Long-term outcome of urea cycle disorders: Report from a nationwide study in Japan

Quo vadis ureagenesis disorders? A journey from 90 years ago into the future

The pathway of ammonia disposal in the mammalian organism has been described in 1932 as a metabolic cycle present in the liver in different compartments. In 1958, the first human disorder affecting this pathway was described as a genetic condition leading to cognitive impairment and constant abnormalities of amino acid metabolism. Since then, defects in all enzymes and transporters of the urea cycle have been described, referring to them as primary urea cycle disorders causing primary hyperammonemia. In addition, there is a still increasing list of conditions that impact on the function of the urea cycle by various mechanisms, hereby leading to secondary hyperammonemia. Despite great advances in understanding the molecular background and the biochemical specificities of both primary and secondary hyperammonemias, there remain many open questions: we do not fully understand the pathophysiology in many of the conditions; we do not always understand the highly variable clinical course of affected patients; we clearly appreciate the need for novel and improved diagnostic and therapeutic approaches. This study does look back to the beginning of the urea cycle (hi)story, briefly describes the journey through past decades, hereby illustrating advancements and knowledge gaps, and gives examples for the extremely broad perspective imminent to some of the defects of ureagenesis and allied conditions.

The efficacy of Carbamylglutamate impacts the nutritional management of patients with N-Acetylglutamate synthase deficiency

BACKGROUND

The autosomal recessive disorder N-acetylglutamate synthase (NAGS) deficiency is the rarest defect of the urea cycle, with an incidence of less than one in 2,000,000 live births. Hyperammonemic crises can be avoided in individuals with NAGS deficiency by the administration of carbamylglutamate (also known as carglumic acid), which activates carbamoyl phosphatase synthetase 1 (CPS1). The aim of this case series was to introduce additional cases of NAGS deficiency to the literature as well as to assess the role of nutrition management in conjunction with carbamylglutamate therapy across new and existing cases.

METHODS

We conducted retrospective chart reviews of seven cases of NAGS deficiency in the US and Canada, focusing on presentation, diagnosis, medication management, nutrition management, and outcomes.

RESULTS

Five new and two previously published cases were included. Presenting symptoms were consistent with previous reports. Diagnostic confirmation via molecular testing varied in protocol across cases, with consecutive single gene tests leading to long delays in diagnosis in some cases. All patients responded well to carbamylglutamate therapy, as indicated by normalization of plasma ammonia and citrulline, as well as urine orotic acid in patients with abnormal levels at baseline. Although protein restriction was not prescribed in any cases after carbamylglutamate initiation, two patients continued to self-restrict protein intake. One patient experienced two episodes of hyperammonemia that resulted in poor long-term outcomes. Both episodes occurred after a disruption in access to carbamylglutamate, once due to insurance prior authorization requirements and language barriers and once due to seizure activity limiting the family's ability to administer carbamylglutamate.

CONCLUSIONS

Follow-up of patients with NAGS deficiency should include plans for illness and for disruption of carbamylglutamate access, including nutrition management strategies such as protein restriction. Carbamylglutamate can help patients with NAGS deficiency to liberalize their diets, but the maximum safe level of protein intake to prevent hyperammonemia is not yet known. Patients using this medication should still monitor their diet closely and be prepared for any disruptions in medication access, which might require immediate dietary adjustments or medical intervention to prevent hyperammonemia.

Impact of citrulline substitution on clinical outcome after liver transplantation in carbamoyl phosphate synthetase 1 and ornithine transcarbamylase deficiency

Carbamoyl phosphate synthetase 1 (CPS1) and ornithine transcarbamylase (OTC) deficiencies are rare urea cycle disorders, which can lead to life-threatening hyperammonemia. Liver transplantation (LT) provides a cure and offers an alternative to medical treatment and life-long dietary restrictions with permanent impending risk of hyperammonemia. Nevertheless, in most patients, metabolic aberrations persist after LT, especially low plasma citrulline levels, with questionable clinical impact. So far, little is known about these alterations and there is no consensus, whether l-citrulline substitution after LT improves patients' symptoms and outcomes. In this multicentre, retrospective, observational study of 24 patients who underwent LT for CPS1 (n = 11) or OTC (n = 13) deficiency, 25% did not receive l-citrulline or arginine substitution. Correlation analysis revealed no correlation between substitution dosage and citrulline levels (CPS1, p = 0.8 and OTC, p = 1). Arginine levels after liver transplantation were normal after LT independent of citrulline substitution. Native liver survival had no impact on mental impairment (p = 0.67). Regression analysis showed no correlation between l-citrulline substitution and failure to thrive (p = 0.611) or neurological outcome (p = 0.701). Peak ammonia had a significant effect on mental impairment (p = 0.017). Peak plasma ammonia levels correlate with mental impairment after LT in CPS1 and OTC deficiency. Growth and intellectual impairment after LT are not significantly associated with l-citrulline substitution.

