Hyperammonaemia

Question 1

Desribe the production and metabolism of ammonia?

Answer 1

• Ammonia is produced from deamination of amino acids in the liver, muscle and kidney and by the action of gut bacteria.
• Blood in hepatic portal vein is very high in ammonia (up to 20-fold systemic concentrations). Liver has critical role in metabolism and detoxification of ammonia.
• In the liver ammonia is converted to urea, via the urea cycle resulting in subsequent excretion by the kidneys.

Question 2

How does pH affect amonia?

Answer 2

• At physiological pH, 95% ammonia is in the ammonium ion form (NH4+) which is less permeable to cell membranes.
• When pH rises, equilibrium shifts in favour of ammonia (NH3), entry into cells and toxicity is enhanced.

Question 3

What are enzymes involved in the urea cycle?

Answer 3

1. Carbamoyl phosphate synthetase
2. Ornithine transcarbamoylase
3. Argininosuccinate synthetase
4. Argininosuccinate lyase
5. Arginase
6. N-acetylglutamate synthetase

Question 4

How is Cabomoyl Phosphate formed?

Answer 4

Carbamoyl phosphate is formed from bicarbonate and ammonia. It can be converted to either Citrulline or Orotic acid.

• Orotic Acid is a Pyrimidine precursor
• Citrulline be formed from ornithine and carbamoyl phosphate with the influence of ornithine transcarbomoylase. Citrulline can leave mitochondria to form ornithine which can re-enter the mitochondria to be substrate for OTC.

Question 5

What do changes in Orotic Acid tell us?

Answer 5

Orotic acid is an extra marker for lesions in the point of entry.

• If it isn’t increased, there is hyperammonamenia and there is no increase in the rest of the amino acids within the circle then its most likely N-acetyl Glutamate and Carbomoyl phosphate affected. NGS is used to investigate.
• Orotic acids is particularly important in OTC disorders and in cases where Carbamoyl Phosphate is affected as it isn’t routinely measured.

Question 6

When are NAGS disorders suspected?

Answer 6

NAGS and Carbomyl Phosphate disorder suspected when:

• Hyperammanaemia where none of the things within the circle and orotic acid isn’t increased.

Next-gen sequencing should be used for NAGS disorders

Question 7

What are reference ranges for Plasma Ammonia?

Answer 7

• Premature Neonate: < 150 µmol/L
• Term Neonate: < 100 µmol/L
• Infant/ Child: < 40 µmol/L

Question 8

How can ammonia be toxic?

Answer 8

• Ammonia increases the transport of tryptophan across the blood-brain barrier. This leads to an increased production and release of serotonin.
• Ammonia also increases synthesis of glutamate and increases intracellular osmolality leading to brain swelling
• Ammonia induces many other electrophysiological, vascular and biochemical changes in experimental systems
• Unknown which factors are most harmful in man

Question 9

What are causes of Hyperamonaemia?

Answer 9

• Artefactual increases
• Transient Hyperammonaemia of the Newborn

Question 10

What are causes of Arterfactual increases in Ammonia?

Answer 10

• Poor specimen quality/ haemolysis
• Difficult venepuncture
• Skin contamination
• Contaminated sample tube
• Delayed analysis

Question 11

What should be done in samples where there is delayed analysis?

Answer 11

• But beware of rejecting samples, unlikely for factitious ammonia to be confused with serious hyperammonaemia.
• Ammonias greater than 350 may constitute a medical emergency

Question 12

What are features of Transient Hyperammonaemia of the Newborn?

Answer 12

• Life-threatening illness presenting in first 48 hours of life
• More likely to occur in pre-term baby – usually as respiratory distress within 24 hours of birth
• Outcome good if treated promptly and aggressively
• Presentation may be indistinguishable from inherited metabolic diseases.

Question 13

How do defects of the urea cucle cause hyperammonaemia?

Answer 13

• The urea cycle is the final common pathway for the excretion of waste nitrogen.
• Plasma ammonia can be raised as a result of metabolic blocks in the urea-cycle.
• Degree of elevation depends on enzyme involved and its residual activity, protein intake and rate of endogenous protein catabolism (particularly if increased because of infection, fever or other metabolic stresses)

Question 14

What is the genetic factos in Urea Cycle defects?

