Clinical manifestations

Presentation and course

Hyperargininemia typically shows a clinical picture distinct from other urea cycle disorders. The prevailing definition used for urea cycle disorders following the disease onset or age at presentation of first symptoms (early- or neonatal-onset [fewer than 28 days] versus late-onset [more than 28 days] urea cycle disorder) does not usually apply to the clinical presentation of arginase 1 deficiency.

Although affected neonates usually do not initially present with hyperammonemic coma, some do develop symptomatic hyperammonemia (65). Often, children appear to develop normally for the first few years of life; however, many show evidence of protein aversion, often associated with anorexia, vomiting, and irritability, as well as developmental delay (15). First symptoms usually occur between ages 2 to 4 years or even later (50; 29) and are mainly nonspecific, such as clumsiness, tripping, or falling. If untreated, symptoms may progress to spastic diplegia (predominantly of lower limbs), seizures, and gradual loss of developmental milestones (eg, motor function, language function) and intellectual disability (20; 71; 08). Other symptoms that may present early in life include episodes of irritability, anorexia, and vomiting. A study on 16 patients with arginase 1 deficiency showed that the first neurologic symptom was lower limb spasticity in 75% of the cases, followed by seizures. Only one patient presented initially with upper limb tremor. Approximately 40% of the patients developed upper limb spasticity during the course of the disease. Visual or hearing impairment does not seem to occur; however, microcephaly and growth deficit are often reported (15). Without therapy, patients usually do not reach normal adult height. Fluctuating hyperammonemia may occur in arginase 1 deficiency, but blood ammonia concentrations are mostly not high enough to provoke acute encephalopathy (15). The clinical features may include progressive gait abnormalities due to spasticity, as well as acute episodes of ataxia, behavior disturbances (aggression, hyperactivity, and irritability), vomiting, lethargy, and seizures. These episodes are often precipitated by intercurrent viral illnesses and are usually associated with moderately elevated plasma ammonia concentrations of three to four times normal and plasma arginine concentrations greater than 5-fold normal, often exceeding 1000 µmol/L (normal is less than 120 µmol/L).

Although hyperammonemia is not a major finding in arginase 1 deficiency, avoidance of precipitating factors for life-threatening hyperammonemic crises is still required, which is supported by several children with arginase 1 deficiency experiencing significant hyperammonemia (42).

Several early-onset patients (neonates and infants with hyperammonemia) with arginase 1 deficiency have been described thus far (68; 67; 42; 69), demonstrating that disease onset may occur earlier than expected and that severe hyperammonemia may be more common than previously reported (42). The manifestations in early-onset arginase 1 deficiency resemble other urea cycle disorders with a sepsis-like presentation, vomiting, lethargy, poor feeding, but also with seizures, cerebral edema, jaundice, and hepatomegaly (42). In such episodes, arginine may, however, be normal or even low, making the exact diagnosis difficult at that time (G. F. Hoffmann, personal experience).

Published data from the Urea Cycle Disorders Consortium (UCDC) report neuroimaging and neurocognitive findings of more than 600 urea cycle disorders patients, including patients suffering from arginase 1 deficiency (ARG1D) (n = 17). Acute hyperammonemic episodes demonstrate a diffuse or focal pattern of cerebral MRI changes and potentially brain edema (19; 09; 28). However, the impact of acute cerebral changes on long-term outcome and, thus, the predictive value of acute MRI changes of the brain is unclear. An analysis of MRIs of the brain during a stable metabolic condition of individuals with ornithine transcarbamylase deficiency showed structural, biochemical, and functional differences in the brain between symptomatic subjects and normal controls (27). The neurocognitive performance showed a considerable variability during infancy, whereas adults received scores on intelligence tests indicating intellectual disability. Most commonly, adults experienced memory and fine motor deficits. The cross-sectional study showed a decline in functioning of adults compared to younger patients; however, it remains unclear whether this is due to decline throughout life or improvements in diagnostics (eg, introduction of newborn bloodspot screening) and treatments (82). Importantly, a descriptive natural history study summarizing the review of the literature has been published (11).

