Molecular Genetics and Metabolism 81 (2004) S86 S91
`
`www.elsevier.com/locate/ymgme
`
`Problems in the management of urea cycle disorders
`
`Bridget Wilcken*
`
`The ChildrenÕs Hospital at Westmead and the Discipline of Paediatrics, University of Sydney, Sydney, Australia
`
`Received 19 September 2003; received in revised form 17 October 2003; accepted 25 October 2003
`
`Abstract
`
`Several recent reviews describe the management of urea cycle disorders. There is much agreement on diet, alternative pathway
`therapy, maintenance of arginine and ornithine levels in acute and chronic management, sick day regimens, and some aspects of
`monitoring. However, differences remain in several areas, and physicians at most treatment centers have relatively little experience,
`because these disorders are rare. Early suspicion of the diagnosis of a urea cycle disorder, and prompt referral to a tertiary center is
`vital. Drug treatment using chronic administration of sodium benzoate has been abandoned by some centers, but the acceptability of
`phenylbutyrate is an issue for many patients. Using citrulline chronically is not always successful in recommended doses, and may
`result in an arginine level too low for maximum control. Appetite and nutrition problems are common. One major concern is the
`early identification and management of chronic catabolism, theoretically easy, but hard in practice. Biochemical measurement
`problems complicate monitoring, and there are disagreements about the optimum way of identifying OTC carriers. It is not always
`clear whom to treat. Within a kindred with an early onset phenotype, an asymptomatic newborn girl may need treatment for some
`undetermined time, but target values for monitoring are not clear. In late onset phenotypes, management of asymptomatic males
`identified by family screening is also difficult. Most centers do not have sufficient cases to solve these conundrums, some of which
`require further multicenter study. This paper examines the recommendations of a consensus conference on management, outlines
`some remaining problems, and incorporates in the text the points raised in open discussion during a session of a symposium held in
`Sydney in 2003 entitled ‘‘New Developments in Urea Cycle Disorders.’’
`Ó 2004 Elsevier Inc. All rights reserved.
`
`Keywords: Urea cycle; Ornithine transcarbamylase; Ammonia-scavengers; Sodium benzoate; Sodium phenylbutyrate; Carrier testing; Heterozygote;
`Nitrogen flux
`
`Introduction
`
`There have been several reviews and position state-
`ments about the diagnosis and management of urea
`cycle disorders (UCDs) [1,2]. In 2000 a consensus con-
`ference was held in Washington DC, to discuss and try
`to come to agreement about the management of patients
`with UCDs. The proceedings were published in January
`2001 [3] and consist of a series of papers tackling various
`aspects of diagnosis and management. The publication
`opens with a number of recommendations reflecting the
`opinion of the 18 participants, all but two of whom were
`from the United States of America. The authors rightly
`point out that many of the relevant aspects have not
`been studied scientifically, and ‘‘. . .cannot be regarded
`
`* Fax: +61-2-9845-3121.
`E-mail address: bridgetw@chw.edu.au.
`
`1096-7192/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved.
`doi:10.1016/j.ymgme.2003.10.016
`
`as evidence-based medicine.’’ Much of what is set out in
`this excellent document is well accepted by everyone in
`the field, but there are remaining concerns and points of
`contention.
`The UCDs are individually quite rare, and many
`otherwise experienced metabolic physicians have man-
`aged far fewer than 50 patients with a UCD. Table 1
`shows the numbers of patients diagnosed in New South
`Wales over a period of nearly 30 years. New South
`Wales has a population of 6 million, and there is a long-
`established central service for both the diagnosis and
`treatment of patients with inborn errors of metabolism.
