Excess phenylalanine, an essential amino acid, is normally eliminated from the body by hydroxylation to tyrosine. The enzyme phenylalanine hydroxylase is essential for this reaction. Symptoms and Signs Symptoms are usually absent in the newborn period. Rarely, an infant may be lethargic or may feed poorly. The most important symptom is mental retardation, usually severe, which occurs in the majority of untreated patients.
Affected infants tend to have lighter colored skin, hair, and eyes than unaffected family members.
Some infants may have a rash similar to infantile eczema. Many neurologic symptoms and signs develop, especially affecting reflexes. For some families, direct mutation studies can also be performed. Because of the usual lack of neonatal symptoms, laboratory testing is mandatory for detection. Early diagnosis depends on detecting a high plasma phenylalanine level and a normal or low plasma tyrosine level.