Semin Neonatol 2002; 7: 27–35
`doi:10.1053/siny.2001.0085, available online at http://www.idealibrary.com on
`
`Urea cycle disorders
`
`J. V. Leonarda and A. A. M. Morrisb
`
`aBiochemistry, Endocrine and Metabolic
`Unit, Institute of Child Health, London,
`UK
`bMetabolic Unit, Great Ormond Street
`Hospital, London, UK
`
`Key words: ammonia, carbamoyl
`phosphate synthetase, ornithine
`transcarbamoylase, citrullinaemia,
`argininosuccinic aciduria, sodium
`benzoate, sodium phenylbutyrate
`
`Most patients with urea cycle disorders who present as neonates, do so with
`deteriorating feeding, drowsiness and tachypnoea, following a short initial period when
`they appear well. The plasma ammonia should be measured at the same time as the
`septic screen in such patients. Ammonia levels above 200 ♯mol/l are usually caused by
`inherited metabolic diseases and it
`is essential
`to make a diagnosis for genetic
`counselling, even if the patients die. The aim of treatment is to lower the ammonia
`concentrations as fast as possible. Sodium benzoate, sodium phenylbutyrate and
`arginine can exploit alternative pathways
`for
`the elimination of nitrogen but
`haemodialysis or haemofiltration should be instituted if ammonia concentrations are
`>500 ♯mol/l or if they do not fall promptly. Long-term management involves drugs,
`dietary protein restriction and use of an emergency regimen during illness. Severe
`hyperammonaemia
`is usually associated with irreversible neurological damage,
`particularly if levels have been above 800 ♯mol/l for >24 hours, and the option of
`withdrawing treatment should be discussed with the family.
` 2002 Published by Elsevier Science Ltd.
`
`Introduction
`
`Clinical presentation
`
`The urea cycle is the final common pathway for the
`excretion of waste nitrogen in mammals (Fig. 1).
`Urea has low toxicity even at high concentrations,
`in contrast to its precursors, particularly ammonia.
`Urea cycle defects presenting in the neonatal
`period are usually associated with severe and
`rapidly worsening hyperammonaemia. This is a
`major emergency in neonates. Early recognition
`and aggressive treatment are essential to achieve a
`good outcome. Even with prompt intervention, the
`prognosis is poor for patients who present with
`symptoms in the neonatal period. A number of
`other disorders besides urea cycle defects can cause
`severe hyperammonaemia but most are inherited
`and every effort must be made to establish a
`diagnosis.
`
`Correspondence to: J. V. Leonard PhD, FRCP, FRCPCH, Biochemistry,
`Endocrine and Metabolic Unit, Institute of Child Health, London, UK.
`Tel: 020 7905 2627; 020 7404 6191; E-mail: j.leonard@ich.ucl.ac.uk
`
`Patients with urea cycle disorders commonly
`present in the neonatal period but the symptoms
`and signs are not specific. Most of these babies are
`of normal birth weight and are initially healthy, but
`then after a short interval, that can be less than 24
`hours, they become unwell. Common early symp-
`toms are poor feeding, vomiting, lethargy and/or
`irritability and tachypnoea. The initial working
`diagnosis is almost invariably sepsis. Rather char-
`acteristically, these babies often have a mild but
`transient respiratory alkalosis at this stage that can
`be a useful diagnostic clue as there are few other
`causes in a baby not on a ventilator. They may also
`have neuro-muscular irritability and stridor but
`all these symptoms are usually only transient as
`generally the patients deteriorate rapidly. They
`develop more obvious neurological and autonomic
`problems, including changes of tone with loss of
`normal reflexes, vasomotor instability and hypo-
`thermia, apnoea and fits. The baby may soon
`become totally unresponsive and require full
`
`1084–2756/02/$-see front matter
`
`© 2002 Published by Elsevier Science Ltd.
`
`Page 1 of 9
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`Horizon Exhibit 2021
`Lupin v. Horizon
`IPR2017-01160
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`

