Secondary Hyperammonemia Research Articles

HYPERAMMONEMIA IN NEONATOLOGY: DIAGNOSTIC DIFFICULTIES, MANAGEMENT FEATURES

There are primary and secondary hyperammonemia. Primary hyperammonemia is a hereditary disorder of the urea formation cycle. Secondary hyperammonemia can occur both in hereditary diseases (mitochondrial diseases, organic aciduria/aminoacidopathies) and in asphyxia suffered in childbirth, liver damage of various etiologies (toxic, infectious, etc.), extrahepatic portal hypertension, renal failure, infections caused by urease-producing microorganisms, diseases accompanied by increased catabolism, etc. transient nature. Regardless of the etiology, hyperammonemia can cause irreversible damage to brain cells, which determines the exceptional importance of early diagnosis and timely therapy.

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

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

Эффективность применения низкобелковой диеты у детей с вторичным синдромом гипераммониемии различного генеза

Objective. To evaluate the efficacy of a low-protein diet (LPD) in patients with secondary hyperammonemia (HA) during the period of clinical manifestation depending on its etiology and the level of ammonia, and to substantiate management tactics. Patients and methods. This study included 29 patients with secondary HA. Patients were divided into two age groups depending on the onset of HA: the neonatal group (group 1; n = 15) and the infant and toddler group (group 2; n = 14). Depending on the causes of HA, two subgroups were identified: patients with HA associated with inherited metabolic diseases (IMD) and patients with HA not associated with IMD. Results. In group 1, 5 patients with mild transient HA (THA) (116.4 [106.9–136.9] μmol/L) did not receive LPD, and HA resolved within a week. Of the remaining 10 patients, five had a positive response to LPD (patients with IMD and THA). HA was severe (1588.5 [1315.0–1862.0] μmol/L), and the normalization of parameters was noted by day 14 [9–19]. Another 5 patients had a wave-like character of HA against the background of LPD (p = 0.350). In group 2, 10 patients with mild HA (87.45 [75.25–107.6] μmol/L) were not treated with LPD, all of them belonged to subgroup 2, and the normalization of parameters was observed on day 4. Another 4 patients with moderate HA (219.15 [188.85–245.6] μmol/L) received LPD, HA had a wave-like character (patients with IMD and HA not associated with IMD). Conclusion. The study showed that the course of HA against the background of LPD varies depending on patient age, disease severity and etiology. Key words: hyperammonemia, newborns, metabolic crisis, inherited metabolic diseases, low-protein diet, valproic acid

2264 Hyperammonemia Secondary to an Acquired Urea Cycle Disorder in a Patient With Metastatic Hepatocellular Carcinoma

INTRODUCTION: Urea cycle disorders are rare but critically important to consider in the differential diagnosis of hyperammonemia. Delayed treatment may result in disastrous neurologic consequences. CASE DESCRIPTION/METHODS: A 45-year-old woman with past history of gastric sleeve, no known liver disease, and biopsy-proven hepatocellular carcinoma (HCC) with peritoneal metastases on sorafenib presented with acute onset confusion and seizures. She was found to have severe hyperammonemia to 331 µMol/L. Labs were also notable for normal urine organic acids and low plasma citrulline (Table 1). She started on emergent dialysis as well as levetiracetam, lactulose, rifaximin, citrulline, and a low protein diet; sorafenib was discontinued. Her ammonia levels and mentation gradually improved over 3 days. She underwent genetic testing for 14 genes associated with primary urea cycle defects and further testing for another 72 genes associated with secondary hyperammonemia, all of which were negative. DISCUSSION: Urea cycle disorders (UCDs), whether genetic or acquired, can lead to life-threatening hyperammonemia. While classically seen in children, an increasing number of late-onset genetic UCDs have been reported. Ornithine transcarbamylase (OTC) deficiency in particular is X-linked and late-onset, and mild forms are more common than in other genetic UCDs. Individuals may also develop acquired urea cycle defects, with symptoms and lab results consistent with the genetic forms but without an identifiable mutation, as seen in this patient. Acquired UCDs have been reported secondary to tyrosine kinase inhibitors such as sorafenib, and a few cases described in untreated fibrolamellar-type HCC (Table 2). Whether these are primary acquired deficiencies in the setting of hypermetabolic states, as seen in advanced tumors, or an unmasking of an underlying genetic predisposition remains unknown. Further, this patient’s bariatric surgery could have resulted in nutritional deficiencies, revealing an occult UCD. Regardless of etiology, treatment for UCDs includes aggressive ammonia control with nitrogen scavengers, amino acid supplements, and appropriate dietary protein intake. Suspected offending agents should be stopped immediately and emergent hemodialysis initiated if hyperammonemia does not improve with conservative measures. Empiric therapy should be initiated even without confirmation of the diagnosis, as prolonged exposure to high ammonia can lead to irreversible neurological ramifications.

