Phenylketonuria (PKU) that is not treated causes several characteristic symptoms that range from mild to severe and differ from child to child. Children with PKU often have a lighter complexion (including blond hair and blue eyes) than their siblings, because phenylalanine contributes to the production of melanin, which is the pigment responsible for hair and skin color. As the derivative phenylacetic acid builds up, it may cause a musty or mousy odor on the child’s breath, skin, sweat and urine. These are the first noticeable symptoms.

Typically, children with PKU appear to behave normally until they are a few months old. At this point, symptoms first appear, and may include loss of interest in their surroundings. By the time a child is one year old, developmental delays typically are obvious.

Older children with untreated PKU may develop other symptoms. These include attention-deficit hyperactivity disorder (ADHD), seizures, movement disorders (athetosis) and rocking.

 Other symptoms associated with PKU include:

• Abnormally small head (microcephaly)
• Delayed mental and social skills
• Heart defects
• Jerking movements of the arms or legs (spasticity)
• Mental retardation
• Skin rashes (e.g., eczema)
• Stunted growth
• Tremors
• Unusual hand posturing
• Vomiting