Prognosis and complications

Although acute death due to arginase 1 deficiency is uncommon, especially when compared to other urea cycle disorders, morbidity is high. Most affected children develop intellectual disability, spastic di/quadriplegia, seizures, and impaired language function with worsening or loss of spoken language. Secondary complications, such as joint restriction and loss of ambulation, develop in nearly all patients with arginase 1 deficiency (15; 11). Brain neuroimaging studies generally show microcephaly and cortical atrophy, with enlargement of the lateral ventricles and sulci. Diffusion tensor imaging (DTI) studies demonstrate alterations in corticospinal tracts (59) consistent with the clinical findings. Moreover, a variable degree of brain and mild cerebellar atrophy has been described in a series of arginase 1 deficient–patients (16).

A detailed overview of organ-specific disease manifestations and complications in patients suffering from urea cycle disorders (eg, ARG1D) was delineated by Kölker and colleagues (51). Importantly, patients with urea cycle disorders suffer from increased frequencies of mental disability and behavioral/emotional problems; however, health-related quality of life of those patients is within the normal range (43).

The overall prognosis is guarded and determined by the degree of spastic diplegia and cognitive deficits. From the available data, it is currently uncertain whether metabolic therapy improves the long-term neurologic outcome for individuals with arginase 1 deficiency (62). Moreover, evidence indicates that individuals with hyperargininemia may experience persistent or progressive disease manifestations despite receiving standard maintenance treatment approaches, ie, protein-restricted diet or scavenger therapy (24). However, liver transplantation can serve as an effective treatment option if conservative management fails to prevent disease progression by improving neurologic status, correcting growth deficits, and increasing the quality of life (21).

Clinical vignette

Patient 1: Typical arginase 1 deficiency presentation. The following is a summary of a case of a 9-year-old boy that was previously published (66). The boy was born at term after an uncomplicated pregnancy to nonconsanguineous African-American parents. Neonatal adaptation, infancy, and early childhood were unremarkable, with normal achievement of gross motor milestones. At 5 years of age, he was brought to medical attention because of toe walking and worsening gait and balance. Cerebral palsy was diagnosed; he was treated with bracing, casting, and physical therapy. At 8 years of age, he underwent bilateral heel cord releases. Postoperatively, he had seizures that were treated with phenytoin and valproic acid. Subsequently, progression of his spasticity and obvious ataxia became apparent.

The maternal family history was remarkable for an uncle and first cousin with seizure disorders, as well as distant cousins who were said to have mental retardation. The paternal family history was not available. The patient preferred a low-protein diet.

At 9 years of age, he was hospitalized for severe vomiting, lethargy, and a generalized tonic-clonic seizure. Weight and head circumference were at the 60th percentile for age, but height was below the fifth percentile. His liver was palpable 4 cm below the inferior costal margin, with a span of 12 cm. He was awake but disoriented. Assessment of cranial nerves and sensation showed good function. Motor strength was normal. His ataxia acutely worsened; he was unable to walk and could only sit with support. He was unable to perform the finger-to-nose maneuver. Deep tendon reflexes were 4+ and brisk, and there was sustained fast-beat ankle clonus. Plasma ammonia concentration was elevated at 287 µmol/L (normal 22 to 48 µmol/L). Plasma amino acid analysis revealed elevated arginine concentration of 334 µmol/L (normal 55 to 145 µmol/L). Erythrocyte arginase activity was 0.04 µmol of urea formed per hour per milligram of hemoglobin (normal controls 2.10 to 6.80; affected controls 0.00 to 0.49).

Medical management included starting a protein-restricted diet and oral sodium phenylbutyrate. His mental status improved with treatment, as did his ataxia; however, his mobility remained impaired. He continued to have a scissoring gait and, when not at home, relied on a wheelchair. He was placed in special education classes and functioned at a first-grade level. His full-scale IQ score on the Wechsler Intelligence Scale for Children, Revised was 62 (66).