`The 56 cases of primary UCD presenting clinically
`equate to an observed birth prevalence in New South
`Wales of 1:44,000, similar to that in British Columbia [4]
`but much lower than the estimate postulated by Brusi-
`low and Maestri [5]. Also, as is found in most other
`centers, a high proportion (57%) of our patients had
`
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`B. Wilcken / Molecular Genetics and Metabolism 81 (2004) S86 S91
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`S87
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`Table 1
`Urea cycle defects diagnosed in New South Wales 1974 2003
`
`Diagnosis
`
`Number
`
`Primary defects
`NAGS
`CPS
`OTC
`Early onset males
`Late onset males
`Female propositi
`ASS
`ASL
`Arginase
`
`Total
`
`Overall incidence, primary UCDs
`excluding asymptomatic
`OTC heterozygotes
`
`Secondary defects
`Lysinuric protein intolerance
`HHH syndrome
`Citrin deficiency (NICCD)
`
`1 (not proven)
`5
`32 (57%)
`19
`7
`6
`7
`11
`1
`
`56
`
`1:44,000 (95% CI
`1:31,000 1:53,000)
`
`4
`0
`1
`
`NAGS, N acetyl glutamate synthase deficiency; CPS, carbamoyl
`phosphate synthase deficiency; OTC, ornithine transcarbamylase
`deficiency; ASS, argininosuccinate synthase deficiency; ASL, argini
`nosuccinate lyase deficiency; HHH syndrome, hyperornithinemia,
`hyperammonemia, homocitrullinemia; and NICCD, neonatal intra
`hepatic cholestasis with citrin deficiency.
`Total population (2003): 6 million; births: c.90,000 per annum.
`
`symptomatic ornithine transcarbamylase (OTC) defi-
`ciency. These data are included to give a flavor of the
`small numbers of patients that will inform the thoughts
`and experience of most laboratory staff and treating
`physicians throughout the world.
`During this satellite meeting on New Developments
`in Urea Cycle Disorders, the final session was devoted to
`a discussion of management problems. This paper at-
`tempts to review the recommendations of the 2001
`consensus paper [3], highlight areas of uncertainty or
`contention that may exist, especially for people working
`‘‘at the coal face,’’ and incorporate points made during
`the prolonged discussion. Not all of the problems are
`answered!
`
`Consensus paper review and discussion
`
`Neonatal presentation
`
`The 2001 consensus statement [3] has seven recom-
`mendations about the neonatal presentation of UCDs,
`and none seems controversial. These can be summarized
`as: (1) always consider the diagnosis of hyperammone-
`mia in a sick neonate; (2) include a plasma ammonia
`measurement in a sepsis work-up; (3) respiratory alka-
`losis is a clue to a UCD; (4) all neonates with symp-
`tomatic hyperammonemia require rapid transfer to a
`tertiary referral center; (5) with the prior initiation of the
`
`administration of intravenous glucose and fluids; (6) if
`possible, collection of samples of plasma and urine for
`analysis; and (7) all feedings containing protein should
`be discontinued.
`It cannot be too strongly emphasized that prompt
`referral should take precedence over everything else, and
`that while stabilization, with intubation and intravenous
`glucose and fluids is important before transport, blood
`and urine samples and placement of a central line can
`readily wait until arrival. As pointed out in the paper by
`Summar [6], specific treatment of hyperammonemia
`should begin before the precise diagnosis is made, so a
`short delay in finalizing the diagnosis is unimportant
`compared with any delay in initiating treatment. In New
`South Wales we have a single center for the investigation
`and treatment of patients with urea cycle disorders (and
`similar acutely presenting inborn errors of metabolism).
`Rapid transport is available from any part of the state,
`using a dedicated helicopter service. Urgent biochemical
`genetics testing is always available, and a firm diagnosis
`for all UCD disorders except N-acetyl glutamate syn-
`thase (NAGS) and carbamyl phosphate synthase (CPS)
`deficiencies is expected within 2 3 h of receipt of plasma
`and urine samples. The appropriate drugs and treatment
`protocols are stored in the intensive care unit, and a
`metabolic physician is on call, as is the renal team for
`dialysis. There is no doubt that a centralized service is
`vital for the treatment of rare and challenging disorders.
`
`Acute hospital management
`
`Here too, the consensus conference recommendations
`[3] are uncontroversial. They can be encompassed in
`three topics: (1) measure the plasma ammonia level; (2)
`start therapy with intravenous ammonia scavenging
`drugs; and (3) make preparation for dialysis.
