Clinical manifestations

Presentation and course

The classic presentation of N-acetylglutamate synthase deficiency is a catastrophic illness in the first week(s) of life, (neonatal-/early-onset).

Typically, the affected neonate is born after an uncomplicated, full-term pregnancy, labor, and delivery with normal Apgar scores. Symptoms have mirrored other urea cycle disorders and remind of a neonatal sepsis-like picture with hyperventilation, respiratory distress, and temperature instability. Poor sucking, vomiting, muscular hypotonia, and abnormal motor functions may be observed (39; 40; 34). Hepatomegaly and abnormal liver function tests are mostly evident during these episodes, and liver biopsy shows microvesicular and macrovesicular fat and mitochondria of irregular size and shape with intracristae crystallizations (67). Typically, symptoms rapidly progress from somnolence and lethargy to hyperammonemic coma (25). Convulsions may already be late complications following alterations in consciousness.

Cases of late-onset disease with partial deficiencies have been less commonly reported (13). Symptoms may develop from infancy to adulthood and are associated with weaning or switching from formula to cow's milk, high-protein intake (eg, barbecue, parenteral nutrition), or triggers of catabolic stress. Such triggers might be fever, infections, gastrointestinal bleeding, vomiting, decreased energy or increased protein intake, seizures, trauma or burns, and surgery. Furthermore, severe exercise, drugs (steroids, valproate, haloperidol, and L-asparaginase/pegaspargase), chemotherapy, and the postpartum period (due to catabolism and the involution of the uterus) are important triggers for late-onset hyperammonemia (49; 41).

Clinically, late-onset N-acetylglutamate synthase deficiency manifests with recurrent episodes of headache, vomiting, nausea, ataxia, irritability, and behavioral changes, and these may progress to severe lethargy or coma (14; 34). Therefore, due to this broad phenotypic spectrum, maintaining a high index of suspicion at any age is needed for early diagnosis (03).

Data from Europe describing the initial phenotypic presentation of urea cycle disorders suggested that the median age of diagnosis for patients suffering from N-acetylglutamate synthase deficiency was 7 days (39). Acute hyperammonemic episodes cause diffuse or focal patterns of cerebral MRI changes and signs of brain edema (15; 24). However, the impact of acute cerebral changes on long-term outcome; thus, the predictive value of acute MRI changes is unclear. An analysis of MRIs during a stable metabolic condition of individuals with ornithine transcarbamylase deficiency, another urea cycle disorder, showed structural, biochemical, and functional changes in the brain between symptomatic subjects as compared to normal controls (22). Published results from the UCDC reported neurocognitive findings in more than 600 urea cycle disorder patients, including patients suffering from NAGSD (68). In some 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 treatments. Patients suffering from NAGSD (n = 4) tended not to have declining scores over time; however, considering the low number of patients, follow-up studies will have to prove this observation.

Prognosis and complications

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, leading to intellectual disability, cerebral palsy, seizure, as well as movement and speech disorders (40). Data demonstrate that neurocognitive outcome does essentially depend on the initial and peak-blood ammonia level (PBAL) (05; 17; 54). During the first hyperammonemic episode, peak-blood ammonia level of less than 180 µmol/L is associated with a good outcome, and peak-blood ammonia level of more than 360 µmol/L is a marker for poor prognosis. Variable outcome is observed when peak-blood ammonia level is between 180 µmol/L and 360 µmol/L (38). A detailed overview of organ-specific disease manifestations and complications in patients suffering from urea cycle disorders (eg, NAGSD) is given by Kölker and colleagues (40). Moreover, individuals with urea cycle disorders suffer from increased frequencies of mental disability and behavioral/emotional problems. Interestingly, health-related quality of life of these patients is within the normal range (33).