Patient 2: Atypical arginase 1 deficiency presentation. The following is a summary of a case of a male that was previously published (42). He was born at term to a first cousin consanguineous couple having two further healthy siblings. He showed normal development until 6 weeks of age when he presented with a 12-hour history of poor feeding progressing to irritability, lethargy, respiratory distress, and seizures, requiring intubation and ventilation. Initial venous plasma ammonia was 537 µmol/L and increased to 739 µmol/L 2 hours later. With continuous veno-venous hemodialysis and intravenous sodium phenylacetate/benzoate (Ammonul ®), the ammonia concentration normalized within 10 hours of admission. The results of plasma amino acid analysis became available as the intravenous arginine solution was being prepared; thus, the patient did not receive intravenous arginine. Glutamine was elevated to 1398 µmol/L (normal 1-746), arginine to 841 µmol/L (normal 53-71), citrulline to 34 µmol/L (normal 0-26), and urinary orotic acid to 228.9 mmol/mol creatinine (normal 1-3.2). Ornithine was reduced to 23 µmol/L (normal 37-61). This constellation of findings pointed to arginase 1 deficiency.

MRI brain scan performed 24 hours after presentation revealed diffuse and abnormally increased signal intensity of the deep and central white matter. The patient recovered from the initial insult and regained oral feeding. A low-protein diet and oral sodium phenylbutyrate (titrated up to a dose of 330 mg/kg/d) were instituted, but he continued to have recurrent hyperammonemic episodes over the next year and half. Neuropsychological assessment at 19 months showed nonverbal and motor development to be equivalent to 15 months and verbal development equivalent to 11 months developmental age.

Axial and limb hypertonia with hyperreflexia progressively developed. Despite dietary regimen, arginine remained elevated at more than 200 µmol/L; seizures disappeared on low-dose phenobarbital. Molecular analysis found a nonsense homozygous mutation in exon 2 (c.61C> T corresponding to p.R21X).

Biological basis

Etiology and pathogenesis

Hyperargininemia is caused by a deficiency of liver arginase (EC 3.5.3.1), also called arginase 1, the final enzyme of the urea cycle.

This cytosolic enzyme catalyzes the cleavage of arginine into urea and ornithine (18). Arginase 1 is also found in red blood cells. In contrast, arginase 2 is found in the mitochondrial matrix and differs from the liver-type enzyme in biochemical, molecular, and immunological properties. Arginase 2 is found in the kidney, prostate, small intestine, and brain. Arginase 2 activities become elevated in patients with argininemia (40). It is possible that the presence of arginase 2 in hyperargininemia provides some degree of protection from nitrogen accumulation, resulting in less severe hyperammonemic episodes than in other urea cycle disorders.

Hyperargininemia is inherited as an autosomal recessive trait. The gene for liver arginase, which is mutated in hyperargininemia, has been localized to chromosome 6q23 (73). There is evidence for multiple point mutations and microdeletions in the arginase 1 gene of patients, indicating extensive genetic heterogeneity (81; 05), and many affected individuals are compound heterozygotes. More than 40 disease-causing mutations have been reported (71). There has been a correlation between the severity of the mutation and the degree of clinical symptoms (80; 79).

The principal biochemical findings are elevated plasma concentrations of arginine, glutamine, and ammonia. These, plus ornithine, aspartate, threonine, glycine, and methionine concentrations are elevated in CSF (17). In addition, there is a generalized diamino aciduria (argininuria, lysinuria, cystinuria, ornithinuria); urinary excretions of orotic acid and guanidine compounds are also markedly increased. Diagnosis can be confirmed by measuring arginase 1 activity in erythrocytes. However, 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 for arginase 1 deficiency.

The mechanisms responsible for the spasticity and cognitive deficits are unknown. It is unlikely that the clinical picture is due to moderately elevated ammonia. Arginine, its guanidine metabolites, and altered biogenic amines are candidate neurotoxins (37; 01). Arginine being substrate for nitric oxide synthetase also raises the possibility that overproduction of nitric oxide may play a role in the neuropathology of hyperargininemia (40). It is theoretically possible that pharmacologic inhibitors of nitric oxide synthetase will be beneficial. A knockout mouse model of arginase 1 was produced (41). The homozygous mice die within 10 to 14 days after birth from hyperammonemia and, therefore, may not be a suitable model for the study of the long-term effects of arginase 1 deficiency on the brain (71). Other animal studies showed that arginine administration in large amounts interferes with antioxidative function in brain cortex and with energy production in the hippocampus (83). It has been suggested that guanidinoacetate might be a marker or even an active compound and that oxidative stress does not play a major role in arginase 1 deficiency. Finally, altered nitric oxide synthesis or downregulation of eNOS may also be involved in the pathophysiology of arginase 1 deficiency (38; 33).