`28
`
`J. V. Leonard and A. A. M. Morris
`
`H EPATIC
`
`“I'll-3:065"
`_
`Glutamlne
`
`Aspartate
`
`Glycine
`
`
`
`Phenylbutyrate
`
`Phenylbutyryl CoA
`\
`
`Phenylacetate
`
`_
`Glutamme
`
`Phenylacetyl-
`glutamine
`
`Benzoate —> Benzoyl CoA
`
`Glycine
`
`Hippurate
`
`Glutamate
`
`Acetyl CoA
`
`N H3
`
`N-acetyl .....
`glutamate
`
`0
`
`Carbamoyl
`phosphate
`
`A
`
`Citrulline
`
`Ornithine
`
`
`I 7
`
`1 r
`
`Citrulline
`
`Aspartate %9
`
`Ornithine
`
`cytosol
`
`A
`
`Orotic acid
`Orotidine
`
`Arginino—
`
`succinate 6
`
`6
`
`Arginine
`
`Urea
`
`Fumarate
`
`I
`
`.
`Urme
`
`Figure 1. Pathways for the disposal of waste nitrogen. The urea cycle and alternative pathways of nitrogen excretion. Steps
`in the urea cycle: 1. Carbamoyl phosphate synthetase; 2. Ornithine transcarbamoylase; 3. Argininosuccinate synthetase;
`4. Argininosuccinate lyase: 5. Arginase; 6. Mitochondrial omithine carrier; 7. N—acetyl glutamate synthetase. A: Allopun'nol
`inhibits the metabolism of the pyrimidines, orotic acid and orotidine, allowing carriers of OTC deficienCy to be detected.
`
`Page 2 of 9
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`

`Urea cycle disorders
`
`29
`
`intensive care. The babies then often develop a
`wide range of secondary complications such as
`disordered liver function that obscures the primary
`condition. Untreated, most babies will die, often
`with complications such as cerebral or pulmonary
`haemorrhage. Some survive but they are invariably
`handicapped, usually severely.
`Patients with arginase deficiency usually present
`after the neonatal period with spasticity in the legs
`and developmental delay but seldom have symp-
`tomatic hyperammonaemia. On the other hand,
`neonatal hyperammonaemia is well recognized in
`patients with defects of the mitochondrial ornithine
`transporter, an essential component of the urea
`cycle
`(Hyperornithinaemia, Hyperammonaemia,
`Homocitrullinuria
`syndrome).
`Severe neonatal
`hyperammonaemia also occurs in patients with
`ornithine aminotransferase deficiency [1,2], a defect
`that more commonly presents in adults with cata-
`racts and gyrate atrophy of the choroid and retina.
`
`Differential diagnosis
`
`The differential diagnosis of hyperammonaemia is
`wide and is summarized in Table 1. The most com-
`mon differential diagnoses of severe hyperammonae-
`mia are organic acidaemias, particularly propionic
`and methylmalonic acidaemia.
`It is important to
`recognize that patients with these disorders may
`have marked hyperammonaemia with a respiratory
`alkalosis without acidosis or ketosis. Transient
`hyperammonaemia of the newborn (THAN) is an
`ill-understood condition, possibly related to imma-
`turity of liver metabolism or hepatic vascular disease.
`Plasma ammonia levels may be very high initially
`but no underlying metabolic disease is found. Al-
`though babies with THAN are often born prema-
`turely with early onset of symptoms [3], it may be
`difficult to distinguish between urea cycle disorders
`and this disorder on clinical grounds. The incidence
`of THAN appears to have been falling over recent
`years in many centres around the world. Less severe
`hyperammonaemia is common, both in other meta-
`bolic disorders and acquired illness such as sepsis and
`perinatal asphyxia. Babies with systemic herpes sim-
`plex, particularly involving the liver, may have
`marked hyperammonaemia without obvious signs.
`
`Investigations
`
`for establishing
`Routine tests are not helpful
`the diagnosis of hyperammonaemia. The most
`
`Table 1. Differential diagnosis of hyperammonaemia
`
`Inherited disorders
`Urea cycle enzyme defects
`Carbamoyl phosphate synthetase deficiency
`Ornithine transcarbamoylase deficiency
`Argininosuccinate synthetase deficiency (Citrullinaemia)
`Argininosuccinate lyase deficiency (Argininosuccinic
`aciduria)
`Arginase deficiency
`N-acetylglutamate synthetase deficiency
`Transport defects of urea cycle intermediates
`Lysinuric protein intolerance
`Hyperammonaemia – hyperornithinaemia –
`homocitrullinuria syndrome
`Organic acidurias
`Propionic acidaemia
`Methylmalonic acidaemia and other organic acidaemias
`Fatty acid oxidation disorders
`Medium chain acyl-CoA dehydrogenase deficiency
`Systemic carnitine deficiency
`Long chain fatty acid oxidation defects and other related
`disorders
`Other inborn errors
`Pyruvate carboxylase deficiency (neonatal form)
`Ornithine aminotransferase deficiency (neonates)
`Acquired disorders
`Transient hyperammonaemia of the newborn
`Any severe systemic illness
`Herpes simplex – systemic infection
`Liver failure (rare in neonates)
`Infection with urease positive bacteria (if urinary tract stasis)
`
`important diagnostic test in urea cycle disorders is
`measurement of
`the plasma ammonia concen-
`tration. In healthy neonates plasma ammonia is
`normally less than 65 ♯mol/l [4], but may be raised
`as a result of a high protein intake, difficult
`venepuncture or a haemolysed blood sample. In
`sick neonates (for example, those with sepsis or
`perinatal
`asphyxia), plasma
`ammonia
`concen-
`trations may increase up to 180 ♯mol/l. Patients
`with inborn errors presenting in the newborn
`period usually have concentrations greater than
`200 ♯mol/l, often very much greater.
`Ammonia levels can rise rapidly in patients with
`urea
`cycle disorders. Thus, plasma
`ammonia
`measurement should be repeated after a few hours,
`even if it is only modestly elevated.
`In cases of significant hyperammonaemia, the
`following investigations
`should be performed
`immediately:
`

