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
CAUSE: 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.
SYMPTOMS: 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. The following describes untreated PKU symptomscurrently a rarity:
About 50% of untreated infants have early symptoms, such as vomiting, irritability, an eczema-like rash, and a musty 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.
Other manifestations may include: Failure to thrive, Hyperactivity and unpredictable erratic behavior.
Fright reactions, screaming episodes, disorientation, Failure to respond to strong stimuli and convulsion
•The objective in diagnosing or treating the disorder is to prevent mental retardation.
•Serum phenylalanine levels greater than 4 mg/dl is abnormal, the normal values is 2 mg/dl. Significant brain damage usually occurs when levels are greater than 10 – 15 mg/dl
Treatment of PKU is dietary, since the genetic enzyme is intracellular. The dietary management must do the following:
•Meet the child’s need from nutrition for optimum growth
•Maintain phenylalanine levels within a safe level.
•Natural food proteins contain about 15% phenylalanine thus specially prepared milk substitutes must be used.
Newborn babies are screened for the blood phenylalanine level 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
PKU and the Pregnant Woman
•Statistics for offspring of untreated women with PKU
–92% risk for mental retardation
–73% risk for microcephaly
–12% risk for congenital heart disease
–Ensuring that the heel stick is performed before discharge
–Properly record the specimen and send for analysis
–After diagnosis, assist with parent education re:proper dietary management
–A diet low in phenylalanine
–If begun before 3 months of age, can limit damage to CNS
–Blood phenylalanine levels should be kept below 15-20mg/dL for life r/t brain myelination continues actively into adulthood
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