Clinical vignette

Patient 1: Late-onset N-acetylglutamate synthase deficiency. The girl was born to nonconsanguineous parents after a normal pregnancy. Delivery, birth weight, length, head circumference, and neonatal period were all normal. Breast and bottle feeding were well-tolerated, and the developmental milestones were normal. At 13 months of age, vomiting, somnolence, and muscular hypotonia were treated by standard intravenous rehydration therapy. The child recovered completely after 24 hours. Despite the absence of evidence, this episode was interpreted as drug intoxication. In childhood, aversion to protein-rich food resulted in a self-chosen lacto-vegetarian diet. Growth was on the 25th percentile despite periods of lack of appetite. At 12 years of age, during an acute illness confusion, aggressiveness and somnolence developed, which led the physician to suspect encephalitis. A normal cerebrospinal fluid cell count, protein level, and absence of infectious agents did not support this diagnosis. Cranial CT was normal. She recovered completely after 12 hours of 5% glucose and electrolyte infusions. At 13 years of age, she was admitted to a hospital after a protein-rich meal. Symptoms included restlessness, disorientation, aggressiveness, ataxia, increased deep tendon reflexes, and dilated pupils with delayed pupillary reflexes. Cerebrospinal fluid analysis was again normal. There were no signs of cerebral edema on neuroimaging. Biochemical workup showed normal glucose, acid-base balance, electrolytes, creatine and creatine kinase, but plasma ammonia was increased to 221 µmol/L (normal less than 50 µmol/L). Orotic acid excretion was not increased. Amino acids in plasma showed glutamine concentration at the upper normal limit with a glutamine to ammonia ratio of 3.8; this rendered a portosystemic bypass unlikely. Citrulline was 12 µmol/L (normal is 15 to 55 µmol/L), and there was no evidence of a respiratory chain defect that can also lead to low citrulline concentration. There was no arginine deficiency. Urinary organic acid analysis was normal. With other metabolic disorders excluded, carbamylphosphate synthetase 1 or N-acetylglutamate synthase deficiencies were considered possible diagnoses. Restriction of natural protein and substitution of L-arginine and sodium benzoate or phenylbutyrate had not completely normalized ammonia or glutamine concentrations; this in part might retrospectively be due to the excessive protein restriction. An open liver biopsy resulted in the diagnosis of a partial N-acetylglutamate synthase deficiency (15% of lower normal limit measured under optimized conditions) with normal carbamylphosphate synthetase 1 and ornithine transcarbamylase activity. A therapeutic trial with N-carbamylglutamate was successful and allowed a normal protein diet and withdrawal of all other medications. She has since done well with no further metabolic crises on daily treatment of N-carbamylglutamate.

Patient 2: Early-onset N-acetylglutamate synthase deficiency. A 7-day-old formula-fed female, the second child of consanguineous parents, born at term without complications with a birth weight of 3070 g was admitted to a peripheral hospital for a 1-day history of lethargy, poor feeding, and vomiting without diarrhea. She was treated with an oral rehydration solution overnight and subsequently developed a brief episode of supraventricular tachycardia. She was then transferred to the intensive care unit where she arrived lethargic, mildly hypotonic, with incomplete Moro and poor sucking reflexes. Her cardio-respiratory and abdominal examinations were uneventful. A metabolic work-up revealed a blood ammonia level of 290 µmol/L (normal < 100 µmol/l in neonates) with a compensated respiratory alkalosis (pH of 7.48, pCO2 of 19 mmHg, bicarbonate of 14 mmol/L and base excess of 6.6 mmol/L). This was accompanied by normal lactate, normal glucose, and the absence of ketones in the urine. Protein intake was stopped, and an infusion of 20 % glucose was started to achieve a high energy intake. Treatment with sodium benzoate and arginine hydrochloride was initiated. After an initial drop of ammonium within 12 hours of treatment, levels started rising again. N-carbamylglutamate was added, which resulted in the normalization of ammonia. Within 24 to 36 hours oral feeding was progressively reintroduced. The closely monitored ammonium levels remained within the normal range. Further metabolic work-up revealed an increased plasma level of glutamine (1004 µmol/L), but normal levels of citrulline, arginine, ornithine, and normal urine argininosuccinic acid. This suggested a proximal block of the urea cycle. Because urinary orotic acid analysis was also normal, ornithine transcarbamylase deficiency could be ruled out. Genetic analysis confirmed N-acetylglutamate synthase deficiency with a homozygous mutation (c.1450C> T; p.W484R). Both parents were heterozygous for the same mutation. L-arginine and sodium benzoate were stopped. At the age of 9 months, the child was on monotherapy with N-carbamylglutamate and on a normal diet. Growth parameters were within normal limits. Her psychomotor development was normal for a 9-month-old infant; she was sitting without support and had started to crawl. She grasped objects with both hands and transferred them from one hand to the other. Overall, her movements were fluent and well-coordinated. She babbled, waved goodbye, and had good social interaction. This case shows that early treatment of N-acetylglutamate synthase deficiency with N-carbamylglutamante can result in a normal neurologic outcome (65).