`The possibility of using ammonia-scavenging com-
`pounds in treatment was first discussed by Brusilow et
`al. [7], and subsequently protocols were developed which
`are still in use, with modifications, in most treatment
`centers. While there was no doubt about the wisdom of
`using ammonia-scavenging drugs, there was some dis-
`cussion about the detail. The advisability of giving a
`priming infusion to a patient with acute decompensation
`but already on treatment was thought to be safe, pro-
`vided that not more than 500 mg/kg/24 h of sodium
`benzoate and phenylacetate was given. This view, how-
`ever, is not universal [8]. The general opinion on the
`safety of using intravenous sodium benzoate and phen-
`ylacetate in the presence of cerebral edema was that
`dialysis, which would be used in the presence of cerebral
`edema, would rapidly remove any excess of these com-
`pounds, and that there was no good reason not to use
`them. However, some participants felt that there was a
`danger of osmotic shifts because of the sodium load.
`The need to monitor potassium levels closely during
`
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`B. Wilcken / Molecular Genetics and Metabolism 81 (2004) S86 S91
`
`intravenous (as well as oral) therapy with ammonium
`scavengers was emphasized in discussion. The acute use
`of phenylbutyrate in argininosuccinate lyase (ASL) de-
`ficiency is not recommended in the ‘‘Brusilow’’ protocols
`[2], but is recommended in the consensus document [6]
`and was again supported at this meeting.
`When to stop treating neonates was a subject of some
`discussion. Factors affecting the neurological outcome
`in UCDs are addressed elsewhere in this issue (Bat-
`shaw). European experience suggests that if the ammo-
`nia level at presentation has been above 400 lmol/L
`there is no normal cognitive outcome, [9] and devas-
`tating deficits can be expected if coma has lasted for
`more than 72 h. Any decision about the withdrawal, or
`non-institution, of treatment must of course be negoti-
`ated with the parents.
`
`Blood sampling, laboratory testing, and the interpretation
`of results
`
`Blood ammonia
`The consensus conference recommendations [3] about
`ammonia measurement relate mainly to blood collec-
`tion. This is appropriate, as with the usual methods of
`ammonia measurement, most inaccuracies are pre-ana-
`lytical in origin. Currently the usual method in Australia
`is enzymatic, and the mean coefficient of variation
`among 60 laboratories using the enzymatic method
`during the first half of 2003 was <5.0%. The paper by
`Summar
`[6]
`suggests
`that an ammonia level of
`>150 lmol/L, together with a normal anion gap and
`blood glucose is a strong indication for the presence of a
`UCD. It must be very uncommon for a symptomatic
`neonate with a UCD to present with a plasma ammonia
`level of less than 200, and in our experience ammonia
`levels have always been above 300 lmol/L. A reported
`ammonia level of >150 lmol/L in a sick neonate is not
`uncommon, and frequently results from a combination
`of pre-analytic problems including a capillary collection.
`
`Confirmation of diagnosis
`In the consensus document, the confirmation of di-
`agnosis of argininosuccinate synthase (ASS) and ASL
`
`deficiencies is said not to require an enzyme analysis, as
`‘‘. . .the AA (amino acid) analysis is definitive and un-
`ambiguous’’ while this would be required for NAGS,
`CPS, and OTC deficiencies. It is true that a liver biopsy
`and enzyme analysis is required for the definitive diag-
`nosis of CPS 1 and NAGS deficiencies, but the plasma
`amino acid findings, together with elevated urinary
`orotic acid is also definitive and unambiguous for
`hemizygous OTC deficiency.
`
`OTC carrier detection
`The detection of carriers of OTC deficiency is another
`matter, and the plasma amino acid profile can indeed be
`normal. Although DNA testing is recommended where
`possible, in approximately 20% of cases no mutation can
`currently be found [10]. Where DNA confirmation is not
`possible, the consensus document comes down in favor
`of the allopurinol-loading test [11], while noting that it
`‘‘. . . is not completely sensitive or specific. . ..’’ Indeed,
`Bonham et al. [12] found very poor specificity with the
`allopurinol load in sick children. We recently published
`our data using an improved protein-loading test [13].