2310 A Mysterious Case of Non-Cirrhotic Recurrent Severe Hyperammonemia: What Urea-lly Need to Know

INTRODUCTION: Ammonia is a well-known toxin, mostly studied in Hepatic encephalopathy. While, it is related to severe liver disease in 90% cases, the remaining 10% is constituted by disturbances in its excretion or related to over production from other etiologies. [1] Awareness of hyperammonemia in absence of severe hepatic disease may lead to lifesaving prompt diagnosis and treatment. We present a mysterious case of recurrent severe hyperammonemia without known liver disease. CASE DESCRIPTION/METHODS: A 53 year old male with prior history of well controlled hypertension presented to the emergency department with unresponsiveness. On exam, he was unconscious with occasional spontaneous eye opening and a left gaze preference. CT head and CT angiogram of neck did not reveal any acute abnormalities. Liver enzymes and Bilirubin were normal. Blood alcohol level was 0.089gm/dl%. EEG showed diffuse encephalopathy. Serum ammonia was 705 umol/L. He was started on lactulose. His mentation improved in a day and repeat serum ammonia was 25umol/L. Ultrasound liver and CT abdomen ruled out cirrhosis and gall bladder disease. He was discharged home with a prescription of lactulose. He returned to the ED with another episode of unresponsiveness a month later and was found to have elevated serum ammonia of 213 umol/L. Levetiracetam was started empirically with suspicion of recurrent seizures causing this unresponsiveness. He continued to have similar episodes of hyperammonemic encephalopathy. Orotic acid in urine was high (>200 mmol/mol creatinine ), clinching the diagnosis of ornithine transcarbamylase (OTC) deficiency. He was advised to consume a low protein diet and was prescribed L-carnitine along with lactulose. DISCUSSION: Ammonia directly affects neuronal electric activity by inhibiting the generation of both excitatory and inhibitory postsynaptic potentials. Secondary hyperammonemia occurs commonly in the presence of hepatic disorders leading to portosystemic encephalopathy. Less common causes include drugs like valproate, post-chemotherapy, gastric bypass surgery. Causes of primary hyperammonemia in adults include milder forms of urea cycle enzyme deficiencies like OTC, argininosuccinate lyase. Noncirrhotic hyperammonemia as a cause of altered mental status remains a diagnosis of exclusion and requires high index suspicion. Treatment options include lactulose, l-carnitine, and drugs to lower cerebral edema if patient has MRI findings. Liver transplantation is an important treatment modality in urea-cycle disorders.

Spanish multicenter study: hyperammonemia not associated with inborn errors of metabolism in children

The hyperammonemic encephalopathy induced by causes different from inborn errors of metabolism is a relatively uncommon but severe complication. To study the characteristics of a secondary hyperammonemia episode to discern the triggering causes to get to the diagnosis, and the development in the therapeutic intervention. A multicenter retrospective study of children with hyperammonemia unrelated to inborn errors of metabolism, conducted in Spanish hospitals. Nineteen patients were selected; hyperammonemia developed in infants under one year old in 47% of them, being diagnosed mostly with two or more symptoms. The most common clinical finding was an altered consciousness level similar to that of intoxication symptoms, followed by seizures. These clinical symptoms were present in 14 patients, with one of them or both. Twelve of the 19 patients were in treatment with more than two antiepileptic drugs routinely. All children were treated with protein restriction (n: 10), scavengers (n: 10) and/or carglumic acid (n: 12) for the treatment of hyperammonemia. This study suggests that secondary hyperammonemia could be underdiagnosed because it is only detected when severe symptoms appear. Risk seems to be higher in those patients receiving antiepileptic drugs or those critically ill with a restricted diet or incremented metabolism. The response to specific treatment is adequate but should be established earlier to avoid neurological sequelae of this entity.

