Urea cycle disorders in adult patients

INTRODUCTION: Urea cycle disorders (UCD) usually present after 24 h to 48 h of life with failure to thrive, lethargy and coma leading to death, but milder forms may occur from infancy to adulthood. STATE OF THE ART: Survival of children with UCD has significantly improved and the need for transitional care to adulthood has emerged. Adult onset UCD present with chronic or acute neurological, psychiatric and digestive symptoms associated with protein avoidance. Ornithine transcarbamylase (OTC) deficiency, which is inherited as an X-linked disorder, is the most well-described UCD in adults. Acute decompensations associate the triad of encephalopathy, respiratory alkalosis and hyperammonemia. Acute encephalopathy is characterized by brain edema, which is life-threatening without treatment. Specific urea cycle enzyme deficiency can be suspected in the presence of abnormal plasma amino acids concentrations and urinary excretion of orotic acid. A measurement enzyme activity in appropriate tissue, or DNA analysis if available, is required for diagnosis. Treatment requires restriction of dietary protein intake and the use of alternative pathways of waste nitrogen excretion with sodium benzoate and sodium phenylbutyrate. Patients with acute forms may need hemodialysis or hemodiafiltration. Therapeutic goals for OTC deficiency are to maintain plasma ammonia<80 micromol/L, plasma glutamine<1,000 micromol/L, argininemia 80-150 micromol/L and branched chain amino acids within the normal range, in order to prevent episodes of potentially lethal acute hyperammonemia. CONCLUSION: Potentially fatal acute hyperammonemia may occur in male or female patients at any age. Ammonia should be measured promptly in case of acute neurological and psychiatric symptoms or coma.     

Magnetic resonance spectroscopy in adult-onset citrullinemia: elevated glutamine levels in comatose patients

BACKGROUND: Adult-onset type II citrullinemia is an inborn error of urea cycle metabolism that can lead to hyperammonemic encephalopathy and coma. However, type II citrullinemia is rare outside Japan, and diagnosis and treatment can be delayed. Magnetic resonance spectroscopy may be a useful adjunct to magnetic resonance imaging, and has been applied to noninvasively study chemical metabolism in the human brain. PATIENTS: We describe 2 patients with type II citrullinemia who presented with episodic postprandial somnolence and coma. Diffusion-weighted magnetic resonance imaging showed bilaterally symmetrical signal abnormalities of the insular cortex and cingulate gyrus. On magnetic resonance spectroscopy, glutamine and glutamate levels were elevated, and choline and myo-inositol levels were decreased. The diagnosis of citrullinemia was confirmed based on elevated plasma ammonia and citrulline levels. CONCLUSION: Characteristic features found at the time of magnetic resonance imaging and magnetic resonance spectroscopy may be helpful for early diagnosis of type II citrullinemia in adult patients who present with hyperammonemic encephalopathy and coma.     

Sodium phenylbutyrate improved the clinical state in an adult patient with arginase 1 deficiency

An adult female patient was diagnosed with arginase 1 deficiency (ARG1-D) at 4 years of age, and had been managed with protein restriction combined with sodium benzoate therapy. Though the treatment was successful in ameliorating hyperammonemia, hyperargininemia persisted. After being under control with a strict restriction of dietary protein, severe fall of serum albumin levels appeared and her condition became strikingly worsened. However, after sodium phenylbutyrate (NaPB) therapy was initiated, the clinical condition and metabolic stability was greatly improved.Current management of ARG1-D is aimed at lowering plasma arginine levels. The nitrogen scavengers, such as NaPB can excrete the waste nitrogen not through the urea cycle but via the alternative pathway. The removal of nitrogen via alternative pathway lowers the flux of arginine in the urea cycle. Thereby, the clinical complications due to insufficient amount of protein intake can be prevented. Thus, NaPB therapy can be expected as a useful therapeutic option, particularly in patients with ARG1-D.     