`The improved features of the test included ensuring an
`orotate-free load (containing no milk, which has a high
`orotate content) of 35 g protein/m2 of body surface area,
`using a more specific method for measuring orotic acid,
`and measuring the rate of orotate excretion as a ratio of
`urinary excretion at 2 4 h/0 2 h after the load. Table 2
`shows a summary of our results, including some previ-
`ously published [13]. All 21 obligate heterozygotes had
`an unequivocally positive result. Among 14 family
`members who were possible heterozygotes, one, the
`mother of an affected male in whom no mutation could
`be found, showed an unequivocally positive result, and
`the remainder were negative. Six had later DNA con-
`firmation of non-carrier status, one was the mother of
`an affected girl, and the remaining six were at 1:4 to 1:16
`risk only. A protein load, given as voluntary oral intake
`of normal food, has appeared completely safe in our
`hands, as protein-averse subjects will refuse to take the
`whole load, and these invariably have a positive result to
`testing, but no, or trivial, adverse symptoms. The reli-
`ability of this improved test seems likely to be better
`
`Table 2
`Protein loading test results—adults
`
`Subjects
`
`Healthy controls
`Obligate carriers
`
`Possible carrier
`Possible carriers (family members)
`
`Number
`
`Observed ratio 2 4 h/0 2 h
`
`18
`21
`
`1
`13
`
`0.8 1.39
`2.28 17.8
`
`5.72
`0.8 1.2
`
`Comment
`Mean 4SD 0.64 1.85
`Three had elevated fasting urinary
`orotate, and no load
`Mother of affected male
`Six—DNA excluded carrier status,
`one—mother of female propositus,
`six—at 1:4 to 1:16 risk
`
`See, http://www.chw.edu.au/prof/services/biogen/ and follow prompts to ‘‘protein load test protocol’’ for details.
`* Observed ratio—ratio of urinary orotic acid in timed urine collections from 0 2 to 2 4 h after a protein load.
`
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`S89
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`than that of an allopurinol load, and is a more direct test
`of functional capacity of the liver in nitrogen disposal.
`Methods using 15N-labeled urea and glutamine are not
`yet readily available [14,15].
`
`Monitoring and laboratory tests
`
`The 2001 consensus document [3,11] identifies plas-
`ma glutamine as a useful marker of metabolic control
`and suggests maintaining the level at <1000 lmol/L.
`The discussion first centered on the correlation between
`plasma glutamine and ammonia levels, which was said
`to be good in CPS and OTC deficiencies, but much less
`so in later disorders. Also, there is patient to patient
`variation, with some patients chronically running glu-
`tamine levels higher than 1000 lmol/L whilst remaining
`well. Elevated glutamine beyond the normal basal level
`for an individual patient was thought likely to be a
`harbinger of a hyperammonemic decompensation, al-
`though there appear to be no publications relating to
`this. Circulating glutamine may not correlate well with
`brain glutamine, and elevated levels may simply indi-
`cate an overload of nitrogen. The meetingÕs consensus
`was that in the absence of studies, glutamine levels
`above the patientÕs usual levels indicate the need for
`intervention. The possibility of glutamine monitoring
`by tandem mass spectrometry, using a blood-spot
`sample taken at home and sent by post to the labo-
`ratory was also raised [16].
`The question was raised as to what time of day to
`measure plasma amino acids. Within the first hour or so
`after a meal plasma amino acids are elevated, and de-
`cline slowly to trough values at 3.5 h after the end of a
`meal. Citrulline levels differ, with a trough at 30 min
`after a meal, and a steady climb thereafter. It is also
`clearly important to relate blood sampling to the timing
`of doses of arginine or citrulline when adjusting medi-
`cation. Monitoring of levels of ammonia scavengers was
`also discussed. A recently developed stable isotope di-
`lution blood-spot assay for phenylbutyrate, phenylace-
`tate and benzoate, demonstrates the immense variability
`in both peak and trough levels, not clearly related to
`weight or dosage [17]. This could be due in part to in-
`complete absorption, or to poor solubility of the medi-
`cations, so that the dosages actually taken could vary.