Blood pH and gases
`

Plasma urea and creatinine, electrolytes, glucose
`
`Page 3 of 9
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`

`

`30
`
`J. V. Leonard and A. A. M. Morris
`
`Table 2. Diagnostic tests in urea cycle defects
`
`Disorder
`
`Alternative
`names
`
`Plasma
`amino acid
`concentrations
`
`Urine
`orotic
`acid
`
`Tissue for
`enzyme
`diagnosis
`
`Genetics
`(chromosome
`localization)
`
`Carbamoyl phosphate
`synthetase deficiency
`
`CPS deficiency
`
`Ornithine
`transcarbamoylase
`deficiency
`
`OTC deficiency
`
`Argininosuccinic acid
`synthetase deficiency
`Argininosuccinic acid
`lyase deficiency
`
`Citrullinaemia
`
`Argininosuccinic
`aciduria (ASA)
`
`Arginase deficiency
`N-acetylglutamate
`synthetase deficiency
`
`Hyperargininaemia
`NAGS deficiency
`
`>glutamine
`>alanine
`?citrulline
`?arginine
`>glutamine
`>alanine
`?citrulline
`?arginine
`>>citrulline
`?arginine
`>citrulline
`>argininosuccinic acid
`?arginine
`>arginine
`>glutamine
`>alanine
`
`AR: autosomal recessive; RBC: red blood cells; N: normal.
`
`N
`
`Liver
`
`AR (chromosome 2p)
`
`>>
`
`Liver
`
`X-linked (Xp21.1)
`
`>
`
`>
`
`>
`N
`
`Liver/Fibroblasts
`
`AR (chromosome 9q)
`
`RBC/Liver/Fibroblasts AR (chromosome 7q)
`
`RBC/Liver
`Liver
`
`AR (chromosome 6q)
`AR (not confirmed)
`
`

