What is Phenylketonuria (PKU)?

Phenylketonuria (PKU) is a rare genetic condition where a child is born unable to properly break down an essential amino acid called phenylalanine. This amino acid is found in protein-containing foods. Without the necessary enzyme to break it down, phenylalanine levels build up in the body, which can damage the central nervous system and cause brain damage if left untreated [1].

Causes of Phenylketonuria

Phenylketonuria is inherited, meaning it is passed down through families. The condition occurs when children lack an enzyme called "phenylalanine hydroxylase," which is essential for breaking down phenylalanine. If both parents carry a non-functional copy of the gene associated with this condition, each child has a 25% chance (1 in 4) of having the disorder. This is known as autosomal recessive inheritance [1].

Symptoms of Phenylketonuria

Phenylalanine plays a role in the body's production of melanin, the pigment responsible for skin and hair color. Therefore, infants with the disorder often have lighter skin, hair, and eyes than siblings without the disorder. If PKU is untreated, or if foods containing phenylalanine are consumed, the breath, skin, earwax, and urine may have a "musty" or "mousy" odor due to the buildup of phenylalanine substances in the body [1]. Other symptoms that may appear if left untreated include [1]:

• Delayed mental and social skills.
• Head size significantly smaller than normal (microcephaly).
• Hyperactivity.
• Jerky movements in the arms or legs.
• Intellectual disability.
• Seizures.
• Skin rashes.
• Tremors.

Diagnosing Phenylketonuria

Phenylketonuria can be easily detected through a simple blood test. In many countries, a PKU screening is required for all newborns as part of a newborn screening panel. The test is usually performed by taking a few drops of blood from the baby before they leave the hospital. If the screening test is positive, further blood and urine tests are required to confirm the diagnosis. Genetic testing is also performed [1]. Early diagnosis is critical, as it allows for prompt initiation of treatment and helps prevent brain damage and intellectual disability [2].

Managing Phenylketonuria

Phenylketonuria is a treatable disease. Treatment involves following a diet that is very low in phenylalanine, especially when a child is in the growth stage. This diet must be strictly followed under the supervision of a registered dietitian or healthcare provider [1]. Lifelong adherence to the diet is the standard recommended by most experts, as those who continue the diet into adulthood have better physical and mental health [1].

For infants with PKU, special formulas are available. Infants with the disorder can be safely breastfed, alongside the use of medical foods and appropriate monitoring of phenylalanine levels. These special medical foods provide a source of protein that is extremely low in phenylalanine and balanced for the remaining essential amino acids. Older children and adults use a different formula that provides protein in the amounts they need [1]. People with PKU must consume these formulas daily for life [1].

For more information on managing this condition, you can visit our article on Phenylketonuria (PKU): Early Diagnosis, Diet, and its Impact on Child Development (Arabic).

When to Call a Doctor

Contact your healthcare provider if your newborn has not been screened for PKU. This is especially important if anyone in your family has this disorder [1].

Medical references

• Phenylketonuria
• Phenylketonuria
• About Phenylketonuria (PKU)