Biological basis

Etiology and pathogenesis

This disorder is caused by a complete or partial deficiency of N-acetylglutamate synthase (EC 2.3.1.1). The human gene including frameshift mutation by one base-pair insertion in a patient was found by Elpeleg and colleagues (16). The human and mouse protein (12) share 84% identity. More than 30 mutations have been described with approximately two thirds being missense mutations (11; 28; 26; 56). In addition, regulatory regions might be affected as molecular causes of N-acetylglutamate synthase deficiency are detected more frequently (69; 27). Disease-causing mechanisms appear to involve decreased solubility/stability, aberrant kinetics/catalysis, and altered arginine modulation (56). Based on a NAGS knockout mouse, it was demonstrated that NAGS deficiency can be rescued by gene therapy and that L-arginine is essential to the NAGS enzyme for normal ureagenesis (59). There are no prevalent mutations, and there is uncertainty about the impact of functional data on clinical long-term outcome variables due to a lack of systematic (functional) genotype-phenotype analyses.

N-acetylglutamate synthase is a mitochondrial hepatic enzyme that catalyzes the formation of N-acetylglutamate from acetyl-CoA and glutamate.

It is also expressed to a minor degree in the intestine. N-acetylglutamate formation depends on substrate availability and is stimulated by mitochondrial arginine. N-acetylglutamate breakdown is decreased by ornithine supplements. N-acetylglutamate serves as an allosteric activator of carbamylphosphate synthetase 1, which is severely reduced in the absence of N-acetylglutamate (less than 5% of normal). N-acetylglutamate synthase deficiency is inherited as an autosomal recessive trait. For accurate diagnosis, molecular confirmation is recommended (34).

In neonates with congenital urea cycle disorders dying of hyperammonemic coma, neuropathologic changes involve prominent cerebral edema and generalized neuronal cell loss. Survivors of prolonged hyperammonemic coma show neuroimaging abnormalities, such as ventriculomegaly with increased sulcal markings, bilateral symmetrical low-density white matter defects, and diffuse atrophy with sparing of the cerebellum (15). The distribution of age-specific MRI lesions is thought to reflect hypoperfusion secondary to hyperammonemia and hyperglutaminemia, predominantly affecting brain regions with a high metabolic rate (21). Neuropathology in those children who died was consistent with the neuroimaging findings and included ulegyria, cortical atrophy with ventriculomegaly, and prominent cortical neuronal loss.

Liver biopsy shows microvesicular and macrovesicular fat and mitochondria of irregular size and shape with intracristae crystallizations (73). The mechanisms of ammonia-induced brain damage are only partly understood. Ammonia is normally detoxified in astrocytes by mitochondrial glutamate dehydrogenase and cytosolic glutamine synthase. The accumulation of ammonia and subsequently increased astrocytic glutamine production in concert with disturbed autoregulation of cerebral blood perfusion results in a number of deleterious effects on the brain. These include depletion of intermediates of cell energy metabolism and of organic osmolytes, altered amino acid and neurotransmitter concentrations, increased extracellular potassium concentrations (75; 10; 42), potentially altered water transport through aquaporin-4 channels (42), and oxidative and nitrosative stress due to increased free radical production and increased nitric oxide synthesis (48). 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 acting overactivation of NMDA receptors and bioenergetic impairment induced by depletion of 2-oxoglutarate. Withdrawal of 2-oxoglutarate from the tricarboxylic acid cycle with consecutive tricarboxylic acid cycle dysfunction ultimately causes impaired oxidative phosphorylation with ATP shortage, decreased ATP/ADP-ratio, and elevated lactate concentrations. Interestingly, inhibition of ornithine aminotransferase is a promising and effective therapeutic approach for preventing neurotoxicity and mortality by hyperammonemia in zebrafish (71; 72).

