From MayoClinic.com 

Phenylketonuria is a hereditary disorder in which the amino acid phenylalanine isn't properly metabolized. This amino acid occurs in significant amounts in all proteins, including breast milk, standard baby formulas, eggs, meat, fish and beans. People with PKU can't process phenylalanine because they lack an enzyme called phenylalanine hydroxylase (PAH). As a result, the amino acid can build up to dangerous levels in the blood and other tissues, causing mental retardation and other serious health problems.

However, mental retardation can be totally prevented - if these babies begin a carefully regulated low-phenylalanine diet just after birth. That's why early detection and diagnosis through newborn screening is critical.

Signs and symptoms

Newborns with PKU don't show any outward indication of this underlying genetic defect. Without treatment, though, these babies typically develop signs and symptoms of PKU within a few months. Signs and symptoms can be mild or severe and may eventually include:

• Mental retardation

• Behavioral or social problems

• Seizures, tremors or jerking movements in the arms and legs

• Rocking and hyperactivity

• Stunted growth

• Skin rashes (eczema)

• Small head size (microcephaly)

• Vomiting

• A musty odor in the child's breathe, skin or urine, caused by too much phenylalanine in the body

• Fair skin and blue eyes, because phenylalanine cannot transform into melanin — the pigment that is responsible for hair and skin tone

• Eczema
• vomiting

The most severe form of the disorder is known as classic PKU. Children with untreated classic PKU usually develop obvious, permanent mental retardation and behavioral problems by their first birthday. However, even in people with exactly the same genetic defect, the severity of the disease may be different. Less severe forms of PKU — sometimes called mild or moderate PKU — have a smaller risk of significant brain damage, but most children with these forms of the disorder still require a special diet to prevent mental retardation and other complications.

Another form of the condition — maternal PKU — also causes signs and symptoms. This term refers to mothers with PKU who don't stick to a low-phenylalanine diet during pregnancy.

However, if these women don't return to the diet during pregnancy, their blood phenylalanine levels can be very high, which can be devastating to a growing fetus. Ninety percent of babies born to mothers with high levels of phenylalanine develop mental retardation or microcephaly. They may also have heart defects, low birth weights and behavioral problems. But most don't actually inherit PKU and don't benefit from following a PKU diet after birth.

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