`Several episodes of toxicity with unintended over-dosage
`have been recorded [18]. Home ammonia monitoring
`may be a useful strategy, but there was little experience
`of this.
`
`Nutritional management
`
`The broad recommendations of the 2001 consensus
`document [3] are not controversial. In essence they are:
`(1) a reduced protein intake, (usually less than the
`Recommended Daily Allowance), (2) giving essential
`
`(3) supplementation with
`amino acid supplements,
`minerals, vitamins, and trace elements, and (4) moni-
`toring growth, hair, skin, nails, and biochemical indices
`of nutritional status. The document does not specifically
`recommend the involvement of a metabolic dietician,
`which in New South Wales we find essential. Some
`practical problems with nutrition include anorexia, and
`the management of nasogastric or gastrostomy tubes
`when these are necessary. A more difficult issue is the
`recognition of chronic catabolism.
`In discussion, it was conceded that it was difficult to
`tell when branched-chain amino acids in a protein-re-
`stricted patient were too low, or whether they should be
`specifically supplemented as treatment with phenylbu-
`tyrate may selectively decrease them (see the paper by
`F. Scaglia et al. in this supplement). Monitoring of lin-
`ear growth will detect a problem of chronic catabolism,
`but a deficit may occur at a late stage, possibly after
`growth in head circumference has slowed. Nutritional
`deficits must be detected at an early stage where possi-
`ble. The role of carnitine was also canvassed. Low car-
`nitine levels were reported to be uncommon in treated
`UCD patients and it was pointed out that carnitine
`administration in patients treated with sodium benzoate
`will favor the formation of benzoyl carnitine, and thus
`negate the benefit of the nitrogen scavenger potential of
`the benzoate.
`
`Pharmacological management and chronic therapy
`
`The consensus document [3] makes several recom-
`mendations, some of which have been discussed above.
`There is an emphasis on safety: double checking of
`written orders for pharmacological scavenging agents;
`monitoring of plasma levels; patients should have a
`written treatment protocol for acute management to
`present to the emergency room. For chronic therapy,
`the document recommends 4 times daily dosage of
`ammonia-scavenging drugs, linked to meals, to maxi-
`mize the effect of ammonia removal. The recommen-
`dation for OTC and CPS deficiencies is for the use of
`oral citrulline rather than arginine, in combination with
`the ammonia scavengers [19]. Our experience with
`several patients with OTC is that divided doses of
`citrulline of 170 mg/kg/day as recommended often re-
`sult in quite low levels of plasma arginine, near the
`bottom of the reference range, usually accompanied by
`elevated levels of glutamine. Additional supplementa-
`tion with arginine lowers the glutamine into the normal
`range. Clearly this needs further investigation to de-
`termine whether there is a particular advantage in
`giving arginine or whether the recommended dosage of
`citrulline is far too low in some patients. The ideal level
`of plasma arginine during treatment is said to be be-
`tween 50 and 200 lmol/L [20], but our experience is
`that levels below 90 lmol/L are not ideal for some
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`patients, and again there need to be studies to explore
`this. The use of benzoate and phenylbutyrate is much
`more complex. Some centers favor the chronic use of
`phenylbutyrate alone, without any benzoate. There was
`no further discussion about this, but an accompanying
`paper in this supplement spells out the pharmacoki-
`netics of these medications.
`The safety of phenylbutyrate in pregnancy was que-
`ried during the discussion. The consensus document
`recommends caution, as the safety of phenylbutyrate
`has not been determined. The discussants noted one
`published and two unpublished pregnancies where the
`offspring have been healthy, but one view was that so-
`dium benzoate might be a better option, since we all
`ingest benzoate in soft drinks and other common foods,
`whereas sodium phenylbutyrate affects many metabolic
`and other functions. Another safety issue mentioned in
`discussion was the danger of phenylbutyrate tablets
`lodging in the esophagus and causing serious tissue
`damage. Patients must be instructed how to ensure
`complete swallowing.
