Phenylketonuria with normal intelligence.

A review was made of the literature with respect to cases of untreated phenylketonuria (PKU) with normal intelligence. Six families were found in which a person with classical PKU and normal intelligence, and a sibling with PKU and severe mental handicap had been reported. Details are given of another similar family, and possible reasons are discussed for the preservation of intelligence in the affected individual with an IQ of 82. Among the reasons proposed are breast feeding, which has a low phenylalanine level and a raised biopterin level, which may help in the production of brain neurotransmitters. Studies of similar cases are suggested.