Signs & Symptoms of Phenylketonuria (PKU)

Phenylketonuria, also known as PKU, is a rare genetic disorder that causes the amino acid phenylalanine to build up in the body. Without treatment, this buildup of phenylalanine can cause a number of symptoms, from cognitive impairment to intellectual disability. Treatment to prevent this buildup includes avoiding the consumption of foods containing protein and aspartame in order to limit the buildup of phenylalanine in the body. Some individuals may be able to take a medicine to help assist their body in the breakdown of phenylalanine, in addition to keeping a low protein and low aspartame diet. PKU is rare, affecting only an estimated 1 in every 10,000 to 15,000 births in the United States each year. The Special Olympics Arizona team is providing more detail on the signs and symptoms of this rare disorder.

What Are the Symptoms of Phenylketonuria?

The severity of the symptoms of phenylketonuria depends on the type of PKY that an individual has. There are two types of PKU: classic PKU and other less severe forms of PKU. Individuals with classic PKU will generally have more severe symptoms due to the enzyme needed to break down phenylalanine either missing completely or being greatly reduced. This results in high levels of the amino acid and potential severe brain damage. In individuals with less severe forms of PKU, the enzyme still has some function, and phenylalanine levels are not as high, which results in a lower possibility of severe symptoms and brain damage. Symptoms of PKU will generally only affect individuals who are not treated or are not properly regulating their diet, and symptoms include:

Mild Symptoms of PKU

Mild symptoms of PKU for individuals who are not being treated or are not regulating their diet include:

Eczema.
Discoloration of the skin or hair, especially when compared to their family members.
A small head size, or microcephaly.
A musty odor present in their breath, skin, or urine.

Severe Symptoms of PKU

More severe symptoms of untreated PKU include:

Behavioral problems.
Developmental delays.
Intellectual disabilities.
Seizures.

Severity of PKU

The severity of PKU will affect the severity of symptoms, and individuals with mild cases of phenylketonuria are much less likely to develop symptoms such as intellectual disability, even when leaving PKU untreated. But, no matter the severity, it is important to continue treatment and diet regulation under the guidance of your doctor to avoid any dangerous symptoms and side effects of PKU.

When Do Symptoms of PKU Appear?

Babies with PKU will generally seem normal and healthy for the first few months of life, but once they reach about six months old without treatment, they will begin exhibiting symptoms of PKU and will need to begin treatments, including diet regulation. These early symptoms of PKU that are common in infants around 6 months old include:

Jerky movements of the arms and legs.
A musty body smell.
Lighter skin in the eyes due to the inability to properly make melanin in babies with PKU.
Seizures.
Frequent rashes on the skin, including eczema.
A smaller than average head size.
Taking longer to reach physical milestones, such as sitting, crawling, or walking.
Losing interest in their surroundings.
Delays in reaching mental and social skill milestones.
Intellectual disabilities.
Behavior problems, especially hyperactivity.

Pregnancy & Phenylketonuria

Women who have PKU and become pregnant may be at risk for developing another form of the condition, referred to as maternal PKU. Maternal PKU required women to follow the strict, special PKU diet before and during pregnancy. If women fail to follow this strict treatment and diet, blood phenylalanine levels can become high and harm the baby’s development. Women with less severe forms of PKU must also follow the strict diet, since they can still be placing their unborn children at risk if they fail to follow the proper PKU regulating diet.

It is common for babies to be born to parents with PKU without inheriting the disorder. But they may still have serious health problems due to a high level of phenylalanine in the blood during fetal development. Babies who are developing in a mother with maternal PKU that is being left untreated may be born with a low birth weight, an unusually small head, or problems with their heart. It can also cause the child to be developmentalally delayed, have an intellectual disability, or display behavioral problems. Pregnancy is safe for mothers with PKU, provided you work closely with your doctor and regulate your diet to ensure safe levels of phenylalanine present in your blood to protect your developing baby.