What Causes High Ammonia Levels in a Child's Blood?

Ammonia is a toxic by-product that results when the body breaks down proteins. A healthy person's liver converts ammonia into urea, a nonpoisonous substance excreted in urine. Several conditions and disorders that afflict children disrupt this process, known as the urea cycle, resulting in excessive levels of ammonia in the blood that can cause brain damage, coma and even death.

A young girl's hand rests on a hospital bed with an IV attached (Image: Paul Burns/Blend Images/Getty Images)

Urea Cycle Disorders

A urea cycle disorder or UCD is an inherited condition in which a person lacks one of the six enzymes necessary for the liver to convert ammonia into urea. Symptoms generally appear during the first 24 hours of an infant's life, according to the Cincinnati Children's Hospital Medical Center, and may include vomiting, irritability and excessive tiredness. Some children, however, do not exhibit symptoms until later in life. A child must inherit a defective gene from both parents to develop a UCD. UCDs are genetic conditions and there is no cure. Treatment focuses on managing the condition and can include a low-protein diet and medications to rid excess ammonia from the blood.

Hemolytic Disease of the Newborn

Transient increases in ammonia blood levels are common in newborns. High concentrations, however, may indicate hemolytic disease of the newborn or HDN, which results from an incompatibility between the Rh factor -- the presence or absence of a blood protein -- of the mother and her baby. When an Rh positive mother carries an Rh negative baby, the former's immune system may consider the latter's red blood cells foreign and produce antibodies that attack them, which releases proteins and ammonia. The antibodies develop in first pregnancies, but generally do not become a problem until a later gestation. Newborns with HDN may suffer from severe anemia. In pregnancies with a known potential for an Rh incompatibility, Rh immune-globulin injections are given to the mother to prevent her body from producing antibodies that can endanger the fetus.

Reye's Syndrome

Reye's syndrome is an extremely rare disease specific to children and teenagers that causes encephalopathy -- brain dysfunction -- and liver degeneration concurrent with high ammonia and low glucose levels. The use of aspirin during a viral infection such as the chickenpox is the suspected cause. No therapy exists for Reye's syndrome, therefore treatment focuses on controlling seizures, intercranial pressure, liver damage and other potential effects. One-third of children and teenagers with Reye's syndrome die or suffer permanent neurological damage, according to the MassGeneral Hospital for Children.

Other Causes

Since ammonia is neutralized in the liver, any kind of liver damage or failure can yield higher than normal levels. Bleeding in the digestive tract, known as gastrointestinal bleeding, can also lead to a build-up of ammonia as red blood cells are broken down in the intestine, thereby releasing protein into the blood. Moreover, in children with kidney failure, the body is not able to excrete urea effectively and ammonia accumulates in the blood as a result.