Clinical manifestations

Presentation and course

The classic presentation of ornithine transcarbamylase deficiency in hemizygous males is as catastrophic illness in the first week of life (neonatal/early onset). This accounts for approximately one third of all cases (102; 76); however, neonatal-onset male patients might be underrepresented in statistical analyses due to early and undiagnosed death. Typically, the affected baby is born after an uncomplicated full-term pregnancy, labor, and delivery with normal Apgar scores. Compared to distal urea cycle disorders (argininosuccinate synthetase 1 deficiency and argininosuccinate lyase deficiency), subjects with carbamoyl phosphate synthetase 1 deficiency (CPS1D) and ornithine transcarbamylase deficiency present earlier, usually within 24 to 72 hours after birth and with a higher initial peak-blood ammonia concentration (02).

Symptoms remind of a neonatal sepsis-like picture with hyperventilation, respiratory distress, and temperature instability. Respiratory alkalosis is often seen. Poor sucking, vomiting, and muscular hypotonia may be observed. Symptoms rapidly progress from somnolence and lethargy to coma (37). Neurologic findings may include increased deep tendon reflexes and papilledema. Convulsions may already be late complications and follow alterations in consciousness. Results from the Urea Cycle Disorders Consortium (UCDC) reported neuroimaging and neurocognitive findings of more than 600 patients with urea cycle disorder, including patients suffering from male and female ornithine transcarbamylase disorder (n = 320). Acute hyperammonemic episodes demonstrate a diffuse or focal pattern of cerebral MRI changes and brain edema (21; 35; 96). However, the impact of (acute) cerebral changes on clinical long-term outcomes; thus, the predictive value of diagnostic imaging, is yet to be determined and subject to future analyses (82; 96). An analysis of MRIs during a stable metabolic condition of individuals with ornithine transcarbamylase deficiency showed structural, biochemical, and functional changes in the brain between symptomatic subjects and normal controls (33).

In symptomatic heterozygous females and in males with partial ornithine transcarbamylase deficiency, symptoms rarely occur in the newborn period. In this group of patients, there is a wide spectrum of presentations after the newborn period even within the same family, with some individuals developing hyperammonemic episodes in infancy or early childhood, others in later childhood, and still others not until adulthood, with pregnancy and diet changes constituting some of the precipitating causes (23; 11). Symptoms may be delayed in onset by avoidance of trigger factors, eg, dietary self-restriction (self-avoidance of meats, fish, eggs, milk, and other high-protein foods). The occurrence of some chronic hyperammonemic symptoms appears to be age-specific. In infants, hepatodigestive (eg, episodes of vomiting, feeding problems, hepatopathy) and recurrent neurologic symptoms (eg, lethargy, irritability, psychomotor/mental retardation, movement disorder, convulsions, and coma) are common. It has been suggested that patients with late-onset urea cycle disorder most often present with progressive intellectual disability, movement disorders, and epilepsy (55). In older children and adults, behavioral abnormalities (eg, biting, self-injury, nocturnal restlessness, hyperactivity) and psychiatric signs (eg, confusion, irritability, agitation, headache, and aggression) can be found (55). Especially in adults, symptoms may mimic specific psychiatric or neurologic disorders (04; 22). In such patients, chronic hyperammonemic symptoms normally precede acute metabolic decompensation and are often unfortunately only identified retrospectively. However, urea cycle disorders in adults still remain a life-threatening condition (107). Asymptomatic female ornithine transcarbamylase deficiency individuals might present clinically with neurologic or psychiatric signs during the postpartum period (29; 59; 37). A study found that maternal ornithine transcarbamylase deficiency is associated with high maternal and neonatal morbidity and mortality when diagnosis is made during the pregnancy compared to when a diagnosis of female ornithine transcarbamylase deficiency is known prior to pregnancy, which shows why gynecologists or general practitioners should be aware of the above-described clinical presentation of chronic hyperammonemia or acute decompensation during pregnancy (108). Stroke-like episodes have also been described (22; 46). Some patients may present with acute liver failure (106). Analysis of a Swiss patient cohort showed that hepatic complications of urea cycle disorders, especially in individuals suffering from ornithine transcarbamylase deficiency, may be underrecognized (58). Liver involvement was a reported complication found in 100% of male and 40% of symptomatic female patients with ornithine transcarbamylase deficiency at least once during their life. However, bleeding complications and vitamin K dependence are rare.

In some urea cycle disorders, adults performed less well than younger patients in neurocognition; however, it remains unclear whether this is due to decline throughout life or improvements in diagnostics and treatment. Patients suffering from ornithine transcarbamylase deficiency tended not to have declined scores over time (117). Another study suggested that intellectual impairment is global rather than domain-specific and is associated with disease onset, gender, maximum ammonia concentration, and number of hyperammonemic events (15).

Triggers for acute metabolic manifestation of late-onset ornithine transcarbamylase deficiency include switching from low-protein breast milk to high-protein formula or cow milk, fever, infections, vomiting, gastrointestinal bleeding, decreased energy or protein intake, and surgery. Importantly, also drugs, especially valproate, steroids, haloperidol, and L-asparaginase/pegaspargase (79), and the postpartum period (due to catabolism and the involution of the uterus) are important trigger factors for late-onset hyperammonemia (29; 37; 59). There is some evidence that carnitine may protect against valproate-induced hyperammonemia in urea cycle disorders (31).

