20 Disorders of the Cycle and Related Enzymes

James V. Leonard

20.1 Clinical Presentation – 265 20.1.1 Neonatal Presentation – 265 20.1.2 Infantile Presentation – 265 20.1.3 Children and Adults – 265

20.2 Metabolic Derangement – 266 20.2.1 The – 266 20.2.2 Toxicity of Ammonia – 267

20.3 Genetics – 267

20.4 Diagnostic Tests – 268 20.4.1 Biochemical Tests – 268 20.4.2 Imaging – 268 20.4.3 Differential Diagnosis – 268 20.4.4 Prenatal Diagnosis – 269

20.5 Treatment – 269 20.5.1 Low-Protein Diet – 269 20.5.2 Essential Amino Acids – 269 20.5.3 Alternative Pathways for Nitrogen Excretion – 269 20.5.4 Replacement of Deficient Nutrients – 270 20.5.5 General Aspects of Therapy – 270 20.5.6 Emergency Management at Home – 270 20.5.7 Emergency Treatment in Hospital – 271

20.6 Prognosis – 272

References – 272 264 Chapter 20 · Disorders of the Urea Cycle and Related Enzymes

The Urea Cycle The urea cycle (. Fig. 20.1) which, in its complete form, are responsible for hyperammonaemia. Genetic defects is only present in the liver, is the main pathway for the of other metabolic pathways may also lead to secondary disposal of excess of nitrogen. This sequence of reac- inhibition of the urea cycle. Alternative pathways for tions, localised in part in the mitochondria and in part nitrogen excretion, namely conjugation of glycine with in the cytosol, converts the toxic ammonia and other benzoate and of glutamine with phenylacetate can be nitrogenous compounds into the non-toxic product, exploited in the treatment of patients with defective urea, which is excreted in the urine. Genetic defects of ureagenesis. IV each enzyme of the urea cycle are recognised and all

. Fig 20.1. The urea cycle and alternative pathways of nitrogen synthetase; 7, glutamine synthetase. 8, Citrin (mitochondrial excretion. Enzymes: 1, synthetase; aspartate-glutamate carrier); + denotes stimulation. Defects are 2, transcarbamoylase; 3, argininosuccinate synthetase; depicted by solid bars across the arrows 4, argininosuccinate lyase; 5, arginase; 6, N-acetylglutamate 265 20 20.1 · Clinical Presentation

20.1.1 Neonatal Presentation Six inherited disorders of the urea cycle are well de- scribed (. Fig. 20.1). These are the deficiencies of car- Most babies with urea cycle disorders that present in the bamoyl phosphate synthetase (CPS), ornithine trans- neonatal period are of normal birthweight and are initially carbamoylase (OTC), argininosuccinate synthetase, argi- healthy but, after a short interval that can be less than ninosuccinate lyase, arginase, and N-acetylglutamate 24 h, they become unwell. Common early symptoms are synthetase (NAGS). Deficiencies of glutamine synthetase poor feeding, vomiting, lethargy and/or irritability and and of citrin have also been described. All these defects tachypnoea. The initial working diagnosis is almost invari- are characterised by hyperammonaemia and disordered ably sepsis. Rather characteristically, these babies may have metabolism. The presentation is highly va- a transient mild respiratory alkalosis, which can be a useful riable: those presenting in the newborn period usually diagnostic clue at this stage. Usually, they deteriorate rapidly, have an overwhelming illness that rapidly progresses with more obvious neurological and autonomic problems, from poor feeding, vomiting, lethargy or irritability and including changes of tone with loss of normal reflexes, vaso- tachypnoea to fits, coma and respiratory failure. In in- motor instability and hypothermia, apnoea and fits. They fancy, the symptoms are less severe and more variable. may soon become unresponsive and may require full inten- Poor developmental progress, behavioural problems, sive care. Untreated, most babies will die, often with com- hepatomegaly and gastrointestinal symptoms are com- plications, such as cerebral or pulmonary haemorrhage, the mon. Children and adults frequently have a chronic underlying metabolic cause for which may not be recog- neurological illness that is characterised by variable be- nised. Some survive neonatal hyperammonaemia but are havioural problems, confusion, irritability and episodic invariably handicapped to a significant degree. vomiting. However, during any metabolic stress the pa- tients may become acutely unwell. Arginase deficiency has more specific symptoms, such as spastic diplegia, 20.1.2 Infantile Presentation dystonia, ataxia and fits. All these disorders have auto- somal-recessive inheritance except ornithine transcar- In infancy, the symptoms are generally rather less acute bamoylase deficiency, which is X-linked. and more variable than in the neonatal period and include anorexia, lethargy, vomiting and failure to thrive, with poor developmental progress. Irritability and behavioural prob- lems are also common. The liver is often enlarged but, as the symptoms are rarely specific, the illness is initially attrib- 20.1 Clinical Presentation uted to many different causes that include gastrointestinal disorders (gastro-oesophageal reflux, cow's milk protein Patients with urea-cycle disorders may present at almost intolerance), food allergies or hepatitis. The correct diagno- any age. However, there are certain times at which they are sis is often only established when the patient develops a more likely to develop symptoms because of metabolic more obvious encephalopathy with changes in conscious- stress, such as infection precipitating protein catabolism. ness level and neurological signs (7 below). These are: 4 The neonatal period. 4 During late infancy. Children are vulnerable during 20.1.3 Children and Adults this period because of the slowing of growth, the change to cow's milk and weaning foods and the declining ma- At these ages, the patients commonly present with a more ternal antibody and consequent development of inter- obviously neurological illness. current infections. 4 Puberty. The changing growth rate and psychosocial Acute Encephalopathy factors may precipitate decompensation. Whilst older patients often present with episodes of acute metabolic encephalopathy, they may also have chronic However, it must be emphasised that many patients may symptoms. Usually, symptoms develop following metabolic present outside these periods. The patterns of the clinical stress precipitated by infection, anaesthesia or protein cata- presentation of hyperammonaemia are rather characteristic bolism, such as that produced by the rapid involution of the and are broadly similar for all the disorders except arginase uterus in the puerperium [1]. However an obvious trigger deficiency, which is discussed separately. The early symp- is not always apparent. The patients first become anorexic, toms are often non-specific and initially, therefore, the lethargic and unwell. Sometimes they are agitated and irri- diagnosis is easily overlooked. The most important points table, with behaviour problems or confusion. Vomiting and in diagnosing hyperammonaemia are to think of it and to headaches may be prominent, suggesting migraine or cycli- measure the plasma ammonia concentration. cal vomiting. Others may be ataxic as though intoxicated. 266 Chapter 20 · Disorders of the Urea Cycle and Related Enzymes