Liver function tests and clotting studies
`

Plasma amino acids
`

Urine organic acids, orotic acid and amino acids
`

Plasma free and acylcarnitines
`The plasma amino acids and urine organic acids are
`very urgent.
`In all urea cycle disorders there is accumulation
`of glutamine and alanine. There are also increased
`concentrations of
`the amino acids immediately
`proximal to the block in the metabolic pathway and
`decreased concentrations of
`those beyond the
`block (Fig. 1). Thus,
`in citrullinaemia, arginino-
`succinic aciduria (ASA) and arginase deficiency, the
`plasma amino acids are usually diagnostic (Table 2).
`Orotic acid and orotidine are excreted in excess
`in the urine if there is a metabolic block distal to the
`formation of carbamoyl phosphate. In these dis-
`orders carbamoyl phosphate accumulates,
`leaves
`the mitochondrion and enters the pathway for the
`de novo synthesis of pyrimidines in the cytosol
`(Fig. 1). Measurement of urinary orotic acid is
`particularly helpful
`for distinguishing ornithine
`transcarbamoylase (OTC) deficiency from car-
`bamoyl phosphate synthetase (CPS) or N-acetyl
`glutamate synthetase (NAGS) deficiencies.
`Diagnoses can generally be confirmed by meas-
`uring the enzyme activity in an appropriate tissue
`(Table 2). This is the only way to distinguish
`
`between CPS and NAGS deficiencies. Assays of
`CPS are well-established but measurement of
`NAGS activity is not straightforward. Patients with
`NAGS deficiency generally show a
`clinical
`response to N-carbamyl glutamate, an orally active
`analogue of N-acetyl glutamate, but this is un-
`reliable for diagnosis because a response is also
`seen in some patients with CPS deficiency [5].
`Other investigations will detect complications.
`In the late stages of hyperammonaemia patients
`may have marked disturbances of liver function
`with disordered clotting, renal failure and hypocal-
`caemia. In the later stages of hyperammonaemic
`encephalopathy, brain imaging may show cerebral
`oedema or intracranial haemorrhage.
`If the patient seems likely to die it is essential to
`collect the appropriate specimens, since otherwise
`the diagnosis cannot be confirmed:
`
`

Plasma (heparinized, separated and deep frozen)
`

Blood spots on filter paper for acylcarnitines
`

Urine (deep frozen in a plain tube)
`

Blood for DNA (anticoagulated with EDTA and
`deep frozen)
`

Skin for fibroblast culture taken with sterile
`precautions into medium and stored at 4–8C,
`not frozen
`

Liver, snap frozen for enzymology.
`
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`

`Urea cycle disorders
`
`31
`
`Table 3. The emergency treatment of neonatal hyperammonaemia
`v General neonatal supportive care e.g. ventilation (particularly prior to transfer) treatment of
`sepsis, seizures etc.
`v
`Stop protein intake
`v Give a high energy intake
`either (a) oral
`(i) 10% soluble glucose polymer (higher concentrations may be given if they are tolerated)
`(ii) protein free formula (80056 (Mead Johnson); Duocal (SHS Ltd))
`or (b) intravenously
`(i) 10% glucose by peripheral infusion
`(ii) 10–25% glucose by central venous line
`Fluid volumes may be restricted if there is concern about cerebral oedema
`v Alternative pathways for nitrogen excretion
`Sodium benzoate up to 500 mg/kg/day – oral or intravenously
`Sodium phenylbutyrate up to 600 mg/kg/day
`L-arginine
`– In citrullinaemia and ASA – up to 700 mg/kg/day
`– In OTC deficiency and CPS deficiency – up to 150 mg/kg/day
`L-citrulline
`– In OTC deficiency and CPS deficiency up to 170 mg/kg/day instead of arginine
`For the emergency treatment of hyperammonaemia before the diagnosis is known, some
`centres consider the following to be a safer alternative:
`L-arginine 300 mg/kg/day
`L-carnitine 200 mg/kg/day
`Both can be given orally or intravenously
`v Dialysis (haemodialysis, haemodiafiltration or haemofiltration)
`Start immediately if plasma ammonia >500 ♯mol/l or if ammonia does not fall with the
`above measures.
`
`Note these regimens are not nutritionally complete and will cause malnutrition. They must not be continued longer than
`absolutely necessary.
`
`Pathogenic mechanisms
`
`Acute management
`
`Ammonia induces many electrophysiological, vas-
`cular and biochemical changes in experimental
`systems but it is not known to what extent these
`are relevant to the problems of hyperammonaemia
`in man [6]. Ammonia increases the transport of
`tryptophan across the blood brain barrier, which
`then leads to an increased production and release of
`serotonin [7]. Some of the symptoms of hyper-
`ammonaemia can be explained on this basis and the
`dietary tryptophan restriction has reversed ano-
`rexia in some patients with urea cycle disorders [8].
`Glutamine has been shown to accumulate in the
`brain during hyperammonaemia in experimental
`animals and in man in vivo, using proton nuclear
`magnetic resonance spectroscopy [9]. The concen-
`trations are such that the increase in osmolality
`could be responsible for cellular swelling and
`cerebral oedema.
`
`In the neonatal period the immediate goal of
`treatment is to control the metabolic derangement.
`Once plasma ammonia concentrations are greater
`that 500 ♯mol/l this is urgent. The strategies used
`are to stop any dietary protein and give a high
`energy intake, reduce plasma ammonia concen-
`trations with dialysis and utilize alternative path-
`ways of nitrogen excretion. The emergency
`management of neonatal hyperammonaemia is
`summarized in Table 3.
`
`Protein and energy intake
`
`As soon as hyperammonaemia is suspected all
`intake of protein should be stopped and a high
`energy intake given, either orally or intravenously.
`
`Page 5 of 9
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`

