Phenylketonuria

Introduction

     Phenylketonuria (PKU) is a rare condition in which the body does not properly break down (metabolize) an amino acid called phenylalanine.

Causes

     Phenylketonuria (PKU) is inherited, which means it is passed down through families. Both parents must pass on the defective gene in order for a child to have PKU. This is called an autosomal recessive trait.

Symptoms 

• Skin rashes (eczema)

• Microcephaly

• Tremors

• Jerking movements of the arms or legs (spasticity)

• Unusual positioning of hands

• Seizures

• Hyperactivity

• Delayed mental and social skills

• Mental retardation

• A distinctive "mousy" odor to the urine, breath, and sweat

• Light complexion, hair, and eyes

Screening and Diagnostics

     Tests include an enzyme assay to determine if the parents carry the defective gene responsible for PKU. During pregnancy, a chorionic villus sample may be taken to screen the unborn baby for PKU.

     PKU is a treatable disease that can be easily detected with a simple blood test. Most states require a PKU screening test for all newborns. The test is generally done with a heelstick shortly after birth.

Treatment

• Treatment involves a diet that is extremely low in phenylalanine, particularly when the child is growing. However, sticking to the diet is difficult because phenylalanine products often have poor taste.

• The diet must be strictly followed to prevent or reduce mental retardation. This requires close supervision by a registered dietitian or doctor, and cooperation of the parent and child. Those who continue the diet into adulthood have better physical and mental health.

• Phenylalanine occurs in significant amounts in milk, eggs and other common foods. The artificial sweetener NutraSweet (aspartame) also contains phenylalanine. Any products containing aspartame should be avoided by children with this disorder.

• A special infant formula called Lofenalac is made for infants with PKU. It can be used throughout life as a protein source that is extremely low in phenylalanine and balanced for the remaining essential amino acids.

• Tetrahydrobiopterin (BH4) has been used in mild cases as an experimental treatment. Taking supplements to replace the long chain fatty acids missing from a standard phenylalanine-free diet may be helpful.

Prognosis

     The outcome is expected to be very good if the diet is closely followed, starting shortly after the child's birth. If treatment is started after 3 years or if the disorder remains untreated, brain damage will occur.

Complications 

     Severe mental retardation occurs if the disorder is untreated. ADHD (attention-deficit hyperactivity disorder) appears to be the most common problem seen in those who do not stick to a very low-phenylalanine diet.

Prevention  

     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 very important that women with PKU closely follow a strict low-phenylalanine diet both before becoming pregnant and throughout the pregnancy, since build-up of phenylalanine will damage the unborn baby even if the child has not inherited the defective gene.