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Urea cycle, asa in kids 09023.pdf


(also known as argininosuccinic acidemia)

This protocol is for the sick infant/child who has been previously diagnosed with either
argininosuccinic acidemia. Hyperammonemic crises in children with urea cycle defects
(UCDs) are medical emergencies and must be treated as such to avoid death or serious
brain injury.
Each of the five bioche mical reactions within the urea cycle is associated with a known
enzyme deficiency and a related clinical disorder as shown in the diagram below
Ammonia + Bicarbonate and Phosphate
Carbamylphosphate synthetase (defective in CPS deficiency)
Ornithine transcarbamylase (defective in OTC deficiency)
Argininosuccinate synthetase
Argininosuccinate lyase (defective in ASL deficiency -
Carbamyl phosphate synthetase (CPS) and ornithine transcarbamylase (OTC) are located in
the mitochondria. Arginase, argininosuccinate synthetase (ASS) and argininosuccinic acid
lyase (ASL), also known as argininosuccinase, are cytosolic in location. The major site of
complete urea cycle activity is the hepatocyte. Argininosuccinic acidemia is autosomal
recessive in inheritance; males and females are equally affected.
Unlike fats and carbohydrates, the body does not store protein. Excess protein is
catabolized, releasing liberated nitrogen as ammonia (NH3). This additional NH3 cannot be
metabolized by a defective urea cycle and so accumulates. In general, protein overload
comes from either dietary protein intake beyond bodily requirements or secondary to
catabolic processes, e.g. stresses of the newborn period, infection, dehydration etc.

Raised ammonia levels appear to be extremely toxic to the central nervous system, causing
cerebral edema. It is not clear whether this is a primary effect and/or secondary to elevated
glutamine (GLN) which, containing two nitrogenous moieties, functions as a temporary
“repository” for ammonia. GLN thus accumulates in excessive quantities in affected
untreated individuals, as does alanine (ALA) in the plasma. Amino acid abnormalities may
precede hyperammonemia and the onset of symptoms.

• Lethargy
• Coma Apart from arginase deficiency, which usually presents neurologically rather than as a hyperammonemic syndrome, the other urea cycle defects often present in the newborn period with catastrophic hyperammonemia, hepatomegaly, seizures and coma secondary to cerebral edema. Typically OTC and CPS have the most severe presentation but citrullinemia and argininosuccinic acidemia may also present with severe illness. However, all the UCD disorders may present later in life with a severe acute onset or a more chronic course.

Assess for cardiorespiratory instability, dehydration, fever, infection or any other physical
stressor (e.g. surgery), as a potential precipitant for metabolic decompensation. Assess
hepatic and neurological status.
Blood glucose
Electrolytes, CO2 and blood gas
Ammonia (1.5 ml blood in sodium-heparin tube sent STAT to lab on ice)
Plasma amino acids
LFTs (AST,ALT,AlkPO4, bilirubin)
Plasma ammonia is a direct index of toxicity, important for acute management.
A level greater than 250 µg/dl (150 µmol/L), typically with the absence of metabolic
acidosis (though may occur secondary to a primary respiratory alkalosis).
Plasma amino acids should be drawn first thing in the morning , calling the metabolic lab in
advance for urgent samples. Glutamine acts as an ammonia buffer and reflects the direction
of control of hyperammonemia. It is therefore essential that amino acids are checked daily
in the acutely sick child with hyperammonemia secondary to a urea cycle defect.
An infant/child at risk from a urea cycle disorder should be treated prospectively. The
rationale of treatment includes –
1. Minimize protein intake. 2. Reverse or minimize catabolism. 3. Promote waste nitrogen excretion.
In argininosuccinic acide mia,
the infant can start with 0.6 grams/kg/day on day 1, using a regular formula. The
administered protein is gradually increased to a maximum of 1.5-2.0 grams/kg/day.
Supplemental calories are provided as Mead- Johnson 80056 formula or equivalent.
Enteral feeds should be started as soon as practical, may even occur concomitant with IV
via NG or NJ tube if necessary. Essential amino acids should not be withheld > 24 hours,
to avoid catabolic breakdown of endogenous proteins. To avoid excess amino acid load
aim for 1.0 - 1.5g protein/kg body weight (50% as essential amino acids). Contact the
metabolic nutritionist (and discuss with the parent) before starting oral diet such as Mead
Johnson 80056 or Ross ProPhree.
Once patient stabilized, feedings established and the ammonia not fluctuating may switch to
oral UCD medications.

