US
`
`Semin Neonatal 2002: 7: 27—35
`doi:10.1053isiny2001.0085. available online at http:iiwww.idealibrary.oom on Inlgl‘i’
`
`Urea cycle disorders
`
`J. V. Leonarda and A. A. M. Morrisb
`
`'“Biorlmiristiy, Endocrine and Metabolic
`Unit, institute of Ciiiiri Health, London,
`UK
`
`J"{\A‘L'trrb'ofic Unit, Grant Ornirmd 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 pmolfl 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 umola’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 umoli'l
`for >24 hours, and the option of
`withdrawing treatment should be discussed with the family,
`:tj 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.
`
`cycle disorders commonly
`Patients with urea
`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
`
`II‘RCPCIL Biochemistry.
`i’liCP.
`I. V. Leonard Phi).
`(.‘orrespondence to:
`Endocrine and Metabolic Unit. Institute of Child Health. London. UK.
`Tel: {120 T905 262.7: 020 'f4ti4 6191'. ii—mail: j.leonandggichuclacuk
`
`1084—2756l02i$—see front matter
`
`1of10
`
`Horizon Exhibit 2012
`Horizon Exhibit 2012
`Lupin v. Horizon
`Lupin v. Horizon
`IPR2018-00459
`lPR2018-00459
`
`1 of 10
`
`

`

`US
`
`Semiri Neonatal 2002: 7: 27—35
`doi:10.1053i’siny2001.0085. available online at httpvi'wwwidealibrarycom on Inn-£9
`
`Urea cycle disorders
`
`J. V. Leonarda and A. A. M. Morrisb
`
`'“Bioclmirisiiy, Endocrine and Metabolic
`Unit, institute it)" Cirr'tii Health, London,
`UK
`
`J"{\A‘L'trrb'ofir Unit, Grant Ormrmd 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 umoli’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 umola’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 umoll'l
`for >24 hours, and the option of
`withdrawing treatment should be discussed with the Family,
`:_:t:l 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. I).
`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 hyperammonaernia. 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.
`
`i’ltCP. FRUIT“. Biochemistry.
`|eonard Phi).
`I. V.
`(.‘orrespondence to:
`Endocrine and Metabolic Unit.
`institute of Child Health. London. UK.
`Tel: {120 T905 262.7: 020 'f4ti4 6191'. ii—mail: j.lminaridficgichuclacuk
`
`1084—2756i02l$—see front matter
`
`20f10
`
`cycle disorders commonly
`Patients with urea
`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
`
`(6) 2002 Published by Elsevier Science Ltd.
`
`2 of 10
`
`

`

`28
`
`J. V. Leonard and A. A. M. Morris
`
`Phenyiburyrate
`
`Phenyibutyryi CoA
`3
`Phenyiacerate
`
`.
`Glutamlne
`
`:
`
`Phenylacetyl-
`glutamine
`
`Benzoais —> Benzoyi CoA
`
`Glycine
`
`Hippurate
`
`H EPAT|C
`NITROGEN
`POOL
`
`-
`Glutamlne
`
`
`
`Aspartate
`
`Glycine
`
`Glutamate
`
`Acetyl 60A
`
`0
`
`NHs
`
`N-acetyl
`glutamate
`
`+
`
`0
`
`Carbamoyl
`phosphate
`
`m
`
`Citrulline
`
`Ornithine
`
`
`
`‘ I
`
`‘ 7
`
`Citrulline
`
`Aspartate \l9
`
`Ornithine
`
`cytosol
`
`A
`
`Orotic acid
`Orotidine
`
`Arginino—
`succinate 9
`
`6
`
`Urea
`
`Arginine
`
`Fumarate
`
`.
`Urine
`
`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 ornithine carrier; 7. N—acetyl glutamate synthetase. A: Allopurinol
`inhibits the metabolism of the pyrimidines, orotic acid and orotidine. allowing carriers of OTC deficiency to be detected.
`
`3of10
`
`3 of 10
`
`

`

`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 omithine
`transporter, an essential component of the urea
`cycle
`(Hyperornithinaemia, Hyperammonaemia,
`Homocitrullinuria
`syndrome).
`Severe
`neonatal
`hyperammonaemia also occurs in patients with
`omithine 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
`
`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 umol/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 umol/l. Patients
`with inborn errors presenting in the newborn
`period usually have concentrations greater than
`200 umol/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.
`the
`In cases of significant hyperammonaemia,
`following investigations
`should be performed
`immediately:
`
`for establishing
`Routine tests are not helpful
`the diagnosis of hyperammonaemia. The most
`
`0 Blood pH and gases
`0 Plasma urea and creatinine, electrolytes, glucose
`
`4of10
`
`4 of 10
`
`