Current status and future directions of liver transplantation for metabolic liver disease in children

Orthotopic liver transplantation (OLT) in the care of children with inborn errors of metabolism (IEM) is well established and represent the second most common indication for pediatric liver transplantation in most centers worldwide, behind biliary atresia. OLT offers cure of disease when a metabolic defect is confined to the liver, but may still be transformative on a patient's quality of life reducing the chance of metabolic crises causing neurological damage in children be with extrahepatic involvement and no "functional cure." Outcomes post-OLT for inborn errors of metabolism are generally excellent. However, this benefit must be balanced with consideration of a composite risk of morbidity, and commitment to a lifetime of post-transplant chronic disease management. An increasing number of transplant referrals for children with IEM has contributed to strain on graft access in many parts of the world. Pragmatic evaluation of IEM referrals is essential, particularly pertinent in cases where progression of extra-hepatic disease is anticipated, with long-term outcome expected to be poor. Decision to proceed with liver transplantation is highly individualized based on the child's dynamic risk-benefit profile, their family unit, and their treating multidisciplinary team. Also to be considered is the chance of future treatments, such as gene therapies, emerging in the medium term.

Anesthesia management protocol for liver transplantation as treatment for ornithine transcarbamylase deficiency

BACKGROUND

Ornithine transcarbamylase deficiency is an X-linked genetic disorder that induces accumulation of ammonia in the liver and is the most common urea cycle disorder. The clinical manifestation of ornithine transcarbamylase deficiency is hyperammonemia that causes irreversible neurological damage. Liver transplantation is a curative therapy for ornithine transcarbamylase deficiency. The aim of this study is to suggest, from our previous experience, an anesthesia management protocol of liver transplantation for ornithine transcarbamylase deficiency, particularly focused on liver transplantation for cases with uncontrolled hyperammonemia.

METHOD

We retrospectively reviewed our anesthesia-related experience in all cases of liver transplantation for ornithine transcarbamylase deficiency in our center.

RESULTS

Twenty-nine liver transplantation cases for ornithine transcarbamylase deficiency were found between November 2005 and March 2021 in our center. Of these, 25 cases were stable through the perioperative period. However, 2 cases with carrier donor graft had hyperammonemia after liver transplantation. Another two cases had uncontrolled hyperammonemia before liver transplantation, even with continuous hemodialysis. They underwent life-saving liver transplantation. Their metabolic status stabilized after the anhepatic phase.

CONCLUSION

Liver transplantation for cases with uncontrolled hyperammonemia can be performed with proper management. Second, liver transplantation with carrier donors should be avoided because of the risk of postoperative recurrence.

Citrulline in the management of patients with urea cycle disorders

BACKGROUND

Treatment recommendations for urea cycle disorders (UCDs) include supplementation with amino acids involved in the urea cycle (arginine and/or citrulline, depending on the enzyme deficiency), to maximize ammonia excretion through the urea cycle, but limited data are available regarding the use of citrulline. This study retrospectively reviewed clinical and biological data from patients with UCDs treated with citrulline and/or arginine at a reference center since 1990. The aim was to describe the prescription, impact, and safety of these therapies. Data collection included patient background, treatment details, changes in biochemical parameters (plasma ammonia and amino acids concentrations), decompensations, and patient outcomes.