Prognosis and complications

Both in males and females, mortality is still an important topic in neonatal-/early-onset ornithine transcarbamylase deficiency. Mortality rates of patients with early-onset ornithine transcarbamylase deficiency who had been diagnosed between the mid-70s and mid-90s of the past century were very poor, with more than 85% of patients dying during the initial episode (114; 77). According to a study from Japan, nowadays survival rates have improved significantly for both early and late-onset ornithine transcarbamylase deficiency, with survival rates of approximately 85% for male neonatal onset ornithine transcarbamylase deficiency and more than 95% for late-onset ornithine transcarbamylase deficiency (49). However, a review and meta-analysis of observational studies spanning a period of more than 35 years demonstrated that early-onset patients, among those with ornithine transcarbamylase deficiency, still have a high risk of early death (16). Survival of males with ornithine transcarbamylase deficiency was only 15% by the end of the first year in that study. Survival of female patients with ornithine transcarbamylase deficiency is unknown because of the lack of sufficient data. Surprisingly, no significant improvement in survival for urea cycle disorders was observed over a period of more than 30 years (16), which is in contrast to the higher survival rate reported in the study from Japan (49).

Morbidity in urea cycle disorders remains high in survivors of neonatal hyperammonemic coma. Frequent comorbidities in urea cycle disorders are associated with the most vulnerable organ (ie, the brain) (56) and lead to intellectual disability, cerebral palsy, seizure disorder, and visual deficits (69; 15). A study of long-term survivors demonstrated that approximately only half of the patients had IQ scores greater than 85, and most of these patients had late-onset disease manifestation or were diagnosed and treated prospectively, ie, before onset of clinical symptoms conceding poor outcome for early-onset disease manifestation (114). Other data, however, demonstrate that neurocognitive outcome is independent of the age at which the first symptoms are noted or the age at diagnosis but does essentially depend on the initial peak-blood ammonia concentration (07; 28). At the first hyperammonemic attack, peak-blood ammonia concentrations of less than 180 µmol/L are associated with good outcomes, and peak-blood ammonia concentrations of more than 360 µmol/L are markers of poor prognoses. Variable outcome is observed when peak-blood ammonia concentration is between 180 and 360 µmol/L (114; 49). The intrinsic disease severity, which is reflected by the onset type and initial peak-blood ammonia concentration, affects the neurologic outcome of individuals with urea cycle disorders (86).

Importantly, current clinical knowledge suggests that cognitive outcome at the last regular visit differs between patients with proximal (ie, carbamoyl-phosphate synthetase 1 deficiency, ornithine transcarbamylase deficiency) and distal defects (ie, argininosuccinate synthetase deficiency, argininosuccinate lyase deficiency); the latter group being more compromised despite lower initial peak blood ammonium concentrations (89). Furthermore, a study demonstrated no difference in movement disorders (ie, dystonia, spasticity, chorea, ataxia) with regard to proximal versus distal urea cycle disorders; however, compared to patients with early-onset disease manifestations, patients with late-onset urea cycle disorders developed movement disorders and motor abnormalities less often (56; 86). A detailed overview of organ-specific disease manifestations and complications in patients suffering from urea cycle disorders (eg, OTCD) has been provided by Kölker and colleagues (56).

In a study of 19 women heterozygous for ornithine transcarbamylase deficiency Gyato and colleagues found normal overall IQ scores, but significant deficiencies in nonverbal intelligence, visual memory, attention or executive skills, and math, suggesting selective vulnerability of white matter and better preservation of gray matter (36). However, this notion is challenged by a study suggesting that impairment of cognitive function in ornithine transcarbamylase deficiency is global rather than domain-specific (15). Individuals with urea cycle disorders suffer from increased frequencies of mental disability and behavioral/emotional problems; however, they experience a normal health-related quality of life (45).

Clinical vignette

Patient 1: Late-onset female OTCD. The following is a summary of a case of a previously reported 3-year-old girl (106). The child was born after an uncomplicated pregnancy and delivery to nonconsanguineous parents, which was succeeded by an uneventful newborn period and early development. The girl had developed aggressive behavior starting in late infancy and intermittently severe sleeping difficulties from 3 years of age. Eight weeks before admission, the child started to vomit and became lethargic without being able to eat or drink intermittently. She recovered from that period, but 1 week before admission, she started to deteriorate and blood tests showed elevated liver enzymes (aspartate aminotransferase (AST) 5756 U/L, alanine aminotransferase (ALT) 4397 U/L). Three days later after a protein-rich meal she developed loss of appetite and drowsiness leading to admission. The initial work-up revealed hyperammonemia of 161 µmol/L. Emergency treatment was initiated and aminotransferases slowly recovered. Elevated orotic acid in urine (1323 mmol/mol creatinine; n < 4.41) and increased plasma glutamine were detected. Emergency treatment was intensified and adapted to ornithine transcarbamylase deficiency-specific requirements. At this point, AST and ALT were below 1000 U/L, and INR and lactate dehydrogenase were elevated. Factor V activity was reduced, whereas fibrinogen was normal. Administration of vitamin K did not improve INR. INR further increased necessitating the substitution with fresh-frozen plasma. The following day, parameters did not improve, and the patient developed hepatic encephalopathy and coma. There were no signs of brain edema on cranial CT, but a slight reduction of brain volume. Differential diagnoses included hepatotropic viruses, alpha-1 antitrypsin deficiency, autoimmune hepatitis, and cholangitis, which could all be excluded. Despite biochemical improvement during emergency treatment (decrease in urinary excretion of orotic acid, decrease in ammonia concentrations), the girl remained encephalopathic, and the hepatic function did not recover. Liver transplantation was considered and the girl was registered with Eurotransplant for high urgency liver transplantation and received a suitable donor organ 19 hours thereafter. Suspected ornithine transcarbamylase deficiency was confirmed by enzymatic analysis of the explanted liver showing a residual ornithine transcarbamylase activity of 14%. Mutation analysis showed a heterozygous mutation for G188fs (c.561delA) in the OTC gene. In contrast, mutation analysis of the OTC gene in her mother was unremarkable. Despite undoubted clinical and biochemical evidence for severe acute liver failure, histological evaluation of the explanted liver showed only mild to moderate abnormalities. Three weeks after admission, the child was discharged from the hospital in good condition and on a normal diet. Eight years after transplantation, the patient is doing well and liver function is normal, plasma amino acids and ammonia concentrations remained constantly normal.

