). PMID: 23324524 ↗

    L4TRIAL_NONRANDOMCited in: Clinical Presentation & Phenotypic Spectrum
  28. [28]

    Burgard P, Kölker S, Haege G et al.. “Neonatal mortality and outcome at the end of the first year of life in early onset urea cycle disorders--review and meta-analysis of observational studies published over more than 35 years.” Journal of inherited metabolic disease (2015). PMID: 26634836 ↗

    L2SR_OBSCited in: Clinical Presentation & Phenotypic Spectrum, History and Evolution of Treatment, Prognosis & Natural History
  29. [29]

    Cen Z, Ge P, Chen Y et al.. “Clinical characteristics and long-term outcomes of 101 patients with urea cycle disorders in China.” Orphanet journal of rare diseases (2025). PMID: 40804416 ↗

    L4OTHERCited in: Clinical Presentation & Phenotypic Spectrum, Variant Interpretation & ACMG/AMP Classification, Complications & Multisystem Surveillance, Special Populations, Prevention & Population Screening
  30. [30]

    Martín-Hernández E, Aldámiz-Echevarría L, Castejón-Ponce E et al.. “Urea cycle disorders in Spain: an observational, cross-sectional and multicentric study of 104 cases.” Orphanet journal of rare diseases (2014). PMID: 25433810 ↗

    L4OTHERCited in: Clinical Presentation & Phenotypic Spectrum, History and Evolution of Treatment, Prognosis & Natural History
  31. [31]

    Sen K, Izem R, Long Y et al.. “Are asymptomatic carriers of OTC deficiency always asymptomatic? A multicentric retrospective study of risk using the UCDC longitudinal study database.” Molecular genetics & genomic medicine (2024). PMID: 38634223 ↗

    L4COHORTCited in: Clinical Presentation & Phenotypic Spectrum, Acute & Crisis Management, Complications & Multisystem Surveillance
  32. [32]

    De Bruyne A, Imbard A, Roux CJ et al.. “Cluster of Severe Arginase 1 Deficiency in the Comoros: Clinical, Neuroimaging, and Molecular Features in 17 Patients From Mayotte Compared With 10 From Paris.” Journal of inherited metabolic disease (2026). PMID: 41684183 ↗

    L2OTHERCited in: Clinical Presentation & Phenotypic Spectrum, Variant Interpretation & ACMG/AMP Classification
  33. [33]

    Zhou Y, Jiang X, Zhang Y et al.. “Variant analysis and PGT-M of OTC gene in a Chinese family with ornithine carbamoyltransferase deficiency.” BMC pregnancy and childbirth (2024). PMID: 39039447 ↗

    L4CASE_REPORTCited in: Diagnosis & Genetic Testing Cascade, Variant Interpretation & ACMG/AMP Classification, Counseling, Cascade Screening & Reproductive Options, Special Populations
  34. [34]

    Vernon HJ. “Inborn Errors of Metabolism: Advances in Diagnosis and Therapy.” JAMA pediatrics (2015). PMID: 26075348 ↗

    L5REVIEW_NARRATIVECited in: Diagnosis & Genetic Testing Cascade, Prevention & Population Screening
  35. [35]

    Jamaleddin T, El-Akouri K, Abiib S et al.. “Qatar's National Expanded Metabolic Newborn Screening Program: Incidence and Outcomes.” International journal of neonatal screening (2025). PMID: 40700042 ↗

    L2OTHERCited in: Diagnosis & Genetic Testing Cascade
  36. [36]

    Vela-Amieva M, Alcántara-Ortigoza MA, González-Del Angel A et al.. “Concordance Between Biochemical and Molecular Diagnosis Obtained by WES in Mexican Patients with Inborn Errors of Intermediary Metabolism: Utility for Therapeutic Management.” International journal of molecular sciences (2024). PMID: 39519275 ↗

    L4OTHERCited in: Diagnosis & Genetic Testing Cascade
  37. [37]

    Lang SH, Lo RS, Cromie GA et al.. “Two commonly reported incidental variants in OTC are associated with late-onset disease.” HGG advances (2025). PMID: 41108081 ↗

    L4OTHERCited in: Diagnosis & Genetic Testing Cascade, Variant Interpretation & ACMG/AMP Classification
  38. [38]

    Zhang G, Chen Y, Ju H et al.. “Carbamoyl phosphate synthetase 1 deficiency diagnosed by whole exome sequencing.” Journal of clinical laboratory analysis (2017). PMID: 28444906 ↗

    L4CASE_REPORTCited in: Diagnosis & Genetic Testing Cascade
  39. [39]