RESULTS

Overall, 79 patients (median age at diagnosis, 0.9 months) received citrulline and/or arginine in combination with a restricted protein diet, most with ornithine transcarbamylase (n = 57, 73%) or carbamoyl phosphate synthetase 1 (n = 15, 19%) deficiencies. Most patients also received ammonium scavengers. Median follow-up was 9.5 years and median exposure to first treatment with arginine + citrulline, citrulline monotherapy, or arginine monotherapy was 5.5, 2.5, or 0.3 years, respectively. During follow-up, arginine or citrulline was administered at least once (as monotherapy or in combination) in the same proportion of patients (86.1%); the overall median duration of exposure was 5.9 years for arginine + citrulline, 3.1 years for citrulline monotherapy, and 0.6 years for arginine monotherapy. The most common switch was from monotherapy to combination therapy (41 of 75 switches, 54.7%). During treatment, mean ammonia concentrations were 35.9 µmol/L with citrulline, 49.8 µmol/L with arginine, and 53.0 µmol/L with arginine + citrulline. Mean plasma arginine concentrations increased significantly from the beginning to the end of citrulline treatment periods (from 67.6 µmol/L to 84.9 µmol/L, P < 0.05). At last evaluation, mean height and weight for age were normal and most patients showed normal or adapted behavior (98.7%) and normal social life (79.0%). Two patients (2.5%) experienced three treatment-related gastrointestinal adverse reactions.

CONCLUSIONS

This study underlines the importance of citrulline supplementation, either alone or together with arginine, in the management of patients with UCDs. When a monotherapy is considered, citrulline would be the preferred option in terms of increasing plasma arginine concentrations.

Arginase deficiency with parotid gland swelling and hyperamylasemia: A case report

Arginase deficiency is a progressive neurological disorder characterized by episodic hyperammonemia crises. Our patient had been diagnosed with cerebral palsy (spastic paraplegia) in childhood and received rehabilitation. She had suffered parotid swelling since the age of 5 years, prior to liver dysfunction becoming apparent, and then developed hyperamylasemia at 8 years of age. At age 25 years, she presented with hyperammonemia and elevations of aspartate aminotransferase and alanine aminotransferase. At age 27 years, she was diagnosed with arginase deficiency due to hyperargininemia and absent arginase activity in erythrocytes. Liver cirrhosis was also present. She was hospitalized several times for management of episodic hyperammonemia due to recurrent viral infections, an unbalanced diet, and poor compliance with medications.

Evaluation of living donors for hereditary liver disease (siblings, heterozygotes)

Living donor liver transplantation (LDLT) is recognised as an alternative treatment modality to reduce waiting list mortality and expand the donor pool. Over recent decades, there have been an increasing number of reports on the use of LT and specifically LDLT for familial hereditary liver diseases. There are marginal indications and contraindications that should be considered for a living donor in paediatric parental LDLT. No mortality or morbidity related to recurrence of metabolic diseases has been observed with heterozygous donors, except for certain relevant cases, such as ornithine transcarbamylase deficiency, protein C deficiency, hypercholesterolemia, protoporphyria, and Alagille syndrome, while donor human leukocyte antigen homozygosity also poses a risk. It is not always essential to perform preoperative genetic assays for possible heterozygous carriers; however, genetic and enzymatic assays must hereafter be included in the parental donor selection criteria in the aforementioned circumstances.

[Expert consensus on the diagnosis and treatment of neonatal hyperammonemia]

Neonatal hyperammonemia is a disorder of ammonia metabolism that occurs in the neonatal period. It is a clinical syndrome characterized by abnormal accumulation of ammonia in the blood and dysfunction of the central nervous system. Due to its low incidence and lack of specificity in clinical manifestations, it is easy to cause misdiagnosis and missed diagnosis. In order to further standardize the diagnosis and treatment of neonatal hyperammonemia, the Youth Commission, Subspecialty Group of Neonatology, Society of Pediatrics, Chinese Medical Association formulated the expert consensus based on clinical evidence in China and overseas and combined with clinical practice experience,and put forward 18 recommendations for the diagnosis and treatment of neonatal hyperaminemia.

Pharmacokinetics, safety, and tolerability of sodium phenylacetate and sodium benzoate in healthy Japanese volunteers: A phase I, single-center, open-label study