`

`30
`
`J. V. Leonard and A. A. M. Morris
`
`Table 2. Diagnostic tests in urea cycle defects
`
`Urine
`Plasma
`,
`.
`.
`.
`Alternative
`,
`Disorder
`names
`ammo acrd
`orotic
`enzyme
`(chromosome
`concentrations
`acid
`diagnosis
`localization)
`
`
`Tissue for
`
`Genetics
`
`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
`
`AR.- autosomal recessive; RBC: red blood cells; N: I'lOI'n'lal.
`
`“glutamine
`“alanine
`vcitrulline
`varginine
`“glutamine
`“alanine
`vcitrulline
`varginine
`ATcitrulline
`varginine
`“citrulline
`“argininosuccinic acid
`varginine
`“arginine
`“glutamine
`“alanine
`
`
`
`N
`
`Liver
`
`AR (chromosome 2p)
`
`TT
`
`Liver
`
`X-linked (Xp21.l)
`
`T
`
`I
`
`I
`N
`
`LiverfFibroblasts
`
`AR (chromosome 9q)
`
`RBCJ’LiveriFibroblasts AR (chromosome 7q)
`
`RBCfLiver
`Liver
`
`AR (chromosome éq)
`AR (not confirmed)
`
`I Liver Function tests and clotting studies
`I Plasma amino acids
`
`between CPS and NAGS deficiencies. Assays of
`CPS are well—established but measurement of
`
`0 Urine organic acids, orotic acid and amino acids
`0 Plasma free and acylcamitines
`
`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
`
`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:
`
`0 Plasma (heparinized, separated and deep frozen)
`0 Blood spots on filter paper for acylcamitines
`
`O Urine (deep frozen in a plain tube)
`0 Blood for DNA (anticoagulated with EDTA and
`deep frozen)
`oSkin for
`fibroblast culture taken with sterile
`
`precautions into medium and stored at 4—8°C,
`not frozen
`
`0 Liver, snap frozen for enzymology.
`
`50f10
`
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`
`

`

`Urea cycle disorders
`
`31
`
`Table 3. The emergency treatment of neonatal hyperammonaemia
`
`' General neonatal supportive care e.g. ventilation (particularly prior to transfer} treatment of
`sepsis, seizures etc.
`' Stop protein intake
`‘ 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
`' Alternative pathways for nitrogen excretion
`Sodium benzoate up to 500 mgr'kgfday 7 oral or intravenously
`Sodium phenylbutyrate up to 600 mgl'kgiday
`L~arginine
`— In citrullinaemia and ASA — up to 700 mgl'kgiday
`— In OTC deficiency and CPS deficiency - up to 150 mgfkgiday
`L—citrulline
`
`— In OTC deficiency and CPS deficiency up to 170 mgl'kgi'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 mgr'kg/day
`L-carnitine 200 mgfkgiday
`Both can be given orally or intravenously
`' Dialysis {haemodialysis, haemodiafiltration or haemofiltration)
`Start immediately if plasma ammonia >SOC! pmolfl 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 16]. 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 umol/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.
`
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`
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`
`

`

`32
`
`Dialysis
`
`are
`Once the plasma ammonia concentrations
`greater that 500 umoI/I, 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/kgfday 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.
`
`Lon g-term treatment
`
`Once the acute illness has been controlled,
`
`it
`
`is
`
`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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`
`7 of 10
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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. Essantial 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 pmol/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 ”mom and
`plasma
`glutamine
`less
`than
`800 umol/l
`[16], but
`in practice 1000 umol/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-
`bom 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
`
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`
`8 of 10
`
`

`

`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 urea cycle enzymes except
`The genes for all
`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
`
`investigation is currently the
`most convenient
`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.
`
`References
`
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`J inherit Metnb Dis 1999: 22(Suppl 1): 80.
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`[wata BA, Kissel R,
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`12 Praphanphoj V, Boyadjiev SA, Waber L], ei oi. Three cases
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`14 Bachmann C, Colombo JP, Jaggi K. N—acetylglutamate
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