Patient 2: Neonatal-/early-onset male OTCD. The following is a summary of a case of a previously reported boy (20). This male patient weighed 3250 g at birth and was delivered after an uncomplicated pregnancy at term by cesarean section. An older brother had died in the neonatal period at day 6 following an illness attributed to neonatal sepsis. His condition at birth was excellent, and he received routine infant care. He thrived until the third day of life, when he was noted to feed slowly. Several hours later, grunting and inspiratory retractions developed, and he became unresponsive to stimulation. There was profuse perspiration over his head and neck, and the rectal temperature was 35.5°C. In view of the identical history of illness in his previous male sibling, a detailed laboratory survey was initiated, including blood ammonia, which was estimated at 860 µmol/L. There was no evidence of generalized hepatic impairment. The child deteriorated rapidly and died. Absent ornithine transcarbamylase activity was found in his liver.

Biological basis

Etiology and pathogenesis

This disorder is caused by a partial or complete deficiency of ornithine transcarbamylase, the second enzyme in the urea cycle.

It is a mitochondrial enzyme composed of three identical subunits each with a molecular weight of 36kD (47). The gene encoding ornithine transcarbamylase is located on the short arm of the X-chromosome at band 21.1 (62), and it has been cloned and sequenced (41).

Ornithine transcarbamylase deficiency is inherited as an X-linked trait. Hemizygous males presenting in the newborn period have virtually no ornithine transcarbamylase activity in liver. The remainder of affected males, and some of the heterozygous females presenting later in life, have some residual enzyme activity (10). These individuals generally have 5% to 30% of normal ornithine transcarbamylase activity. However, there is uncertainty about the impact of functional data on clinical long-term outcome variables. Whereas an analysis of females with ornithine transcarbamylase deficiency found no clear genotype-phenotype correlation based on a large cohort of individuals (32). For male individuals with OTC-D, a genotype-phenotype relation between the onset time, the severity, and the underlying pathogenic variant was observed (54). In-depth analysis applying a newly established monoallelic in vitro expressions system found a correlation between the residual enzymatic OTC activity of male individuals and relevant clinical long-term outcome parameters, offering the potential for early and precise prediction of phenotypic severity (94). Moreover, a yeast growth assay has been developed to evaluate the pathogenicity of (unknown) OTC variants in a high-throughput manner (63).

Rarely, a balanced translocation involving an X chromosome in a female can result in the deletion of the OTC locus and cause consistent, non-random inactivation of the X-chromosome with the normal OTC gene leading to complete OTC deficiency and severe phenotype in a female (121). Approximately 560 disease-causing mutations (110; 05) have been described in patients. Frequently, the mutation appears de novo in female, and sometimes in male patients, and the mother is often not a disease-carrier (37). Generally, mutations causing neonatal disease affect amino acid residues that are in the interior of the enzyme, especially near the active site, whereas those associated with late-onset and milder phenotypes tend to be located on the surface of the protein (112). Conventional mutation analysis methods detect mutations in approximately 80% of patients with a clinical diagnosis of OTCD (67; 37). In approximately half of the patients in whom no mutation is identified by conventional methods, microarray analysis detects large deletions or rearrangements involving the OTC locus (98). Nowadays diagnosis is confirmed through molecular studies more frequently than through enzymatic activity (67; 87).

There are several disease genes very close to the OTC locus, including the Duchenne muscular dystrophy gene and the chronic granulomatous disease gene. Patients with these large deletions may have up to five severe genetic diseases making them extremely difficult to manage (25).

Neuropathologic changes in neonates dying of hyperammonemic coma involve intracerebral hemorrhage, prominent cerebral edema, and generalized neuronal cell loss (27). In survivors of prolonged hyperammonemic coma, changes observed on neuroimaging studies obtained months later include ventriculomegaly with increased sulcal markings, bilateral symmetrical low-density white matter defects, and diffuse atrophy with sparing of the cerebellum (69; 21). Neuropathology in those children who subsequently died was consistent with the neuroimaging findings and included ulegyria, cortical atrophy with ventriculomegaly, and prominent cortical neuronal loss (18), which, according to a study, occurred especially in the cingulate gyrus and insular cortex, with sparing of the perirolandic and occipital cortices (105).

Mechanisms of ammonia-induced brain damage in ornithine transcarbamylase deficiency are only partly understood. Ammonia is normally detoxified in astrocytes by glutamate dehydrogenase and glutamine synthetase. The accumulation of ammonia and glutamine has a number of toxic effects on the brain, including depletion of intermediates of cell energy metabolism and of organic osmolytes, the disaggregation of microtubules, altered amino acid and neurotransmitter concentrations, and alteration in water and potassium homeostasis (60; 60). Nitric oxide concentrations were found to be low in plasma and urine of females with ornithine transcarbamylase deficiency raising the possibility that reduced nitric oxide synthesis (due to reduced arginine synthesis) has a role in brain pathophysiology (73). Animal studies provided evidence for forebrain cholinergic neuronal loss in congenital ornithine transcarbamylase deficiency (19). Neurotransmitter alterations have been explored and altered serotonin metabolism has been put forward to explain some of the unusual behavior patterns, including anorexia and sleep abnormalities (44). Hepatic pathology investigations show diffuse microvesicular steatosis, periportal nuclear glycogen, variable portal fibrosis, and inflammation with occasional portal-to-portal bridging (106).