In vitro experiments demonstrated that hyperammonemia observed in patients receiving valproate may result from a direct inhibition of the N-acetylglutamate synthase activity by valproyl-CoA. Consequently, reduced availability of N-acetylglutamate will impair activation of carbamylphosphate synthetase 1 activity and compromise ammonia detoxification through the urea cycle (02).

Epidemiology

N-acetylglutamate synthase deficiency was increasingly reported after mutation analysis became available. Molecular assays have led to correcting the diagnosis in previously misdiagnosed patients, which may account, in part, for the increased reporting of this deficiency (29). However, N-acetylglutamate synthase deficiency is, along with hyperornithinemia-hyperammonemia-homocitrullinuria syndrome and citrullinemia type 2 (CTLN2), among the rarest of the urea cycle disorders reported in the United States, with a calculated overall incidence of fewer than 1 in 2,000,000 people (62). The estimated overall prevalence of urea cycle disorders is approximately 1 in 35,000 to 52,000 newborns (46).

Prevention

No method is known for preventing N-acetylglutamate synthase deficiency. Successful prenatal diagnosis of N-acetylglutamate synthase deficiency by gene mutation analysis using chorionic villus samples or amniotic fluid cells is available (26). According to a guideline for the diagnosis and management of urea cycle disorders, women should be informed prior to prenatal testing that in N-acetylglutamate synthase deficiency the phenotype can be normalized completely with life-long substitutive therapy (25). This is shown by a case series in three infants from an inbred family (50).

Considering proximal urea cycle disorders (eg, NAGS deficiency, carbamylphosphate synthetase 1-deficiency, ornithine transcarbamylase-deficiency) for expanded newborn screening programs bears the potential of a disease-changing intervention for affected individuals. This is controversial because it is unclear whether newborn screening results of severely affected individuals are available before initial symptoms occur, making presymptomatic initiation of treatment and a potential favorable clinical outcome possible (52; 66). More evidence will be needed in order to evaluate whether prenatal testing and newborn screening for proximal urea cycle disorders will be useful future tools for preventing affected individuals from serious neurologic long-term sequelae.

Differential diagnosis

Disorders of fatty acid beta-oxidation, organic acid metabolism (including valproate treatment), lactic acidosis, and respiratory chain disorders with increased nicotinamide adenine dinucleotide to nicotinic acid dehydrogenase ratio lead to functional N-acetylglutamate synthase deficiency by depriving the enzyme of its substrate acetyl-CoA, by direct inhibition of the enzyme, or by competing with N-acetylglutamate at the carbamoylphosphate synthetase 1 binding site. Benzoate or phenylbutyrate (phenylacetate) depletes coenzyme A and might also lead to secondary deficiency of N-acetylglutamate synthesis. In acetyl-CoA deficiency, concomitant hypoglycemia occurs because acetyl-CoA stimulates pyruvate carboxylase.

Confusing conditions

A number of inborn errors of metabolism can have a similar clinical presentation to N-acetylglutamate synthase deficiency, especially in the newborn period. These include other urea cycle disorders and amino acidopathies, mitochondriopathies, congenital lactic acidoses, defects in fatty acid beta-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 with similar symptoms. Differentiation between these depends first on identifying hyperammonemia associated with specific metabolic patterns of amino acids, acylcarnitines, or organic acid abnormalities. Mitochondrial carbonic anhydrase VA deficiency should also be included in the differential diagnosis (64).

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

Diagnostic workup

In N-acetylglutamate synthase deficiency, the leading biochemical feature is hyperammonemia. Plasma ammonia levels are elevated from 3- to 20-fold during acute episodes. The workup of a suspected case of N-acetylglutamate synthase deficiency should include measurement of plasma ammonia, lactate, (pyruvate) and amino acids, urinary organic acids, and orotic acid. Defects of the other urea cycle enzymes, related transport proteins, organic acidurias, as well as fatty acid beta-oxidation defects or carnitine deficiency have to be excluded because they lead secondarily to a functional N-acetylglutamate synthase deficiency. The differentiation from carbamoylphosphate synthetase 1 deficiency can be achieved by the difficult enzyme assay in a representative liver biopsy sample (with simultaneous assay of carbamoylphosphate synthetase 1 activity). However, also because insufficient sensitivity of enzyme analysis to discriminate between homo- and heterozygosity of N-acetylglutamate synthase deficiency was reported, molecular diagnostics rather than enzyme analysis should be primarily used in patients suspected of N-acetylglutamate synthase deficiency (25).

