Eur J Pediatr (2011) 170:21–34
`DOI 10.1007/s00431-010-1369-2
`
`REVIEW
`
`Clinical practice
`The management of hyperammonemia
`
`Johannes Häberle
`
`Received: 20 October 2010 / Revised: 22 November 2010 / Accepted: 24 November 2010 / Published online: 17 December 2010
`# Springer-Verlag 2010
`
`Abstract Hyperammonemia is a life-threatening condition
`which can affect patients at any age. Elevations of ammonia
`in plasma indicate its increased production and/or decreased
`detoxification. The hepatic urea cycle is the main pathway
`to detoxify ammonia; it can be defective due to an inherited
`enzyme deficiency or secondary to accumulated toxic
`metabolites or substrate depletion. Clinical signs and
`symptoms in hyperammonemia are unspecific but they are
`mostly neurological. Thus, in any unexplained change in
`consciousness or
`in any unexplained encephalopathy,
`hyperammonemia must be excluded as fast as possible.
`Any delay in recognition and start of treatment of hyper-
`ammonemia may have deleterious consequences for the
`patient. Treatment largely depends on the underlying cause
`but
`is, at
`least
`in pediatric patients, mainly aimed at
`establishing anabolism to avoid endogenous protein break-
`down and amino acid imbalances. In addition, pharmaco-
`logical
`treatment options exist
`to improve urea cycle
`function or to remove nitrogen, but their use depend on
`the underlying disorder. To improve the prognosis of acute
`hyperammonemia, an increased awareness of this condition
`is probably more needed than anything else. Likewise, the
`immediate start of appropriate therapy is of utmost
`importance. This review focuses on a better understanding
`of factors leading to ammonia elevations and on practical
`aspects related to diagnosis and treatment
`in order to
`improve clinical management of hyperammonemia.
`
`J. Häberle (*)
`Kinderspital Zurich, Division of Metabolism,
`University Children’s Hospital Zurich,
`Steinwiesstrasse 75,
`8032 Zurich, Switzerland
`e-mail: Johannes.Haeberle@kispi.uzh.ch
`
`Keywords Ammonia . Glutamine . Urea cycle . Nitrogen
`metabolism . Awareness . Neurotoxicity . Cerebral edema
`
`Introduction
`
`Hyperammonemic disorders not due to general liver failure
`are rare and the symptoms non-specific. The clinical
`presentation varies depending on age of the patient and on
`type and severity of the underlying disorder. In all age
`groups, loss of appetite and then vomiting are early and
`reversible findings if treated. In newborns, first symptoms
`are poor feeding, vomiting, seizures, unstable body tem-
`perature,
`respiratory distress or poor peripheral blood
`circulation leading to an initial suspicion of intracranial
`bleeding, septicemia or meningitis;
`in infants, vomiting
`may evoke pyloric stenosis, cow milk intolerance or
`infectious enteritis;
`in children, adolescents and adults,
`vomiting, ataxia, confusion, disorientation, hallucinations
`or abnormal behavior point to central nervous system or
`psychiatric disorders. In all age groups,
`the change of
`consciousness should shift
`the search to intoxications,
`encephalitis or metabolic disorders. Since more common
`disorders are considered first, valuable time is often lost
`when hyperammonemia has already reached levels above
`400–500 μmol/L thus increasing the risk of irreversible
`brain damage, of neurodevelopmental retardation or even
`death.
`The first goal of this review is to stimulate the reader to
`consider the rare metabolic disorders in presence of non-
`specific symptoms in order to rule out hyperammonemia
`and—if present—to prevent
`irreversible damage to the
`brain by timely action. The second goal is to outline the
`principles of the available treatments and necessary controls
`in order to empower the pediatrician to follow and guide
`
`Horizon Exhibit 2007
`Lupin v. Horizon
`IPR2018-00459
`
`1 of 14
`
`