    Nguyen HH, Khanh Nguyen N, Dung Vu C et al.. “Late-Onset Ornithine Transcarbamylase Deficiency and Variable Phenotypes in Vietnamese Females With OTC Mutations.” Frontiers in pediatrics (2020). PMID: 32793520 ↗

    L4CASE_REPORTCited in: Diagnosis & Genetic Testing Cascade
  40. [40]

    van Kuilenburg ABP, Mandel H, Moady TA et al.. “Glutaminase deficiency provides insight to the role of glutamine accumulation and neurotoxicity.” Molecular genetics and metabolism (2026). PMID: 41865506 ↗

    L4CASE_REPORTCited in: Diagnosis & Genetic Testing Cascade
  41. [41]

    Burrage LC, Thistlethwaite L, Stroup BM et al.. “Untargeted metabolomic profiling reveals multiple pathway perturbations and new clinical biomarkers in urea cycle disorders.” Genetics in medicine : official journal of the American College of Medical Genetics (2019). PMID: 30670878 ↗

    L4OTHERCited in: Variant Interpretation & ACMG/AMP Classification
  42. [42]

    Kido J, Sugawara K, Tavoulari S et al.. “Deciphering the Mutational Background in Citrin Deficiency Through a Nationwide Study in Japan and Literature Review.” Human mutation (2025). PMID: 40309478 ↗

    L4REVIEW_NARRATIVECited in: Variant Interpretation & ACMG/AMP Classification, Special Populations
  43. [43]

    Jang YJ, LaBella AL, Feeney TP et al.. “Disease-causing mutations in the promoter and enhancer of the ornithine transcarbamylase gene.” Human mutation (2018). PMID: 29282796 ↗

    L4OTHERCited in: Variant Interpretation & ACMG/AMP Classification
  44. [44]

    Diez-Fernandez C, Rüfenacht V, Gemperle C et al.. “Mutations and common variants in the human arginase 1 (ARG1) gene: Impact on patients, diagnostics, and protein structure considerations.” Human mutation (2018). PMID: 29726057 ↗

    L4OTHERCited in: Variant Interpretation & ACMG/AMP Classification, Special Populations
  45. [45]

    Bijarnia-Mahay S, Häberle J, Jalan AB et al.. “Urea cycle disorders in India: clinical course, biochemical and genetic investigations, and prenatal testing.” Orphanet journal of rare diseases (2018). PMID: 30285816 ↗

    L4OTHERCited in: Variant Interpretation & ACMG/AMP Classification, Counseling, Cascade Screening & Reproductive Options, Special Populations
  46. [46]

    Lu D, Han F, Qiu W et al.. “Clinical and molecular characteristics of 69 Chinese patients with ornithine transcarbamylase deficiency.” Orphanet journal of rare diseases (2020). PMID: 33272297 ↗

    L4OTHERCited in: Variant Interpretation & ACMG/AMP Classification
  47. [47]

    Schreiner F, Vollbach H, Sonntag N et al.. “Phenylbutyrate Treatment in a Boy With MCT8 Deficiency: Improvement of Thyroid Function Tests and Possible Hepatotoxicity.” The Journal of clinical endocrinology and metabolism (2025). PMID: 38781537 ↗

    L4CASE_REPORTCited in: Variant Interpretation & ACMG/AMP Classification
  48. [48]

    Rokicki D, Pajdowska M, Trubicka J et al.. “3-Methylglutaconic aciduria, a frequent but underrecognized finding in carbamoyl phosphate synthetase I deficiency.” Clinica chimica acta; international journal of clinical chemistry (2017). PMID: 28526534 ↗

    L4CASE_REPORTCited in: Variant Interpretation & ACMG/AMP Classification
  49. [49]

    Choi JH, Kim H, Yoo HW. “Two cases of citrullinaemia presenting with stroke.” Journal of inherited metabolic disease (2006). PMID: 16601887 ↗

    L4CASE_REPORTCited in: Variant Interpretation & ACMG/AMP Classification
  50. [50]

    Ramosaj A, Borsuk M, Underhaug J et al.. “Novel Treatment Strategy for Patients With Urea Cycle Disorders: Pharmacological Chaperones Enhance Enzyme Stability and Activity in Patient-Derived Liver Disease Models.” Journal of inherited metabolic disease (2025). PMID: 40421674 ↗

    L5OTHERCited in: Genotype-Phenotype Correlation, Severity & Risk Stratification
  51. [51]

    Burrage LC, Madan S, Li X et al.. “Chronic liver disease and impaired hepatic glycogen metabolism in argininosuccinate lyase deficiency.” JCI insight (2020). PMID: 31990680 ↗