Answer 14

• Most common disorder is OTC deficiency (prevalence approx. 1 in 40,000) X-linked.
• All other conditions have autosomal recessive inheritance.

Question 15

What are some examples of rare urea cycle disorders?

Answer 15

• HHH syndrome (hyperammonaemia, hyperornithinaemia, homocitrullinaemia): Defect of ornithine carrier that imports ornithine into mitochondrion. Reduces flux into urea cycle
• Citrin Deficiency (Citrullinaemia Type II): Citrin is a mitochondrial aspartate-glutamate carrier – both of which are key components in the urea cycle.
• Lysinuric Protein Intolerance: Dibasic amino acid transport defect. Decreased uptake of ornithine, lysine, arginine from gut and increased losses from renal tubule. Reduces metabolite availability for urea cycle
• Carbonic anhydrase deficiency, CA-VA: Mitochondrial enzyme that enables synthesis of bicarbonate as substrate for Carbamoyl phosphate synthetase (CPS). CAVA also supplies HCO3 to three other mitochondrial enzymes, pyruvate carboxylase (gluconeogenesis), propionyl carboxylase and 3methyl crotonyl carboxylase (BCAA catabolism). Hence lactataemia and ketonaemia and hypoglycaemia

Question 16

What are other metabolic disorders associated with Hyperammonaemia?

Answer 16

Organic Acidurias

• Mechanism – accumulation of CoA derivatives of organic acids, inhibit formation of N-acetylglutamate which is required for urea cycle

Disorders of Fatty Acid Oxidation

• Some patients have modest hyperammonaemia secondary to hepatic dysfunction.

Pyruvate Dehydrogenase Deficiency

• Reduction in acetyl CoA inhibits formation of N-acetylglutamate which is required for urea cycle

Hyperinsulinaemic hyperammonaemia (glutamate dehydrogenase gene defect causing up regulation)

Question 17

How does Hyperammonaemia present clinically in babies?

Answer 17

• Most babies are normal birthweight and initially healthy.
• After a short interval (can be <24 hours, usually 2-3 days), they become unwell.
• However, initial working diagnosis is usually sepsis

Question 18

What are symptoms of Hyperammonaemia in babies?

Answer 18

Early Symptoms

• Poor Feeding
• Vomiting
• Lethargy and/or irritability
• Tachypnoea. May present with respiratory alkalosis as a result of increased breathing due to increased ammonia

Severe Symptoms

• Untreated hyperammonaemia leads to neurological damage and death (can cause changes of tone with loss of normal reflexes, hypothermia, apnoea, fits)

Question 19

What are some infantile presentations of Hyperammonaemia?

Answer 19

Symptoms generally less acute and more variable than in the neonatal period

• Anorexia
• Lethargy
• Vomiting
• Failure to thrive with poor developmental progress
• Irritability and behavioural problems.

Correct diagnosis may only be established when encephalopathy develops with changes in consciousness and neurological signs.

Question 20

What is the presentation of Hyperammonaemia within Children and Adults?

Answer 20

• Most commonly associated with females with OTC
• At these ages, patients commonly present with a more obvious neurological illness.
• Symptoms may be precipitated by an event – e.g. protein load (dietary or catabolic).
• More common in the teenage years or post-partum. May occur outside of these times

Question 21

What should be done when checking ammonia?

Answer 21

• Commonest cause of a mildly raised plasma ammonia is contamination or sample deterioration.
• All elevated ammonia results should be confirmed on a second sample to exclude artefactual increases.
• Ammonia should be measured on a free-flowing venous sample or arterial stab – capillary samples should be avoided.

Question 22

How should ammonia be checked during in hyperammonaemias?