`

`32
`
`Dialysis
`
`Once the plasma ammonia concentrations are
`greater that 500 ♯mol/l, it is essential to take steps
`to reduce this as quickly as possible. Haemodialysis
`or haemofiltration should be used rather
`than
`peritoneal dialysis, which is much less effective.
`The exact management will depend on the facili-
`ties and experience available. The easiest and
`most widely used is
`continuous veno-veno-
`haemofiltration. Although there may be theoretical
`concerns
`about
`rapid removal of
`ammonia
`and other metabolites, there are no reports of
`complications from rapid fluid shifts or by other
`mechanisms.
`
`Alternative pathways for nitrogen
`excretion
`
`A major advance in this field has been the devel-
`opment of compounds that are conjugated to
`amino acids and rapidly excreted [10,11]. The effect
`of the administration of these substances is that
`nitrogen is excreted as compounds other than urea
`and hence the load on the urea cycle is reduced
`(Fig. 1). The first compound introduced was sodium
`benzoate. Benzoate is conjugated with glycine to
`form hippurate, which is rapidly excreted. For each
`mol of benzoate given, 1 mol of nitrogen is
`removed. The major side effects are nausea, vom-
`iting and irritability. In the newborn, conjugation
`may require enzyme induction – hence, conju-
`gation may be incomplete at the very time when it
`is needed most (C. Bachmann, personal communi-
`cation). There is also an increased risk of toxicity.
`Theoretically, sodium benzoate might precipitate
`kernicterus and it is particularly important to take
`into account the high sodium content in neonates.
`The next drug used was phenylacetate but this has
`now been superseded by phenylbutyrate, because
`the former has a peculiarly unpleasant clinging
`mousy odour. Phenylbutyrate is oxidized in the
`liver to phenylacetate, which is then conjugated
`with glutamine. The
`resulting phenylacetyl-
`glutamine is excreted in the urine and hence 2 mol
`of nitrogen are lost for each mol of phenylbutyrate
`given. Accidental overdoses of sodium benzoate
`and sodium phenylbutyrate have caused metabolic
`acidosis, cerebral oedema and circulatory collapse
`[12].
`In patients with citrullinaemia and ASA, nitro-
`gen can be excreted in the form of citrulline and
`
`J. V. Leonard and A. A. M. Morris
`
`argininosuccinic acid, respectively. The formation
`of these metabolites is limited by the low ornithine
`levels that result from the metabolic block (Fig. 1).
`Arginine supplements can replenish the supply of
`ornithine, maximizing the excretion of citrulline
`and argininosuccinic acid. Arginine doses of up to
`700 mg/kg/day may be used. Though the concen-
`trations of citrulline or argininosuccinate rise, these
`compounds are thought to have less adverse effects
`than the accumulation of ammonia and glutamine.
`Alternative treatment regimens have been pro-
`posed because of concerns about the potential
`toxicity of sodium benzoate and sodium phenyl-
`butyrate. Some authorities, for example, advocate
`giving only arginine and carnitine before the diag-
`nosis is known, if the ammonia concentration does
`not warrant dialysis (Table 3). No studies have
`been done comparing these different regimens.
`
`Long-term treatment
`
`it is
`Once the acute illness has been controlled,
`necessary to reintroduce an oral feed containing
`protein and energy. The aim of long-term treat-
`ment is to correct the biochemical disorder and yet
`ensure that all the nutritional needs are met. For
`severely affected patients this can be difficult. The
`major strategies used are to give a low protein diet,
`to utilize alternative pathways of nitrogen excre-
`tion and to replace nutrients that are deficient.
`
`Low protein diet
`
`All patients with urea cycle disorders presenting in
`the newborn period require a strict low protein
`diet. The protein tolerance of patients varies con-
`siderably and depends on factors such as age and
`growth rate as well as the residual enzyme activity.
`When protein is first introduced there is often a rise
`in the plasma ammonia concentration and it is
`necessary to persist with the feeds to get the baby
`anabolic. Once this is achieved, metabolic control
`during early infancy is often straightforward and
`the patients may need 1.8–2 g/kg/day of protein or
`sometimes even more during very rapid growth.
`All diets must, of course, be nutritionally complete
`and meet the requirements of growth and normal
`development.
`
`Essential amino acids
`
`In the most severe variants it may not be possible
`to achieve good metabolic control and satisfactory
`
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`