`

`22
`
`Eur J Pediatr (2011) 170:21–34
`
`the patient when the disorder has been diagnosed. An
`increased awareness of all medical professionals towards
`the possibility of hyperammonemia is needed for an
`improved prognosis of affected patients.
`
`Background
`
`Physiology
`
`About 90% of nitrogen-containing compounds are normally
`excreted as urea. They originate from the obligate oxidation
`of amino acids and from excess waste nitrogen, mainly
`from amino acids not used for protein neo-synthesis, from
`cell turnover of hydrolyzed body protein and from protein
`intake. The bulk of protein mass is in the skeletal and
`visceral musculature. The rate of protein breakdown is
`relatively constant while the synthesis is regulated e.g. by
`hormones, cytokines and available substrates. The capacity
`of urea formation adapts normally within a few days to
`changes of the amount of protein intake.
`Ammonia (NH3) is a constituent of all human body
`fluids and at neutral pH present mostly (>98%) in its
`+) [8]. This is physiologi-
`ionized form, ammonium (NH4
`cally advantageous because ammonium, in comparison to
`ammonia, barely permeates cell membranes. Mainly for
`convention, “ammonia” is used in this review although
`“ammonium” would be the correct biochemical term. The
`concentration of ammonia in human plasma is micromolar
`and varies in venous, arterial or capillary blood as well as
`depending on the time and mode of sampling (see below).
`Tissue ammonia concentrations are higher and ammonia is
`trapped as ammonium in compartments with lower pH such
`as in lysosomes and renal tubules [3]. Plasma ammonia
`concentrations depend on the age of the patient and assay
`method used but
`it should be noted that well-defined
`reference limits for ammonia do not exist (limits for use in
`clinical practice are depicted in Table 1 [18]).
`Hyperammonemia indicates an elevation of ammonia in
`blood and tissues by its increased production and/or
`decreased detoxification and is a strong indicator of
`
`Table 1 Plasma ammonia concentrations depending on age of
`patients (adapted from [18])
`
`Newborns (arterial cord blood)
`Infants and children
`Adults (female)
`Adults (male)
`
`μmol/l
`
`50–159
`24–48
`11–48
`15–55
`
`μg/dl
`
`85–271
`41–82
`19–82
`26–94
`
`Conversion μg/dl×0.5872=μmol/l. The levels given are decision
`limits which should be interpreted together with the clinical situation
`
`abnormal nitrogen homeostasis. Since in clinical practice
`ammonia can be determined very fast but also because of
`the associated neurotoxicity, ammonia belongs to the core
`parameters of metabolic medicine together with blood
`gases, glucose, lactate and ketone bodies. However, the
`clinical condition of a patient should guide the management
`rather than solely ammonia concentrations because they can
`be fluctuating and may not entirely correlate with already
`impaired brain function.
`In mammals, skeletal muscle and intestinal mucosa are
`mainly responsible for ammonia production (Fig. 1a).
`Many of the reactions of amino acid metabolism take place
`in skeletal muscle, where protein is broken down and where
`single amino acids are transaminated for new protein
`synthesis or to form glutamine from glutamate and/or
`alanine from pyruvate [28]. Glutamine is not only the most
`abundant amino acid in the human organism, the temporary
`storage form of waste nitrogen and the main transport form
`of amino groups between organs but also a major source of
`ammonia if deaminated by glutaminase [48]. Also in
`skeletal muscle, deamination of adenosine monophosphate,
`particularly during physical exercise, results in ammonia
`production. In intestinal mucosa, ammonia is produced after
`uptake of amino acids as a result of glutamine deamination.
`In colon and bladder, microorganisms expressing enzymes
`enabling protein and urea degradation, respectively, can
`lead to hyperammonemia [40, 41, 66, 67]. About 25% of
`endogenous ammonia is derived from intestinal production
`[46, 63].
`For the final transformation of glutamine/glutamate to
`ammonia and for detoxification of the portal ammonia and
`export of ammonia, the liver and to a certain extent the kidney
`play the central role. In liver, the urea cycle is located in
`periportal hepatocytes and provides a high capacity for
`detoxification of the vast amount of surplus nitrogen while
`glutamine synthetase, expressed only in perivenous hepato-
`cytes, serves as back-up system with high affinity (but low
`capacity) to ammonia (Fig. 1b) [36]. Accordingly, hyper-
`ammonemia can occur in many acquired and inherited
`hepatic disorders. In kidney, ammonia is formed from
`glutamine deamination. However, renal ammonia production
`mainly contributes to buffering H+ ions while excretion of
`ammonia in urine plays only a minor role in overall
`ammonia detoxification (Fig. 1a and b show the key players
`of ammonia production and detoxification).
`
`Practical points
`➢ Ammonia is neurotoxic itself and indicates increased ammonia
`production or decreased ammonia detoxification
`➢ Ammonia determination is crucial for many metabolic disorders
`➢ The neurological condition of the patient should guide the clinical
`management rather than ammonia levels alone
`
`2 of 14
`
`