A zebrafish model was used to analyze the effects of hyperammonemia, demonstrating strongly enhanced transamination-dependent formation of osmolytic glutamine and excitatory glutamate, thereby inducing neurotoxicity and death via synergistically resulting in overactivation of NMDA receptors and bioenergetic impairment induced by depletion of 2-oxoglutarate. Withdrawal of 2-oxoglutarate from the tricarboxylic acid (TCA) cycle with consecutive tricarboxylic acid cycle dysfunction ultimately causes impaired oxidative phosphorylation with ATP shortage, decreased ATP/ADP-ratio, and elevated lactate concentrations. Interestingly, in zebrafish, inhibition of ornithine aminotransferase is a promising and effective therapeutic approach for preventing neurotoxicity and mortality by hyperammonemia (122; 123).

Epidemiology

The estimated cumulative prevalence of urea cycle disorders is 1 in 35,000 to 52,000 newborns (78). Ornithine transcarbamylase deficiency is the most common inborn error of urea synthesis. Data provided by the UCDC (95) calculated the overall incidence of ornithine transcarbamylase deficiency in the United States as 1 in 56,500 people (104). Similar estimates have been calculated for Finland, Japan, and Germany. However, the true prevalence is probably higher as it is suspected that a significant proportion of cases remain undiagnosed and die shortly after birth.

Prevention

No method is known for preventing ornithine transcarbamylase deficiency. However, prenatal diagnosis is available using DNA techniques in approximately 80% of affected families (48). If the family’s disease-causing mutation is known, carrier testing and prenatal diagnosis are possible (38). Identification of common intragenic polymorphisms allows tracking of the mutant allele even when the deleterious mutation is unknown (84). Preimplantation genetic diagnosis in couples at risk is now feasible (90), and prenatal DNA diagnosis has even been reported using a single fetal nucleated erythrocyte isolated from maternal blood (118).

Among males with ornithine transcarbamylase deficiency, only 7% have been found to have spontaneous mutations, whereas 80% of females with ornithine transcarbamylase deficiency have spontaneous mutations or inherited mutations from the father’s mutated sperm (111). Therefore, the chance of a mother having a second affected child depends on whether she has previously had an affected male or female (111). Prospective treatment of hemizygous males has been associated with an improved prognosis (66).

There is still no satisfactory method of newborn screening that enables the attenuation or even prevents the initial hyperammonemic decompensation in many identified individuals. A promising approach combines the determination of citrulline and orotate (101). Future long-term studies will need to evaluate the clinical impact of this finding, especially with regard to mortality, as well as long-term cognitive outcome and quality of life of survivors.

Differential diagnosis

Confusing conditions

A number of inborn errors of metabolism can have similar clinical presentations to ornithine transcarbamylase deficiency in the newborn period. These include other urea cycle disorders and amino acidopathies, mitochondriopathies, defects in fatty acid oxidation, and organic acidurias. In addition, a number of acquired conditions including transient hyperammonemia of the newborn, sepsis, intracranial hemorrhage, and cardiorespiratory disorders can present similarly. Differentiation depends on identifying hyperammonemia associated with specific pathological patterns of amino acids, acylcarnitines, or organic acids. Another disorder to be included in the differential diagnosis is mitochondrial carbonic anhydrase VA deficiency (115; 37).

In older children and adults, a number of acquired disorders can also present with hyperammonemia, including liver disease, Reye syndrome, drug toxicity, and hepatotoxins. Historical information, prothrombin time, a urinary toxic screen, and plasma amino acid pattern should help to differentiate these disorders.

Associated or underlying disorders

Clinically, individuals suffering from neonatal-onset ornithine transcarbamylase deficiency, carbamoyl phosphate synthetase 1 deficiency, or N-acetylglutamate synthase deficiency are hardly distinguishable. Importantly, biochemical investigation will provide more insight and help in differentiating these disorders.

Diagnostic workup

In ornithine transcarbamylase deficiency, the principal biochemical feature is hyperammonemia. The most common inborn errors of metabolism that can present as a catastrophic illness in the newborn period are urea cycle disorders, organic acidemias, fatty acid oxidation disorders, mitochondriopathies, and maple syrup urine disease. Of these, only maple syrup urine disease is consistently associated with normal plasma ammonia concentrations.

In organic acidemias (methylmalonic acidemia, propionic acidemia, isovaleric acidemia, glutaric acidemia type II, and multiple carboxylase deficiency), marked metabolic acidosis, ketosis, and an increased anion gap can be found. Hypoglycemia and pancytopenia may also be present. Acylcarnitine profiles by tandem mass spectrometry from simple blood spots, collected on a Guthrie card or plasma, identify disease-characteristic acylcarnitines, gas chromatography-mass spectrometry of urine diagnostic organic acids. Plasma, but not CSF, amino acids may show an elevated glycine concentration.

Fatty acid oxidation defects present with hypoglycemia, decreased ketones, and increased free fatty acids. Acylcarnitine analyses reveal disease-specific profiles. Urinary organic acids often detect dicarboxylic aciduria, which is less sensitive than blood acylcarnitine profiling.