The caloric intake for these infants should run at least 120-130 kcal/g/day. Accurate
records of intake and output should be kept to monitor hydration. Infection as a potential
but severe catabolic stressor should be considered early (when clinical signs are apparent)
and managed vigorously. Avoid valproic acid, as it decreases urea cycle function and
accentuates hyperammonemia.
To help facilitate the excretion of waste nitrogen, the following medications are employed. Sodium benzoate – conjugates with glycine to form hippuric acid which
bypasses the urea cycle and is excreted in urine.
Sodium phenylacetate – conjugates with glutamine to form
phenylacetylglutamine which bypasses the urea cycle and is excreted in the
Arginine – to prevent ARG deficiency and prime any residual OTC activity
but must NOT BE used in arginase deficiency where there is already an
excess of arginine.
Avoid carnitine as it has not been shown to be helpful. Although UCD infants are often low in carnitine, it is known to conjugate with sodium benzoate. Also avoid citrulline as it will will further exacerbate citrullinemia and ASA in which there already is an excess of citrulline If an IV is required, that solution should NOT contain sodium as plenty will be provided by the sodium benzoate and sodium phenylacetate.

If the blood ammonia is > 100 – 125 ug/dl (60-75 µmol/L), repeat the level. If confirmed: discontinue oral feedings and oral medication administer a 10% (or higher) glucose solution and Intralipid. administer the urea cycle medications as an IV bolus.

Sodium benzoate (250 mg/kg/day or 5.5g/m2) Sodium phenylacetate (250 mg/kg/day or 5.5g/m2 ) 10% Arginine HC1 (600 mg/kg/day) (Surface area for the benzoate and phenylacetate should provide a more accurate dose in
adolescents and adults)
Mix this in 35 cc/kg of 10% dextrose (no sodium) and run as a bolus over 90 minutes. This
is then followed by the same solution administered as a 24 hour infusion.

• These infusions should begin during acute illness regardless of the amount of oral UCD
medication already provided. Monitor ammonia levels every 4 hours, amino acids daily. Electrolytes, acid-base status and the anion gap should be monitored regularly. If another IV is required, that solution should not contain sodium. • Glucose levels should be kept between 120-170 mg/dl. If necessary for control of hyperglycemia can use insulin (remains controversial) bearing in mind that wide swings in glucose levels affect brain osmolarity. • Cerebral edema; Oncotic agents such as albumin will increase the overall nitrogen load but may in selected cases be considered. Mannitol has not been found to be helpful for edema secondary to hyperammonemia and steroids should not be used. Hyperventilation is recommended, but only under close appropriate supervision.
Potential side effects of sodium benzoate/phenylacetate regime
Increased incidence of nausea and vomiting with bolus.
Overdoses (3-5x recommended dose) can lead to symptoms reminiscent of
hyperammonemia, specifically agitation, confusion and hyperventilation. Death has
occurred (associated with cerebral edema, hypotension and cardiovascular collapse)
If the ammonia continues to rise >200-250 µg/dl (120-150 µ mol/L)
Suggest transfer to PICU with metabolic and hemodialysis facilities and alert pediatric
nephrology team. Remember placement of access lines for dialysis takes time so do not
If dialysis is not immediately available, give a loading dose of sodium
benzoate/phenylacetate, to slightly retard ammonia rise and in anticipation of dialysis
If the ammonia continues to rise >300 µg/dl (175 µmol/L) CONSIDER DIALYSIS
Dialysis will clear ammonia at :-
170-200ml/min for ECMO based dialysis. Osmotic shifts have NOT been observed with
this rapid rate of clearance. Additionally a hemofilter in the circuit will continue to remove
ammonia between dialysis cycles.
10-30 ml/min hemodialysis
3-5 ml/min peritoneal dialysis (this rate will however take several days to significantly
reduce the ammonia load, at a time when brain damage is related to duration of
hyperammonemia toxicity)
*note that dialysis itself is associated with significant morbidity/mortality, particularly in
the neonate, and decisions to consider using dialysis must balance the risk:benefit ratio for
each child.
As ammonia falls below 125-150mg/dl (60-75µmol/L) and clinical status returns to
Can switch to oral medications and gradual reintroduction of diet in conjunction with the
metabolic dietician as described above (in section “therapy”) . The use of oral sodium
benzoate and sodium phenylbutyrate (the much less odiferous oral form of sodium
phenylacetate) is determined, dependent on the patient, either on body weight or body
surface area. The dose should be decided in conjunction with a metabolic physician if the
patient does not have an up to date regimen.
NOTE that there may be a rebound hyperammonemia initially with the efflux of
intracellular ammonia into the ‘relatively’ ammonia depleted blood. THUS it is important
to continue closely monitoring ammonia levels until they remain stable in the normal range.
Adapted from
Proceedings of a consensus conference for the management of patients with Urea Cycle
disorders. J Peds. Suppl. Vol. 138 (1), 2001
This protocol should be used ONLY in conjunction with metabolic consultation. For this
please call or have paged the Genetics/Metabolism Fellow-on-call or, failing this, the
Metabolic attending on call at your hospital or nearest pediatric tertiary care center (click on
“metabolic consultation” at the top of the page to find local contact information [ in the
New England area ] ).


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