On examination, hepatomegaly may be present, particularly 20.2 Metabolic Derangement in those with argininosuccinic aciduria. The patients may then recover completely but, if not, they may then develop 20.2.1 The Urea Cycle neurological problems, including a fluctuating level of con- sciousness, fits and (sometimes) focal neurological signs, The urea cycle is the final common pathway for the excre- such as hemiplegia [2] or cortical blindness. Untreated, they tion of waste nitrogen in mammals. The steps in the urea continue to deteriorate, becoming comatose, and they may cycle are shown in . Fig. 20.1. Ammonia is probably derived die. The cause of death is usually cerebral oedema. Alterna- principally from glutamine and glutamate and is converted tively, they may recover with a significant neurological to carbamoyl phosphate by carbamoyl phosphate synthe- IV deficit. tase (CPS). This enzyme requires an allosteric activator, Between episodes, the patients are usually relatively N-acetylglutamate, for full activity. This compound is well, although some, particularly younger ones, may con- formed by the condensation of acetyl coenzyme A (acetyl tinue to have problems, such as vomiting or poor develop- CoA) and glutamate in a reaction catalysed by N-acetyl mental progress. Some patients may voluntarily restrict glutamate synthetase (NAGS). Carbamoyl phosphate their protein intake. In addition to those disorders already condenses with ornithine to form in a reaction mentioned, the illness may be attributed to a wide variety catalysed by ornithine transcarbamoylase (OTC). The of other disorders, including Reye syndrome, encephalitis, product, citrulline, condenses with aspartate to produce poisoning and psychosocial problems. argininosuccinate in a reaction catalysed by arginino- succinate synthetase, and the argininosuccinate is then Chronic Neurological Illness hydrolysed to and fumarate by argininosuccinate Learning difficulties or more obvious mental retardation lyase. The supply of aspartate is dependent on the mito- are common, and some patients, particularly those with chondrial shuttles, particularly the aspartate-glutamate argininosuccinic aciduria, may present with relatively few carrier, SLC25A13 [5]. symptoms apart from mental retardation and fits. About half The arginine is itself cleaved by arginase, releasing urea the patients with argininosuccinic aciduria have brittle hair and re-forming ornithine. Within the urea cycle itself, (trichorrhexis nodosa). Patients may present with chronic ornithine acts as a carrier; it is neither formed nor lost. ataxia, which is worse during intercurrent infections. Although all the enzymes are present in the liver, there are large inter-organ fluxes. Citrulline is synthesised in the gut Arginase Deficiency and metabolised to arginine in the kidney. The arginine is Arginase deficiency commonly presents with spastic di- taken up by the liver and hydrolysed to urea and orni- plegia and, initially, a diagnosis of cerebral palsy is almost thine. always suspected. However, the neurological abnormali- Each molecule of urea contains two atoms of waste ties appear to be slowly progressive, although it may be nitrogen, one derived from ammonia and the other from difficult to distinguish this from an evolving cerebral aspartate. Regulation of the urea cycle is not fully under- palsy. During the course of the disease, fits, ataxia and stood, and it is likely that there are several mechanisms dystonia may develop. Occasionally, patients may present controlling flux through this pathway [6]. These include with an acute encephalopathy or anticonvulsant-resistant enzyme induction, the concentrations of substrates, inter- fits [3]. mediates and N-acetyl glutamate, and hormonal effects. Defects of each step have now been described and are listed Glutamine Synthetase Deficiency in . Table 20.1. Two neonates have recently been described who presented The plasma ammonia concentration is raised as a result with convulsions on day 1. During the second week one of metabolic blocks in the urea-cycle. The degree to which patient developed a necrolytic skin disorder. Both had mild it is elevated depends on several factors, including the en- hyperammonaemia (140 Pmol/l) and very low glutamine zyme involved and its residual activity, the protein intake con centrations in plasma, urine and cerebrospinal fluid and the rate of endogenous protein catabolism, particularly (CSF). A homozygous mutation was identified in the if this is increased because of infection, fever or other meta- glutamine synthetase gene [4]. bolic stresses. The values may also be falsely elevated if the specimen is not collected and handled correctly. Citrin Deficiency The concentrations of the amino acids in the metabolic This disorder, also called citrullinaemia type II, is a defi- pathway immediately proximal to the enzyme defect will ciency of the mitochondrial aspartate-glutamate carrier. increase, and those beyond the block will decrease (. Table The result is an intramitochondrial deficiency of aspartate 20.1). In addition, plasma alanine and particularly glutamine (. Fig. 20.1). The disorder presents at two ages: in the neo- accumulate in all the disorders. The concentration of citrul- natal period with liver disease, and in adulthood with line can be helpful, but it may not always be reliable, par- typical symptoms of hyperammonaemia [5]. ticularly during the newborn period [7]. 267 20 20.3 · Genetics