`Urea cycle disorders
`
`33
`
`nutrition with restriction of natural protein alone.
`In these patients some of the natural protein may
`be replaced with an essential amino acid mixture,
`giving up to 0.7 g/kg/d. Essential amino acid mix-
`tures ensure that there are adequate precursors for
`protein synthesis whilst minimizing the nitrogen
`load to be excreted.
`
`Arginine and citrulline
`
`Arginine is normally a non-essential amino acid
`because it is synthesized within the urea cycle. For
`this reason, all patients with urea cycle disorders
`except those with arginase deficiency are likely to
`need a supplement of arginine to replace that which
`is not synthesized [13]. The aim should be to
`maintain plasma arginine concentrations between
`50 and 200 ♯mol/l.
`In citrullinaemia and ASA,
`patients will need up to 500 mg/kg/day. For most
`patients with OTC and CPS deficiencies, a dose of
`100–150 mg/kg/day appears
`to be
`sufficient.
`Severely affected patients with these disorders may
`profit from using citrulline (up to 170 mg/kg/day)
`instead of arginine as this will utilize an additional
`molecule of nitrogen.
`
`Alternative pathways for nitrogen
`excretion
`
`Patients continue to need alternative pathway
`therapy to maintain good metabolic
`control,
`although full doses may not be necessary during
`the phases of rapid growth. For each patient there
`is a balance between the protein intake and the
`dose of their medicines to achieve good metabolic
`control. If patients can take large doses of sodium
`benzoate and sodium phenylbutyrate,
`it will
`increase their protein tolerance but if they only
`manage small doses, their diet will have to be
`stricter.
`
`should not be used as this drug may precipitate
`fatal decompensation particularly in OTC deficient
`patients [15].
`
`Monitoring
`
`treatment must be monitored with regular
`All
`estimations of plasma ammonia and quantitative
`amino acids, paying particular attention to the
`concentration of glutamine and essential amino
`acids. The aim is to keep plasma ammonia less than
`80 ♯mol/l
`and
`plasma glutamine
`less
`than
`800 ♯mol/l
`[16], but in practice 1000 ♯mol/l
`is
`probably more realistic,
`together with concen-
`trations of essential amino acids within the normal
`range.
`
`Management of acute illness
`
`All patients with urea cycle disorders are at risk of
`acute decompensation with acute hyperammonae-
`mia. This can be precipitated by metabolic stresses,
`such as fasting, a large protein load,
`infection,
`anaesthesia and surgery but in patients with severe
`variants there may be no very obvious reason. All
`patients should have detailed instructions of what
`to do when they are at risk. We routinely use a
`three-stage procedure. If the patient is off colour,
`the protein is reduced and more carbohydrate
`given. If symptoms continue, protein should be
`stopped and a high energy intake given together
`with their medication both day and night. If they
`refuse or vomit their emergency drinks or medi-
`cines, or show any signs of encephalopathy, they
`should go to hospital urgently for assessment and
`intravenous therapy. For further practical details
`see [17].
`
`Prognosis
`
`Other medication
`
`N-carbamyl glutamate can be used in NAGS
`deficiency to replace the missing compound, as it is
`active orally. The dose is 100 mg/kg/day [14].
`Patients who respond may require treatment only
`with this compound.
`Anticonvulsants may be needed for patients
`with urea cycle disorders but sodium valproate
`
`The prognosis in these disorders is closely related
`to the age of the patient and their condition at the
`time of diagnosis. For those patients who present
`with symptomatic hyperammonaemia in the new-
`born period, the outlook is very poor. Even with
`the most aggressive treatment, the majority of the
`survivors will be handicapped. Those who are
`treated prospectively do much better but there
`may still be significant complications [18]. For
`these patients there remains a serious risk of
`
`Page 7 of 9
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`