Cantero and coworkers found that loss of arginase 1 expression in a mouse model altered dendritic complexity as well as synapse numbers and synaptic transmission in layer 5 motor cortical neurons (14). Furthermore, they demonstrated that hepatic arginase 1 gene therapy with adeno-associated virus rescued nearly all abnormalities when applied at a neonatal stage to homozygous knock-out animals, supporting the concept that neonatal gene therapy might be beneficial to prevent functional abnormalities in the brain. A similar approach provided indications that hepatocyte transplantation corrects liver disease in arginase-deficient mice (02). Furthermore, arginase 1 mRNA therapy might be a novel therapeutic approach for arginase 1 deficiency (04; 78). It was demonstrated that arginase-1 deficiency might also be a leukodystrophy affecting the central nervous system, and neonatal AAV hepatic gene therapy can prevent the sequelae resulting in dysmyelinated axons (53). These data are supported by another study with neuronal arginase-1 knock-out mice (nARG1 KO mice) indicating that the neurologic clinical manifestations of arginase-1 deficiency are the consequence of liver but not neuronal arginase-1 deficiency. Thus, restoration of hepatic ARG1 expression and careful lowering of blood arginine concentrations may offer the best chance of avoiding neurologic sequelae (63). However, recent data also support the importance of arginase 1 in microglia as microglial-specific knockdown of arginase 1 causes impaired dendritic spine maturation in the hippocampus where cholinergic neurons project (74).

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 TCA 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 (86; 87).

Epidemiology

The estimated cumulative prevalence of urea cycle disorders is 1 in 35,000 to 52,000 newborns (58). Hyperargininemia is one of the rarest inborn errors of urea synthesis. Data provided by the UCDC (70) calculated the overall incidence of arginase 1 deficiency in the United States as 1 in 950,000 people (76).

Prevention

There is no known method for prevention of hyperargininemia. If the mutation of the patient is known, prenatal diagnosis can be accomplished by mutation analysis in chorionic villous tissue and in amniotic fluid cells (30; 20; 29). Prenatal diagnosis can also be performed on red blood cells from cord blood (44; 29). Preimplantation genetic diagnosis in couples at risk is a possibility. Newborn screening of dried blood spots by tandem mass spectrometry is an established method and allows early detection of elevated arginine or arginine to ornithine ratio and subsequent treatment of this condition (89; 35). Newborn screening for arginase 1 deficiency is rarely performed; however, if newborn screening for arginase 1 deficiency is available, a trend for earlier detection of arginase 1-deficient individuals was reported (38; 62). Furthermore, preliminary neurologic outcome studies suggest that early detection of the disorder by newborn screening might have a positive effect on motor function of arginase 1 deficiency patients. However, this investigation requires reevaluation in a larger sample and at a more advanced age group using additional (neurologic) outcome parameters.

Differential diagnosis

Because seizures are common in this disorder, their association with progressive spasticity and intellectual disability should lead to further diagnostic testing. Measuring plasma ammonia and amino acid concentrations in the context of (progressive) spastic paraparesis may be the key to detection of hyperargininemia and may save the patient from years of inadequate or no treatment as well as progressive neurologic sequelae (15). Plasma amino acids will discriminate hyperargininemia from other disorders because of the markedly elevated arginine concentrations usually associated with a moderate degree of hyperammonemia.

Importantly, Yahyaoui and colleagues reported a new disorder (human cationic amino acid transporter 2) that biochemically mimics arginase 1 deficiency. Newborn screening centers that analyze for arginase 1 deficiency should be aware of this disorder (84).