. Table 20.1. Diagnostic tests in urea cycle defects

Disorder Alternative names Plasma amino Urine orotic Tissue for Genetics – gene acid concen- acid enzyme (chromosome localisation) trations diagnosis

N-acetylglutamate NAGS deficiency n glutamine N Liver AR – NAGS (chromosome synthetase deficiency n alanine 17q 21.31)

Carbamoyl phosphate CPS deficiency n glutamine N Liver AR – CPS1 (chromosome synthetase deficiency n alanine 2p 35) p citrulline p arginine

Ornithine transcar- OTC deficiency n glutamine nn Liver X-linked – OTC (Xp21.1) bamoylase deficiency n alanine p citrulline p arginine

Argininosuccinic acid Citrullinaemia nn citrulline n Liver/ AR – ASS (chromosome synthetase deficiency p arginine fibroblasts 9q 34)

Argininosuccinic acid Argininosuccinic n citrulline n RBC/Liver/ AR – ASL (chromosome lyase deficiency aciduria (ASA) n arginino- fibroblasts 7cen-q11.2) p arginine

Arginase deficiency Hyperargininaemia n arginine n RBC/Liver AR – ARG1 (chromosome 6q 23)

AR, autosomal recessive; RBC, red blood cells; N, normal

Orotic acid and orotidine are excreted in excess in the centrations, both in experimental models and in vivo in urine if there is a metabolic block distal to the formation man [11]. The concentrations are such that the increase in of carbamoyl phosphate, as is the case in OTC deficiency, osmolality could be responsible for cellular swelling and citrullinaemia, argininosuccinic aciduria and arginase de- cerebral oedema. ficiency (. Fig. 20.1). In these disorders, carbamoyl phos- phate accumulates, leaves the mitochondrion and, once in the cytosol, enters the pathway for the de novo synthesis of 20.3 Genetics pyrimidines. The urea cycle is also closely linked to many other pathways of intermediary metabolism, particularly The genes for all the urea-cycle enzymes including NAGS the citric-acid cycle through the glutamate-aspartate shuttle have now been mapped, isolated and fully characterised. (citrin). Many mutations have been described. The most common urea cycle disorder is OTC deficiency, which is X-linked disorder and in which molecular genetic studies are par- 20.2.2 Toxicity of Ammonia ticularly helpful. When the diagnosis of OTC deficiency is established, it is necessary to take a careful family history Ammonia increases the transport of tryptophan across and for the mother's carrier status to be assessed, most the bloodbrain barrier, which then leads to an increased reliably by mutation analysis. However, if the mutation is production and release of serotonin. Some of the symptoms not known, the most convenient investigation is the allo- of hyperammonaemia can be explained on this basis, and purinol test, which is used to detect increased de novo the dietary tryptophan restriction reverses anorexia in some synthesis of pyrimidines (7 Chap. 35). This is easier than the patients with urea cycle disorders [9]. Ammonia induces protein- or alanine-loading tests and carries no risk of many other electrophysiological, vascular and biochemical hyperammonaemia . However, recent studies suggest that changes in experimental systems, but it is not known to the sensitivity and specificity are not as good as was once what extent all of these are relevant to the problems of thought [8]. clinical hyperammonaemia in man [10]. All the other conditions have autosomal-recessive in- Using proton nuclear magnetic resonance spectroscopy, heritance. glutamine can also be shown to accumulate at high con- 268 Chapter 20 · Disorders of the Urea Cycle and Related Enzymes