Congenital lactic acidosis or mitochondriopathies can be the result of a genetic defect in pyruvate metabolism or the mitochondrial respiratory chain. The principal biochemical finding is lactic acidosis. In primary defects of pyruvate metabolism, the ratio of lactate to pyruvate is usually between 10 to 1 and 20 to 1, whereas in secondary lactic acidosis (shock, sepsis, heart failure) and mitochondrial defects, this ratio is significantly increased.

These findings are in contrast to inborn errors of urea synthesis, including ornithine transcarbamylase deficiency, where acylcarnitine profiles and urinary organic acid profiles are normal and plasma lactate concentration is normal to mildly increased. In other inborn errors of metabolism, elevated peak-blood ammonia concentration is generally in the range of 150 to 500 µmol/L (normally 15 to 40 µmol/L). In neonatal-onset urea cycle disorders, however, peak-blood ammonia concentration is likely to increase above 500 up to 5000 µmol/L.

Plasma amino acid patterns are distinct within the urea cycle disorders, with elevated concentrations of glutamine, alanine, and asparagine and low concentrations of arginine and ornithine, and abnormal citrulline (113). Citrulline is the product of carbamoyl phosphate synthetase 1 and ornithine transcarbamylase activity and the substrate for argininosuccinic synthetase and argininosuccinic lyase.

Thus, the concentration of citrulline is absent or markedly reduced in carbamoyl phosphate synthetase 1 and ornithine transcarbamylase deficiencies but markedly elevated in citrullinemia and argininosuccinic aciduria. This contrasts with transient hyperammonemia of the newborn, which is not associated with a congenital urea cycle defect and has normal glutamine, arginine, as well as normal or only slightly elevated citrulline concentrations. Differentiation of carbamoyl phosphate synthetase from ornithine transcarbamylase deficiency depends on detecting excessive urinary orotic acid excretion in ornithine transcarbamylase deficiency and decreased or normal excretion in carbamoyl phosphate synthetase 1 deficiency. A deficiency of N-acetylglutamate synthase, the enzyme needed for the production of the cofactor (N-acetylglutamate) for carbamoyl phosphate synthetase 1 is also associated with low-normal orotic acid excretion. Very similar is finally the mitochondrial carbonic anhydrase VA deficiency, which is also associated with low-normal orotic acid excretion (115).

Diagnosis of ornithine transcarbamylase deficiency in older children and adults with partial deficiencies may be less straightforward than in neonatal cases. The plasma ammonia concentration may be in the range of 150 to 250 µmol/L rather than above 500 µmol/L during symptomatic episodes and normal when the individual is clinically stable. Arginine and citrulline concentrations are often low normal in partial ornithine transcarbamylase deficiency rather than absent to trace as in a complete defect. Orotic aciduria can be provoked by an allopurinol loading test (39). It is reliable for identifying carriers with severe mutations but may be less reliable in milder variants. A subsequent study confirmed the high sensitivity of the test but documented a much lower specificity (34). The diagnosis of ornithine transcarbamylase deficiency is best confirmed by DNA studies in blood. Molecular genetic analysis is currently used more often in North America and Europe to confirm diagnosis than measurement of enzymatic activity (87). In vivo assays of residual urea synthetic capacity have also been developed (93; 81). This involves the oral or intravenous administration of stable isotope (nitrogen-15 or carbon-13) ammonia, glutamine, or sodium acetate and the subsequent measurement by mass spectrometry of labeled urea and glutamine synthesized. These methods may have the potential to differentiate urea cycle disorder patient subgroups and may be helpful in monitoring novel therapies for urea cycle disorders (81).

Management

For a detailed discussion see “Suggested guidelines for the diagnosis and management of urea cycle disorders” (37).

Management of acute hyperammonemia in OTCD.

Table 1. Protein, Liquid, and Glucose Management

Table 2. First-Line Medication for OTCD****

In the newborn period, management of acute hyperammonemia may be anticipatory or reactive. In families who have had an index patient, the birth of an at-risk or prenatally diagnosed infant provides the opportunity for prospective management. Within hours after birth, the child can be placed on oral therapy with appropriate ammonia scavengers and amino acids as subsequently described.

For newborns or infants who have been diagnosed during hyperammonemia or coma, therapy must not be delayed because coma duration of fewer than 1.5 days (83) and timely start of treatment are crucial determinants of outcome. In fact, current knowledge proposes that noninterventional variables, such as disease onset and initial peak-blood ammonia concentration, are of utmost importance for the neurologic outcome of individuals with urea cycle disorders (86). Large ongoing studies from the E-IMD and UCDC consortia aim to investigate the effects of (early) diagnosis and current treatment principles on the neurologic and cognitive outcome of affected individuals. Specialized pediatric hospitals should have first-line medications and consensus-based treatment protocols, and they must act according to the following principles:

(1) Stop protein intake (see Table 1)
(2) Start intravenous fluid and glucose substitution (see Table 1)
(3) Start first-line medication (see Table 2)

Suggestions for a consensus-based treatment protocol, following the above-outlined principles, are depicted in Tables 1 and 2. Each (specialized) pediatric hospital should be able to adapt these recommendations to facility-specific conditions to provide best-care medicine for their patients and prevent delay of treatment.