Confusing conditions

This disorder has a unique set of clinical and biochemical findings that should differentiate it from other inborn errors of metabolism, seizure disorders, and causes of cerebral palsy (66; 60). An inborn error of intermediary metabolism may be suspected because of the episodic nature of clinical symptoms (eg, vomiting and behavioral changes), and because of the progressive neurologic symptoms. However, there may be a long period between the manifestation of the first classical clinical symptoms and age at diagnosis. The reason for the delay in diagnosis and, thus, the delay in adequate treatment, is diverse. The atypical presentation as a urea cycle disorder and the rarity of arginase 1 deficiency may result in physicians failing to consider hyperargininemia as a putative differential diagnosis for the clinical picture, as outlined in the clinical vignette (patient 1). Patients may be initially diagnosed with cerebral palsy or autosomal recessive hereditary spastic paraplegia. The following features may help to distinguish hyperargininemia from cerebral palsy: patients suffering from arginase 1 deficiency show (1) progression of spasticity, (2) deterioration of cognitive and language function, (3) avoidance of high-protein foods, and (4) usually absence of a clear history of hypoxia at birth or during the neonatal period (15). Differentiating hyperargininemia from hereditary spastic paraplegia is more difficult, but the following clinical features may point to hyperargininemia: (1) progression of spasticity, (2) avoidance of high-protein foods, and (3) infrequent hypertonic urinary bladder disturbances (15). Thus, arginase 1 deficiency must be considered in the differential diagnosis of hereditary spastic paraplegia and should be included in hereditary spastic paraplegia gene panels (55).

Diagnostic workup

Plasma ammonia and amino acids should be obtained in suspected cases. The principal biochemical features of this disorder are hyperammonemia and hyperargininemia.

Plasma ammonia concentrations are usually normal when arginase 1 deficiency patients are well; however, glutamine concentration may be increased. Interestingly, increased plasma arginine concentrations are relatively constant for most patients, barely responding to protein intake variation within a normal and growth sustaining range. However, plasma urea concentrations rise with increased protein intake and are higher than in patients with other urea cycle defects, which is due to the existence of an isoenzyme (20).

Furthermore, there is diamino-aciduria, orotic aciduria (probably due to a functional decrease of ornithine transcarbamylase activity as a result of low ornithine) and increased urinary excretion of guanidino compounds.

Cerebrospinal fluid (CSF) arginine, glutamine, other amino acids (see above), and guanidino compounds are usually markedly elevated. Neurologic changes in arginase 1 deficiency may be due to increases in CSF arginine and CSF guanidino compounds that can lead to seizures and demyelination (22; 42). In North America and Europe, molecular genetic analysis rather than measurement of enzymatic activity is currently more often used to confirm the diagnosis (61).

Management

For a detailed discussion see “Suggested guidelines for the diagnosis and management of urea cycle disorders” (29).

Management of acute hyperammonemia in arginase 1 deficiency.

Table 1. Protein, Liquid, and Glucose Management

Escalation level	NH3 (µmol/L)	Protein	Liquid IV (ml/kg/d)	Glucose IV (mg/kg/min)	Insulin	Comments*****
1	< 100	Stop*	100 - 150**	10***	i.n.****
2	100 - 250	Stop*	100 - 150**	10***	i.n.****	Inform metabolic clinic
3	250 - 500	Stop*	100 - 150**	10***	i.n.****	Inform dialysis clinic
4	> 500	Stop*	100 - 150**	10***	i.n.****	Hemodialysis
Consensus-based treatment protocol for pediatric (specialized) hospitals treating ARG1D patients with acute hyperammonemia according to “Suggested guidelines for the diagnosis and management of urea cycle disorders” (29). *Stop protein intake for 24 hours (maximum 48 hours) **Liquid management might be adapted to hospital- and age-specific requirements ***Glucose management might be adapted to age-specific requirements ****If necessary (i.n.) give 0.05 units/kg/h *****Hyperglycemia can be dangerous, monitor glucose every 0.5 - 1 hour; monitor blood ammonia levels every 3 hours; monitor electrolytes, blood gases and lactate regularly, eg, every 3 hours