Astrocyte glutamine synthetase: Importance in hyperammonemic syndromes and potential target for therapy

Many theories have been advanced to explain the encephalopathy associated with chronic liver disease and with the less common acute form. A major factor contributing to hepatic encephalopathy is hyperammonemia resulting from portacaval shunting and/or liver damage. However, an increasing number of causes of hyperammonemic encephalopathy have been discovered that present with the same clinical and laboratory features found in acute liver failure, but without liver failure. Here, we critically review the physiology, pathology, and biochemistry of ammonia (i.e., NH₃ plus NH₄ ⁺) and show how these elements interact to constitute a syndrome that clinicians refer to as hyperammonemic encephalopathy (i.e., acute liver failure, fulminant hepatic failure, chronic liver disease). Included will be a brief history of the status of ammonia and the centrality of the astrocyte in brain nitrogen metabolism. Ammonia is normally detoxified in the liver and extrahepatic tissues by conversion to urea and glutamine, respectively. In the brain, glutamine synthesis is largely confined to astrocytes, and it is generally accepted that in hyperammonemia excess glutamine compromises astrocyte morphology and function. Mechanisms postulated to account for this toxicity will be examined with emphasis on the osmotic effects of excess glutamine (the osmotic gliopathy theory). Because hyperammonemia causes osmotic stress and encephalopathy in patients with normal or abnormal liver function alike, the term “hyperammonemic encephalopathy” can be broadly applied to encephalopathy resulting from liver disease and from various other diseases that produce hyperammonemia. Finally, the possibility that a brain glutamine synthetase inhibitor may be of therapeutic benefit, especially in the acute form of liver disease, is discussed.     

Restoration of learning ability in hyperammonemic rats by increasing extracellular cGMP in brain

Intellectual function is impaired in patients with hyperammonemia and hepatic encephalopathy. Chronic hyperammonemia with or without liver failure impairs the glutamate-nitric oxide-cGMP pathway function in brain in vivo and reduces extracellular cGMP in brain as well as the ability of rats to learn a Y maze conditional discrimination task. We hypothesized that the decrease in extracellular cGMP may be responsible for the impairment in learning ability and intellectual function and that pharmacological modulation of the levels of cGMP may restore learning ability. The aim of this work was to try to reverse the impairment in learning ability of hyperammonemic rats by pharmacologically increasing extracellular cGMP in brain. We assessed whether learning ability may be restored by increasing extracellular cGMP in brain by continuous intracerebral administration of: (1) zaprinast, an inhibitor of the phosphodiesterase that degrades cGMP or (2) cGMP. We carried out tests of conditional discrimination learning in a Y maze with control and hyperammonemic rats treated or not with zaprinast or cGMP. Learning ability was reduced in hyperammonemic rats, which needed more trials than control rats to learn the task. Continuous intracerebral administration of zaprinast or cGMP restored the ability of hyperammonemic rats to learn this task. Pharmacological modulation of extracellular cGMP levels in brain may be a useful therapeutic approach to improve learning and memory performance in individuals in whom cognitive abilities are impaired by different reasons, for example in patients with liver disease who present hyperammonemia and decreased intellectual function.     

Quinolinic acid in children with congenital hyperammonemia

Levels of the excitotoxin quinolinic acid (QUIN) were measured in the cerebrospinal fluid of infants and children with congenital hyperammonemia. Twofold to tenfold elevations of QUIN were found in 4 neonates in hyperammonemic coma (QUIN range, 250–990 nM; control mean, 110 ± 90 nM; p < 0.005). Similar elevations of neopterin were found (range, 24–75 nM; control mean, 9.0 ± 4.9 nM; p < 0.005). In addition, significant elevations of QUIN were found in 14 older children with congenital hyperammonemia (mean, 50 ± 20 vs 17 ± 6 nM; p < 0.05). Neopterin levels were not elevated in these children. The QUIN may originate from an increase in tryptophan transport across the blood-brain barrier or from induction of indolamine-2,3-dioxygenase activity. These findings support a role for QUIN in the neuropathology of congenital hyperammonemia. They also suggest the potential utility of N-methyl-D -aspartate receptor–-blocking agents or inhibitors of QUIN synthesis in the treatment of hyperammonemic coma.     