Ammonia scavengers (sodium benzoate, sodium phenylacetate/-butyrate) provide alternate pathways to eliminate waste nitrogen (14). A few studies demonstrate that treatment with sodium benzoate and sodium phenylbutyrate is safe and effective for the treatment or prevention of hyperammonemia in urea cycle disorders (43; 12). Sodium benzoate is conjugated with glycine to form hippurate, and sodium phenylbutyrate is conjugated with glutamine to form phenylacetylglutamine, both of which are cleared by the kidneys. Glutamine contains two nitrogen atoms. Thus, two moles of waste nitrogen are removed for each mole of phenylacetate/-butyrate administered. Theoretically, on a mole-per-mole basis, nitrogen-disposing efficacy of sodium phenylbutyrate should be twice that of sodium benzoate, and although biochemical superiority of sodium phenylbutyrate has been demonstrated (72), no systematic studies regarding the effects of long-term pharmacotherapy on neurologic or cognitive outcome, as defined by clinical endpoints, exists thus far.

Currently, a multicenter, randomized, double-blind, placebo-controlled trial of Carbaglu® in North America investigates if this drug is helpful for individuals suffering from ornithine transcarbamylase deficiency with regard to 1) improved neurocognitive outcome and 2) more rapid reduction of ammonium concentrations, improved function, and shortened hospitalization compared to standard therapy alone (01).

Energy should be constantly supplemented via oral, nasogastric, or intravenous routes by 20% to 100% above the recommended daily requirements using carbohydrates (such as glucose orally or dextrose 20% orally or glucose intravenously) and fat (intralipid 20%) starting at 1 g/kg and increased up to 3 g/kg per day. However, a multicenter study showed that during the first 24 hours of emergency treatment, caloric intake was lower than during maintenance treatment and was below age-adapted recommendations (86). Carbohydrates were the primary, if not sole, energy source, whereas fats were often omitted from initial emergency treatment. Soluble insulin is provided to support intracellular glucose uptake and to avoid hyperglycemia. The intake of natural protein is stopped for 24 hours to maximally 48 hours and is then reintroduced gradually as tolerated (42). In the event that ammonia concentrations do not respond to this conservative management, and biochemical or clinical symptoms worsen, continuous venovenous hemodiafiltration (CVVHDF) must be started immediately (planned and organized early, ie, at concentrations greater than 400 µmol/L) in neonates or children with ammonia concentrations of greater than 500 µmol/L (see Table 1) or at lower concentrations if response to medical treatment is inadequate. Notably, even though continuous venovenous hemodiafiltration is the optimal modality for extracorporal ammonia detoxification (83), prognosis is not related to dialysis modality, but primarily to the duration of coma before the start of treatment, confirming the necessity for rapid and aggressive management. In fact, a retrospective data analysis of 202 published cases revealed that current practice of dialysis does not affect outcomes. Only 20% of all investigated individuals had a normal clinical outcome. In conclusion, dialysis along with conservative pharmacotherapeutic treatment is recommended to be initiated as early as possible and at even lower concentrations of ammonium (40); however, long-term data analyzing these recommendations are not available.

The dietary aim is to minimize external protein (and, thus, nitrogen) intake and at the same time to prevent endogenous protein catabolism by meeting high-energy demands of the patient. The initial dietary emergency regimen should be protein-free, but protein or essential amino acids must be reintroduced after 24 to 48 hours or once blood ammonia concentration has fallen below 100 µmol/L (37). In addition to close-meshed control of laboratory parameters like ammonia, electrolytes, glucose, etc., plasma amino acids must be determined and the results must be available daily to safely adjust management. The reduction of protein intake must be carefully monitored to prevent over-restriction. A diet with inadequate intake can impair protein synthesis and lead to catabolic metabolic decompensation or failure to thrive (42).

According to the physician’s advice, an oral dietary emergency regimen might be applicable to treat milder hyperammonemic episodes, as long as the patient is not at risk of developing catabolism due to insufficient energy supply as a consequence of vomiting, loss of appetite, or diarrhea. Protein-, liquid-, and glucose management is identical to the protocol in Table 1.

Patients and families might start with an oral emergency dietary regimen at home. Antipyretic measurements must be consequently carried out if temperature exceeds 38°C; however, patients and parents must be aware that these measures must not postpone or replace adequate emergency treatment in the hospital (42; 126).

Patients should be supplied with an emergency card, letter, or bracelet containing instructions for emergency measures and phone numbers. Logistics of rational therapeutic measures should be repeatedly evaluated by the specialist team with the family and the primary care physicians.

Long-term management of OTCD.

Table 3. Long-Term Medication for OTCD

Long-term management of ornithine transcarbamylase deficiency relies on the goals of preventing recurrent hyperammonemia and neurologic sequelae and improving quality of life by the following principles (42; 37):

(1) Long-term medication (see Table 3)
(2) If individually necessary, avoidance of catabolism
(3) Suitable emergency regimens in intercurrent illness (see Tables 1 and 2)

Catabolism must be avoided as much as possible. In addition to intercurrent illnesses, especially if associated with high fever and decreased intake of food and fluids, dangerous triggers are severe exercise, seizures, trauma or burns, steroid administration, chemotherapy, and gastrointestinal hemorrhage.

Dietary treatment is an essential anchor point of long-term management and requires the knowledge of a specialist metabolic dietitian. For infants and older children, nutritional management involves the use of a high-caloric, low-protein diet supplemented with essential amino acids and, if necessary, vitamins and minerals. This is most readily accomplished by using small amounts of natural protein, an essential amino acids formula, and supplemental calories provided by a special formula that does not contain protein. The goal of long-term management is based on minimizing the nitrogen load on the urea cycle. The FAO/WHO/UNO 2007 report (37) can be used as age- and gender-dependent recommendations for energy intakes. Especially in young infants and children, fasts should be avoided and snacks given to reduce the possibility of (overnight) catabolism.