`34
`
`J. V. Leonard and A. A. M. Morris
`
`decompensation and careful consideration should
`be given to early liver transplantation, which may
`offer
`the hope of a better
`long-term outlook
`[19,20].
`
`Genetics and prenatal diagnosis
`
`The genes for all the urea cycle enzymes except
`N-acetyl glutamate synthetase have been mapped,
`isolated and fully characterized. Many mutations
`have been described. The commonest urea cycle
`disorder is OTC deficiency. This is an X-linked
`disorder in which molecular genetic studies are
`particularly helpful. When the diagnosis of OTC
`deficiency is established, a careful family history
`should be taken and the mother’s carrier status
`should be assessed. If the mutation is unknown, the
`most convenient
`investigation is currently the
`allopurinol test. This detects increased synthesis of
`orotic acid and orotidine; allopurinol inhibits the
`metabolism of these pyrimidines, enhancing their
`excretion and allowing the detection of even
`asymptomatic carriers (Fig. 1)
`[21,22]. The allo-
`purinol test is easier than protein or alanine loading
`tests and carries no risk of hyperammonaemia.
`Prenatal diagnosis is possible in most families using
`informative polymorphisms if the mutation itself
`has not been identified. Whilst the phenotype of
`the males can be predicted, that of the females
`cannot because of the random inactivation of the
`X chromosome. This presents a problem when
`counselling families, but the prognosis for females
`who are treated prospectively from birth is good.
`All the other urea cycle disorders have auto-
`somal recessive inheritance and prenatal diagnosis
`is possible for all except NAGS deficiency. For CPS
`deficiency, prenatal diagnosis using closely linked
`gene markers is now possible in a substantial
`proportion of
`families.
`If the molecular genetic
`studies are uninformative, prenatal liver biopsy is
`an alternative. Citrullinaemia and ASA can both be
`diagnosed on chorionic villus biopsy. Arginase
`deficiency can be diagnosed either by molecular
`genetic studies or, if they are not informative, on a
`fetal blood sample.
`
`Conclusions
`
`It is important to have a low threshold for meas-
`uring the plasma ammonia in neonates. Severe
`hyperammonaemia is a neonatal emergency with a
`
`high risk of neurological damage. Unless the par-
`ents wish treatment to be withdrawn, ammonia
`levels should be lowered as fast as possible, usually
`by haemofiltration or dialysis. Most causes of
`significant hyperammonaemia are genetic and it is
`important to make a diagnosis even if the patient
`dies.
`
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