`

`23
`
`Eur J Pediatr (2011) 170:21–34
`
`Fig. 1 a, b. Illustration of the
`key players of ammonia pro-
`duction and detoxification. a
`Simplified graph showing the
`interplay of intestine and skele-
`tal muscle (organs that produce
`ammonia and glutamine) with
`liver and kidney (organs that
`detoxify ammonia). Ammonia is
`detoxified in liver while gluta-
`mine is not. b This simplified
`graph focuses on periportal and
`perivenous hepatocytes as the
`two ammonia detoxifying com-
`partments in liver. Ammonia is
`metabolized with high capacity
`but low affinity in the urea cycle
`which is solely expressed in
`periportal hepatocytes. As
`back-up, ammonia is detoxified
`by the action of glutamine syn-
`thetase that is solely expressed
`in perivenous hepatocytes and
`has a low capacity but high
`affinity towards ammonia. Urea
`and glutamine re-enter the cir-
`culation to be excreted in urine
`or further metabolized in the
`kidney, respectively. Urea cycle
`enzymes abbreviated: NAGS N-
`acetylglutamate synthase, CPS1
`carbamoylphosphate synthetase
`1, OTC ornithine transcarbamy-
`lase, ASS argininosuccinate syn-
`thetase, ASL argininosuccinate
`lyase, ARG1 arginase 1
`
`Neurotoxicity of ammonia
`
`The brain is the main organ affected by hyperammonemia
`[21]. Ammonia enters the brain mainly by diffusion, but it
`is to a lesser extent also produced by brain metabolism [47].
`A number of reversible and irreversible metabolic and
`neurotransmitter disturbances and ensuing morphologic
`changes add up to severe brain toxicity but
`the exact
`
`pathogenic mechanisms still need to be unraveled [4, 15,
`29, 31]. Depending on age as well as duration and level of
`hyperammonemia severe cerebral edema, brain stem herni-
`ation and death can result. In acute hyperammonemia,
`astrocytes are swollen as observed by microscopy. One
`important factor in this pathology is the osmotic effect of
`newly synthesized brain glutamine on astrocytes which is
`taken up together with water [13, 69] but many other
`
`3 of 14
`
`

