If the urea cycle fails, the level of ammonia in the blood rises, resulting in hyperammonemia. Ammonia has the ability to pass through the blood-brain barrier. If a child has a urea cycle disorder, their liver is unable to produce one of the enzymes required by the cycle. When their bodies are unable to remove nitrogen, ammonia forms and accumulates in their blood. It's toxic and can damage their brains or put them in a coma. If not treated promptly, it can be fatal.