Phenylketonuria Definition

An inherited genetic disorder in which the enzyme phenylalanine hydroxylase is missing or severely deficient, preventing the metabolism of the essential amino acid (one the body must acquire from dietary sources) phenylalanine. Phenylalanine is common in all foods that contain protein (such as meats, dairy products, fish, and legumes) and in artificial sweeteners such as aspartame. Avoiding foods that contain phenylalanine, which means following a strict low-protein diet, prevents phenylalanine accumulations and the resultant damage that affects primarily the nervous system. The most significant consequence of undiagnosed Phenylketonuria (PKU) is irreversible, and usually severe, intellectual impairment.

Symptoms of Phenylketonuria (PKU) and Diagnostic Path

Early symptoms of PKU appear soon after birth and include restlessness, irritability, stunted growth, and a characteristic musty smell to the breath. The appearance of symptoms means neurologic damage is already occurring, however. Newborn screening to identify Phenylketonuria (PKU) before symptoms appear is essential to prevent intellectual impairment. Hospitals in the United States routinely screen newborns, typically within two days of birth, to detect elevated levels of phenylalanine in the blood. Further testing can confirm the diagnosis, and immediate dietary restrictions can prevent the disorder from causing permanent damage.

Phenylketonuria (PKU) Treatment Options and Outlook

Treatment is stringent restriction of dietary phenylalanine, which includes breast milk. Infants require special phenylalanine-free formulas. Dietary restrictions are lifelong. Many food products contain labeling information that states their phenylalanine content, and a number of food manufacturers produce low-phenylalanine versions of popular foods such as cereals as well as phenylalanine-free protein substitutes. Fruits, vegetables, breads, and pastas contain very low amounts of phenylalanine. In the United States foods that contain aspartame must state on the label that they contain phenylalanine.

Women who have PKU can safely carry a pregnancy to term though must be especially diligent to maintain a low phenylalanine diet because excessive phenylalanine in the mother’s blood circulation also affects the developing fetus and can cause permanent neurologic and other damage before birth. Because the inheritance pattern for Phenylketonuria (PKU) is autosomal recessive, women who have PKU will pass the disorder to their children only if the father carries the mutated gene or also has PKU.

Risk Factors and Preventive Measures

PKU is an autosomal recessive, single-gene mutation. Both parents must carry the PKU mutation for a child to have the disorder. However, PKU carriers often do not know they have the mutated gene because they do not show any indications of the disorder. People who know they are PKU carriers or who have PKU should consider genetic counseling as an element of their family planning.


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