`

`24
`
`Eur J Pediatr (2011) 170:21–34
`
`mechanisms contributing to brain toxicity of ammonia have
`been suggested and the reader is referred to recent reviews
`[4, 10, 11, 14, 44, 62].
`
`optimally prevent its rise to such levels; a rapid diagnosis
`should be reached to allow the application of more efficient
`measures [6, 43]. Otherwise, the risk of irreversible damage
`to the brain is high.
`
`Metabolic crisis
`
`Metabolic crises occur whenever the load of waste nitrogen
`exceeds the detoxification capacity. In the periportal liver
`(and to a lesser extent in the intestinal mucosa), the main
`part of ammonia from the gut is handled by urea synthesis
`[36]. All enzymes and membrane transporters needed are
`expressed in these cell systems [17, 48, 51]. In all other
`organs and cell systems including perivenous hepatocytes,
`amino groups and ammonia are detoxified by glutamine
`formation [37].
`If
`the capacity for detoxification of ammonia is
`insufficient a vicious cycle can lead to crises. This occurs
`when the increasing systemic ammonia leads to loss of
`appetite and vomiting. Rapid intervention is needed to
`avoid a further increase of ammonia,
`i.e. when protein
`synthesis is reduced and catabolism prevails like during
`postpartum physiologic weight loss in neonates, infections
`(even minor ones) or increase of nutritional protein supply
`beyond the actual needs.
`Therapeutic measures are initially non-specific in order
`to reduce hyperammonemia below 400–500 μmol/L or
`
`Biochemical basis of primary hyperammonemia
`
`In the mammalian organism, the major part of ammonia is
`detoxified by the urea cycle (Figs. 1a, b and 2). This cycle
`is fully expressed only in liver and intestinal mucosa and
`comprises six enzymatic steps of which three are intra-
`mitochondrial and three cytosolic [36]. The urea cycle has a
`second role—the synthesis of arginine—which is important
`for the treatment of hyperammonemia [12].
`A defect in one of the six urea cycle enzymes and two
`membrane transporters results in so called primary hyper-
`ammonemia, while metabolic defects outside the urea cycle
`as well as side effects of drugs can lead to secondary
`hyperammonemia. This classification is by no means purely
`academic but
`is part of any differential diagnosis of
`unexplained hyperammonemia (Table 3).
`The single most important of the urea cycle disorders
`(UCDs) is ornithine transcarbamylase (OTC) deficiency
`because it is the most common one and the only X-linked
`[57]. While male patients are often affected by severe
`neonatal metabolic decompensation, females with OTC
`
`Fig. 2 Influence of metabolic disorders on function of urea cycle
`leading to secondary hyperammonemia. Figure showing sites of action
`of various compounds on urea cycle function by either leading to
`inhibition of enzymes (NAGS or CPS1) or
`to a decrease in
`intermediate substrates (both negative actions are depicted as
`- ).
`HIHA hyperinsulinism-hyperammonemia syndrome, FAOD fatty acid
`oxidation defects, PDHCD pyruvate dehydrogenase complex disor-
`
`ders, OA organic acidemias, HHH hyperornithi nemia-
`hyperammonemia-homocitrullinuria syndrome, PC pyruvate carbox-
`ylase defect, Citrin Citrullinemia type 2, LPI
`lysinuric protein
`intolerance, P5CS pyrroline-5-carboxylate synthetase defect. The site
`of action of valproate has also been added. + depicts the stimulatory
`effect of NAG on CPS1. Figure adapted from [53]
`
`4 of 14
`
`