Answer 22

• Commonest cause of a mildly raised plasma ammonia is contamination or sample deterioration. All elevated ammonia results should be confirmed on a second sample to exclude artefactual increases.
• Ammonia should be measured on a free-flowing venous sample or arterial stab – capillary samples should be avoided.
• Samples should be sent to the lab within 30 minutes, ideally on ice.
• Do not reject results unless way outside time but report the result with a. warning of when how late a sample was. Report in case factitious ammonia is confused with serious hyperammonaemia.
• Ammonias greater than 350 may constitute a medical emergency

Question 23

What are some cut offs that determine treatment in Hyperammonaemia?

Answer 23

• Levels <200 µmol/L are more likely to be acquired.
• Levels >200 µmol/L suggest a metabolic cause.
• Levels >350 umol/L may be metabolic emergency
• Levels >500 umol/L if persistent, may require dialysis

Question 24

What are first line blood investigations for Hyperammonaemia?

Answer 24

• Urea and Electrolytes: Urea may be inappropriately low compared to other markers of renal function/dehydration in urea cycle disorder.
• Liver Function Tests: Severely deranged in some acquired causes of hyperammonaemia. May be mild elevations in urea cycle defects and organic acidurias.
• Clotting Studies
• Glucose: Hypoglycaemia may occur in e.g. fatty acid oxidation disorders, hyperinsulinism and liver failure.
• Lactate: May be raised in a number of metabolic conditions and also liver failure.
• Calcium: Hypocalcaemia is a feature of some organic acid disorders.

Question 25

What are first line Urine investigations for Hyperammonaemia?

Answer 25

• Ketones: Differentiate organic acid disorders (increased) from fatty acid oxidation disorders and liver failure (not present)

Results of these investigations may support an acquired cause or provide evidence for an underlying metabolic condition.

Question 26

What are some blood gas investigations used in Hyperammonaemia?

Answer 26

• Ammonia stimulates the respiratory centre in the medulla – therefore hyperammonaemia causes a respiratory alkalosis.
• The presence of a metabolic acidosis may suggest an organic acid disorder or fatty acid oxidation defect.

Question 27

What are some specialist investigations for Hyperammonaemia?

Answer 27

To be performed where a metabolic cause of hyperammonaemia is suspected:

• Urine and plasma amino acids
• Urine organic acids
• Urine orotic acid
• Blood spot or plasma acylcarnitines

Question 28

How can specialist investigations for hyperammonaemia be investigated?

Answer 28

• Urine orotic acid and plasma amino acids are useful in the differential diagnosis of urea cycle defects.
• Citrullinaemia, arginosuccinic aciduria and argininaemia can all be diagnosed on the basis of amino acid results.
• Glutamine, alanine and asparagine may be non-specifically increased in all urea cycle disorders.
• Orotic acid is excreted in excess in urine if there is a metabolic block distal to the formation of carbamoyl phosphate, as is the case in OTC deficiency, citrullinaemia, argininosuccinic aciduria and arginase deficiency.
• Urine organic acid analysis is diagnostic of organic acid disorders, each of which results in excretion of characteristic metabolites.
• Acyl carnitines are diagnostic of fatty acid oxidation disorders which are differentiated according to the pattern of elevated acylcarnitines. Some acyl carnitines and other carnitine conjugates are elevated in organic acid disorders.

Question 29

What is the emergency management of Hyperammonaemia?

Answer 29

• Protein intake should be stopped.
• Calories should be supplied by giving hypertonic glucose. Can also give insulin
• Ammonia levels >500 µmol/l at presentation or failure to lower ammonia significantly within 4 hours can be an indication for dialysis.
• Intravenous infusion of benzoate (conjugates with glycine) and phenylacetate (conjugates with glutamine). Provide alternative pathways for nitrogen excretion
• Intravenous arginine (improves flux through the urea cycle)
• CarbaGlu –NAGS analogue (stimulates CPS)

Question 30

What is the prognosis for patients with Hyperammonaemia?

Answer 30

• Better outcomes rely on early recognition and diagnosis of hyperammonaemia.
• Good developmental outcome if initial ammonia levels reduced to less than 250µmol/l and/or hyperammonaemic coma resolved within 48 hours of the onset of symptoms.
• Aggressive treatment with full intensive care support and intravenous therapy with sodium benzoate and/or sodium phenylbutyrate/ phenylacetate and arginine recommended