Table 2. First-Line Medication for ARG1D****

Esca-lation level	NH3 (µmol/L)	Sodium benzoate IV***		Sodium benzoate/-phenylacetate (Ammonul ®) IV***		L-Arginine hydro-chloride 21% IV		Carbamyl-glutamate by mouth
		Bolus (mg/kg) in 90 – 120 min	Maintenance (mg/kg/d)**	Bolus (mg/kg) in 90 – 120 min	Maintenance (mg/kg/d)**	Bolus (mg/kg) in 90 – 120 min	Maintenance (mg/kg/d)
1	< 100	/	/	/	/	/	/	/
2	100 - 250	250	250 – 500 5.5 g/m2/d*	250	250 – 500	/	/
3	250 - 500	250	250 – 500 5.5 g/m2/d*	250	250 – 500	/	/
4	> 500	250	250 – 500 5.5 g/m2/d*	250	250 – 500	/	/
Treatment protocol for pediatric (specialized) hospitals treating ARG1D patients with acute hyperammonemia modified from “Suggested guidelines for the diagnosis and management of urea cycle disorders” (29). *If patient weighs more than 20 kg **If on dialysis maintenance dose should increase to 350 mg/kg/d ***First-line medications must be diluted in 10% glucose prior to IV application ****Control electrolytes due to possible danger of hypernatremia and hypokalemia; overall low risk of hyperammonemic decompensation in ARG1D

Although patients with arginase 1 deficiency are less prone to acute hyperammonemia and plasma ammonia is usually only moderately elevated (20), episodes must be treated promptly to prevent additional brain damage. In families with an index patient, the birth of an at-risk or prenatally diagnosed infant provides the opportunity for prospective management.

Specialized pediatric hospitals should have first-line medications, consensus-based treatment-protocols, and 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 (12). 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 (36; 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, 2 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 (57), no systematic studies regarding the effects of long-term pharmacotherapy on neurologic or cognitive outcome, as defined by clinical endpoints, exist thus far.

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 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 below age-adapted recommendations; carbohydrates were the primary, if not sole, energy source; and fat was often omitted from initial emergency treatment (62). Soluble insulin may become necessary to avoid hyperglycemia and to support intracellular glucose uptake. The intake of natural protein is stopped for 24 hours to maximally 48 hours and is then reintroduced gradually as tolerated (34). In the event that ammonia concentrations do not respond to this management and biochemical or clinical symptoms worsen, continuous veno-venous hemodiafiltration should be started immediately (planned and organized earlier, ie, at concentrations ≥ 400 µmol/L) in patients with ammonia concentrations of more than 500 µmol/L (see Table 1) or at lower concentrations if response to medical treatment is inadequate. Of note, even though continuous veno-venous hemodiafiltration is by far the most efficient method for extracorporal ammonia elimination, prognosis is not related to dialysis modality but primarily to the duration of coma before the initiation of treatment, confirming the need for rapid and aggressive management. In fact, a retrospective data analysis of 202 published cases revealed that current practice in dialysis does not impact outcome. 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 concentrations of ammonium (32); 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 prevent endogenous protein catabolism by exceeding the energy demand 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 (29). 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 modify 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 (34).

According to the physician’s advice, an oral dietary emergency regimen may be applicable to prevent and 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, fluid, and glucose management is identical to the protocol in Table 1.

In mild intercurrent illnesses, patients and families may start with an oral emergency dietary regimen at home. Temperatures should be monitored and those above 38°C should be treated with antipyretics; however, patients and parents must be aware that these measures must not postpone or replace adequate emergency treatment in the hospital (34; 88).

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 arginase 1 deficiency.

Table 3. Long-Term Medication for ARG1D

	Sodium benzoate by mouth	Sodium phenylbutyrate by mouth*		L-Arginine by mouth		L-Citrulline by mouth	Carbamyl-glutamate by mouth
	(in mg/kg/d)**	< 20 kg (in mg/kg/d)**	> 20 kg (in g/m2/d)**	< 20 kg (in mg/kg/d)	> 20 kg (in g/m2/d)		/
Dose	≤ 250	≤ 250	5	/	/	/	/
Maximum	12 g/d	12 g/d		/		/	/
Consensus-based long-term treatment protocol for pediatric (specialized) hospitals treating patients with ARG1D according to “Suggested guidelines for the diagnosis and management of urea cycle disorders” (29). *Second choice that should be given together with sodium benzoate in patients in which benzoate alone is not enough. **In some patients, higher doses may be needed (< 20 kg: 450 – 600 mg/kg/d and > 20 kg: 9.9 – 13 g/m2/d).