    L2OTHERCited in: Genotype-Phenotype Correlation, Severity & Risk Stratification
  52. [52]

    Makris G, Lauber M, Rüfenacht V et al.. “Clinical and structural insights into potential dominant negative triggers of proximal urea cycle disorders.” Biochimie (2020). PMID: 33309754 ↗

    L4TRIAL_NONRANDOMCited in: Genotype-Phenotype Correlation, Severity & Risk Stratification
  53. [53]

    Coughlin CR, Barber J, Murali CN et al.. “Self-Reported Health-Related Quality of Life (HRQoL) in Adults With Urea Cycle Disorders.” Journal of inherited metabolic disease (2026). PMID: 41954389 ↗

    L2OTHERCited in: Genotype-Phenotype Correlation, Severity & Risk Stratification
  54. [54]

    Marín-Valencia I, Vilaseca MA, Thió M et al.. “Assessment of the perimortem protocol in neonates for the diagnosis of inborn errors of metabolism.” European journal of paediatric neurology : EJPN : official journal of the European Paediatric Neurology Society (2009). PMID: 19515591 ↗

    L4OTHERCited in: Genotype-Phenotype Correlation, Severity & Risk Stratification
  55. [55]

    Gerstein MT, Markus AR, Gianattasio KZ et al.. “Choosing between medical management and liver transplant in urea cycle disorders: A conceptual framework for parental treatment decision-making in rare disease.” Journal of inherited metabolic disease (2020). PMID: 31883128 ↗

    L5OTHERCited in: Genotype-Phenotype Correlation, Severity & Risk Stratification
  56. [56]

    Husson MC, Schiff M, Fouilhoux A et al.. “Efficacy and safety of i.v. sodium benzoate in urea cycle disorders: a multicentre retrospective study.” Orphanet journal of rare diseases (2016). PMID: 27663197 ↗

    L2COHORTCited in: Acute & Crisis Management
  57. [57]

    Lee B, Diaz GA, Rhead W et al.. “Blood ammonia and glutamine as predictors of hyperammonemic crises in patients with urea cycle disorder.” Genetics in medicine : official journal of the American College of Medical Genetics (2014). PMID: 25503497 ↗

    L2OTHERCited in: Acute & Crisis Management
  58. [58]

    Hediger N, Landolt MA, Diez-Fernandez C et al.. “The impact of ammonia levels and dialysis on outcome in 202 patients with neonatal onset urea cycle disorders.” Journal of inherited metabolic disease (2018). PMID: 29520739 ↗

    L2SR_OBSCited in: Acute & Crisis Management, Prognosis & Natural History
  59. [59]

    Spada M, Porta F. “Pre-emptive treatment in later-onset urea cycle disorders: a clinical perspective on glycerol phenylbutyrate.” Orphanet journal of rare diseases (2026). PMID: 42351235 ↗

    L5REVIEW_NARRATIVECited in: Acute & Crisis Management, Complications & Multisystem Surveillance
  60. [60]

    Stepien KM, McSweeney M, Ochoa-Ferraro A et al.. “Perspectives on long-term medical management of urea cycle disorders: insights from a survey of UK healthcare professionals.” Orphanet journal of rare diseases (2025). PMID: 40102865 ↗

    L4OTHERCited in: Acute & Crisis Management
  61. [61]

    Lindner M, Gramer G, Haege G et al.. “Efficacy and outcome of expanded newborn screening for metabolic diseases--report of 10 years from South-West Germany.” Orphanet journal of rare diseases (2011). PMID: 21689452 ↗

    L2OTHERCited in: Acute & Crisis Management, Prevention & Population Screening
  62. [62]

    Teufel U, Burgard P, Meyburg J et al.. “High blood pressure, a red flag for the neonatal manifestation of urea cycle disorders.” Orphanet journal of rare diseases (2019). PMID: 30961665 ↗

    L3OTHERCited in: Acute & Crisis Management, Special Populations
  63. [63]

    Unsinn C, Das A, Valayannopoulos V et al.. “Clinical course of 63 patients with neonatal onset urea cycle disorders in the years 2001-2013.” Orphanet journal of rare diseases (2016). PMID: 27538463 ↗

    L4OTHERCited in: Acute & Crisis Management, Prognosis & Natural History
  64. [64]

    Yahyaoui R, Quijada-Fraile P, Blasco-Alonso J et al.. “Health Outcomes of Patients with Distal Urea Cycle Disorders Detected by Newborn Screening: Data from the Spanish National Registry.” International journal of neonatal screening (2026). PMID: 42346733 ↗

