Phenylketonuria Symptoms

Phenylketonuria (PKU) is a rare birth defect that can be detected by a heel prick blood test at birth. Left undetected PKU can cause mental retardation.

PKU is a genetic birth defect inherited from both parents. Babies who have PKU need a special formula (low-phenylalanine) to meet its nutritional needs that will compensate for the deficient or missing enzyme that is needed to process the essential amino acid called phenylalanine. The child will need to continue to a special diet as a child and as an adult.

Phenylketonuria is rare, but it still needs to be tested for because left untreated, serious health problems can result.


    Mental retardation

    Behavioral or social problems

    Jerking, tremors or seizures in the arms or legs

    Rocking motions


    Stunted growth

    Skin rashes

    Small head size (microcephaly)


    Musty breath, skin or urine, which is caused by too much phenylalanine in the body.

    Fair skin and blue eyes

Those children with “classic PKU”, which is the severest form, usually develop obvious and permanent mental retardation and behavioral problems before their first birthday.

There is also mild and moderate forms of PKU. These children have a smaller risk for brain damage. The special diet will prevent the mental retardation.

A woman who herself had PKU and becomes pregnant is at risk for another form of the condition called “maternal PKU”. She will need to follow the PKU diet during pregnancy, otherwise the phenylalanine levels can become dangerously high for her fetus. Individuals are recommended to follow the PKU diet for life.

Babies born to mothers with PKU may not inherit the defect, they may however have complications at birth. They may not need to follow the PKU diet if they test normal at the PKU screening. They are however at risk for being born with microcephaly (small head) and mental retardation, low birth weight, heart defects and behavioral problems due to the high levels of phenylalanine levels in the bloodstream due to the mother’s PKU.

Causes of Phenylketonuria

A single mutated gene causes PKU. This mutated gene is suppose to carry instructions for making the enzyme needed to process amino acid (phenylalanine). Those who have PKU are missing this gene or have a defective gene. Without this gene to process phenylalanine, a dangerous level of phenylalanine can build up in the body when the person with PKU eats foods that are high in protein such as milk, cheese, nuts or meats.

Serious health problems can result when a person with PKU consumes these foods high in protein. A person can have the defective gene but not have the disorder, this is called being a carrier. In order for a baby to have PKU, both parents must have the defective or missing gene.


An unborn baby is at risk if both parents have the missing or defective PKU gene. If only one parent has the missing or defective PKU gene than there is no risk for passing along the defect to the unborn baby.

Babies of mothers who have PKU and who did not follow the PKU diet during pregnancy are at risk for the consequences of high levels of phenylalanine in the mother’s blood.

Seek Medical Attention:

Newborn – if routine PKU tests show the baby has PKU; the baby will need to be started on a special formula and may be referred to a specialist in genetics and a dietician or nutritionist who will help with the diet guidelines.

Adults – doctors now recommend that all individuals with PKU continue the diet for life as it may improve mental functioning and behavior and slow any damage to the central nervous system.

Women – It is important that all women especially who are of child producing age follow the diet as having high levels of phenylalanine can be dangerous to a developing fetus and not all women realize when they are pregnant until after the fetus has developed most of the major organs.