A novel UPLC-MS/MS based method to determine the activity of N-acetylglutamate synthase in liver tissue.

N-acetylglutamate synthase (NAGS) plays a key role in the removal of ammonia via the urea cycle by catalyzing the synthesis of N-acetylglutamate (NAG), the obligatory cofactor in the carbamyl phosphate synthetase 1 reaction. Enzymatic analysis of NAGS in liver homogenates has remained insensitive and inaccurate, which prompted the development of a novel method. UPLC-MS/MS was used in conjunction with stable isotope (N-acetylglutamic-2,3,3,4,4-d5 acid) dilution for the quantitative detection of NAG produced by the NAGS enzyme. The assay conditions were optimized using purified human NAGS and the optimized enzyme conditions were used to measure the activity in mouse liver homogenates. A low signal-to-noise ratio in liver tissue samples was observed due to non-enzymatic formation of N-acetylglutamate and low specific activity, which interfered with quantitative analysis. Quenching of acetyl-CoA immediately after the incubation circumvented this analytical difficulty and allowed accurate and sensitive determination of mammalian NAGS activity. The specificity of the assay was validated by demonstrating a complete deficiency of NAGS in liver homogenates from Nags -/- mice. The novel NAGS enzyme assay reported herein can be used for the diagnosis of inherited NAGS deficiency and may also be of value in the study of secondary hyperammonemia present in various inborn errors of metabolism as well as drug treatment.

Urea cycle disorders revisited – clinical, biochemical and therapeutical aspects

Urea cycle disorders (UCDs) are inherited defects of nitrogen metabolism that can result in life-threatening hyperammonemia and severe neurological disease [1]. The overall incidence of the entire group of disorders is estimated at about 1 in 35.000 to 40.000 [2, 3] but there may still be underreporting due to undiagnosed patients. The affected pathway, the urea cycle, is only expressed in the liver and is the main route for detoxification of ammonia in the human body [4]. There are six enzymes and two transporters directly involved in this pathway but, in addition, several inherited and acquired conditions can lead to impaired urea cycle function. Accordingly, there is a long list of differential diagnoses that can result in primary or secondary hyperammonemia, i.e. direct or indirect defects of the urea cycle [5]. UCDs are panethnic diseases but the distribution of disorders around the world is heterogeneous resulting in different frequencies of single UCDs in Japan, Europe or the US. 

Inhibition of N-acetylglutamate synthase by various monocarboxylic and dicarboxylic short-chain coenzyme A esters and the production of alternative glutamate esters

Hyperammonemia is a frequent finding in various organic acidemias. One possible mechanism involves the inhibition of the enzyme N-acetylglutamate synthase (NAGS), by short-chain acyl-CoAs which accumulate due to defective catabolism of amino acids and/or fatty acids in the cell. The aim of this study was to investigate the effect of various acyl-CoAs on the activity of NAGS in conjunction with the formation of glutamate esters. NAGS activity was measured in vitro using a sensitive enzyme assay with ultraperformance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) product analysis. Propionyl-CoA and butyryl-CoA proved to be the most powerful inhibitors of N-acetylglutamate (NAG) formation. Branched-chain amino acid related CoAs (isovaleryl-CoA, 3-methylcrotonyl-CoA, isobutyryl-CoA) showed less pronounced inhibition of NAGS whereas the dicarboxylic short-chain acyl-CoAs (methylmalonyl-CoA, succinyl-CoA, glutaryl-CoA) had the least inhibitory effect. Subsequent work showed that the most powerful inhibitors also proved to be the best substrates in the formation of N-acylglutamates. Furthermore, we identified N-isovalerylglutamate, N-3-methylcrotonylglutamate and N-isobutyrylglutamate (the latter two in trace amounts), in the urines of patients with different organic acidemias. Collectively, these findings explain one of the contributing factors to secondary hyperammonemia, which lead to the reduced in vivo flux through the urea cycle in organic acidemias and result in the inadequate elimination of ammonia.

