Citrullinemia Type I
An inherited urea cycle disorder caused by argininosuccinate synthetase deficiency, leading to elevated ammonia and citrulline levels that require prompt recognition and specialized metabolic management.
- Identified by Newborn Screening
- Autosomal Recessive (ASS1 Gene)
- Managed with Diet & Ammonia Scavengers
- Estimated Incidence
- ~1 in 57,000–100,000 births
- Inheritance Pattern
- Autosomal Recessive
- Gene Involved
- ASS1
- Management Anchor
- Protein Restriction & Arginine Supplementation
Condition Overview
Citrullinemia type I is a rare inherited urea cycle disorder caused by deficiency of the enzyme argininosuccinate synthetase, which catalyzes a key step in converting ammonia into urea for excretion. Reduced enzyme activity leads to elevated blood citrulline and ammonia levels.
The classic neonatal-onset form typically presents within the first days of life with hyperammonemia and is a medical emergency. A milder, late-onset form may present later in childhood or adulthood, often triggered by illness, high protein intake, or other metabolic stress.
Because hyperammonemia can rapidly affect the brain, early diagnosis — increasingly achieved through newborn screening — and prompt treatment are central to a good long-term outcome.
Types and Subtypes
Citrullinemia type I is classified by age of onset and residual enzyme activity.
Symptoms and Signs
Symptoms reflect the effects of elevated ammonia on the brain and vary by age of onset.
Causes and Risk Factors
Citrullinemia type I is caused by biallelic pathogenic variants in the ASS1 gene, which encodes argininosuccinate synthetase.
Diagnosis and Investigations
Diagnosis is based on recognizing hyperammonemia with markedly elevated citrulline, followed by confirmatory genetic testing.
Disease Classification and Risk Stratification
Citrullinemia type I is not staged like cancer; classification is based on age of onset and residual enzyme activity, which guide treatment urgency.
Standard Treatment Options
Treatment combines emergency ammonia reduction during crises with long-term dietary and pharmacologic management.
Advanced and Emerging Approaches
For patients with recurrent severe hyperammonemia despite medical management, advanced options may be considered.
Transplantation
Liver Transplantation
Considered in selected severe cases with recurrent life-threatening hyperammonemia refractory to medical therapy; provides functional urea cycle capacity.
Gene Therapy
Investigational Gene-Based Approaches
Early-stage research is exploring gene therapy and mRNA-based approaches for urea cycle disorders, including citrullinemia type I.
Biomarkers and Laboratory Monitoring
Laboratory monitoring guides treatment adjustments and crisis recognition rather than prognostic staging.
When a Second Opinion May Be Important
Given the complexity and rarity of citrullinemia type I, expert input is valuable at several decision points.
Clinical Trials and Research
Outlook and Key Outcome Factors
Outlook in citrullinemia type I depends heavily on how quickly hyperammonemia is recognized and treated, particularly in the neonatal period.
Supportive Care and Living with Citrullinemia Type I
Day-to-day management extends well beyond medication, requiring coordinated dietary, educational, and emotional support.
How CancerFax Helps You Explore Treatment Options
CancerFax can help families coordinate medical report review, connect with metabolic and transplant specialists for second opinions, and identify experienced centers for ongoing management of citrullinemia type I.
Get a free case reviewFrequently Asked Questions
It is a rare inherited urea cycle disorder caused by deficiency of argininosuccinate synthetase, leading to elevated citrulline and ammonia levels.
It is caused by inherited variants in the ASS1 gene, passed down from two carrier parents in an autosomal recessive pattern.
In most countries with universal newborn screening, it is detected through markedly elevated citrulline on a dried blood spot test.
Yes, through dietary protein restriction, arginine supplementation, ammonia-scavenging medications, and, in select severe cases, liver transplantation.
Ammonia rises rapidly and can cause lethargy, vomiting, seizures, or coma; it requires emergency medical treatment.
Yes, milder or late-onset forms can present later in life, sometimes triggered by illness, fasting, or pregnancy.
No, transplantation is reserved for severe, recurrent cases that do not respond adequately to medical and dietary management.
Parents of an affected child are carriers, with an approximate 25% recurrence risk in future pregnancies; genetic counseling is recommended.
No, it is a urea cycle metabolic disorder and is not associated with increased cancer risk.
Yes. CancerFax can help coordinate medical report review, facilitate second opinions with metabolic and transplant specialists, and assist with matching families to experienced centers for ongoing care.
Get Expert Guidance for Citrullinemia Type I
CancerFax can help connect you with metabolic specialists, support second opinions, and guide you through ongoing management decisions.