`

`Eur J Pediatr (2011) 170:21–34
`
`25
`
`deficiency can present with a broad clinical picture [57, 59].
`Depending on random X-inactivation, the resulting OTC
`phenotype is a continuum and affected females may remain
`asymptomatic but may also resemble hemizygous males
`[50]. All other UCDs are autosomal recessively inherited.
`
`Biochemical basis of secondary hyperammonemia
`
`Many inborn errors of metabolism result in accumulation of
`toxic products which can lead to inhibition of other
`metabolic pathways (Fig. 2). This is also the case for the
`urea cycle which can loose its ammonia detoxifying
`capacity because accumulating metabolites can impair the
`synthesis of N-acetylglutamate (NAG), the obligate activa-
`tor of CPS1. Furthermore, some organic acids (e.g.
`methylcitrate in case of propionic acidemia or methylma-
`lonic acidemia) inhibit the mitochondrial Krebs cycle and
`thus the availability of α-ketoglutarate as ammonia accep-
`tor for glutamate synthesis. In addition, deficiency of
`acetyl-CoA or glutamate in the mitochondria or of
`substrates required for normal urea cycle function can lead
`to secondary hyperammonemia (see also Table 3).
`Increased ammonia production caused by bacterial
`overgrowth can occur in bladders, uretero-sigmoid shunts
`or within the intestine. This can be seen in intestinal
`hypomotility of any cause, e.g. postoperative, in diabetic
`gastroparesis or myotonic muscular dystrophy. Besides,
`hyperammonemia can result if ammonia does not reach the
`detoxifying hepatocytes, e.g. in open Ductus venosus or
`portocaval shunting, and this might also contribute to the
`unclear phenomenon of “transient hyperammonemia of the
`newborn” (THAN). In addition, drugs can lead to hyper-
`ammonemia which was mostly reported secondary to
`valproate but other antiepileptic agents, L-asparaginase,
`furosemide and salicylic acid are also possible triggers of
`severe ammonia elevations [4, 7, 35].
`
`Key points
`➢ Primary hyperammonemia results from a defect of one of the urea
`cycle enzymes or transporters of ornithine or aspartate/glutamate
`➢ Secondary hyperammonemia is caused by a defect outside the urea
`cycle that indirectly affects urea cycle function via inhibition or
`substrate deficiency
`
`even true for “vomiting” as one of the most common signs
`of hyperammonemia at all ages which is not a pure
`abdominal sign but also a neurological sign.
`Although non-specific at all ages, the clinical presen-
`tation will now be discussed in relation to certain age
`groups.
`
`Signs and symptoms in neonates
`
`Neonates have long been regarded as the group of patients
`most affected by hyperammonemia. This is not true with
`regard to the proportion of patients beyond the neonatal
`period but still partly true in clinical practice. In primary
`and secondary defects of the urea cycle, the pregnancy and
`first days of life will be uneventful because the maternal
`urea cycle will clear off any surplus nitrogen from the fetus.
`Depending on the specific defect, postnatal catabolism can
`lead to a clinically relevant ammonia increase within days.
`In severe primary defects of the urea cycle, e.g. if the
`intramitochondrial enzymes are defective, the asymptomat-
`ic interval may be as short as 24 h. Milder variants might
`only decompensate during severe states of catabolism in
`later life. Up to 50% of urea cycle patients present with
`respiratory alkalosis. Since septicemia is the most common
`differential diagnosis in a sick neonate and is in general
`accompanied by metabolic acidosis, presence of respiratory
`alkalosis should alert the clinician to perform an immediate
`re-evaluation including ammonia determinations. Con-
`firmed septicemia does not exclude a primary hyper-
`ammonemic defect, since the catabolism associated with
`infection can provoke the manifestation of the genetic
`defect.
`
`Signs and symptoms in infants and young children
`
`Despite an uneventful postnatal period, affected infants
`and young children can manifest during any catabolic
`state. Especially in late infancy, protein anabolism is
`decreasing when postnatal growth slows down. This can
`be estimated from levels of urea production which are
`very low during rapid growth but
`increase after
`late
`infancy [13]. Any imbalance in energy demands, e.g.
`during febrile illness when nutritional intake is decreased,
`will result in endogenous protein catabolism and risk for
`hyperammonemia.
`
`Clinical signs and symptoms
`
`Signs and symptoms in older children and adults
`
`General aspects of signs and symptoms
`of hyperammonemia
`
`Since ammonia is toxic mainly to the brain, most signs and
`symptoms of hyperammonemia are neurological. This is
`
`Hyperammonemia can manifest for the first time at any age.
`Even an uneventful history with many catabolic situations but
`no signs of metabolic decompensation must not be interpreted
`as an exclusion of a primary or secondary urea cycle
`dysfunction. A very early in life self-chosen vegetarian diet
`
`5 of 14
`
`