Dietary measures are very important for safely steering the conservative management in urea cycle disorders; however, growth impairment is a common and considerable burden for affected individuals (51). It was unknown thus far, if and to which extent a protein-controlled or protein-reduced diet does iatrogenically contribute to growth retardation in urea cycle disorders. A study showed that growth impairment was determined by disease severity and associated with reduced or borderline plasma branched-chain amino acid concentrations but was not associated with the degree of natural protein restriction (88), which is important for the consultation and reassurance of the affected urea cycle disorders individual.

Periodic measurement of plasma amino acids, which includes glutamine, and blood ammonia may permit adjustment of therapy before clinical symptoms appear. Long-term medication comprises the use of sodium benzoate and sodium phenylbutyrate as well as the essential amino acids L-arginine or L-citrulline. A detailed overview is provided in Table 3. Drugs may not be well tolerated by the child or family. Sodium phenylbutyrate tastes and smells unpleasant and may be irritating to the stomach.

Glycerol phenylbutyrate (RAVICTI®) has the same mechanism of action as sodium phenylbutyrate but is a sodium- and sugar-free prepro-drug of phenylacetic acid that has little odor or taste. Phenylbutyrate may deplete branched-chain amino acid concentrations and cause menstrual dysfunction/amenorrhea in up to 25% of postpubertal females (17; 37). Acute toxicity of benzoate and phenylbutyrate has been rare, but severe overdoses (2 to 10 times recommended) have led to symptoms that may be clinically mistaken for hyperammonemic episodes, including lethargy, hyperventilation, metabolic acidosis, cardiopulmonary collapse, and death (09; 85). To avoid complications, eg, mucositis or gastritis, sodium benzoate and sodium phenylbutyrate should be administered several times daily during meals with abundant fluids (37). In addition, supplementations of L-arginine or L-citrulline as essential amino acids aim at maximizing ammonia excretion through the urea cycle (13). For long-term treatment, sodium benzoate is used more often than sodium phenylbutyrate in both male and female patients with ornithine transcarbamylase deficiency, whereas L-arginine and L-citrulline are used with equal frequency (86).

Plasma carnitine deficiency may develop in patients with urea cycle disorder who are on a low-protein diet or receive treatment with ammonia scavengers. Carnitine supplementation (50-100 mg/kg/d) can be helpful in such conditions (80). Neomycin, colistin, and metronidazole have formerly been put forth as means of decreasing intestinal ammonia in hepatic encephalopathy; however, there is still no good evidence to support this use and pharmacological gastrointestinal decontamination is only sporadically used (86; 37). The benefit of vaccinations outweighs the risk of decompensations. They are recommended at the same schedule as for healthy children (68; 37).

By now a significant number of patients with various urea cycle disorders have received partial or total orthotopic liver transplants to provide enzyme replacement therapy (119; 71). In all successful cases, this procedure has cured hyperammonemia and permitted a normal protein intake. However, its effectiveness is hampered by expense, limited availability of donor organs, and significant morbidity from complications of transplantation or immunosuppression. Long-term survival is over 95% in proximal urea cycle disorders. Liver transplantation does not normalize citrulline concentration, which is primarily produced in the gut; thus, following transplantation, supplementation with citrulline or arginine may need to be continued (109).

Ideally, in patients with severe neonatal-onset disease, in patients with progressive liver disease, or in patients suffering from recurrent sever decompensations despite intensive medical treatment, orthotopic liver transplantation should be carried out between (3 to) 6 and 12 months of age before irreversible neurologic damage has occurred (37). A retrospective survey study investigated if patients would profit from liver transplantation by preventing further recurrent decompensations. Individuals with urea cycle disorders had an improved neurocognitive outcome if they received a liver graft, suggesting they should undergo liver transplantation after comparatively low maximum ammonium concentrations in order to protect the patient’s neurocognitive abilities (53; 52). However, patients with urea cycle disorder receiving a liver transplant are mainly between 1 and 6 years of age. An analysis evaluating the outcome of pediatric and adult patients with urea cycle disorder who underwent liver transplantation showed that approximately two thirds were transplanted before 5 years of age and that ornithine transcarbamylase deficiency was the most common urea cycle disorder in pediatric and adult patients receiving a new liver (120). Overall 1-, 5-, and 10-year survival was excellent, with 93%, 89%, and 87% survival, respectively. Two analyses, including one study from the United Network for Organ Sharing database comprising all pediatric urea cycle disorders candidates receiving liver transplantation between 2002 and 2020, found that delayed transplantation was associated with a long-term risk of cognitive delay and that early liver transplantation may prevent progressive neurologic injury and optimize cognitive outcomes (89; 124). However, further data suggest limited impact of liver transplantation on the neurodevelopmental outcomes of individuals with a severe disease burden (50).

Some medications are contraindicated in urea cycle disorders because of secondary inhibition of the urea cycle or inducing catabolism, most importantly valproic acid, which inhibits NAGS (03). Even in well-controlled and managed patients, peracute deadly coma can occur. Systemic steroid treatment can have the same result. Less often, but also to be considered, is the potential development of hyperammonemic crises by the treatment with carbamazepine, the use of asparaginase or 5-fluorouracil in cancer therapy, or bladder, uterine, or joint irrigation with glycine solution during surgery.

Hepatocyte transplantation is an interesting therapeutic option being investigated for patients with urea cycle disorder who are waiting for liver transplantation (70). New (genetic) therapies might become an option in the near future (24; 26). Intravenous application of an AAV-vector harboring human codon-optimized OTC gene showed no adverse clinical events, predominant hepatic biodistribution, and sustained supra-physiological OTC overexpression in OTC-deficient cynomolgus monkeys (08).

