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Metabolic Disorders
Calcium Imbalance
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Syndrome of Inappropriate Antidiuretic Hormone Secretion
Tay-Sachs Disease


What is Phenylketonuria ?

Phenylketonuria (PKU) is an inborn error in amino acid (specifically phenylalanine) metabolism. It results in high serum levels of phenylalanine, increased urine concentrations of phenylalanine and its by-products, cerebral damage, and mental retardation. It's also called phenylalaninemia and phenylpyruvic oligophrenia. The disorder occurs in about 1 in 14,000 births in the United States. (About 1 person in 60 is an asymptomatic carrier.) PKU has a low incidence in Finland and among Ashkenazic Jews and American Blacks. Incidence is high among the Irish and Scottish.

Although blood phenylalanine levels approach normal at birth, they begin to increase within a few days. By the time they reach significant levels (about 30 mg/dl), cerebral damage has begun. Such irreversible damage probably is complete by age 2 or 3. Early detection and treatment can minimize cerebral damage.

Causes of Phenylketonuria

PKU is transmitted through an autosomal recessive gene. Patients with classic PKU, the most common and clinically important of the hyperphenylalaninemias, have almost totally deficient activity of phenylalanine hydroxylase, an enzyme that acts as a catalyst in the conversion of phenylalanine to tyrosine. As a result, phenylalanine accumulates in the blood and urine, and reduced tyrosine formation results.

Signs & Symptoms of Phenylketonuria

Signs and symptoms of phenylketonuria include:

  • Skin problems, such as sensitivity to light (photosensitivity), dry skin, or itchy skin rashes.
  • Vomiting and diarrhea, leading to weight loss.
  • A musty odor to the skin, hair, and urine.
  • Irritability.
  • Jerking movements of the arms or legs (spasticity).
  • Delayed mental and social skills.
  • A distinctive "mousy" odor to the urine and sweat.
  • Hyperactivity

Diagnostic Tests

Most states require screening for PKU at birth; the Guthrie test on a capillary blood sample (bacterial inhibition assay) is used to reliably detect the disorder. Because phenylalanine levels may be normal at birth,the infant should be evaluated after he has begun protein feedings. With PKU, levels are usually abnormally high by day 4.

More quantitative fluorometric or chromatographic assays provide additional diagnostic information.


To prevent or minimize brain damage, phenylalanine blood levels are kept between 3 and 9 mg/dl by restricting dietary intake of the amino acid phenylalanine. During the first month of life, a special, low-phenylalanine, amino acid mixture is substituted for most of the protein in the diet, supplemented with a small amount of natural foods. An enzymatic hydrolysate of casein, such as Lofenalac Powder or Progestimil Powder, is substituted for milk in the diets of affected infants. Dietary restrictions will probably continue throughout life.

Such a diet calls for close monitoring. The body doesn't make phenylalanine, so overzealous dietary restrictions can induce phenylalanine deficiency, causing lethargy, anorexia, anemia, rashes, and diarrhea.

Prevention Tips

Genetic counseling is recommended for prospective parents with a family history of PKU. The carrier state for PKU can be detected by enzyme assays, and PKU can be diagnosed prenatally.

It is imperative that women with PKU who becomes pregnant adhere closely to the special low-phenylalanine diet, since accumulation of phenylalanine will damage the unborn baby even if the baby has not inherited the abnormality.

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