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Phenylketonuria Definition:

Phenylketonuria (PKU) is a genetic disorder that is characterized by an inability of the body to utilize the essential amino acid, phenylalanine. Amino acids are the building blocks for body proteins. 'Essential' amino acids can only be obtained from the food we eat as our body does not normally produce them. In 'classic PKU', the enzyme that breaks down phenylalanine phenylalanine hydroxylase, is completely or nearly completely deficient. This enzyme normally converts phenylalanine to another amino acid, tyrosine. Without this enzyme, phenylalanine and its' breakdown chemicals from other enzyme routes, accumulate in the blood and body tissues. Although the term 'hyperphenylalaninemia' strictly means elevated blood phenylalanine, it is usually used to describe a group of disorders other than classic PKU. These other disorders may be caused by a partial deficiency of the phenylalanine breakdown enzyme or the lack of another enzyme important to the processing of this amino acid. A normal blood phenylalanine level is about 1 mg/dl. In classic PKU, levels may range from 6 to 80mg/dl, but are usually greater than 30mg/dl. Levels are somewhat less in the other disorders of hyperphenylalaninemia. Chronically high levels of phenylalanine and some of its breakdown products can cause significant brain problems. Classic PKU is the most common cause of high levels of phenylalanine in the blood and will be the primary focus of this topic sheet.


Classic PKU and the other causes of hyperphenylalaninemia affect about one of every 10,000 to 20,000 Caucasian or Oriental births. The incidence in African Americans is far less. These disorders are equally frequent in males and females.


PKU and the other causes of hyperphenylalaninemia are inherited in a recessive fashion. This means an affected person inherited two traits for the disorder (e.s., one from each parent). A person with one trait for the disorder, is called a 'carrier' for PKU. Carriers do not have symptoms of the disorder.


Infants with PKU appear normal at birth. Many have blue eyes and fairer hair and skin than other family members. Currently, most symptoms of untreated PKU are avoided by newborn screening, early identification, and management. (see Treatment Section) The following describes untreated PKU symptoms-currently a rarity: About 50% of untreated infants have early symptoms, such as vomiting, irritability, an eczema-like rash, and a mousy odor to the urine. Some may also have subtle signs of nervous system function problems, such as increased muscle tone, and more active muscle tendon reflexes. Later, severe brain problems occur, such as mental retardation and seizures. Other commonly noted features in untreated children include: microcephaly (small head), prominent cheek and upper jaw bones with widely spaced teeth, poor development of tooth enamel, and decreased body growth.


Every state now screens the blood phenylalanine level of all newborns at about 3 days of age. This test is one of several newborn screening tests performed before or soon after discharge from the hospital. Usually, a few drops of blood are obtained by a small prick on the heel, placed on a card, and then sent for measurement. If the screening test is abnormal, other tests are needed to confirm or exclude PKU. Newborn screening allows early identification and early implementation of treatment. The goal of PKU treatment is to maintain the blood level of phenylalanine between 2 and 10 mg/dl. Some phenylalanine is needed for normal growth. This requires a diet that has some phenylalanine but in much lower amounts than normal. High protein foods, such as: meat, fish, poultry, eggs, cheese, milk, dried beans, and peas are avoided. Instead, measured amounts of cereals, starches, fruits, and vegetables, along with a milk substitute are usually recommended. Phenylalanine free formulas are available for all age groups. In some clinics, a phenylalanine 'challenge' may be suggested to evaluate whether or not the child continues to require a low phenylalanine diet. This test identifies those few persons with a transient or 'variant' form of the disorder. However, most authorities currently recommend lifelong dietary restriction of phenylalanine for individuals with classic PKU, in order to promote maximal development and cognitive abilities. Trying

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to reinstitute the PKU diet after a period of 'relaxation' to a regular diet, has been difficult for many individuals. Periodic phenylalanine blood level measurement, and the guidance of a nutritionist and other members of the health care team, allow individuals and families to work toward consistently maintaining the blood level in the desirable range. Fever and illness can cause normal body proteins to break down, the liberation of the body's own amino acids, and thus, a rise of the blood phenylalainine level. The physician and nutritionist can suggest dietary changes to help maintain levels in the desirable range during illness. Medical follow-up often involves periodic developmental screening. This checks for the expected normal development over time, and allows early recognition and intervention for problems.


None for treated individuals.