    L2OTHERCited in: Acute & Crisis Management
  65. [65]

    Nagamani SCS, Agarwal U, Tam A et al.. “A randomized trial to study the comparative efficacy of phenylbutyrate and benzoate on nitrogen excretion and ureagenesis in healthy volunteers.” Genetics in medicine : official journal of the American College of Medical Genetics (2017). PMID: 29693650 ↗

    L1RCTCited in: Targeted, Disease-Modifying & Supportive Therapy, History and Evolution of Treatment
  66. [66]

    Feierman ER, Whittaker MN, Quigley A et al.. “Implications of the FDA's new plausible mechanism framework for the development of a personalized in vivo prime editing platform.” American journal of human genetics (2026). PMID: 41923647 ↗

    L5REVIEW_NARRATIVECited in: Targeted, Disease-Modifying & Supportive Therapy
  67. [67]

    McNutt M, Rutsch F, Russo RS et al.. “Long-Term Efficacy and Tolerability of Pegzilarginase in Arginase 1 Deficiency: Results of Two International Multicentre Open-Label Extension Studies.” Journal of inherited metabolic disease (2025). PMID: 40714964 ↗

    L4RCTCited in: Targeted, Disease-Modifying & Supportive Therapy, History and Evolution of Treatment, Prognosis & Natural History
  68. [68]

    Diaz GA, Krivitzky LS, Mokhtarani M et al.. “Ammonia control and neurocognitive outcome among urea cycle disorder patients treated with glycerol phenylbutyrate.” Hepatology (Baltimore, Md.) (2013). PMID: 22961727 ↗

    L1RCTCited in: Targeted, Disease-Modifying & Supportive Therapy, History and Evolution of Treatment
  69. [69]

    Kurtz CB, Millet YA, Puurunen MK et al.. “An engineered E. coli Nissle improves hyperammonemia and survival in mice and shows dose-dependent exposure in healthy humans.” Science translational medicine (2019). PMID: 30651324 ↗

    L1RCTCited in: Targeted, Disease-Modifying & Supportive Therapy, History and Evolution of Treatment
  70. [70]

    McGuire BM, Zupanets IA, Lowe ME et al.. “Pharmacology and safety of glycerol phenylbutyrate in healthy adults and adults with cirrhosis.” Hepatology (Baltimore, Md.) (2010). PMID: 20512995 ↗

    L2RCTCited in: Targeted, Disease-Modifying & Supportive Therapy, History and Evolution of Treatment
  71. [71]

    Sim E, Gregor A, MacLeod E et al.. “Measuring dietary intake among participants with a urea cycle disorder using standard diet records or a novel food photography app.” Molecular genetics and metabolism (2025). PMID: 41242089 ↗

    L2RCTCited in: Targeted, Disease-Modifying & Supportive Therapy, History and Evolution of Treatment
  72. [72]

    Osaka S, Nakano S, Mizuno T et al.. “A randomized trial to examine the impact of food on pharmacokinetics of 4-phenylbutyrate and change in amino acid availability after a single oral administration of sodium 4-phenylbutyrarte in healthy volunteers.” Molecular genetics and metabolism (2021). PMID: 33648834 ↗

    L1RCTCited in: Targeted, Disease-Modifying & Supportive Therapy
  73. [73]

    Mokhtarani M, Diaz GA, Rhead W et al.. “Urinary phenylacetylglutamine as dosing biomarker for patients with urea cycle disorders.” Molecular genetics and metabolism (2012). PMID: 22958974 ↗

    L2RCTCited in: Targeted, Disease-Modifying & Supportive Therapy
  74. [74]

    Scaglia F. “New insights in nutritional management and amino acid supplementation in urea cycle disorders.” Molecular genetics and metabolism (2010). PMID: 20299258 ↗

    L5RCTCited in: Targeted, Disease-Modifying & Supportive Therapy
  75. [75]

    Maestro S, Weber ND, Zabaleta N et al.. “Novel vectors and approaches for gene therapy in liver diseases.” JHEP reports : innovation in hepatology (2021). PMID: 34159305 ↗

    L5REVIEW_NARRATIVECited in: Targeted, Disease-Modifying & Supportive Therapy
  76. [76]

    Richard E, Martínez-Pizarro A, Desviat LR. “Exploring RNA therapeutics for urea cycle disorders.” Journal of inherited metabolic disease (2024). PMID: 39449289 ↗

    L5REVIEW_NARRATIVECited in: Targeted, Disease-Modifying & Supportive Therapy
  77. [77]