Clinical and biochemical aspects of primary and secondary hyperammonemic disorders

An increased concentration of ammonia is a non-specific laboratory sign indicating the presence of potentially toxic free ammonia that is not normally removed. This does occur in many different conditions for which hyperammonemia is a surrogate marker. Hyperammonemia can occur due to increased production or impaired detoxification of ammonia and should, if associated with clinical symptoms, be regarded as an emergency.The conditions can be classified into primary or secondary hyperammonemias depending on the underlying pathophysiology. If the urea cycle is directly affected by a defect of any of the involved enzymes or transporters, this results in primary hyperammonemia. If however the function of the urea cycle is inhibited by toxic metabolites or by substrate deficiencies, the situation is described as secondary hyperammonemia. For removal of ammonia, mammals require the action of glutamine synthetase in addition to the urea cycle, in order to ensure lowering of plasma ammonia concentrations to the normal range.Independent of its etiology, hyperammonemia may result in irreversible brain damage if not treated early and thoroughly. Thus, early recognition of a hyperammonemic state and immediate initiation of the specific management are of utmost importance. The main prognostic factors are, irrespective of the underlying cause, the duration of the hyperammonemic coma and the extent of ammonia accumulation.This paper will discuss the biochemical background of primary and secondary hyperammonemia and will give an overview of the various underlying conditions including a brief clinical outline and information on the genetic backgrounds.

Urinary 2-hydroxy-5-oxoproline, the lactam form of α-ketoglutaramate, is markedly increased in urea cycle disorders

α-Ketoglutaramate (KGM) is the α-keto acid analogue of glutamine, which exists mostly in equilibrium with a lactam form (2-hydroxy-5-oxoproline) under physiological conditions. KGM was identified in human urine and its concentration quantified by gas chromatography/mass spectrometry (GC/MS). The keto acid was shown to be markedly elevated in urine obtained from patients with primary hyperammonemia due to an inherited metabolic defect in any one of the five enzymes of the urea cycle. Increased urinary KGM was also noted in other patients with primary hyperammonemia, including three patients with a defect resulting in lysinuric protein intolerance and one of two patients with a defect in the ornithine transporter I. These findings indicate disturbances in nitrogen metabolism, most probably at the level of glutamine metabolism in primary hyperammonemia diseases. Urinary KGM levels, however, were not well correlated with secondary hyperammonemia in patients with propionic acidemia or methylmalonic acidemia, possibly as a result, in part, of decreased glutamine levels. In conclusion, the GC/MS procedure has the required lower limit of quantification for analysis of urinary KGM, which is markedly increased in urea cycle disorders and other primary hyperammonemic diseases.

Necrotizing Enterocolitis in the Asphyxiated Full-Term Infant

Five full term infants with necrotizing enterocolitis, confirmed histologically, presented in the first five days of life. All passed grossly bloody stools and three mucosal casts. Severe perinatal asphyxia with accompanying secondary hyperammonemia was present in all, and abnormal liver function studies were present in three. An awareness of the clinical characteristics of these infants may lead to their early identification and conservative management.