Tolerance of neonatal rat brain to acute hyperammonemia

The aim of the present work was to study the effects of hyperammonemia on brain energy metabolism in neonatal rats. Rats were rendered hyperammonemic by ammonium acetate administration. This decreased brain ATP concentrations but enhanced brain ammonia and lactate levels in both adult and neonatal rats. In adult rats, the decrease in brain ATP concentrations was accompanied by a plunge in the respiratory control rate (RCR) of brain mitochondria. However, the ammonia-induced effect on RCR was not observed in neonatal rats, suggesting that the fall in ATP levels observed in neonatal rats would not be due to an impairment of mitochondrial respiratory efficiency. However, in neonatal rats the increase in blood and brain ammonia concentrations did not change brain glutamate concentrations but decreased glutamine contents. These results may be of relevance for the understanding of the resistance of neonatal rats observed in this work to acute ammonia toxicity     

Episodic hyperammonemia in adult siblings with hyperornithinemia, hyperammonemia, and homocitrullinuria syndrome

A 39-year-old man and his 42-year-old sister, both vegetarians, had episodic confusion for many years, but their mental function was normal between those episodes. They were recently diagnosed with hyperornithinemia, hyperammonemia, and homocitrullinuria syndrome. Hyperammonemia was documented during an episode of confusion in the male sibling but not in his sister. Both had elevated plasma ornithine, glutamine, and alanine levels and persistently low plasma lysine levels. Homocitrulline was present in their urine, and orotic aciduria and orotidinuria developed in the male sibling following ingestion of allopurinol. Studies on their cultured skin fibroblasts showed deficient metabolism of ornithine, indicating a defect in ornithine transport across the mitochondrial membrane. During therapy with citrulline and phenylbutyrate sodium, plasma ornithine levels increased in both patients, while plasma levels of glutamine and alanine decreased to normal. Since therapy started, their clinical conditions have also improved, and no recurrent neurologic dysfunction has occurred during a follow-up period of 20 months.     

Unusual biochemical and clinical features in a girl with ornithine transcarbamylase deficiency

A girl, ultimately diagnosed as having profound ornithine transcarbamylase (OTC) deficiency, presented as a neonate with feeding intolerance, irritability, and seizures without concurrent hyperammonemia. Developing normally until ten months of age, the girl subsequently experienced two episodes of hyperammonemia, which were associated with focal seizures and residual hemiparesis. She continued to have profound neurologic impairment and seizures and died at 26 months of age, despite appropriate dietary protein restriction, sodium benzoate, and arginine supplementation. Symptomatic OTC deficiency has not been previously reported unassociated with hyperammonemia. The recurrent cerebrovascular episodes are distinctly uncommon in patients with urea cycle enzymopathies.     

Antioxidant defence of the neonatal rat brain against acute hyperammonemia

Oxidative stress associated with the presence of elevated concentrations of ammonia in the brain has been proposed as one possible mechanism involved in ammonia toxicity. In a previous study [Brain Res.973 (2003) 31], we reported that neonatal rats are more resistant to acute ammonia toxicity than adult rats. In the present work, we studied the antioxidant status of the brain in hyperammonemic neonatal rats. Increased activities of the antioxidant enzymes and enhanced glutathione content were found in the brains of the hyperammonemic neonatal rats as compared to the controls. In addition, no changes in brain reactive oxygen species (ROS) levels and lipid peroxidation due to hyperammonemia were found. Therefore, acute ammonia intoxication does not induce oxidative stress in neonatal rats, a fact that may explain the resistance against hyperammonemia shown by neonatal rats.     

Citrullinemia presenting as uncontrollable epilepsy

We report the case of a 16-year-old girl with a variant form of citrullinemia who had been treated with anticonvulsants for uncontrolled epilepsy during the last 4 years. The diagnosis of citrullinemia was made because she had elevated values for serum citrulline (about 10 times control levels), elevated blood ammonia (over 400 micrograms/dl) and reduced activity of argininosuccinate synthetase in the biopsied liver tissue. Her EEG showed high voltage slow activity, but not triphasic waves, when she had high concentrations of blood ammonia. Treatment with a low-protein diet and sodium benzoate resulted in a normalized blood ammonia level, but her plasma citrulline levels remained unchanged. After the therapy she had neither convulsions nor seizure discharges on EEG, even when all anticonvulsant drug therapy was stopped. Thus it is suggested that hyperammonemia may account for the observed abnormal EEG findings, and triphasic waves on EEG are not always recorded in cases of hyperammonemia.     