`

`26
`
`Eur J Pediatr (2011) 170:21–34
`
`is a striking finding in many patients with hyperammonemia.
`Along this, in any unexplained neurological symptoms and
`especially in any unexplained encephalopathy hyperammo-
`nemia should be excluded at the very beginning of the
`diagnostic evaluation.
`
`Practical points
`➢ Determine plasma ammonia in all neonates with suspected
`septicemia
`➢ Be alert if respiratory alkalosis is present in neonatal septicemia
`➢ Take a good history including information regarding a self-chosen
`vegetarian diet
`➢ Be alert if loss of appetite or vomiting are accompanied by change
`in consciousness
`➢ Exclude hyperammonemia in every unexplained encephalopathy
`➢ Exclude hyperammonemia in every unexplained change in
`consciousness
`
`Signs and symptoms of acute and chronic hyperammo-
`nemia are, irrespective of the age of the patient, summa-
`rized in Table 2.
`
`Differential diagnosis
`
`The list of differential diagnoses of hyperammonemic
`disorders is long. The most
`important disorders can be
`grouped as listed in Table 3 and as shown in Fig. 2.
`
`Laboratory work-up
`
`is to suspect and to rule in or out hyper-
`The goal
`ammonemia without delay. If hyperammonemia is con-
`firmed, other laboratory investigations should be done
`including blood gases, glucose, creatinine, electrolytes,
`
`plasma acylcarnitines and amino acids, coagulation factors,
`albumin, pre-albumin, AST, ALT, CRP, as well as spot
`urine analysis for organic acids and orotic acid. For reliable
`results the blood should ideally be taken from a central
`vessel, especially in patients with poor peripheral circula-
`tion or after seizures (see below). In a comatose patient,
`preservation of EDTA-blood for later DNA analysis is
`recommended. The list of investigations should be estab-
`lished together with the specialized center to which hyper-
`ammonemic patients will be sent. All samples collected
`should be transferred together with the patient
`to the
`metabolic center taking in charge the confirmation of
`diagnosis and treatment of the patient. Valuable time can
`be gained by such a procedure.
`
`How to diagnose hyperammonemia?
`
`In the emergency situation, blood should be drawn at once for
`a rapid ammonia determination but the following preanalyt-
`ical aspects should still be taken into consideration [8, 23]:
`■ Measurement of ammonia requires free flowing
`venous or arterial blood while capillary blood is only
`useful for excluding hyperammonemia [3].
`■ Except in emergency situations, sampling should be
`done in fasting state or at least 4–6 h after a meal.
`■ Struggling of the child or physical exercise before
`the blood is taken can lead to false high ammonia
`concentrations.
`■ To prevent false high ammonia levels secondary to
`hemolysis blood must be collected with the use of an
`anticoagulant (EDTA or heparin) and preferably in a
`chilled tube.
`■ It has become practice in many hospitals to keep the
`sample on ice directly after blood is drawn. However, it
`should be noted that even short-time storage on ice can
`
`Table 2 Signs and symptoms
`of acute and chronic
`hyperammonemia
`
`Acute hyperammonemia
`
`Chronic hyperammonemia
`
`Lethargy
`Somnolence
`Coma
`Vomiting (metabolic alkalosis)
`Seizures
`Peripheral circulatory failure (metabolic acidosis)
`Cerebral edema (respiratory alkalosis)
`Liver failure
`Multiorgan failure
`Post-partum psychosis
`In neonates, sepsis-like picture
`In neonates, respiratory distress
`In neonates, hypo/hyperthermia
`
`Protein aversion and self-chosen vegetarian diet
`Headaches and migraine
`Tremor, ataxia, dysarthria, and asterixis
`Confusion, lethargy, and dizziness
`Hyperactive, aggressive, or self-injurious behavior
`Cognitive deficits and learning disabilities
`Abdominal pain and vomiting
`Failure to thrive
`Elevated liver enzymes
`Seizures
`Psychiatric symptoms
`Stroke-like episodes
`Episodic character of signs and symptoms
`
`6 of 14
`
`