20.4 Diagnostic Tests or, if the patient is very seriously ill, widespread cerebral oedema [13]. 20.4.1 Biochemical Tests Focal areas of altered signal may be identified and need to be distinguished from herpes simplex encephalitis. Routine tests are not helpful for establishing the diagnosis A careful history revealing previous episodes of encephalo- of hyperammonaemia. Plasma transaminases may be ele- pathy, albeit mild, may provide vital clues. Imaging in pa- vated; combined with hepatomegaly, this may lead to the tients who have recovered from a severe episode of hyper- erroneous diagnosis of hepatitis. ammonaemia usually show cerebral atrophy that may be The most important diagnostic test in urea cycle disor- focal, particularly in those areas in which there were altered IV ders is measurement of the plasma ammonia concentration. signals during the acute illness. Normally, this is less than 50 Pmol/l but may be modestly raised as a result of a high protein intake, exercise, strug- gling or a haemolysed blood sample. Generally, patients 20.4.3 Differential Diagnosis who are acutely unwell with urea cycle disorders have plasma ammonia concentrations greater than 150 Pmol/l, The differential diagnosis of hyperammonaemia is wide, and often significantly higher. However, the concentrations and the most common conditions are summarised in may be near normal when patients are well, are early in an . Table 20.2. In the neonatal period, the most common dif- episode of decompensation or if they have been on a low- ferential diagnoses are organic acidaemias, particularly pro- protein, high-carbohydrate intake for some time. pionic and methylmalonic acidaemia. Patients with these Healthy neonates have slightly higher values than older disorders may have had marked hyperammonaemia with infants. If they are ill (sepsis, perinatal asphyxia etc.), plasma minimal metabolic acidosis or ketosis. Although babies ammonia concentrations may increase to 180 Pmol/l. Pa- tients with inborn errors presenting in the newborn period usually have concentrations greater than 200 Pmol/l, often . Table 20.2. Differential diagnosis of hyperammonaemia very much greater. In that case, further investigations (par- ticularly of the plasma amino acid and urine organic acid Inherited Disorders levels) are urgent. The following investigations should be Urea cycle enzyme defects performed: Carbamoyl phosphate synthetase deficiency 4 Ornithine transcarbamoylase deficiency  Repeat plasma ammonia Argininosuccinate synthetase deficiency (citrullinaemia) 4 Blood pH and gases Argininosuccinate lyase deficiency (argininosuccinic 4 Plasma chemistry: urea, electrolytes, glucose and aciduria) creatinine Arginase deficiency 4 Liver-function tests and clotting studies N-acetylglutamate synthetase deficiency 4 Transport defects of urea cycle intermediates  Plasma amino acids Lysinuric protein intolerance 4 Urine organic acids, orotic acid and amino acids Hyperammonaemia – hyperornithinaemia – homocitrul- 4 Plasma free and acyl carnitines linuria syndrome Citrin deficiency (citrullinaemia type II) In all urea-cycle disorders, there is accumulation of gluta- Organic acidaemias Propionic acidaemia mine and alanine and, in citrullinaemia, argininosuccinic Methylmalonic acidaemia and other organic acidaemias aciduria and arginase deficiency, the changes in the amino Fatty acid oxidation disorders acids are usually diagnostic (. Table 20.1). Orotic aciduria Medium chain acyl-CoA dehydrogenase deficiency with raised plasma glutamine and alanine concentrations Systemic carnitine deficiency suggests OTC deficiency. The diagnosis of this and the Long chain fatty acid oxidation defects and other related disorders other disorders can be confirmed by measuring enzyme Other inborn errors activity in appropriate tissue (. Table 20.1) or by molecular Pyruvate carboxylase deficiency (neonatal form) genetic studies. The enzyme diagnosis of NAGS deficiency Ornithine aminotransferase deficiency (neonates/infants) is not straightforward and the diagnosis is best made by Acquired Disorders molecular genetic studies [12]. Transient hyperammonaemia of the newborn Any severe systemic illness particularly in neonates Herpes simplex – neonates with systemic infection 20.4.2 Imaging Liver failure Infection with urease positive bacteria (with urinary tract stasis) Reye syndrome Patients who present with an acute encephalopathy com- Valproate therapy monly receive brain imaging at an early stage. This may Leukaemia therapy including therapy with asparaginase (rare) show no abnormality, a localised area of altered signal 269 20 20.5 · Treatment

