TRANSIENT HYPERAMMONEMIA OF THE NEWBORN (THAN)
 
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TRANSIENT HYPERAMMONEMIA OF THE NEWBORN (THAN)

 

DEFINITION:

A disorder of unknown etiology characterized by transient hyperammonemia in the newborn period.

EPIDEMIOLOGY:

  • incidence: rare
  • age of onset:
    • newborn
  • risk factors:
    • premature infants with low birth weight
    • M > F (3.5:1)

PATHOGENESIS:

1. Background

  • etiology is unknown and is considered a diagnosis of exclusion (of urea cycle defects and organic acidopathies)
  • first described by Ballard et al., NEJM 299(17):920 (1978)
  • clinical differences between THAN and Urea Cycle Disorders:
    • gestational age <36 weeks
    • birth weight <2.5 kg
    • respiratory distress within 24 hours of birth
    • coma within 48 hours of birth
    • initial ammonia >1500 uM
    • abnormal chest x-ray (RDS-1)
  • the earlier onset of symptoms in THAN is due to a more rapid rise in ammonia levels
  • THAN can also occur in term infants

CLINICAL FEATURES:

1. Mild Transient Hyperammonemia (40-50 uM)

  • asymptomatic
  • hyperammonemia lasts 6-8 weeks
  • no neurological sequelae

2. Severe Transient Hyperammonemia (as high as 4000 uM)

1. Neurolgical Manifestations

  • lethargy -> coma
  • infantile hypotonia
  • neonatal seizures

2. Respiratory Manifestations

  • respiratory distress syndrome (RDS-1)

INVESTIGATIONS:

1. Serum

  • venous hyperammonemia (1800-4000 uM)
  • no respiratory alkalosis
  • amino acids:
    • slightly elevated citrulline and arginine
    • other urea cycle intermediate amino acids are normal
    • elevated glutamine and alanine

2. Chest X-Ray

  • RDS-1

MANAGEMENT:

1. Goal of Therapy

  • symptomatic control of initial episode

2. Diet

1. Protein Restriction

1. Exogenous

  • IV D10W
  • avoid amino acids in TPN

2. Endogenous

  • High Caloric Diet
    • use to minimize tissue catabolism and thus the breakdown of endogenous protein

3. Convert Nitrogen to an Excretable Compound

1. Dialysis

  • Peritoneal or Hemodialysis

2. Sodium Benzoate

  • conjugates with glycine and excreted as hippuric acid

3. Sodium Phenylacetate

  • conjugates with glutamine and excreted as phenylacetyl-glutamine

4. Prognosis

  • 30% mortality rate
  • there is a direct correlation between the duration of hyperammonemic coma and morbidity (mental retardation, developmental delays, cortical atrophy)
  • there is no risk of hyperammonemic episodes after the initial episode and the patients may eat a diet normal in protein

 

 

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