Phenylketonuria (PKU)

What Is It?

Phenylketonuria (PKU) is a rare, genetic (inherited) disorder that can cause abnormal mental and physical development if not detected promptly and treated appropriately. Normally, when a person eats protein-containing foods, special chemicals (enzymes) in the body break down (digest) these proteins into amino acids and even smaller parts, which are the body's building blocks for growth and repair. A person with PKU does not have enough of the specific enzyme that breaks down the amino acid phenylalanine. Therefore, any phenylalanine in the food someone eats cannot be digested properly and collects in the body.

Too much phenylalanine in the body causes problems with the brain and other organs. Damage from a buildup of phenylalanine can begin as early as 3 weeks old and, if undetected and untreated, PKU results in severe mental retardation, hyperactivity and seizures.

PKU affects about one baby in every 10,000 or 15,000 births. To be born with PKU, a baby has to have inherited the PKU gene from both parents. Most often, the parents do not know that they carry the gene.

Symptoms

Newborn babies with PKU do not show any symptoms. At about 3 to 6 months, babies with untreated PKU will begin to be less active, showing little interest in everything around them. By age 1, severe developmental delays can be noted.

Children with PKU have lower levels of melanin, the substance that gives color to hair and skin. That's because when phenylalanine is broken down, one of its products is used to make melanin. As a result, children with PKU often will have pale skin, blond hair and blue eyes. Dry skin, eczema and a "mousy" odor resulting from the buildup of phenylalanine in hair, skin and urine are also common. Other signs and symptoms may include irritability, muscle stiffness, seizures, a small head (microcephaly), and short stature.

Diagnosis

In the United States, newborn babies are screened for PKU routinely within seven to 10 days after birth. Infants must have had a protein meal before being tested for PKU. A very small amount of blood is taken by pricking the baby's heel. If abnormally high amounts of phenylalanine are found, additional blood tests may be necessary to confirm the diagnosis.

Expected Duration

PKU is an inherited disorder that lasts throughout a person's lifetime.

Prevention

PKU occurs when a baby inherits two copies of the specific gene that causes PKU, one from each parent. Each parent usually has only one copy of the gene and therefore doesn't have PKU. Because the parents don't know they are carrying the gene, there is nothing they can do to prevent their babies from having the disorder.