Long-term management of arginase 1 deficiency relies on the goals of preventing recurrent hyperammonemia and neurologic sequelae and improving quality of life by the following principles (34; 29):

(1) Avoidance of catabolism
(2) Low-protein diet
(3) Supplementation with essential amino acids, vitamins, and minerals
(4) Long-term ammonia scavenger medication (see Table 3)
(5) Suitable emergency regimens in intercurrent illness (see Tables 1 and 2)

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

Dietary treatment is essential for 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 arginine-free amino acids formula, and supplemental calories provided by a formula that does not contain protein. The goal of long-term management is based upon minimizing the nitrogen-load on the urea cycle. The FAO/WHO/UNO 2007 report (29) 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. Unfortunately, only few patients can adhere to a diet rigorous enough to get arginine concentrations into or near the normal range (20).

Periodic measurement of plasma amino acids, which include glutamine, and blood ammonia may permit adjustment of therapy before clinical symptoms appear. Long-term medication includes sodium benzoate or sodium phenylbutyrate. 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. However, phenylbutyrate may deplete branched-chain amino acid concentrations and may cause menstrual dysfunction/amenorrhea in up to 25% of postpubertal females (64; 13; 29). To avoid complications (eg, mucositis or gastritis), sodium benzoate and sodium phenylbutyrate should be administered several times daily during meals with abundant fluids (29). 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 (06). For long-term medication, sodium benzoate and sodium phenylbutyrate are used equally frequently (62).

Carnitine deficiency may occur in patients with urea cycle disorder that are on a low-protein diet or receive treatment with ammonia scavengers. This rarely needs supplementation. The benefit of vaccinations outweighs the risk of decompensations. They are recommended on the same schedule as for healthy children (56; 29). In terms of symptomatic treatment, baclofen has been found to be helpful in decreasing spasticity. Seizures usually respond to phenytoin or carbamazepine.

A number of patients with various urea cycle disorders have received partial or total orthotopic liver transplants to provide enzyme replacement therapy (31; 39; 54). In all successful cases, this has cured hyperammonemia and permitted a normal protein intake. However, its effectiveness is hampered by expense, limited availability of donor organs, and significant morbidity and mortality from complications of transplantation or immunosuppression. Furthermore, it is still controversial as to whether the neurologic phenotype is positively influenced. An analysis evaluating the outcome of pediatric and adult urea cycle disorder patients who underwent a liver transplantation showed that approximately two thirds of the patients were transplanted before 5 years of age (85). Overall, 1-, 5-, and 10-year survival was very good with 93%, 89%, and 87% survival, respectively. Another retrospective survey study investigated whether individuals with urea cycle disorders would benefit from liver transplantation by preventing further recurrent decompensations (48; 47). Individuals with these disorders had an improved neurocognitive outcome if they received a liver graft, suggesting that they should undergo liver transplantation after exhibiting comparatively low maximum ammonium concentrations in order to protect the individual’s neurocognitive abilities. However, further data suggest a rather limited impact of liver transplantation on the neurodevelopmental outcomes of individuals with a severe disease burden, whereas obviously for arginase 1-deficient individuals, early intervention by liver transplantation might positively affect the neurologic status, growth deficits, and quality of life (46; 21).

Some medications are contraindicated in urea cycle disorders because of secondary inhibition of the urea cycle, most importantly valproic acid. Even in well controlled and managed patients, acute deadly coma can be provoked. 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.

Case studies have suggested the use of L-ornithine in the long-term therapy of certain patients with arginase 1 deficiency, especially in those who show an inverse relationship between plasma ornithine concentration and urinary orotic acid excretion. Another interesting finding was the dramatic reduction of plasma arginine after red blood cell transfusion, lasting for approximately 6 weeks (which is the expected half-life of transfused red blood cells). Further studies might open new possibilities for application of red blood cells in the therapy of arginase 1 deficiency, especially because hyperargininemia may play an essential role in the neuropathogenesis (42). Furthermore, newborn screening is being used in the detection of arginase 1 deficiency in some countries, leading to early initiation of treatment and possibly to reduction of neurologic sequelae of this very difficult to treat condition (42; 62).