Serum and CSF glutamine levels in valproate-related hyperammonemic encephalopathy

PURPOSE: To investigate ammonia and glutamine levels in valproate (VPA)-related hyperammonemic encephalopathy (VHE). METHODS: We reviewed the medical records and EEG recordings of seven adults diagnosed with VHE. RESULTS: Venous ammonia levels were elevated in five (71%) of the seven patients. Elevated serum or cerebrospinal fluid (CSF) glutamine levels were found in four (80%) of five cases tested, including two who had normal ammonia levels. Initial behavioral signs included violent outbursts in three patients, paranoid ideation severe enough to require restraint in two cases, and milder abnormalities in two instances. The severity of encephalopathy was not related to any particular serum VPA level. In four patients serum VPA levels did not exceed 100 microg/ml, and in one case, VHE developed after taking only one 250-mg dose. Symptoms eventually cleared after reducing the dose of, or discontinuing, VPA. Liver-function tests were normal. Each of six patients tested had EEG findings that supported the diagnosis of VHE and excluded nonconvulsive status epilepticus. The rate of normalization of one patient's serum glutamine level and the EEGs of two cases correlated better with the timing of their delayed clinical recovery than did the more rapid rate of decline of the serum ammonia levels. CONCLUSIONS: Serum or CSF glutamine levels are initially elevated in a majority of patients with suspected VHE, sometimes in the absence of hyperammonemia. Glutamine levels may be useful adjunctive laboratory tests for the diagnosis of VHE.     

Electroencephalographic findings in ornithine transcarbamylase deficiency.

A 3-day-old infant presented with anorexia, irritability, hypotonia, and seizures. Blood ammonia was 2115 micromol/L and amino and organic acid analyses were consistent with ornithine transcarbamylase deficiency. Liver biopsy confirmed only 1% enzyme activity. The patient was treated with hemodialysis. An electroencephalogram (EEG) revealed multifocal independent spike-and-sharp-wave discharges. After initial stabilization he was placed on a low-protein diet with citrulline and phenylbutyrate. Conjugating agents (arginine, sodium benzoate, and sodium phenylacetate) have been added during periods of metabolic decompensation. Although developmentally delayed, the patient has shown signs of clinical improvement and EEG activity has likewise improved with only mild background slowing and no evidence of epileptogenic activity at 4 years of age. A second infant presented at 3 days of age with a similar history, blood ammonia of 1382 micromol/L, and metabolic studies indicative of ornithine transcarbamylase deficiency. EEG showed multifocal independent ictal and interictal discharges. Electrographic abnormalities persisted despite lowering of blood ammonia with hemodialysis and conjugating agents. The patient continued to decline clinically and died on the 7th hospital day. EEG changes parallel the clinical course of ornithine transcarbamylase deficiency and may serve as an objective marker of the effectiveness of therapeutic interventions.     

Catecholamine and octopamine concentrations in brains of patients with Reye syndrome.

Dopamine, norepinephrine, and octopamine levels were estimated in regions of brains obtained postmortem from children who died with Reye syndrome and from age-matched controls. Hypothalamic norepinephrine levels were greatly decreased (to 30 percent of control, p less than 0.02) and octopamine levels were increased (to 700 percent of control, p less than 0.01). Levodopa had little effect on the physiologic condition of the patients. However, CNS dopamine and homovanillic concentrations were not elevated by levodopa, indicating that in the present cases levodopa was not metabolized to its catecholamine products. The findings indicate that the encephalopathy of Reye syndrome (as in other types of hepatic coma) may be linked to the presence of false transmitters in the brain and that levodopa is a rational therapy if administered before irreversible CNS changes occur.     