A therapeutic trial of N-carbamylglutamate with arginine supplementation has been used as a diagnostic test (23). After an adequate dose of N-carbamylglutamate, a decrease of ammonia can be observed in N-acetylglutamate synthase deficiency, accompanied by an increase of plasma citrulline level, urinary orotic acid excretion, and a marked improvement in protein tolerance (16). N-carbamylglutamate has, therefore, been suggested as an emergency medication in neonatal hyperammonemia of unknown etiology. However, the test is not specific for N-acetylglutamate synthase deficiency (25). It may also be positive in organic acidurias, CPS1D, or other defects, leading to low mitochondrial acetyl-CoA or glutamate (causing a secondary malfunction of the enzyme), and the test may even be negative in N-acetylglutamate synthase deficiency (47). N-carbamylglutamate has been used as symptomatic therapy in organic acidurias (19). It is important to exclude organic acidurias so that appropriate treatment is not delayed when hyperammonemia is corrected by N-carbamylglutamate. Moreover, carbamoylphosphate synthetase 1 deficient patients have been reported to respond to N-carbamylglutamate therapy (34). A positive N-carbamylglutamate test should, therefore, be followed by molecular confirmation of N-acetylglutamate synthase deficiency and exclusion of organic acidurias; otherwise, erroneous treatment might harm the patient. Molecular genetic analysis is nowadays more often used in North America and Europe to confirm diagnosis than measurement of enzymatic activity (52; 34). Because N-acetylglutamate synthase deficiency and carbamoylphosphate synthetase 1 deficiency have identical clinical and laboratory manifestations, Haberle and colleagues have provided an algorithm on how to proceed for differential diagnosis between N-acetylglutamate synthase deficiency and carbamoylphosphate synthetase 1 deficiency (25).

Very similar is, finally, the mitochondrial carbonic anhydrase VA deficiency, which is also associated with low-normal orotic acid excretion (64).

Management

For a detailed discussion see “Suggested guidelines for the diagnosis and management of urea cycle disorders” (25).

Management of acute hyperammonemia of N-acetylglutamate synthase deficiency.

Table 1. Protein, Liquid, and Glucose Management

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

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. When N-acetylglutamate synthase deficiency is known, suspected, or even before a definite diagnosis has been made, in cases of acute hyperammonemia, N-carbamylglutamate should be administered (25). N-carbamylglutamate (Carbaglu ®) should be applied initially as bolus at a dose of 100 mg/kg and subsequently every 6 hours at a dose of 25 mg/kg to 62.5 mg/kg. Unfortunately, there is no parenteral application available thus far, so N-carbamylglutamate has to be administered orally or via nasogastric tube if oral intake is not possible.

For newborns or infants who have been diagnosed with hyperammonemic coma, therapy must not be delayed because coma duration of fewer than 1.5 days (51), and timely start of treatment are very important determinants of outcome. An analysis showed that noninterventional variables (eg, disease onset and initial peak-ammonium concentrations) are of utmost importance for neurologic outcomes in individuals with urea cycle disorders (53). Large ongoing studies from the E-IMD and UCDC consortia aim to investigate the effect of (early) diagnosis and treatment principles on the neurologic and cognitive outcomes of affected individuals (54). Specialized pediatric hospitals should have first-line medications, consensus-based treatment protocols, and must act according to the following principles:

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 and sodium phenylacetate/-butyrate) provide alternate pathways to eliminate waste nitrogen (08). 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 (32; 07). 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 (45), no systematic studies regarding the effects of long-term pharmacotherapy on neurologic or cognitive outcome, as defined by clinical endpoints, exist thus far for NAGS-deficiency (45). Energy is 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. Carbohydrates were the primary, if not sole, energy source, but were below age-adapted recommendations, and fat was often omitted from initial emergency treatment (53). It may become necessary to provide soluble insulin to support intracellular glucose uptake and to avoid hyperglycemia. The intake of natural protein is stopped for 24 to maximally 48 hours and is then reintroduced gradually as tolerated (31). In the event that ammonium concentrations do not respond to this management and biochemical or clinical symptoms worsen, continuous venovenous hemodiafiltration (CVVHDF) should be started immediately (planned and organized earlier, ie, at levels > 400 µmol/L) in neonates or children with ammonia levels of more than 500 µmol/L or at lower levels if response to medical treatment is inadequate. Note, even though CVVHDF is by far the most efficient method for extracorporal ammonia elimination (51), 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 dialysis does not impact outcome, and 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 lower ammonium concentrations (30); however, long-term data analyzing these recommendations is 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 the 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 level has fallen below 100 µmol/L (25). In addition to close-meshed control of laboratory parameters like ammonia, electrolytes, glucose, etc., plasma amino acids must be determined and the results available daily to safely stir such management. The reduction of protein intake must be carefully monitored to prevent overrestriction. A diet with inadequate intake can impair protein synthesis and lead to catabolic metabolic decompensation or failure to thrive (31).

After diagnosis of N-acetylglutamate synthase deficiency, no special emergency treatment may be necessary during intercurrent illnesses except continuing the regular administration of N-carbamylglutamate. If necessary, protein, liquid, and glucose management to treat intercurrent hyperammonemic episodes 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; patients and parents must be aware that these measures must not postpone or replace adequate emergency treatment in the hospital (31; 74).

Patients should be supplied with an emergency card, letter, or bracelet containing instructions for emergency measures and phone numbers. The 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 NAGSD.

Table 3. Long-Term Medication for NAGSD

Consensus-based long-term treatment protocol for pediatric (specialized) hospitals treating patients with NAGSD according to suggested guidelines for the diagnosis and management of urea cycle disorders (25).

Long-term management of N-acetylglutamate synthase deficiency relies on the goals of preventing recurrent hyperammonemia and neurologic sequelae and improving quality of life by the following principles (31; 25):

Early identification and treatment of intercurrent hyperammonemic episodes is essential both because treatment is more effective at lower ammonium concentrations and because neurologic outcome appears to be a function of duration of severe hyperammonemia (50). Obviously, the subtle regulation of the urea cycle is not fully restored by treatment with carbamylglutamate.

Catabolism must be avoided to trigger metabolic decompensation. 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.

Unlike other urea cycle disorders, long-term treatment of N-acetylglutamate synthase deficiency does usually not require dietary management except during metabolic crises (see above). The standard N-carbamylglutamate maintenance dosage of approximately 100 mg/kg/d (given in 3-4 doses) should be adjusted individually by down-titration to the minimum dose required (usually 10-15 mg/kg/d (Table 3)) (25). N-carbamylglutamate is an analogue of N-acetylglutamate but unlike N-acetylglutamate, can readily cross the mitochondrial membrane and activate carbamoylphosphate synthase 1. The in vivo effect on ureagenesis in patients with N-acetylglutamate synthase deficiency and propionic acidemia has been confirmed in tracer studies (12; 63; 70). Doses greater than 650 mg/kg can lead to the clinical picture of glutamate intoxication, causing tachycardia, sweating, bronchial hypersecretion, elevated body temperature, and restlessness (57). N-carbamylglutamate treatment leads to a moderate increase of glutamine concentration in plasma. In cases of low plasma L-arginine (or L-citrulline) levels, the essential amino acid L-arginine (or L-citrulline, which is converted to L-arginine) should be provided at a dose of 100 to 200 mg/kg/d both as an essential missing product and to activate or stabilize residual N-acetylglutamate synthase activity to produce N-acetylglutamate.

However, a study demonstrated that all individuals with N-acetylglutamate synthase receiving long-term medication were treated solely with N-carbamylglutamate, unlike L-arginine or L-citrulline, which were only sporadically administered (53). This observation was confirmed by a published review, demonstrating that L-arginine, L-citrulline, or nitrogen scavengers were only rarely used in addition to L-carbamylglutamate with or without a protein-restricted diet (34).