TAK-123, a combination of sodium phenylacetate (NaPA) and sodium benzoate (NaBZ), is an intravenously administered drug developed for the treatment of acute hyperammonemia in infants, children, and adults with urea cycle enzyme deficiencies. The aim of the current study was to evaluate the pharmacokinetics, safety, and tolerability after intravenous infusion of TAK-123 in Japanese healthy adult volunteers. Ten volunteers received a 3.75 g/m² loading dose of TAK-123 over a period of 1.5 h followed by a maintenance infusion of the same dose over 24 h. Phenylacetate (PA) and benzoate (BZ) and their respective metabolites, phenylacetylglutamine (PAG) and hippurate (HIP) were measured over a 24-h period using a high-performance liquid chromatography/tandem mass spectrometry method. Non-compartmental analysis was performed using WinNonlin® Professional. During the loading dose, plasma levels of both PA and BZ peaked at 1.5 h. Plasma PA levels plateaued and were maintained up to 6.5 h, whereas plasma BZ levels declined rapidly after switching to maintenance infusion. Urinary excretion ratios of PAG and HIP at 48 h after the administration were 99.3% and 104%, respectively, suggesting that almost all NaPA and NaBZ were metabolized and excreted into urine. Overall, TAK-123 was well-tolerated in healthy Japanese adults.

Pathogenic variants of ornithine transcarbamylase deficiency: Nation-wide study in Japan and literature review

Ornithine transcarbamylase deficiency (OTCD) is an X-linked disorder. Several male patients with OTCD suffer from severe hyperammonemic crisis in the neonatal period, whereas others develop late-onset manifestations, including hyperammonemic coma. Females with heterozygous pathogenic variants in the OTC gene may develop a variety of clinical manifestations, ranging from asymptomatic conditions to severe hyperammonemic attacks, owing to skewed lyonization. We reported the variants of CPS1, ASS, ASL and OTC detected in the patients with urea cycle disorders through a nation-wide survey in Japan. In this study, we updated the variant data of OTC in Japanese patients and acquired information regarding genetic variants of OTC from patients with OTCD through an extensive literature review. The 523 variants included 386 substitution (330 missense, 53 nonsense, and 3 silent), eight deletion, two duplication, one deletion-insertion, 55 frame shift, two extension, and 69 no category (1 regulatory and 68 splice site error) mutations. We observed a genotype-phenotype relation between the onset time (neonatal onset or late onset), the severity, and genetic mutation in male OTCD patients because the level of deactivation of OTC significantly depends on the pathogenic OTC variants. In conclusion, genetic information about OTC may help to predict long-term outcomes and determine specific treatment strategies, such as liver transplantation, in patients with OTCD.

Epidemiology, methods of diagnosis, and clinical management of patients with arginase 1 deficiency (ARG1-D): A systematic review

BACKGROUND

Arginase 1 Deficiency (ARG1-D) is a rare, progressive, metabolic disorder that is characterized by devastating manifestations driven by elevated plasma arginine levels. It typically presents in early childhood with spasticity (predominately affecting the lower limbs), mobility impairment, seizures, developmental delay, and intellectual disability. This systematic review aims to identify and describe the published evidence outlining the epidemiology, diagnosis methods, measures of disease progression, clinical management, and outcomes for ARG1-D patients.

METHODS

A comprehensive literature search across multiple databases such as MEDLINE, Embase, and a review of clinical studies in ClinicalTrials.gov (with results reported) was carried out per PRISMA guidelines on 20 April 2020 with no date restriction. Pre-defined eligibility criteria were used to identify studies with data specific to patients with ARG1-D. Two independent reviewers screened records and extracted data from included studies. Quality was assessed using the modified Newcastle-Ottawa Scale for non-comparative studies.

RESULTS

Overall, 55 records reporting 40 completed studies and 3 ongoing studies were included. Ten studies reported the prevalence of ARG1-D in the general population, with a median of 1 in 1,000,000. Frequently reported diagnostic methods included genetic testing, plasma arginine levels, and red blood cell arginase activity. However, routine newborn screening is not universally available, and lack of disease awareness may prevent early diagnosis or lead to misdiagnosis, as the disease has overlapping symptomology with other diseases, such as cerebral palsy. Common manifestations reported at time of diagnosis and assessed for disease progression included spasticity (predominately affecting the lower limbs), mobility impairment, developmental delay, intellectual disability, and seizures. Severe dietary protein restriction, essential amino acid supplementation, and nitrogen scavenger administration were the most commonly reported treatments among patients with ARG1-D. Only a few studies reported meaningful clinical outcomes of these interventions on intellectual disability, motor function and adaptive behavior assessment, hospitalization, or death. The overall quality of included studies was assessed as good according to the Newcastle-Ottawa Scale.

CONCLUSIONS

Although ARG1-D is a rare disease, published evidence demonstrates a high burden of disease for patients. The current standard of care is ineffective at preventing disease progression. There remains a clear need for new treatment options as well as improved access to diagnostics and disease awareness to detect and initiate treatment before the onset of clinical manifestations to potentially enable more normal development, improve symptomatology, or prevent disease progression.