Another interesting approach for the treatment of acute hyperammonemia is inhibition of ornithine aminotransferase, leading to transamination-dependent decrease of glutamate and glutamine. Mammalian transfer will provide more insight into determining if this approach is indeed a potential new therapeutic principle for individuals with urea cycle disorders (122). Lately, it has been suggested that autophagy cooperates with the urea cycle in ammonia homeostasis. Selective activation of hepatic autophagy might, therefore, be used to treat hyperammonemia due to acquired or inherited diseases. However, it remains to be elucidated if autophagy also plays an important role during hyperammonemic conditions in the brain in order to determine if autophagy is a suitable target for treatment of hyperammonemic conditions (100).

Outcomes

Prior to the development of alternate pathway therapy using ammonia scavengers (eg, sodium benzoate, sodium phenylacetate/-butyrate), virtually all children with neonatal ornithine transcarbamylase deficiency died in the newborn period or during infancy. Between the 1980s and mid-1990s, nearly all children with early-onset type ornithine transcarbamylase deficiency still died during the initial episode (114). This, however, has slowly changed with the widespread availability of ammonia measurement in hospitals, growing knowledge about the disease, and the use of alternate pathway therapy. Survival rates for patients with early-onset and late-onset ornithine transcarbamylase deficiency have significantly improved (male neonatal-onset OTCD: approximately 85%; male late-onset OTCD: > 95%; female late-onset OTCD: > 95%) (49). In contrast, a review and metaanalysis showed less convincing results, indicating no improvement in survival of urea cycle disorders over more than 3 decades, between 1978 and 2014 (16). Furthermore, additional studies demonstrated that noninterventional variables reflecting disease severity, such as disease onset and initial peak-blood ammonia concentration, are associated with the highest risk of mortality and poor neurologic outcome (28; 86). Growth retardation is not affected by high initial peak-blood ammonia concentration; however, 32% of male and 52% of female patients over 1 year of age with ornithine transcarbamylase deficiency were below the tenth percentile for height (75). Growth retardation is rather associated with high intrinsic disease severity and reduced plasma branched-chain amino acid levels (88).

Long-term morbidity is still substantial in patients with urea cycle disorder. Cognitive outcome is better in prospectively treated infants and in children with partial defects (65; 74). It could be shown that females with partial ornithine transcarbamylase deficiency had IQ scores in the low average range that remained stable during 5-year follow-up on alternate pathway therapy (64). Overall, 50% of patients with urea cycle disorders suffer from intellectual disability (57). A study on individuals with ornithine transcarbamylase deficiency showed that intellectual impairment is global rather than domain-specific (15). Whereas Bachmann suggests that neurocognitive outcome does essentially depend on the initial and peak-blood ammonia concentration (07), investigations show that cognitive outcome does inversely correlate with the height of initial peak-blood ammonia concentration in proximal rather than distal urea cycle disorders (89).

Considering proximal urea cycle disorders in expanded newborn screening is an interesting approach; however, citrulline and glutamine alone are not specific, although early identification and, thus, a potential attenuation of the initial decompensation might open the possibility for an improved clinical outcome (87; 116). Combined levels of orotic acid and citrulline in routine newborn screening might enhance the detection of proximal urea cycle disorders in newborn screening and could possibly foster the discussion on screening for proximal urea cycle disorders in routine newborn screening in the future (101).

Special considerations

Pregnancy

The puerperium in ornithine transcarbamylase-deficient heterozygotes has been repeatedly associated with hyperammonemic crises (04). Langendonk studied a series of pregnancies in women with inherited metabolic diseases and suggested that special care must be taken not to confuse behavioral changes of hyperammonemia for symptoms of postpartum psychosis or depression (59). Because late-onset disease manifestation of urea cycle disorders appears to comprise more than 50% of all urea cycle disorders (77; 102; 76; 55) and because there is a high possibility for missing cases presenting with symptoms of postpartum psychosis, depression, or severe mood disorder, routine monitoring of plasma ammonia concentrations in those women was suggested (29). A special risk period for acute hyperammonemic decompensation is between 3 to 14 days postpartum. The relative metabolic stress during this episode is thought to be due to changes of the puerperium and an increased protein load following involution of the uterus (59; 37). Additionally, nausea and vomiting during pregnancy might lead to severe problems because of catabolism and, thus, should always be taken seriously and treated effectively with antiemetics. Pregnancy in women with ornithine transcarbamylase deficiency should, therefore, be monitored and escorted in close contact with a metabolic center (59).

Anesthesia

There has been one report of hyperammonemia induced by enflurane in argininosuccinic aciduria (06). It is prudent to use anesthetics with low toxicity to the liver. In addition, surgery requires the stopping of oral medication and may be associated with a catabolic condition, both of which may induce hyperammonemia in ornithine transcarbamylase deficiency. Therefore, it is important to provide alternate pathway therapy intravenously until the patient is able to tolerate oral medication. The patient must receive adequate glucose to reduce catabolic stress. One report of two siblings with severe ornithine transcarbamylase deficiency describes the perioperatively uneventful use of midazolam, s-ketamine, fentanyl, and isoflurane with local injection of ropivacaine along with intravenous infusion of glucose and alternate pathway drugs (97). Surgery should only be carried out in centers prepared for dealing with acute hyperammonemic episodes. After surgery, close monitoring of the clinical status and ammonia and glutamine concentrations as well as shifting to oral medications and diet are required (37). A case was reported emphasizing that appropriate guidelines for the pre- and postoperative care of patients with inherited metabolic diseases of the urea cycle are indispensable (30).