Neonatal Screening by Tandem Mass Spectrometry

After completing this article, readers should be able to: 1. List the major categories of diseases detectable by expanded newborn screening. 2. Give examples of the benefits and limitations of screening by tandem mass spectrometry. 3. Describe the appropriate follow-up for an abnormal screening result. The screening of newborns for inherited metabolic disorders is a well-established public health activity, first implemented in the early 1960s for the presymptomatic identification of patients who have phenylketonuria (PKU). Since then, significant technological advances, most importantly the development of tandem mass spectrometry (MS/MS), have enabled the detection of an increasing number of metabolic disorders in newborn blood. This article reviews the basic technology of MS/MS and its application to newborn screening, with the aim of highlighting the benefits and pointing out some limitations of this powerful technology. Early screening for PKU used the bacterial inhibition assay of Guthrie, in which inhibition of bacterial growth on an agar medium, introduced by the addition of a phenylalanine analog, was overcome by the presence of dried blood discs containing high levels of phenylalanine (such as from a PKU patient). This approach later was adapted for the detection of leucine in patients who have maple syrup urine disease (MSUD), methionine in patients who have classic homocystinuria, and tyrosine in patients who have tyrosinemia. Over the years, other techniques have been implemented to screen for additional disorders, including congenital hypothyroidism, biotinidase deficiency, galactosemia, hemoglobinopathies, and congenital adrenal hyperplasia. The classic criteria used to determine whether a disorder is suitable for screening include: 1) the disorder is clinically and biochemically well defined, 2) it is associated with significant morbidity or mortality, 3) an effective treatment is available, and 4) there is a simple and safe screening test. (1) By the early 1990s, all states offered screening for at least PKU and congenital hypothyroidism, …

Continuous Renal Replacement Therapy in the Initial Management of Neonatal Hyperammonemia Due to Urea Cycle Defects

After completing this article, readers should be able to: 1. Describe the metabolic acidosis that may be seen in urea cycle defects. 2. Delineate the most important factor in determining neurologic outcome in urea cycle defects. 3. Explain the factors that affect the blood ammonia concentration in continuous renal replacement therapy (CRRT). 4. Delineate the most difficult issue in pursuing CRRT in neonates. 5. Describe the morbidities associated with CRRT. MP was born at 39 weeks' estimated gestational age by vaginal delivery to a 35-year-old gravida 3 para 2 woman who had an uncomplicated pregnancy. The 3- and 6-year-old female siblings of the patient were healthy. Apgar scores were 8 at 1 minute and 9 at 5 minutes. The newborn was discharged to home after a 2-day stay in the well baby nursery during which time she appeared to be alert and was breastfeeding appropriately. She presented to the emergency department less than 1 day later after her parents noted increasing lethargy and very poor feeding at home. She was admitted to the neonatal intensive care unit (NICU). Results of a complete blood count with differential count and C-reactive protein evaluation were unremarkable, but the patient progressed to worsening lethargy, respiratory failure, and abdominal distension requiring intubation within hours of admission. The anterior fontanelle was soft on admission physical examination. Further testing revealed a base deficit of 6.1 on arterial blood gas, serum ammonia level of 810 mcmol/L (1,134 mcg/dL), serum lactate level of 5.6 mmol/L (5.6 mEq/L), and serum pyruvate level of 0.06 mmol/L. Results of liver function tests were abnormal, and the infant had a prothrombin time of 35.9 seconds, a partial thromboplastin time of 54.3 seconds, and a fibrinogen concentration of 1.75 g/L (175 mg/dL). Genetic and nephrology consultations were requested immediately. Additional metabolic studies were obtained, including measurement of plasma amino …

Decompensation of hepatic and cerebral acyl-CoA metabolism in BALB/cByJ mice by chronic riboflavin deficiency: restoration by acety1-L-carnitine