Maintaining phenylalanine blood levels in the recommended range maximizes the ability of individuals with PKU to reach their potential for normal development and lifespan. Authorities currently recommend that women with PKU who are of child bearing age, closely adhere to the low- phenylalanine diet and monitor phenylalanine levels before conception and throughout pregnancy. The risk of spontaneous abortion, mental retardation, microcephaly, and/or congenital heart disease in the child is high if Mother's blood phenylalanine is poorly controlled.


The presence of a chronic condition may influence one's emotional development. In addition, lifestyle adjustments to accommodate more frequent doctor visits, and dietary restrictions may be required and impact day to day activities. These and other general issues are important to understand and are discussed in the accompanying topic sheet for "Chronic Illness".

The following information addresses PKU specifically:

Untreated PKU can cause significant brain problems including retarded mental development. Current screening for PKU in the newborn period has made unrecognized PKU a rare occurance. With good dietary control, the potential effects of PKU on development are minimized. However, recent studies indicate that loss of dietary control in childhood can interfere with cognitive development. These studies support the idea that individuals with PKU should maintain good dietary control throughout their lifetime. If problem areas in development are suspected, this should be shared with the parent so that an evaluation may be done and intervention can be instituted. It is important that all community personnel who work with individuals with PKU be aware of the dietary requirements and restrictions. The parent and/or nutritionist can suggest appropriate and allowable snack and mealtime foods outside the home. Supporting appropriate dietary habits, while minimizing attention to dietary differences is important. It is also important to be available to the individual should he or she wish to share concerns or feelings about being 'different' from peers. If persistent difficulty adhering to the dietary regimen is noted, the parent should be informed so that he/she and the medical team can address this issue further. Because blood phenylalanine levels are dependent on dietary control, insufficient phenylalanine intake may cause levels to be too low for growth and body functions. Low levels can cause mental and physical sluggishness, loss of appetite, anemia, rashes, and diarrhea. If these symptoms are suspected, the parent should be contacted and arrangements made for a physician evaluation.




"Living with PKU" A publication from:

Inherited Metabolic Diseases Clinic University of Colorado Health Sciences Center Denver, Colorado 80260 Distributed by: Mead Johnson Nutritionals Evansville, Ill. 47721

This booklet contains an extensive bibliography for parents and children with PKU.

National PKU News
6869 Woodlawn Ave. NE #116
Seattle, WA 98115

Pediatric Endocrinology and Metabolism
Rainbow Babies and Childrens Hospital
Cleveland, Ohio 44106
(216) 844-3661

Dr. D. Kerr M.D. and Ms. J. McConnell - Nutritionist

Provides medical evaluation and coordinates long term follow-up for children and families with PKU.

Referrals to clinics in the Cleveland metropolitan area that are nearer to you may be obtained from your physician, Interlink at the Achievement Center for Children (216) 795-7100, or through Pediatric Endocrinology at Rainbow.

Rudolph, A.M., Hoffman, JIE, Pediatrics, 18. Appleton and Lange, Norwalk, Conn., Los Altos, Calif. 1987, p. 239-242.

Behrman, R.E., Vaughan, V.C. and Nelson, W.E., Pediatrics, 13. W.B. Saunders, Philadelphia, PA. 1987, p. 280-282. Roh, F., Friedman, E.G., Koch, R.: Maternal PKU. Metabolic Currents. Vol I. No I: 1-8, 1988. Smith, I., Beasley, M.G., Wolff, OH, Ades, E.A.: Behavior Disturbance in 8-year-old children with early treated phenylketonuria. The Journal of Pediatrics, 112:403-408, March 1988. Seashore, M.R., and others: Loss of Intellectual Foundation in children with phenylketonuria After Relaxation of Dietary Phenylalanine Restriction. Pediatrics 75:226-232, February 1985. Michals, K, and others: Return to Diet Therapy in Patients with Phenylketonuria. The Journal of Pediatrics, 106:933-

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936, June 1985.

Holtzman, N.A., and others: Loss of Dietary Control in Phenylketonuria. The New England Journal of Medicine 314:593-598, March 6, 1986.

Information contained in this topic sheet from PIRC is available to the families and community personnel who care for children with special needs, with the intent to provide general topic information only. We strongly encourage direct contact with each child's physician for specific questions regarding care, medications, activity limitations and other concerns that may arise.

PIRC is a service provided by Rainbow Babies and Childrens Hospital, funded by the following foundations:

Bingham Foundation Cleveland Foundation George Gund Foundation Thomas White Foundation and NEC Computer INC.