Identification of a novel homozygous intron 3 splice site (A>T) mutation in the ARG1 gene in cerebral palsy pediatric cases from Odisha, India

BACKGROUND

Arginase enzyme is essential for the catalysis of the last step of the urea cycle, resulting in the conversion of L-arginine to L-ornithine and urea. Arginase deficiency could lead to hyperarginemia, an autosomal recessive disorder of the urea cycle that could result in developmental manifestations after the first year of life, followed by gradually progressive atonic cerebral palsy, spastic quadriplegia, and mental decline. ARG1 mutations have been reported in hyperarginemia patients of Western countries because they exhibited reduced arginase activity. Hence, it is important to assess ARG1 mutations in cerebral palsy cases with hyperarginemia in different populations.

METHODS AND RESULTS

This study involved two unrelated pediatric patients from two non-consanguineous East Indian families, exhibiting a range of manifestations, including hypotonia of all limbs, mental retardation, and multiple episodes of seizure. The onset of the disease ranged from 1 to 3 years of age. Hyperammonemia (> 250 micromoles) and serum hyperarginemia (> 350 micromoles) were observed in both the patients. Whole-genome sequencing, followed by Sanger sequencing of both the patients confirmed the presence of a homozygous 3' splice site variation in intron 3 of the ARG1 gene (chr6: g.131902357A>T) that affects the invariant AG acceptor splice site of exon 4 (c.330-2A>T; ENST00000356962.2).

CONCLUSION

The study reported the identification of a novel ARG1 mutation in two different unrelated pediatric cases from Odisha, India associated with hyperarginemia. The pathogenicity of the mutation was robustly supported by the clinical phenotype, complete co-segregation with the disease, and biochemical observations.

Clinical manifestation and long-term outcome of citrin deficiency: Report from a nationwide study in Japan

Citrin deficiency is an autosomal recessive disorder caused by mutations in the SLC25A13 gene. The disease can present with age-dependent clinical manifestations: neonatal intrahepatic cholestasis by citrin deficiency (NICCD), failure to thrive, and dyslipidemia by citrin deficiency (FTTDCD), and adult-onset type II citrullinemia (CTLN2). As a nationwide study to investigate the clinical manifestations, medical therapy, and long-term outcome in Japanese patients with citrin deficiency, we collected clinical data of 222 patients diagnosed and/or treated at various different institutions between January 2000 and December 2019. In the entire cohort, 218 patients were alive while 4 patients (1 FTTDCD and 3 CTLN2) had died. All patients <20 years were alive. Patients with citrin deficiency had an increased risk for low weight and length at birth, and CTLN2 patients had an increased risk for growth impairment during adolescence. Liver transplantation has been performed in only 4 patients (1 NICCD, 3 CTLN2) with a good response thereafter. This study reports the diagnosis and clinical course in a large cohort of patients with citrin deficiency and suggests that early intervention including a low carbohydrate diet and MCT supplementation can be associated with improved clinical course and long-term outcome.

Case Report: Juvenile Myelomonocytic Leukemia Underlying Ornithine Transcarbamylase Deficiency Safely Treated Using Hematopoietic Stem Cell Transplantation

BACKGROUND

Juvenile myelomonocytic leukemia (JMML), which is predominantly found in infants, is a clonal abnormality of pluripotent hematopoietic stem cells and presents with the symptoms of both myeloproliferative tumors and myelodysplastic syndromes. Estimates have shown that ~20 cases of JMML occur annually in Japan. Ornithine transcarbamylase deficiency (OTCD), the most common among all urea cycle disorders (UCDs), occurs in 1 of 80,000 people in Japan.