`Another aspect of long-term therapy that was dis-
`cussed was the influence of menses on the exacerba-
`tion of hyperammonemia. Although there are no good
`published studies, there seems no doubt from indi-
`vidual cases that menses can adversely affect control in
`some patients. The mechanism is not clear but it was
`postulated that cytokine release might be involved.
`The menstrual periods were suppressed by both es-
`trogen and progesterone separately in one patient, and
`both strategies led to marked improvement.
`
`Long-term correction
`
`The consensus document recommends consideration
`of liver transplantation for severe CPS and OTC defi-
`ciency patients, and for others whose disease is not well
`controlled by medical means. This is dealt with else-
`where in this publication.
`
`Whom to treat?
`
`There are issues regarding treatment for patients with
`less than the severe phenotype, or those detected when
`still asymptomatic, either through family studies or by
`newborn screening. This aspect was not covered in the
`consensus document, but attracted quite a lot of dis-
`cussion. Asymptomatic neonates known to be OTC
`carriers may or may not need treatment. In New South
`Wales we think it wise to avoid any, even mild, hyper-
`ammonemia in the first two years of life, when the brain
`is growing rapidly, and accordingly we offer mild pro-
`tein restriction and some oral arginine to known affected
`baby girls. During the discussion it was pointed out that
`only 15% of carrier females ever have symptoms, and
`almost all who do will have a severe mutation. However,
`
`mutation analysis is not useful for decisions about
`treatment because of the inherent mosaicism. It was
`thought helpful to measure glutamine and other ami-
`no acids, ammonia, and urinary orotic acid at a time
`of catabolic stress. So far there are no normative data
`for 15N studies in babies, so this approach to decision-
`making about
`the need for
`treatment
`is not yet
`available.
`Newborn screening by tandem mass spectrometry
`makes possible the diagnosis of UCDs in neonates, some
`of whom may be asymptomatic, which opens up new
`problems. In New South Wales, 10 patients with UCDs
`have been born during our tandem mass spectrometry
`screening program [21]. Four were known to be affected
`with OTC, in two of whom both treatment and the
`screening test were refused. The other two had levels of
`citrulline below our low cut-off level (i.e., a positive test
`result). One patient with severe ASL deficiency pre-
`sented symptomatically before the screening test result
`was known, and one girl with OTC deficiency presented
`clinically at 5 months, but was not detected by screen-
`ing. The remaining patients, all detected by newborn
`screening, had mild ASL deficiency, mild citrullinemia
`type I, citrullinemia type II, and apparent CPS defi-
`ciency (not yet confirmed), and all are receiving appro-
`priate treatment or monitoring. The boy with mild ASL
`deficiency was quite well when first seen at 14 days of
`age, but had an elevated plasma glutamine level of
`1313 lmol/L and a confirmed ammonia of 167 lmol/L.
`It could be that chronic mild hyperammonemia in the
`first year of life causes intellectual deficits. This boy has
`normal development at the age of 4 years, on protein
`restriction and arginine supplementation. None of our
`other mild but late-detected ASL patients had normal
`development at that age. It is too early yet to say what
`will be the overall effect of newborn screening for UCDs,
`and the sensitivity for detecting CPS and OTC defi-
`ciencies is not yet clear.
`
`Summary
`There are still many unsolved problems in the
`management of UCDs, and the small number of
`patients makes appropriate investigations difficult, but
`not impossible. Some of the most pressing problems
`are the optimization of medical treatment regimens
`and determination of the best methods of monitoring
`patients, including prediction of decompensations. In
`the longer term, advances in liver transplantation
`and hepatocyte therapy offer the best hope for UCD
`patients.
`
`Acknowledgments
`
`Thanks are due to all the participants, but especially
`to the major discussants: Mark Batshaw (chairman),
`
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`S91
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`Mendel Tuchman, Claude Bachmann, Stephen Ceder-
`baum, and James Leonard.
`
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