    Nagamani SCS, Ali S, Izem R et al.. “Biomarkers for liver disease in urea cycle disorders.” Molecular genetics and metabolism (2021). PMID: 33846069 ↗

    L5TRIAL_NONRANDOMCited in: Complications & Multisystem Surveillance
  78. [78]

    Selvanathan A, Hertzog A, Lemberg DA et al.. “Ornithine Transcarbamylase Deficiency Presenting as Acute Liver Failure in Girls: A Paediatric Case Series.” Journal of pediatric gastroenterology and nutrition (2020). PMID: 32265410 ↗

    L4CASE_REPORTCited in: Complications & Multisystem Surveillance
  79. [79]

    Choi JJ, Kim HS, Lee KC et al.. “Anesthetic experience of an adult male with citrullinemia type II: a case report.” BMC anesthesiology (2016). PMID: 27724842 ↗

    L4CASE_REPORTCited in: Complications & Multisystem Surveillance
  80. [80]

    Ruder J, Legacy J, Russo G et al.. “Neonatal citrullinemia: novel, reversible neuroimaging findings correlated with ammonia level changes.” Pediatric neurology (2014). PMID: 25266618 ↗

    L4CASE_REPORTCited in: Complications & Multisystem Surveillance
  81. [81]

    Tihtonen K, Uotila J, Lähde J et al.. “Risk of hyperammonemic coma in the puerperium: two cases of women with diagnosed and undiagnosed deficiency of urea cycle enzymes.” Acta obstetricia et gynecologica Scandinavica (2010). PMID: 20102292 ↗

    L4CASE_REPORTCited in: Complications & Multisystem Surveillance
  82. [82]

    Enns GM, Berry SA, Berry GT et al.. “Survival after treatment with phenylacetate and benzoate for urea-cycle disorders.” The New England journal of medicine (2007). PMID: 17538087 ↗

    L4TRIAL_NONRANDOMCited in: Special Populations
  83. [83]

    Ilgaz F, Höller A, Marsaux C et al.. “Human Milk Feeding in Inherited Metabolic Disorders: A Systematic Review of Growth, Metabolic Control, and Neurodevelopment Outcomes.” Journal of inherited metabolic disease (2025). PMID: 39912448 ↗

    L2SR_OBSCited in: Special Populations
  84. [84]

    Gündüz M, Yüksel Güdek Y, Kasapkara ÇS. “Out-of-pocket health expenditures in patients living with ınborn errors of metabolism.” Orphanet journal of rare diseases (2023). PMID: 37415155 ↗

    L2OTHERCited in: Special Populations
  85. [85]

    Busiah K, Roda C, Crosnier AS et al.. “Pubertal origin of growth retardation in inborn errors of protein metabolism: A longitudinal cohort study.” Molecular genetics and metabolism (2024). PMID: 38219674 ↗

    L2COHORTCited in: Special Populations
  86. [86]

    Zhou Y, Dou X, Zhang C et al.. “Hyperammonemia in a pregnant woman with citrullinemia type I: a case report and literature review.” BMC pregnancy and childbirth (2022). PMID: 36536326 ↗

    L4CASE_REPORTCited in: Special Populations
  87. [87]

    Koç Yekedüz M, Şen Akova B, Köse E et al.. “Early neuroimaging findings of infants diagnosed with inherited metabolic disorders in neonatal period: A case-control study.” Clinical neurology and neurosurgery (2022). PMID: 36270219 ↗

    L3CASE_CONTROLCited in: Special Populations
  88. [88]

    Wang S, Hamadeh A, Mian P et al.. “Predicting Cumulative Neonatal Sodium Benzoate Exposures During Breastfeeding with Physiologically Based Pharmacokinetic Modelling.” Clinical pharmacokinetics (2026). PMID: 41758461 ↗

    L5OTHERCited in: Special Populations
  89. [89]

    Mütze U, Scharré S, Schnabel-Besson E et al.. “Newborn screening for neuro-metabolic disorders: Strategies, clinical benefits, and prerequisites for program expansion.” European journal of paediatric neurology : EJPN : official journal of the European Paediatric Neurology Society (2025). PMID: 40339400 ↗

    L5REVIEW_NARRATIVECited in: Prevention & Population Screening
  90. [90]

    Merritt JL, Askenazi D, Hahn SH. “Newborn screening and renal disease: where we have been; where we are now; where we are going.” Pediatric nephrology (Berlin, Germany) (2011). PMID: 21947256 ↗

    L5REVIEW_NARRATIVECited in: Prevention & Population Screening

Revision History

All updates applied to this page

Loading revisions…