with transient hyperammonaemia of the newborn are often sised that there is considerable variation in the needs of born prematurely, with early onset of symptoms [14], it may individual patients [15]. be difficult to distinguish between this condition and urea- cycle disorders. All patients in whom a tentative diagnosis of Reye syndrome is made should be investigated in detail 20.5.2 Essential Amino Acids for inherited metabolic disorders, including urea-cycle disorders. In the most severe variants, it may not be possible to achieve good metabolic control and satisfactory nutrition with re- striction of natural protein alone. Other patients will not 20.4.4 Prenatal Diagnosis take their full protein allowance. In both these groups of patients, some of the natural protein may be replaced with OTC deficiency is an X-linked disorder and prenatal diag- an essential amino acid mixture, giving up to 0.7 g/kg/day. nosis is done either by identifying the mutation or using Using this, the requirements for essential amino acids can an informative polymorphism. However, whilst the pheno- be met; in addition, nitrogen is re-utilised to synthesise type of the males can be predicted, that of the females can- non-essential amino acids, hence reducing the load of waste not because of the random inactivation of the X chromo- nitrogen. some. This presents a problem when counselling families, but the prognosis for females who are treated prospectively from birth is good. 20.5.3 Alternative Pathways All the other urea cycle disorders have autosomal-reces- for Nitrogen Excretion sive inheritance and prenatal diagnosis can help most fam- ilies. For CPS deficiency, prenatal diagnosis using closely In many patients, additional therapy is necessary. A major linked gene markers is now possible for a substantial pro- advance in this field has been the development of com- portion of families. If the molecular-genetic studies are pounds that are conjugated to amino acids and rapidly uninformative, prenatal liver biopsy is a possible alternative. excreted [16, 17]. The effect of the administration of these Mutation analysis has to be used for NAGS deficiency. substances is that nitrogen is excreted in compounds other Citrullinaemia and argininosuccinic aciduria can both than urea; hence, the load on the urea cycle is reduced be diagnosed by enzyme or molecular genetic studies on (. Fig. 20.1). The first compound introduced was sodium chorionic villus biopsy. Argininosuccinic aciduria can also benzoate. Benzoate is conjugated with glycine to form hip- be diagnosed by measuring the argininosuccinate concen- purate, which is rapidly excreted. For each mole of benzoate tration in amniotic fluid. Arginase deficiency can be diag- given, if conjugation is complete, 1 mol of nitrogen is lost. nosed either with molecular-genetic studies or, if they are Sodium benzoate is usually given in doses up to 250 mg/kg/ not informative, with a fetal blood sample. day but, in acute emergencies, this can be increased to 500 mg/kg/day. The major side effects are nausea, vomiting and irritability. In neonates, conjugation may be incom- 20.5 Treatment plete, with increased risk of toxicity [C. Bachmann, per- sonal communication]. The aim of treatment is to correct the biochemical disorder The next drug used was phenylacetate, but this has and to ensure that all the nutritional needs are met. The now been superseded by phenylbutyrate, because the major strategies used are to reduce protein intake, to utilise former has a peculiarly unpleasant, clinging, mousy odour. alternative pathways of nitrogen excretion and to replace In the liver phenylbutyrate is oxidised to phenylacetate, nutrients that are deficient. which is then conjugated with glutamine. The resulting phenylacetylglutamine is rapidly excreted in urine; hence, if this reaction was complete 2 mol of nitrogen would be 20.5.1 Low-Protein Diet excreted for each mol of phenylbutyrate given. However recent studies indicated that phenylbutyrate is metabolized Most patients require a low-protein diet. The exact quantity via several different pathways so that for each mol of so dium will depend mainly on the age of the patient and the severity phenylbutyrate only approximately 1 mol of nitrogen is of the disorder. Many published regimens suggest severe lost [18]. Phenylbutyrate is usually given as the sodium salt protein restriction but, in early infancy, patients may need in doses of 250 mg/kg/day, but has been given in doses of >2 g/kg/day during phases of very rapid growth. The pro- up to 650 mg/kg/day [19]. In emergencies sodium benzoate, tein intake usually decreases to approximately 1.2–1.5 g/kg/ sodium phenylacetate (in USA Ammonul) and sodium day during pre-school years and 0.8–1 g/kg/day in late phenylbutyrate can all be given intravenously in the same childhood. After puberty, the quantity of natural protein doses as oral. In a study of the side effects [20], there was a may be less than 0.5 g/kg/day. However, it must be empha- high incidence of menstrual disturbance in females. O ther 270 Chapter 20 · Disorders of the Urea Cycle and Related Enzymes