A strategy for the treatment of arginase 1 deficiency was suggested by Amayreh and coworkers (01). They showed that guanidinoacetate was not only elevated in guanidinoacetate methyltransferase deficiency, but also in arginase deficiency, thereby focusing on reducing guanidinoacetate as a novel therapeutic approach in arginase 1 deficiency. Similar to the treatment in guanidinoacetate methyltransferase deficiency, a 9-year-old boy with arginase 1 deficiency and elevated guanidinoacetate concentration received creatine, L-ornithine, and sodium benzoate along with an arginine-restricted diet. This regimen resulted in lowered guanidinoacetate in blood and clinical improvement with reduced seizure frequency and improved alertness (01). Other new treatment strategies might be hepatocyte transplantation, mRNA therapy, or inhibition of ornithine aminotransferase (02; 04; 86; 87). Future studies will show if these principles might indeed be transferred to clinical practice. 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 (72).

A new treatment modality for individuals with hyperargininemia was evaluated with potentially promising results from a phase 1/2 trial. Pegzilarginase, which has been developed as a novel arginine lowering approach, was analyzed in 16 individuals with arginase-1 deficiency. Despite state-of-the art, long-term management, a significant number of individuals with hyperargininemia still suffer from long-term neurologic sequelae and high plasma L-arginine concentrations, which is thought to be one of the major causes of clinical deterioration in hyperargininemia. Importantly, treatment with pegzilarginase reduced plasma L-arginine concentrations in all patients; 50% were in the normal range. Moreover, nearly 80% of individuals treated with pegzilarginase were defined as clinical responders based on improved mobility assessment scores. This improvement in the neurologic outcome of clinically symptomatic individuals with hyperargininemia, who received long-term metabolic maintenance treatment before the start of pegzilarginase, might support this as potentially powerful future treatment. This will be evaluated in a future phase 3 trial (24).

Outcomes

Prior to the development of alternate pathway therapy using ammonia scavengers (eg, sodium benzoate, sodium phenylacetate/-butyrate), urea cycle disorders mostly led to acute or premature death. Now, the 1-year survival rate for patients with early-onset as well as late-onset urea cycle disorder can reach up to 90% to 95% (49).

Unfortunately, very few specific data are available for the outcome of patients suffering from arginase 1 deficiency; however, the first promising results for early diagnosis by newborn screening have been reported (38; 62). As it is obviously determined by pathophysiological sequences different from the other urea cycle disorders leading to a distinct clinical course, arginase 1 deficiency should not be counseled in a manner identical to other urea cycle disorders. Plasma arginine concentrations, natural protein, or amino acid intake had no significant effect on the clinical phenotype per se; however, patients with arginase 1 deficiency under treatment had a stable disease course (38). This suggests that early diagnosis and adherence to metabolic treatment are important for a favorable long-term outcome in arginase 1 deficiency (62). To our current knowledge, the long-term prognosis of patients is determined by the degree of spastic diplegia and by their cognitive deficits.

Interestingly, two new reviews discuss the effects and limitations of the current standard of care (10; 23). Further studies are needed to evaluate possible interventional variables as well as the neurologic and intellectual long-term outcome of patients suffering from arginase 1 deficiency. Moreover, new treatment principles, earlier access to diagnostics, and more disease awareness might help decreasing plasma L-arginine concentrations earlier in life and, thus, may improve clinical outcomes in the future (24; 23; 10).

Special considerations

Pregnancy

It is unclear what the effects of pregnancy would be for a woman with hyperargininemia or whether arginine would be teratogenic to her fetus. There have been no published reports of pregnancies in affected females.

Langendonk 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 (52), and routine monitoring of plasma ammonia concentrations in those women was suggested (25). Pregnancy in women with inherited metabolic disease should, therefore, be monitored in or in close cooperation with an appropriate metabolic center (52).

Anesthesia

There has been one report of hyperammonemia induced by enflurane in argininosuccinic aciduria (03). Another work reported the successful anesthetic management of a patient with arginase-1 deficiency (45). It is prudent to use anesthetics with low toxicity to the liver. However, surgery requires the stopping of oral medication and results in a catabolic condition, both of which may induce hyperammonemia. It is important to continue alternate pathway therapy intravenously until the patient is able to tolerate oral medication. The patient should also receive adequate glucose infusion to prevent catabolism. Surgery should only be performed in centers experienced in and 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 (29).

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 (26).