CASE PRESENTATION

A 10-month-old infant who had fever, vomiting, and diarrhea for 2 days was referred to our hospital for the following abnormalities in blood tests: white blood cell count, 48,200/μL; hemoglobin, 9.0 g/dL; and platelet count, 135,000/μL. Bone marrow examination showed a nucleated cell count of 396,000/mm³ and blast cell count of 5.0%, as well as decreased mature granulocyte count and slightly myeloperoxidase stain-negative blasts but no monoclonal cell proliferation on May-Giemsa staining. Colony assay showed the proliferation of spontaneous colony and high sensitivity to granulocyte-macrophage colony-stimulating factor. Genetic analysis of peripheral blood mononuclear cells showed that the patient was positive for neuroblastoma RAS (NRAS) mutation. The patient was ultimately diagnosed with JMML. Approximately 170 days after his first hematopoietic stem cell transplantation (HSCT), the patient's JMML relapsed. Shortly after the recurrence, nausea, vomiting, hyperventilation, and decreased vitality were observed, followed by a decrease in the level of consciousness. The patient's ammonia level was 472 μmol/L. A test for seven different genetic mutations for the UCD showed the presence of c. 119G>A (amino acid change p. Arg40His). As such, late-onset OTCD was added to his diagnosis. Administration of sodium phenylacetate, l-arginine hydrochloride, and carnitine was continued following the diagnosis of OTCD, after which hyperammonemia was not observed. Regarding JMML relapse, HSCT was performed on day 405 after the first transplantation.

CONCLUSION

Hyperammonemia should be considered a differential diagnosis when unexplained and non-specific symptoms occur during the treatment of hematologic malignancies. Patients should be tested for UCD as a cause of hyperammonemia, and treatment for hyperammonemia should be continued until the cause is identified. The patient shows normal developmental progress, has an intact neurological status, and has not experienced another hyperammonemia attack. His JMML has remained in remission for over 3 years.

Role of liver transplantation in urea cycle disorders: Report from a nationwide study in Japan

Urea cycle disorders (UCDs) are inherited metabolic diseases causing hyperammonemia by defects in urea cycle enzymes or transporters. Liver transplantation (LT) currently is the only curative treatment option until novel therapies become available. We performed a nationwide questionnaire-based study between January 2000 and March 2018 to investigate the effect of LT in patients with UCDs in Japan. A total of 231 patients with UCDs were enrolled in this study. Of them, a total of 78 patients with UCDs (30 male and 16 female ornithine transcarbamylase deficiency (OTCD), 21 carbamoyl phosphate synthetase 1 deficiency (CPSD), 10 argininosuccinate synthetase deficiency (ASSD) and 1 arginase 1 deficiency (ARGD)) had undergone LT. Concerning the maximum blood ammonia levels at the onset time in the transplanted male OTCD (N = 28), female OTCD (N = 15), CPSD (N = 21) and ASSD (N = 10), those were median 634 (IQR: 277-1172), 268 (211-352), 806 (535-1382), and 628 (425-957) μmol/L, respectively. The maximum blood ammonia levels in female OTCD were thus significantly lower than in the other UCDs (all P < .01). LT was effective for long-term survival, prevented recurrent hyperammonemia attack, and lowered baseline blood ammonia levels in patients with UCDs. LT had limited effect for ameliorating neurodevelopmental outcome in patients with severe disease because hyperammonemia at the onset time already had a significant impact on the brain. Patients with ASSD may be more likely to survive without cognitive impairment by receiving early LT despite severe neonatal hyperammonemia ≥ 360 μmol/L. In patients with neonatal onset OTCD or CPSD, there may be additional factors with adverse effects on the brain that are not improved by LT.

Hyperammonemia in Inherited Metabolic Diseases

Ammonia is a neurotoxic compound which is detoxified through liver enzymes from urea cycle. Several inherited or acquired conditions can elevate ammonia concentrations in blood, causing severe damage to the central nervous system due to the toxic effects exerted by ammonia on the astrocytes. Therefore, hyperammonemic patients present potentially life-threatening neuropsychiatric symptoms, whose severity is related with the hyperammonemia magnitude and duration, as well as the brain maturation stage. Inherited metabolic diseases caused by enzymatic defects that compromise directly or indirectly the urea cycle activity are the main cause of hyperammonemia in the neonatal period. These diseases are mainly represented by the congenital defects of urea cycle, classical organic acidurias, and the defects of mitochondrial fatty acids oxidation, with hyperammonemia being more severe and frequent in the first two groups mentioned. An effective and rapid treatment of hyperammonemia is crucial to prevent irreversible neurological damage and it depends on the understanding of the pathophysiology of the diseases, as well as of the available therapeutic approaches. In this review, the mechanisms underlying the hyperammonemia and neurological dysfunction in urea cycle disorders, organic acidurias, and fatty acids oxidation defects, as well as the therapeutic strategies for the ammonia control will be discussed.