problems included anorexia and vomting, but it is not easy and essential amino acids. The aim is to keep plasma am- to distinguish between the effects of the disorder and those monia levels below 80 Pmol/l and plasma glutamine levels of the medicine. Phenylbutyrate may cause a mucositis and below 800 Pmol/l [25]. In practice, a glutamine concentra- one patient developed an oesophageal stricture (unpub- tion of 1000 Pmol/l together with concentrations of essen- lished observation). Patients are often reluctant to take the tial amino acids within the normal range (7 the algorithm, medicines, and considerable ingenuity is sometimes needed . Fig. 20.2) is probably more realistic. All diets must be to ensure that they do. nutritionally complete and must meet requirements for growth and normal development. The concept of balance of diet and medicine is impor- IV 20.5.4 Replacement of Deficient Nutrients tant. The protein intake of the patients varies considerably, and the figures that have been given should be regarded Arginine and Citrulline only as a guide. The variation reflects not only the residual Arginine is normally a nonessential amino acid, because it enzyme activity but also many other factors, including is synthesised within the urea cycle. For this reason, all pa- appetite and growth rate. Some patients have an aversion to tients with urea-cycle disorders (except those with arginase protein, so it can be difficult to get them to take even their deficiency) are likely to need a supplement of arginine to recommended intake. Consequently, they are likely to replace that which is not synthesised [21]. The aim should need smaller doses of sodium benzoate and phenylbutyrate. be to maintain plasma arginine concentrations between Others take more protein, and this has to be balanced by an 50 Pmol/l and 200 Pmol/l. For OTC and CPS deficiencies, increase in the dosages of benzoate and phenylbutyrate. a dose of 50–150 mg/kg/day appears to be sufficient for Some will not take adequate quantities of sodium benzoate most patients. or sodium phenylbutyrate and, therefore, their protein in- However, in severe variants of OTC and CPS, citrulline takes necessarily have to be stricter than would be needed if may be substituted for arginine in doses up to 170 mg/kg/ they took the medicines. Hence, for each patient, a balance day, as this will utilise an additional nitrogen molecule. must be found between their protein intake and the dose of Patients with citrullinaemia and argininosuccinic aciduria their medicines to achieve good metabolic control. have a higher requirement, because ornithine is lost as a Tube feeding, either by naso-gastric or gastrostomy, result of the metabolic block; this is replaced by administer- is often an integral part of the management to ensure a ing arginine. Doses of up to 700 mg/kg/day may be needed, balanced diet and to give the medicines. but this does have the disadvantage of increasing the con- centrations of citrulline and argininosuccinate, respectively. The consequences of this are thought to be less important 20.5.6 Emergency Management at Home than those caused by the accumulation of ammonia and glutamine but the poor outcome of argininosuccinic acid- All patients with urea cycle disorders are at risk of acute uria may lead to a review of this therapy. decompensation with acute hyperammonaemia. This can be precipitated by any metabolic stress, such as fasting, a Other Medication large protein load, infection, anaesthesia or surgery. For this Citrate has long been used to provide a supply of Krebs- reason, all patients should have detailed instructions of cycle intermediates. It is known to reduce postprandial what to do when they are at risk. We routinely use a three- elevation of ammonia and may be helpful in the manage- stage procedure [26]. If the patient is off-colour, the protein ment of argininosuccinic aciduria [22]. is reduced, and more carbohydrate is given. If symptoms N-carbamyl glutamate can be used in NAGS deficiency continue, protein should be stopped and a high-energy to replace the missing compound, as it is active orally. The intake given with their medication by day and night. dose is 100–300 mg/kg/day [23]. Patients who respond may However, if they cannot tolerate oral drinks and medicines, only require treatment with this compound. are vomiting or are becoming progressively encephalo- Anticonvulsants may be needed for patients with urea- pathic, they should go to a hospital for assessment and cycle disorders, but sodium valproate should not be used, intravenous therapy without delay. For further practical as this drug may precipitate fatal decompensation, partic- details, see Dixon and Leonard [26]. Patients should also ularly in OTC patients [24]. have a high carbohydrate intake before any anaesthesia or surgery. For patients who are seriously ill with hyperammon- 20.5.5 General Aspects of Therapy aemia, treatment is urgent. The most important and useful signs are any degree of encephalopathy and the speed of All treatments must be monitored with regular quantitative onset. It may be initially minor symptoms such as irritabil- estimation of plasma ammonia and amino acids, paying ity but for any further alteration in conscious state treat- particular attention to the concentrations of glutamine ment is urgent. Plasma ammonia concentration is not a 271 20 20.5 · Treatment

