OTC deficiency is a rare but serious inherited metabolic disorder that impairs the body's ability to break down protein and remove ammonia. Early diagnosis and treatment are critical to long-term health.
Ornithine Transcarbamylase (OTC) Deficiency is the most common urea cycle disorder. The urea cycle is a series of chemical reactions in the liver that converts toxic ammonia, a by-product of protein metabolism, into urea, which is then safely excreted in urine.
In OTC deficiency, a mutation in the OTC gene causes a deficiency or complete absence of the OTC enzyme. Without this enzyme, ammonia builds up in the blood (hyperammonaemia), which is toxic to the brain and nervous system.
OTC deficiency is X-linked, which has historically led to the assumption that it primarily affects males and that females who carry the mutation are largely unaffected. This is misleading and potentially dangerous. Research published in 2024 by the Urea Cycle Disorders Consortium found that female carriers face significantly more symptoms and serious health risks than previously understood. Nearly half of those studied showed symptoms, and a significant number developed life-threatening hyperammonaemia. Fatal episodes have occurred in females who did not know they were at risk. The term "carrier" can create a false sense of security.
Crucially, a lack of known family history does not rule OTC deficiency out. The condition can arise from a brand-new (de novo) mutation with no prior cases in the family at all. Just as often, a family history is there but hidden. Relatives may have died of "unexplained" causes, been misdiagnosed with psychiatric or neurological conditions, or lost children without ever knowing why. If the symptoms on this page sound familiar, ask for a metabolic assessment regardless of family history. Do not let yourself be reassured and sent away.
Symptoms vary significantly depending on age of onset and the degree of enzyme deficiency. Prompt recognition is essential, because hyperammonaemic crises can be life-threatening.
Medical emergency: A hyperammonaemic crisis requires immediate hospital treatment. If you or a child shows sudden confusion, vomiting, or loss of consciousness, especially following illness or high protein intake, call emergency services immediately.
There is no cure for OTC deficiency, but the condition can be effectively managed through a combination of dietary modification, medications, and in severe cases, surgery.
Restricting dietary protein reduces the production of ammonia. Patients work with a specialist metabolic dietitian to meet nutritional needs safely, often using protein substitutes.
Medications such as sodium benzoate and sodium phenylbutyrate provide alternative pathways for nitrogen excretion, reducing ammonia build-up in the blood.
A liver transplant can correct the metabolic defect and prevent hyperammonaemic crises. It is typically considered for those with severe disease, though it does not reverse prior neurological damage.
Seeking a diagnosis? If you recognise these symptoms in yourself or a family member, contact your GP and request a referral to a specialist in inherited metabolic diseases. Be persistent, as awareness of OTC deficiency among general practitioners can be limited. Visit our Resources page for specialist contacts in the UK.
Access a list of UK metabolic specialists, patient organisations, and educational resources about urea cycle disorders.
View Resources Watch the Urea Cycle Video ↗