`

`Eur J Pediatr (2011) 170:21–34
`
`27
`
`Table 3 Differential diagnosis of hyperammonemic disorders
`
`Primary hyperammonemia caused by defects of urea cycle enzymes or transporters
`■ N-acetylglutamate synthase deficiency
`■ Carbamoylphosphate synthetase 1 deficiency
`■ Ornithine transcarbamylase deficiency
`■ Argininosuccinate synthetase deficiency
`■ Argininosuccinate lyase deficiency
`■ Arginase 1 deficiency
`■ Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome leading to intramitochondrial ornithine deficiency
`■ Citrin deficiency (citrullinemia type 2) leading to aspartate deficiency
`Secondary hyperammonemia caused by inhibition of the urea cycle
`■ Propionic acidemia leading to glutamate deficiency
`■ Methylmalonic acidemias leading to production of methylcitrate
`■ 3-Hydroxy-3-methylglutaryl-CoA-lyase deficiency
`■ ATP deficiency of any cause leading to secondary CPS1 deficiency
`■ Valproate
`Secondary hyperammonemia caused by deficiency of substrates
`■ Fatty acid oxidation defects leading to CoA deficiency
`■ Carnitine cycle disorders leading to CoA deficiency
`■ Pyruvate dehydrogenase complex disorders leading to CoA deficiency
`■ Acute or chronic liver failure leading to CoA deficiency
`■ Lysinuric protein intolerance leading to citrulline, arginine, and ornithine deficiency
`■ Hyperinsulinism-hyperammonemia syndrome leading to glutamate deficiency
`■ Pyruvate carboxylase deficiency leading to aspartate deficiency
`■ Pyrroline-5-carboxylate synthetase deficiency leading to citrulline, arginine, and ornithine deficiency
`Liver bypass
`■ Open ductus venosus
`■ Vascular malformations resulting in portocaval shunting
`Unclear
`■ Transient hyperammonemia of the newborn
`
`lead to partial freezing resulting in hemolysis; there-
`fore, ice-water should be used instead.
`■ Plasma should be separated as soon as possible but
`certainly within 15–30 min.
`
`Practical points
`➢ Get an immediate ammonia analysis if hyperammonemia is
`suspected
`➢ Obtain free flowing blood
`➢ Use capillary blood only for exclusion of hyperammonemia
`➢ Separate plasma within 15–30 min
`➢ Use ice-water for transport of the sample instead of storage on ice
`
`Laboratory tests
`
`There are different analytical methods of ammonia
`determination including titration, colorimetric/fluorimet-
`
`ric, electrode-based and enzymatic methods [8]. It should
`be noted that the normal values given by the manufac-
`turers of some methods are unfortunately sometimes
`inaccurate. In clinical practice, a widely used method
`utilizing glutamate dehydrogenase (GLDH) determines
`ammonia concentrations by analyzing the decrease in
`absorption at 340 nm caused by oxidation of NADPH as
`shown in the following equation: α-ketoglutarate þ
`NH3 þ NADPH!GLDH glutamate þ NADP
`þ
`.
`
`Bedside tests
`
`The major advantage of bedside ammonia determination is
`to get a rapid result and thus fast information on nitrogen
`homeostasis. If bedside tests are done correctly, i.e. from
`free flowing blood drops (no massage or squeezing of heels
`and fingertips) in warm heels or fingers picked in non-
`traumatized tissue, the result will be probably reliable and
`available within 3 min. However, the result will only allow
`to exclude hyperammonemia.
`
`7 of 14
`
`