. Fig. 20.2. Guidelines for the manage- ment of patients with urea cycle disorders, primarily with OTC deficiency (not arginase deficiency). This is intended for use in pa- tients who have been stabilised previously and should only be regarded as a guide, as many patients have in dividual require- ments. For more detail and information about doses, please refer to the text. EAAs, essential aminoacids; N, normal; n/p in- crease/decrease

reliable guide as it may be normal in the early stages of 4. Give sodium phenylbutyrate up to 600 mg/kg/day – orally encephalopathy and still increased when the patient is or intravenously. If the patient has not received any me- clearly improving. The steps in an emergency are listed dication recently, give a priming dose of 250 mg/kg in below, and early treatment is essential (7 Chap. 4). 2–4 hours then up to 350 mg/kg in the next 20–22 hours. 5. Give L-arginine – orally or intravenously: A. Up to 700 mg/kg/day in citrullinaemia and argi nino- 20.5.7 Emergency Management succinic aciduria. in Hospital B. Up to 150 mg/kg/day in OTC and CPS deficiencies. 6. Dialysis. If hyperammonaemia is not controlled or the The volumes that are given are related to age and the con- medicines are not immediately available, haemofiltra- dition of the patient. Fluid volumes should be restricted if tion (or haemodialysis/haemodiafiltration) should be there is any concern about cerebral oedema. started without delay. Alternatively, peritoneal dialysis 1. Stop protein intake. may be used, but this is a less effective method for re- 2. Give a high energy intake either orally or intravenously. ducing hyperammonaemia. A. Orally: (a) 10–20% soluble glucose polymer or (b) 7. Treat other conditions (sepsis, fits etc.). protein-free formula 8. Reduce intracranial pressure with the usual measures B. Intravenously: (a) 10% glucose by peripheral infu- and maintain perfusion pressure. sion or (b) 10–25% glucose by central venous line 3. Give sodium benzoate up to 500 mg/kg/day – orally For the emergency treatment of hyperammonaemia before or intravenously. If the patient has not received any diagnosis is known, the plan outlined may be replaced by medication recently, give a priming dose of 250 mg/kg 1. L-arginine 300 mg/kg/24 h – orally or intravenously in 2–4 hours then 250 mg/kg in the next 20–22 hours. 2. L-carnitine 200 mg/kg/24 h – orally or intravenously 272 Chapter 20 · Disorders of the Urea Cycle and Related Enzymes