Cerebral ammonia metabolism in normal and hyperammonemic rats

Brain ammonia is generated from many enzymatic reactions, including glutaminase, glutamate dehydrogenase, and the purine nucleotide cycle. In contrast, the brain possesses only one major enzyme for the removal of exogenous ammonia, i.e., glutamine synthetase. Thus, following administration of [¹³N]ammonia to rats [via either the carotid artery or cerebrospinal fluid (csf)], most metabolized label was in glutamine (amide) and little was in glutamate (plus aspartate). Since blood-and csf-borne ammonia are converted to glutamine largely, if not entirely, in the astrocytes, it is not possible from these types of experiments to predict with certainty the metabolic fate of the bulk of endogenously produced ammonia. By comparing the specific activity ofl-[¹³N]glutamate to that ofl-[amine-¹³N]glutamine following intracarotid [¹³N]ammonia administration it was concluded that metabolic compartmentation is no longer intact in the brains of rats treated with the glutamine synthetase inhibitorl-methionine-SR-sulfoximine (MSO) and that blood and brain ammonia pools mix in such animals. In MSO-treated animals, recovery of label in brain was low (～20% of controls), and of the label remaining, a prominent portion was in glutamine (amide) (despite an 87% decrease in brain glutamine synthetase activity). These data are consistent with the hypothesis that glutamine synthetase is the major enzyme for metabolism of endogenously—as well as exogenously—produced ammonia. The rate of turnover of blood-derived ammonia to glutamine in normal rat brain is extremely rapid (t _1/2≤3 s), but is slowed in the brains of chronically (12–14-wk portacaval-shunted) or acutely (urease-treated) hyperammonemic rats (t _1/2≤10 s). The slowed turnover rate may be caused by an increased astrocytic ammonia, decreased glutamine synthetase activity, or both. In the hyperammonemic rat brain, glutamine synthetase is still the only important enzyme for the removal of blood-borne ammonia. Hyperammonemia causes an increase in brain lactate/pyruvate ratios and decreases in brain glutamate and brainstem ATP, consistent with an interference with the malate-aspartate shuttle. In vitro, pathological levels of ammonia also inhibit brain α-ketoglutarate dehydrogenase complex and, less strongly, pyruvate dehydrogenase complex. The rat brain does not adapt to prolonged hyperammonemia by increasing its glutamine synthetase activity. Therefore, since the brain only has a limited capacity to buffer against excess ammonia, it is important that diseases in which hyper-ammonemia is a prominent feature be treated to reduce the associated hyperammonemia as much as possible.     

Differential response of glutamine in cultured neurons and astrocytes

Glutamine, a byproduct of ammonia detoxification, is found elevated in brain in hepatic encephalopathy (HE) and other hyperammonemic disorders. Such elevation has been implicated in some of the deleterious effects of ammonia on the central nervous system (CNS). Recent studies have shown that glutamine results in the induction of the mitochondrial permeability transition (MPT) in cultured astrocytes. We examined whether glutamine shows similar effects in cultured neurons. Both cultured astrocytes and neurons were exposed to glutamine (6.5 mM) for 24 hr and the MPT was assessed by changes in cyclosporin A (CsA)-sensitive inner mitochondrial membrane potential (DeltaPsi(m)) using the potentiometric dye tetramethylrhodamine ethyl ester (TMRE). Glutamine significantly dissipated the DeltaPsi(m) in astrocytes as demonstrated by a decrease in mitochondrial TMRE fluorescence, a process that was blocked by CsA. On the other hand, treatment of cultured neurons with glutamine had no effect on the DeltaPsi(m). Dissipation of the DeltaPsi(m) in astrocytes by glutamine was blocked by treatment with 6-diazo-5-oxo-L-norleucine (DON; 100 microM), suggesting that glutamine hydrolysis and the subsequent generation of ammonia, which has been shown previously to induce the MPT, might be involved in MPT induction by glutamine. These data indicate that astrocytes but not neurons are vulnerable to the toxic effects of glutamine. The selective induction of oxidative stress and the MPT by glutamine in astrocytes may partially explain the deleterious affects of glutamine on the CNS in the setting of hyperammonemia, as well as account for the predominant involvement of astrocytes in the pathogenesis of HE and other hyperammonemic conditions.     

Inborn errors of urea synthesis

Inborn errors of urea synthesis can present in the newborn period as a catastrophic illness or later in childhood or adulthood with an indolent course punctuated by hyperammonemic episodes. Because symptoms mimic other neuropsychiatric disorders, it is common for there to be a delay in diagnosis, often with dire consequences. Diagnosis relies on the combination of clinical suspicion and the measurement of ammonium, lactate, and amino acids in plasma and organic acids and orotic acid in urine. Treatment involves nitrogen restriction combined with the stimulation of alternate pathways of waste nitrogen excretion. More recently liver transplantation has been performed as enzyme replacement therapy. The outcome is poor in children who survive prolonged neonatal hyperammonemic coma, with most manifesting developmental disabilities. The etiology of neuronal injury in this disorder is unclear but may involve some combination of ammonia/amino acid accumulation, neurotransmitter alterations, and excitotoxic injury. Gene therapy holds the promise of improved treatment in the future.     

