Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome
Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome
Also known as:Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome, Ornithine Transporter Deficiency
HHH Syndrome is a rare autosomal recessive inherited disorder caused by mutations in the SLC25A15 gene, which leads to impaired mitochondrial ornithine transport, resulting in elevated blood ammonia, ornithine, and homocitrulline, affecting the nervous system and liver.
Start Here
A quick guide to the next step: which department to start with, what to prepare, and what to ask.
When newborns or children develop hyperammonemia, drowsiness, vomiting, seizures, developmental delay, spasticity, or unexplained liver function abnormalities, they should be evaluated promptly in the emergency department, pediatrics, metabolic genetics, or pediatric neurology.
HHH Syndrome, or Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome, is a rare inherited metabolic disorder caused by mutations in the SLC25A15 gene, which codes for the mitochondrial ornithine transporter 1 (ORNT1). This defect prevents ornithine from properly entering the mitochondria to participate in the urea cycle, leading to accumulation of blood ammonia, elevated blood ornithine, and excretion of homocitrulline in urine. It follows an autosomal recessive inheritance pattern and was included in China's first national list of rare diseases in 2018.
Yes, management approaches related to urea cycle disorders are available, including acute hyperammonemia management, protein intake management, ammonia-scavenging medications, and amino acid supplementation assessment, to be developed by metabolic genetics specialists.
Yes, autosomal recessive inheritance
Symptoms resemble encephalopathy, epilepsy, or liver disease, leading to misdiagnosis; the wide age range of onset (from newborn to adulthood) increases diagnostic difficulty
This page helps patients and families organize care leads. It does not replace a clinician’s diagnosis or treatment plan. For testing, medication, referrals, emergency care, and support applications, follow qualified clinicians, medical institutions, support organizations, and official sources.
Diagnosis Path
Organized around the practical patient journey: identify clues, avoid common delays, then prepare for care.
When to Suspect It
- Drowsiness, refusal to feed, vomiting, coma with respiratory alkalosis appearing after 48 hours of birth in the neonatal period
- Intellectual and motor developmental delay or regression in childhood
- Unexplained liver function abnormalities with neuropsychiatric symptoms
- Marked aversion to or poor tolerance of high-protein foods
- Recurrent seizures, ataxia, or consciousness disturbances
Common Wrong Turns
- Treated only for epilepsy or cerebral palsy without checking blood ammonia
- Attributing liver function abnormalities to viral hepatitis or other liver diseases
- Failing to perform urine metabolic screening, missing key diagnostic clues
- Adult-onset cases often misdiagnosed as psychiatric disorders
Departments to Start With
- Pediatrics or Pediatric Neurology (preferred first visit)
- Metabolic Genetics
- Hepatology or Gastroenterology (for assessment of liver damage)
- Emergency Department (during acute episodes)
Before the Visit
- Blood ammonia, blood glucose, liver and kidney function
- Blood ornithine concentration (persistent elevation is a key clue)
- Urine homocitrulline test (urine metabolic screening)
- Combined testing of blood ammonia, ornithine, and homocitrulline
- SLC25A15 gene molecular testing (important confirmatory evidence)
- ORNT1 activity measurement in skin fibroblasts (auxiliary method)
- Brain MRI or CT (to assess neurological complications)
- Electroencephalogram (EEG) (to assess epileptiform activity)
Tests to Ask About
- Blood ammonia test (whether elevated)
- Blood ornithine concentration
- Urine amino acid analysis (including homocitrulline)
- SLC25A15 gene testing
- Liver function and coagulation function
Questions for the Doctor
- What is the current blood ammonia level? How often should it be checked?
- How should my child's diet be adjusted specifically? How should protein intake be controlled?
- Is arginine, citrulline, or ammonia-scavenging treatment currently needed? Who will develop the plan, and how will treatment be monitored?
- How often should my child's developmental level be assessed?
- What acute triggers should I watch out for?
- If my parents want to have another child, what genetic preparations are needed?
Basic Information
Medical Notes
More complete medical explanations are kept here for discussion with clinicians.