This is a dangerous but treatable hereditary condition affecting one out of 10,000 children. Until recent decades, children with PKU always suffered brain damage and mental retardation. Now, however, the disease usually is detected and treated by means of a special diet before such damge can occur.
All infants born in the United States routinely undergo blood tests to diagnose PKU. If the test shows phenylketonuria, treatment is initiated immediately, in most cases preventing the serious, permanent effects of the disease.
The newest concern regarding PKU is the prevention of complications in the children of women who have been treated for the condition.
WHAT CAUSES PKU?
PKU results from an excess of an amino acid called phenylalanine, which is essential to the body's normal growth and development. Too much phenylalanine, however, damages the central nervous system.
Phenylalanine is present in high protein foods. After the body has absorbed enough for its needs, an enzyme normally converts some phenylalanine to tyrosine, another essential amino acid; in PKU, however, that vital enzyme is absent or insufficient, so conversion to tyrosine does not take place. Instead, unused phenylalanine builds up in the bloodstream. The danger is greatest during the first year of life, when the brain is still developing. However, even if PKU develops later (which may happen if the child abandons the special diet), brain damage will occur.
PKU is a genetic disorder with a recessive inheritance pattern. If both parents are carriers (they have the gene, but not the disease), each baby has a 25 percent chance of inheriting the disease, a 50 percent chance of being a carrier, and a 25 percent chance of freedom from the defective gene. Prospective parents who already have a child with PKU or who have any reason to suspect that they are PKU carriers should consider genetic counseling.
HOW DOES PKU DEVELOP?
A child with untreated PKU will develop the following manifestations:
• Musty odor of urine and sweat
• Failure to produce the pigment melanin, resulting in light-colored hair, skin and eyes (this sign is not always present)
• An eczemalike rash
• Seizures
• Behavioral problems, including hyperactivity or excessive aggressiveness
• Abnormally small head after age six months
• Uncontrollable twitching
• Vomiting
• Decreased growth rate
• Brain damage and progressive mental retardation (symptoms may not appear for a few months)
WHAT TREATMENTS ARE AVAILABLE?
Immediately upon diagnosis of PKU, a strict diet plan must be initiated and maintained indefinitely. The child must avoid foods high in phenylalanine (even a small excess can be damaging) but take in enough for normal development; balancing the level and assuring the child's compliance are the most difficult aspects of the diet. In the first few years of life, frequent visits to the doctor or a metabolic clinic are necessary. Periodic testing will be scheduled to monitor the child's phenylalanine level.
Up to age six months, infants with PKU are given formula low in phenylalanine. Limited amounts of breast feeding also may be incorporated into the infant's diet. The baby can start eating solid foods at the usual time, but the diet must exclude certain high-protein foods, such as animal proteins (meat, fish, eggs and dairy products); high-protein grain products; and nuts. Be sure the child (as well as all caregivers, teachers and school health personnel) know what foods to avoid when he is outside the home. A registered dietician can help you follow the diet with as little disruption in normal family eating patterns as possible.
In the past, children were taken off the special PKU diet sometime between the ages of five and eight. However, recent studies suggest that abandoning the diet that early may lead to learning and behavioral problems. Consequently, many authorities now suggest that the diet be continued indefintely.
PREVENTING COMPLICATIONS IN INFANTS OF MOTHERS WITH PKU
There is no way to prevent or cure PKU. Infants of mothers who have had the disease risk mental retardation even if they do not inherit the abnormal genes. This is because extremely high levels of phenylalanine, which may persist in adults with PKU, can reach the fetus and interfere with its development. To avoid this outcome, the mother must begin the restrictive PKU diet before conception and maintain it throughout pregnancy.
Be aware that a physical trauma or minor illness can elevate the amount of phenylalanine in the child's blood even with strict compliance with the dietary restrictions. In such circumstances, seek medical guidance immediately.