`

`28
`
`Eur J Pediatr (2011) 170:21–34
`
`In addition, there are some concerns towards the use of
`ammonia bedside tests especially outside the hospital: one
`is related to the fact
`that due to contamination by
`intracellular fluid, capillary blood ammonia levels might
`be elevated. Another problem is the limited capability to
`precisely measure high ammonia levels (the upper limit of
`ammonia bedside tests is often below 300 μmol/L) [24]
`which might lead to underestimation and thus a dangerous
`delay. Furthermore, ammonia measurements at home can,
`opposite to their intention, lead to increased stress within
`the family caused by repetitive analyses. However, if used
`in a hospital setting, ammonia bedside tests can speed up
`the time to diagnosis in emergency situations both of
`known and not yet known patients.
`
`Outlook: continuous monitoring of ammonia
`
`Analyzing ammonia continuously instead of only a few
`times a day would allow for better understanding of
`fluctuating levels and closer regulation of therapies but this
`is not yet feasible. Recently, measurement of human breath
`ammonia using a liquid-film conductivity sensor was
`reported [56], but it remains to be seen whether this or
`yet another method for continuous monitoring will come
`into clinical practice.
`
`Thus, amino acids must be controlled and interpreted during
`the emergency phase at least daily and later in the course less
`frequently but at least every 3 months.
`Of utmost importance is the concentration of glutamine
`which is less fluctuating than ammonia and most other
`amino acids and plays a central role in hyperammonemic
`neurotoxicity [1, 22]. The major part of glutamine in the
`CNS is newly synthesized within the brain while the
`transport system at the blood–brain barrier has an only
`low affinity for glutamine. At the same time, transamina-
`tions by glutaminase in endothelial cells [68] can diminish
`differences of arterio-venous glutamine concentrations even
`in presence of experimental hepatic coma. Thus, increased
`plasma glutamine levels are an indicator of the increased
`nitrogen load of the organism but are by no means a
`predictor or even precursor of brain glutamine.
`In addition, essential amino acids need to be closely
`monitored with special emphasis on branched chain amino
`acids (BCAA) [3, 54, 55, 58]. These are a major nitrogen
`source for endogenous glutamine synthesis and might
`become depleted in hyperglutaminemia. Also, the nitrogen
`scavenger drug phenylacetate is known to aggravate BCAA
`deficiency because nitrogen molecules lost by excretion of
`phenylacetylglutamine originate in relevant amounts from
`BCAA [55].
`
`What to do with slightly raised plasma ammonia
`concentrations?
`
`Neuroimaging
`
`it might be
`If plasma ammonia is only slightly raised,
`difficult
`to decide how to proceed. Considering the
`preanalytical pitfalls of ammonia determination, slight
`increases of ammonia might well be artificial. The
`following approach might be helpful in clinical practice:
`■ Repeat the plasma ammonia immediately.
`■ Find out the conditions under which the blood was
`collected, in particular, was the sample obtained after a
`meal and was the child struggling?
`■ Find out the conditions the sample was handled, in
`particular, how long was the time to analysis and
`storage?
`■ Review the clinical and family history very carefully.
`In particular, is there any evidence of previous episodes
`of encephalopathy—mild or severe?
`
`Amino acids
`
`Ammonia levels cannot replace close observation of plasma
`amino acid profiles including their careful interpretation [60].
`Therefore, in any patient with hyperammonemia amino acid
`profiles are necessary not only for differential diagnosis in
`newly diagnosed patients but also for optimal treatment.
`
`To estimate the effect of acute or chronic ammonia on the
`brain, neuroimaging can provide additional
`information.
`Unfortunately, widely available techniques such as ultrasound
`(by scanning through the open fontanel in neonates and young
`infants) and computed tomography cannot detect acute or
`moderate changes of brain morphology except very severe
`brain edema. Color duplex sonography might help to detect
`brain edema but there are no studies on the sensitivity of this
`method in hyperammonemia. However, the role of neuro-
`imaging in management of hyperammonemia has been
`evolving in recent years mainly using functional magnetic
`resonance imaging (fMRI), diffusion tensor imaging (DTI)
`and 1H/13C magnetic resonance spectroscopy (MRS) [33].
`While ammonia cannot be followed directly by MRS,
`hyperammonemia leads to a number of secondary changes.
`One is the change in glutamine and myoinositol
`levels
`detected by MRS [20, 33]. Studies have shown that both
`acute and chronic hyperammonemia result in an increase of
`glutamine and at the same time in a decrease in myoinositol
`which functions as a compensatory organic osmolyte in
`astrocytes [32, 33, 69].
`Although the role of neuroimaging needs further
`evaluation,
`it will hopefully become an important
`tool
`mainly to define the effect of subclinical hyperammonemic
`
`8 of 14
`
`

`

`Eur J Pediatr (2011) 170:21–34
`
`29
`
`episodes and of chronic moderate hyperammonemia and
`hyperglutaminemia on the brain.
`
`Management of hyperammonemic crisis
`
`Management of hyperammonemic crises is beyond the
`resources of non-specialized hospitals and transfer to a
`metabolic center is needed to allow efficient interventions.
`
`How to proceed if hyperammonemia is suspected?
`
`The result of plasma ammonia should be available within
`30-(60) min at all times. Meanwhile, protein supply should
`be stopped and glucose supplied (see below and Fig. 3).
`Any somnolent or pre-comatose patient suspected to be
`
`hyperammonemic should be transferred to a specialized
`center without delay,
`together with urine and plasma
`samples (kept on ice or frozen) and this will allow to gain
`time. The center should have the resources for ruling out or
`establishing the diagnosis within 12 h and for starting the
`specific therapy of a metabolic crisis.
`
`Practical points
`➢ Actively seek the result of plasma ammonia within 30–(60) min
`➢ Stop protein supply
`➢ Start glucose infusion±insulin (control blood lactate after 2 h)
`➢ Contact metabolic center for advice on management
`➢ Transfer any somnolent or pre-comatose patient (together with
`urine and plasma samples for further analyses) to the experienced
`center
`
`Fig. 3 Algorithm for the management of hyperammonemia in a
`newly recognized patient. Algorithm for the management of hyper-
`ammonemia in a newly recognized patient and for interpretation of
`first laborat