Overview
Transient hyperammonemia of the newborn (THAN) is a condition that occurs in some newborns, most often premature infants, where ammonia levels in the blood become dangerously high shortly after birth. Ammonia is a waste product that the body normally processes through the liver. In THAN, this process temporarily fails, leading to a buildup of ammonia that can be toxic to the brain and other organs. Unlike inherited urea cycle disorders, THAN is not caused by a permanent genetic defect — it is a temporary condition, though it can be life-threatening if not treated quickly. Symptoms usually appear within the first 24 to 72 hours of life and can include poor feeding, vomiting, rapid breathing, lethargy, seizures, and in severe cases, coma. The exact cause of THAN is not fully understood, but it is thought to be related to the immaturity of the liver's ability to handle ammonia, possibly combined with other stresses of early life such as low oxygen levels or infection. Treatment focuses on rapidly lowering ammonia levels through medications, dialysis, and supportive care in a neonatal intensive care unit (NICU). With prompt and aggressive treatment, many babies recover fully, and the condition does not recur. However, if ammonia levels remain very high for too long, there can be lasting effects on brain development. Early recognition and treatment are critical to achieving the best possible outcome.
Key symptoms:
Extreme sleepiness or difficulty waking the babyPoor feeding or refusal to eatVomitingRapid or abnormal breathingSeizuresLow body temperatureMuscle floppiness or low muscle toneComa or unresponsivenessIrritabilitySwelling of the brainLow blood pressure
Sporadic
Usually appears on its own, not inherited from a parent
Neonatal
Begins at or shortly after birth (first 4 weeks)
Treatments
No FDA-approved treatments are currently listed for Transient hyperammonemia of the newborn.
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Specialists
View all specialists →No specialists are currently listed for Transient hyperammonemia of the newborn.
Treatment Centers
8 centersBaylor College of Medicine Rare Disease Center ↗
Baylor College of Medicine
📍 Houston, TX
🏥 NORDStanford Medicine Rare Disease Center ↗
Stanford Medicine
📍 Stanford, CA
🔬 UDNNIH Clinical Center Undiagnosed Diseases Program ↗
National Institutes of Health
📍 Bethesda, MD
🔬 UDNUCLA UDN Clinical Site ↗
UCLA Health
📍 Los Angeles, CA
🔬 UDNBaylor College of Medicine UDN Clinical Site ↗
Baylor College of Medicine
📍 Houston, TX
🔬 UDNHarvard/MGH UDN Clinical Site ↗
Massachusetts General Hospital
📍 Boston, MA
🏥 NORDMayo Clinic Center for Individualized Medicine ↗
Mayo Clinic
📍 Rochester, MN
👤 Mayo Clinic Center for Individualized Medicine
🏥 NORDUCLA Rare Disease Day Program ↗
UCLA Health
📍 Los Angeles, CA
Travel Grants
No travel grants are currently matched to Transient hyperammonemia of the newborn.
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Caregiver Resources
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Questions for your doctor
Bring these to your next appointment
- Q1.How high were my baby's ammonia levels, and how quickly were they brought down?,Has genetic testing been done to rule out an inherited urea cycle disorder?,Is there any risk that this condition could come back?,What follow-up testing or monitoring does my baby need?,Should my baby see a developmental specialist for ongoing assessments?,Are there any signs I should watch for at home that would require emergency care?,Could this condition affect future pregnancies?
Common questions about Transient hyperammonemia of the newborn
What is Transient hyperammonemia of the newborn?
Transient hyperammonemia of the newborn (THAN) is a condition that occurs in some newborns, most often premature infants, where ammonia levels in the blood become dangerously high shortly after birth. Ammonia is a waste product that the body normally processes through the liver. In THAN, this process temporarily fails, leading to a buildup of ammonia that can be toxic to the brain and other organs. Unlike inherited urea cycle disorders, THAN is not caused by a permanent genetic defect — it is a temporary condition, though it can be life-threatening if not treated quickly. Symptoms usually app
How is Transient hyperammonemia of the newborn inherited?
Transient hyperammonemia of the newborn follows a sporadic inheritance pattern. Genetic counseling can help families understand recurrence risk and testing options.
At what age does Transient hyperammonemia of the newborn typically begin?
Typical onset of Transient hyperammonemia of the newborn is neonatal. Age of onset can vary across affected individuals.