20.6 Prognosis 13. Kendall B, Kingsley DPE, Leonard JV, Lingam S, Oberholzer VG (1983) Neurological features and computed tomography of the The prognosis in these disorders is closely related to the age brain in children with ornithine carbamyl transferase deficiency. J Neurol Neurosurg Psychiatr 46:28-34 of the patient and their condition at the time of diagnosis. 14. Hudak ML, Jones MD, Brusilow SW (1985) Differentiation of tran- For those patients who present with symptomatic hyperam- sient hyperammonaemia of the newborn and urea cycle enzyme monaemia in the newborn period, the outlook is not good. defects by clinical presentation. J Pediatr 107:712-719 Even with the most aggressive treatment, the majority of the 15. Leonard JV (2001) Nutritional treatment of urea cycle disorders. survivors will be handicapped. Those who are treated pro- J Pediatr 138[Suppl 1]:S40-44 16. Brusilow SW, Valle DL, Batshaw ML (1979) New pathways of nitro- spectively do better, but there may still be significant com- gen excretion in inborn errors of urea synthesis. Lancet II:452-454 IV plications [27]. The main factors that determine outcome 17. Feillet F, Leonard JV (1998) Alternative pathway therapy for urea are not clear but both the duration and the peak of hyper- cycle disorders. J Inherit Metab Dis 21[Suppl 1]:101-111 ammonaemia are likely to be important [28, 29]. For these 18. Kasumov T, Brunengraber LL, Comte B et al (2004) New secondary patients, there remains a serious risk of decompensation, metabolites of phenylbutyrate in humans and rats. Drug Metab Dispos 32:10-19 and careful consideration should be given to early liver 19. Brusilow SW (1991) Phenylacetylglutamine may replace urea as a transplantation, which may offer the hope of a better long- vehicle for waste nitrogen excretion. Pediatr Res 29:147-150 term outlook [30–32]. Of those who present later, their neu- 20. Wiech NL, Clissold DM, MacArthur RB (1997) Safety and efficacy of rological problems at the time of diagnosis are critical, as buphenyl (sodium phenylbutyrate) tablets and powder (abstract). most will have already suffered neurological damage. At Advances in inherited urea cycle disorders, satellite to the 7th inter- national congress for inborn errors of metabolism, Vienna, p 25 best, this may apparently resolve, but almost all are left with 21. Brusilow SW (1984) Arginine, an indispensible aminoacid for pa- some degree of learning and neurological problems. Pa- tients with inborn errors of urea synthesis. J Clin Invest 74:2144- tients who have widespread cerebral oedema almost all die 2148 or survive with severe handicaps. By contrast, those who are 22. Iafolla AK, Gale DS, Roe CR (1990) Citrate therapy in arginosuccinate treated prospectively have a better outcome. lyase deficiency. J Pediatr 117:102-105 23. Bachmann C, Colombo JP, Jaggi K (1982) N-acetylglutamate syn- thetase (NAGS) deficiency: diagnosis, clinical observations and treatment. Adv Exp Med Biol 153:39-45 References 24. Tripp JH, Hargreaves T, Anthony PP et al (1981) Sodium valproate and ornithine carbamyl transferase deficiency (letter). Lancet 1. Arn PH, Hauser ER, Thomas GH et al (1990) Hyperammonemia 1:1165-1166 in women with a mutation at the ornithine carbamoyltransferase 25. Maestri NE, McGowan KD, Brusilow SW (1992) Plasma glutamine locus. A cause of postpartum coma. N Engl J Med 322:1652-1655 concentration: a guide to the management of urea cycle disorders. 2. Christodoulou J, Qureshi IA, McInnes RR, Clarke JT (1993) Ornithine J Pediatr 121:259-261 transcarbamylase deficiency presenting with strokelike episodes. 26. Dixon MA, Leonard JV (1992) Intercurrent illness in inborn errors J Pediatr 122:423-425 of intermediary metabolism. Arch Dis Child 67:1387-1391 3. Patel JS, Van't Hoff W, Leonard JV (1994) Arginase deficiency pre- 27. Maestri NE, Hauser ER, Bartholomew D, Brusilow SW (1991) Pro- senting with convulsions. J Inherit Metab Dis 17:254 spective treatment of urea cycle disorders. J Pediatr 119:923-928 4. Häberle J, Gőrg B, Rutsch F et al (2005) Congenital glutamine 28. Picca S, Dionisi-Vici C, Abeni D et al (2001) Extracorporeal dialysis in deficiency with glutamine synthetase mutations. N Engl J Med neonatal hyperammonaemia: modalities and prognostic indica- 353:1926-1933 tors. Paediatr Nephrol 16:862-867 5. Saheki T, Kobayashi K,Iijima M et al (2004) Adult-onset type II citrul- 29. Bachmann C (2003) Outcome and survival of 88 patients with urea linaemia and idiopathic neonatal hepatitis caused by citrin defi- cycle disorders: a retrospective evaluation. Eur J Pediatr 162:410- ciency: involvement of the aspartate glutamate carrier for urea 416 synthesis and maintenance of the urea cycle. Mol Genet Metab 30. Todo S, Starzl TE, Tzakis A et al (1992) Orthotopic liver transplanta- 81[Suppl 1]:SS20-26 tion for urea cycle enzyme deficiency. Hepatology 15:419-422 6. Newsholme EA, Leech AR (1983) Biochemistry for the medical 31. Saudubray J-M, Touati G, DeLonlay P et al (1999) Liver transplanta- sciences. Wiley, Chichester, pp 491-494 tion in urea cycle disorders. Eur J Pediatr 158[Suppl 2]:S55-59 7. Batshaw ML, Brusilow SW (1978) Asymptomatic hyperammon- 32. Sokal E (2006) Transplantation for inborn errors of liver metabo- aemia in low birthweight infants. Pediatr Res 12:221-224 lism. J Inherit Metab Dis 29:426-430 8. Grünewald S, Fairbanks L, Genet S et al (2004) How reliable is the allopurinol load in detecting carriers for ornithine transcarbamy- lase deficiency? J Inherit Metab Dis 27:179-186 General Reference 9. 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