EARLY SIGNS
Different forms of phenylketonuria vary in their severity of signs. Classic phenylketonuria (PKU) is the most severe form. Babies with PKU usually seem healthy at birth. Signs of PKU begin to appear around six months of age.
Signs of classic PKU include:
- Irritability
- Seizures (epilepsy)
- Dry, scaly skin (known as eczema)
- “Musty” or "mouse-like” body odor
- Pale hair and skin
- Developmental delays
Many of these signs may occur when your baby eats foods that his or her body cannot break down. They can be triggered by long periods of time without eating, illnesses, and infections.
CAUSES
When we eat food, enzymes help break it down. Some enzymes break down protein into its building blocks, called amino acids. Other enzymes break down these amino acids. In classic phenylketonuria (PKU), the enzyme phenylalanine hydroxylase (PAH) is not working correctly.
PAH’s job is to break down the amino acid phenylalanine. Babies with PKU either do not make enough or make non-working PAH. When PAH does not work correctly, the body cannot break down phenylalanine and it builds up in the blood. Everyone has some phenylalanine in their blood, but high levels can be toxic.
PKU is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for PKU, one from each parent, in order to have the condition. The parents of a child with an autosomal recessive condition each carry one copy of the non-working gene, but they typically do not show signs and symptoms of the condition. While having a child with PKU is rare, when both parents are carriers, they can have more than one child with the condition.
TREATMENT
Dietary Treatment
Your baby will need to be on a restricted diet to avoid phenylalanine, a building block of proteins that individuals with phenylketonuria (PKU) cannot break down. Phenylalanine is found in all foods that contain protein and also in artificial sweeteners. The dietary treatment for PKU includes specific medical foods: a phenylalanine-free medical formula that is given regularly throughout the day, as well as foods modified to be low in protein. A registered dietician will help you plan a low-protein diet, that avoids high levels of phenylalanine, and gives your baby the nutrients he or she needs for healthy growth.
Supplements and Medications
Your baby’s doctor may prescribe a low protein formula. As your baby gets older, their doctor may prescribe a medication that contains BH4. BH4 is a substance naturally produced by the body, but your baby’s body might not make enough of it. Taking BH4 supplements may help break down the phenylalanine that builds up. Your baby’s doctor will need to write a prescription for these supplements. BH4 does not work in everyone with PKU. A trial period on the drug with evaluation by a physician is necessary.
EXPECTED OUTCOMES
If phenylketonuria (PKU) is detected at birth, early treatment can prevent the signs of the condition mentioned in the Early Signs section. This is why it is so important to screen for PKU at birth.
If babies start treatment several weeks after birth, some signs of PKU can be avoided.
If treatment is started after six months of age, babies are at risk for severe intellectual disabilities.
It is important to treat PKU, even if treatment is started after noticing signs and symptoms, in order to help prevent permanent brain damage.