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phenylhydrazine

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(fen″ĭl-hī′dră-zēn″) [phenyl + hydrazine] An oily nitrogenous base used as a test for the presence of sugar in the urine.

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phenylketonuria

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(fen″ĭl-kēt″ŏn-ū′rē-ă, fēn″) [phenyl + ketone + -uria] ABBR: PKU. A congenital autosomal recessive disease that can cause irreversible neurological damage in infancy if it is not promptly detected. SYN: Folling disease.

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INCIDENCE: PKU is present in about 3.5 in 10,000 newborns in the U.S.

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CAUSES: It is caused by an enzyme deficiency that prevents the metabolism of the amino acid phenylalanine to tyrosine.

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SYMPTOMS AND SIGNS: Phenylalanine and its by-products accumulate in the body, esp. in the nervous system, where they cause severe mental retardation (IQ test results often below 40), seizure disorders, tremors, gait disturbances, coordination deficits, and psychotic or autistic behavior. Eczema and an abnormal skin odor also are characteristic.

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DIAGNOSIS: The U.S. Preventive Services Task Force recommends that all newborns be screened for PKU before discharge from the nursery or in the first 2 weeks of life. The accuracy of the test is highest if it is performed no sooner than 24 hr after birth. Mass screening for the disease began in the 1960s.

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PREVENTION: The consequences of PKU can be prevented if it is recognized in the first weeks of life and a phenylalanine-restricted (very low protein) diet is maintained throughout infancy, childhood, and young adulthood. Some women with PKU are now of childbearing age. During their pregnancies, strict adherence to a low-phenylalanine diet will help to prevent fetal malformations. Chorionic villus sampling can detect PKU prenatally.

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PATIENT CARE: Testing newborns for PKU is typically performed with a heel-stick specimen of blood, which is allowed to dry on blotting paper before being sent to the lab for the Guthrie test. The presence of phenylpyruvic acid in the urine also confirms the diagnosis (a few drops of 10% ferric acid are added to a wet diaper, resulting in a deep, bluish coloration if the test is positive. Since urinary levels of the acid vary according to the amount of protein ingested, testing should be repeated 4 to 6 weeks after birth. If elevated levels of phenylalanine are found, additional tests are performed to confirm the diagnosis.

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Effective treatment requires that phenylalanine blood levels be maintained between 3 and 9 mg/dL. This means severely limiting natural proteins in the diet, as most contain 5% phenylalanine. An enzymatic hydrolysate of casein (Lofenalac or Pregestimil) is substituted for milk for affected infants, providing normal amounts of protein other than phenylalanine, plus additional carbohydrates and fat. As the child grows, breads, cheese, eggs, flour, meat, poultry, fish, nuts, milk, legumes, and phenylalanine-type sugar substitutes must be avoided. Frequent blood and urinary testing evaluates dietary ...

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