BALB/cByJ mice have an autosomal recessive deficiency of short-chain acyl-CoA dehydrogenase (SCAD) and show elevated excretion of urinary butyrylglycine, ethylmalonate, and methylsuccinate, which resembles the SCAD deficiency disorder in children. These mice are clinically normal, perhaps because of an efficient acyl-CoA conjugation system. We attempted to decompensate the acyl-CoA metabolism in mutant mice by chronic treatment with a riboflavin-deficient diet for 3 weeks to potentiate the SCAD deficiency. We studied the urinary profiles of organic acids, acylglycines, hepatic and cerebral profiles of carnitines, and ammonia to assess the potentiation of this disorder. Cerebral activity of choline acetyltransferase (ChAT) was determined to study the effects of acyl-CoA accumulation on the cholinergic system. The results indicate that in riboflavin-deficient mutant mice, the excretion of ethylmalonate, methylsuccinate, butyrylglycine, and dicarboxylic acids was enhanced. Hepatic and cerebral free and esterified carnitines were reduced, indicating a potentiation of the secondary carnitine deficiency. Hepatic ammonia levels, but not cerebral ammonia or glutamine levels, were elevated, indicating a tendency towards secondary hyperammonemia. Brain choline acetyltransferase activity was significantly reduced in striatum, implying a reduced availability of cerebral acetyl-CoA or a decreased cerebral transport of choline: Most of these changes were partially or completely restored by a concomitant treatment with acetyl-L-carnitine (ALCAR). In summary, we conclude that BALB/cByJ mice with SCAD deficiency, but with a functional urea cycle, might have an adequate adaptive mechanism to adjust to an excessive acyl-CoA load without hyperammonemia at the cerebral level. However, any deficiency of vitamins or cofactors in the diet could disturb an adaptation to this disorder and produce an effect on the cholinergic system.

Valproate-associated encephalopathy

Valproate (VPA) has been infrequently associated with encephalopathy. This was possibly the effect of secondary hyperammonemia or co-medications such as phenobarbital or phenytoin. We describe several patients with primary generalized seizures who had acute febrile delirium with alternating periods of stupor, confusion, and agitation shortly after initiating VPA therapy. They were without signs of hepatic dysfunction or medication toxicity, and they had no evidence of acute cerebral lesions. These observations suggest that VPA may produce delirium by direct central nervous system effects, possibly mediated by γ-aminobutyric acid pathways. Young adults with pre-existing encephalopathy may be at greater risk, especially those with generalized epilepsy.

Valproic acid encephalopathy

Valproic acid, when used in combination with phenobarbital or diazepam, may produce drowsiness and sometimes stupor and coma. We describe four cases in which valproate therapy was initiated due to inadequate control with other drugs. Our cases include two patients intoxicated on 40 mg/kg/day of valproate monotherapy, one following abrupt initiation of this dosage level and the other after 6 months of gradual titration. Although the other two patients were on phenobarbital and valproate, discontinuation of valproate resulted in resolution of the EEG and clinical findings of encephalopathy. In three cases, the EEG findings included diffuse moderate voltage slowing in the theta and delta range. The fourth case also showed paroxysmal generalized spike-and-wave discharges that had remained unchanged since baseline. Various mechanisms have been proposed including: (a) elevation of the unbound portion of phenobarbital, (b) an additive or synergistic effect caused by valproate and phenobarbital, (c) direct intrinsic depressant effect of valproate on the central nervous system, and (d) secondary hyperammonemia, probably caused by inhibition of carbamyl phosphate synthetase I. Analysis of these cases leads to the conclusion that valproate used in combination with phenobarbital, valproate monotherapy titrated to therapeutic levels, or abrupt initiation of a full dosage schedule may be associated with the development of acute or subacute encephalopathy in certain susceptible individuals.

Dysautonomia in an Infant with Secondary Hyperammonemia Due to Propionyl Coenzyme A Carboxylase Deficiency

A male infant who had vomiting and coma in the absence of ketoacidosis was initially thought to have dysautonomia because of abnormal responses to methacholine and histamine, as well as abnormal urinary catecholamine excretion. Following an episode of hyperammonemia, a liver biopsy was performed which revealed a partial deficiency of carbamyl phosphate synthetase activity. The patient was treated with a protein-restricted diet supplemented with a mixture of ketoacid analogues of the essential amino acids, which precipitated ketosis and acidosis. A primary deficiency of propionyl coenzyme A (CoA) carboxylase was subsequently demonstrated. Because disorders of propionate metabolism may not initially present with ketoacidosis, we recommend examination of both plasma and urine for metabolites of this pathway, as well as direct measurement of propionyl CoA carboxylase activity in peripheral blood leukocytes, before performing a liver biopsy to evaluate urea cycle enzyme activities, and particularly before adding keto acid/amino acid mixtures to a protein-restricted diet.