Valproate-induced hyperammonemic encephalopathy

Valproate-induced hyperammonemic encephalopathy (VHE) is an unusual complication characterized by a decreasing level of consciousness, focal neurological deficits, cognitive slowing, vomiting, drowsiness, and lethargy. We have thoroughly reviewed the predisposing factors and their screening, the biochemical and physiopathological mechanisms involved, the different treatments described, and those that are being investigated. Etiopathogenesis is not completely understood, although hyperammonemia has been postulated as the main cause of the clinical syndrome. The increase in serum ammonium level is due to several mechanisms, the most important one appearing to be the inhibition of carbamoylphosphate synthetase-I, the enzyme that begins the urea cycle. Polytherapy with several drugs, such as phenobarbital and topiramate, seems to contribute to hyperammonemia. Hyperammonemia leads to an increase in the glutamine level in the brain, which produces astrocyte swelling and cerebral edema. There are several studies that suggest that treatment with supplements of carnitine can lead to an early favorable clinical response due to the probable carnitine deficiency induced by a valproate (VPA) treatment. Development of the progressive confusional syndrome, associated with an increase in seizure frequency after VPA treatment onset, obliges us to rule out VHE by screening for blood ammonium levels and the existence of urea cycle enzyme deficiency, such as ornithine carbamoyltransferase deficiency. Electroencephalography (EEG) is characterized by signs of severe encephalopathy with continuous generalized slowing, a predominance of theta and delta activity, occasional bursts of frontal intermittent rhythmic delta activity, and triphasic waves. These EEG findings, as well as clinical manifestations and hyperammonemia, tend to normalize after VPA withdrawal.     

Ammonia: Key factor in the pathogenesis of hepatic encephalopathy

There is substantial clinical and experimental evidence to suggest that ammonia toxicity is a major factor in the pathogenesis of hepatic encephalopathy associated with subacute and chronic liver disease. Ammonia levels in patients with severe liver disease are frequently found to be elevated both in blood and cerebrospinal fluid (csf). Hepatic encephalopathy results in neuropathological damage of a similar nature (Alzheimer type II astrocytosis) to that found in patients with congenital hyperammonemia resulting from inherited defects of urea cycle enzymes. Following portocaval anastomosis in the rat, blood ammonia concentration is increased 2-fold, and brain ammonia is found to be increased 2–3-fold. Administration of ammonia salts or resins to rats with a portocaval anastomosis results in coma and in Alzheimer type II astrocytosis. Since the CNS is devoid of effective urea cycle activity, ammonia removal by brain relies on glutamine formation. Cerebrospinal fluid and brain glutamine are found to be significantly elevated in cirrhotic patients with encephalopathy and in rats following portocaval anastomosis. In both cases, glutamine is found to be elevated in a region-dependent manner. Several mechanisms have been proposed to explain the neurotoxic action of ammonia. Such mechanisms include: (i) Modification of blood-brain barrier transport; (ii) alterations of cerebral energy metabolism; (iii) direct actions on the neuronal membrane; and (iv) decreased synthesis of releasable glutamate, resulting in impaired glutamatergic neurotransmission.     

Metabolic response to hypertonic glucose administration in Reye syndrome

Blood substrate and hormone concentration were determined in 16 children with Reye syndrome prior to and following administration of hypertonic glucose. Baseline concentrations of lactate, pyruvate, alanine, glutamine, glutamate, proline, hydroxyproline, lysine, and aspartate were elevated (p less than 0.01), whereas citrulline and arginine were low. All substrate concentrations were below or within the normal range following 36 hours of therapy except those of lactate, pyruvate, and aspartate. Urea nitrogen excretion was reduced (p less than 0.05) on the second day of therapy. Plasma concentrations of insulin and growth hormone increased and glucagon decreased during the first day. Cortisol remained elevated throughout the study period. We conclude that the high circulating concentrations of substrates are the result of both increased mobilization and decreased clearance and that hypertonic glucose infusion suppresses substrate mobilization. A primary abnormality of the mitochondria could explain the metabolic perturbations that occurred. A possible relationship between the encephalopathy in this disorder and an insult to both brain and brain capillary mitochondria is discussed.