Occasionally, protein intake restriction may be necessary, depending on the response to N-carbamylglutamate. However, the stimulation of alternate pathways of waste nitrogen excretion (sodium benzoate, sodium/glycerol phenylbutyrate) is usually unnecessary and would consume coenzyme A, one of the substrates of N-acetylglutamate synthase and ATP, which is needed for CPS1 activation (55). Transporters involved in renal excretion of N-carbamylglutamate have been identified. Schwob and colleagues proposed that N-carbamylglutamate is taken up at the basolateral membrane of kidney proximal tubule renal cells by the organic anion transporter 1 (OAT1) in cooperation with the sodium dicarboxylate cotransporter 3 (NaDC3). Efflux across the luminal membrane into the tubular lumen probably occurs by OAT4, completing renal secretion of N-carbamylglutamate (58). Periodic measurements of plasma amino acids (which include glutamine) and blood ammonia may permit adjustment of therapy before clinical symptoms appear. The benefit of vaccinations outweighs the risk of decompensations. They are recommended at the same schedule as for healthy children (44; 25).

Some medications are contraindicated in urea cycle disorders because of secondary inhibition of the urea cycle or inducing catabolism, most importantly valproic acid and systemic steroids. Even in well-controlled and managed patients, peracute deadly coma can occur. 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.

Liver transplantation might also be a suitable treatment for individuals with N-acetylglutamate synthase deficiency that do not reach a stable metabolic condition despite intensive conservative treatment. A study suggested that individuals with urea cycle disorders had an improved neurocognitive outcome if they received a liver graft, suggesting that they should undergo liver transplantation already after comparatively low maximum ammonium concentrations to protect the patient’s neurocognitive abilities (37; 36). However, further data suggest a rather limited impact of liver transplantation on the neurodevelopmental outcomes of individuals with a severe disease burden (35). Hepatocyte transplantation is an interesting therapeutic bridging option in patients with urea cycle disorder awaiting liver transplantation (43) and is investigated in studies.

An interesting approach for treatment of acute hyperammonemia is the inhibition of ornithine aminotransferase, leading to a transamination-dependent decrease of glutamate and glutamine. Mammalian transfer will provide more insight if this approach is indeed a potential therapeutic principle for individuals with urea cycle disorders (71). Lately, it was 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 might also play an important role during hyperammonemic conditions in the brain in order to determine if autophagy might be a suitable target for treatment of hyperammonemic conditions (60).

Outcomes

Although some publications suggest an improved outcome (eg, neurologic manifestation, mortality) of individuals with urea cycle disorders, a review and metaanalysis showed less convincing results, suggesting that no substantial improvement of survival for urea cycle disorders was observed over more than 3 decades between 1978 and 2014 (09). Some studies showed that noninterventional variables reflecting disease severity are associated with the highest risk of mortality and poor neurologic outcome (17; 53). Long-term morbidity is still substantial in individuals with urea cycle disorders. A transatlantic study with more than 500 individuals evaluated the impact of the initial peak blood ammonium concentration on their cognitive outcome, revealing that high initial peak blood ammonium concentrations adversely affect cognition in proximal (including NAGS-deficiency) rather than distal urea cycle disorders (54). More research is needed in order to evaluate the pathomechanistic basis for this finding and to study further predictors of good and poor neurocognitive outcomes in individuals with urea cycle disorders.

Special considerations

Pregnancy

It is unclear how pregnancies would affect women with N-acetylglutamate synthase deficiencies. There have been no published reports of pregnancies in affected females.

Langendonk and colleagues studied a series of pregnancies in women with inherited metabolic disease and suggested that special care must be taken not to confuse behavioral changes of hyperammonemia for symptoms of postpartum psychosis or depression (41). Routine monitoring of plasma ammonia levels in those women was suggested (18). Therefore, pregnancies in women with inherited metabolic disease should be monitored and escorted in close contact with or better in a medical center that includes a metabolic center (41).

Anesthesia

There has been one report of hyperammonemia induced by enflurane in argininosuccinic aciduria (04). It is prudent to use anesthetics with low toxicity to the liver. However, surgery requires stopping of oral medication and may be associated with a catabolic condition, both of which may induce hyperammonemia. It is important to resume oral medication of N-carbamylglutamate as soon as possible and, if necessary, start alternate pathway therapy intravenously until the patient is able to tolerate his oral medication. The patient should also receive adequate glucose infusion to prevent catabolism. 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 levels as well as shifting to oral medications are required (25).

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 (20).