PKU is inherited as a single-gene disorder, which is a condition caused by a mutant or abnormal gene. It is an autosomal recessive disorder, which means that each parent of a child with PKU carries one defective gene for the disorder and one normal gene. In a recessive condition, an individual must have two defective genes in order to have the disorder. Individuals with only one copy of a defective gene are called “carriers,” show no symptoms of having the disease, and usually remain unaware of their status until they have an affected child. In order for a child to inherit PKU, both parents must be PKU carriers. When this occurs, there is a one in four chance of their producing an affected child with each pregnancy. Boys and girls are equally at risk of inheriting this disorder.
Before the 1960s, most infants born with PKU developed mental retardation and CEREBRAL PALSY. Although treatment for PKU using a low phenylalanine diet was first described in the 1950s, the inability to detect PKU early in the child’s life limited effective treatment. The first newborn screening test was developed in 1959 specifically to test for PKU. This simple yet effective and economical test was developed to screen newborn infants for PKU before they leave the hospital. Today all states routinely screen newborns for PKU. The American Academy of Pediatrics recommends that infants receiving the test during the first 24 hours of life be retested at two to three weeks of age during their first postnatal pediatric visit.
To test for PKU, a few drops of blood are taken from the infant’s heel and then tested in a state laboratory for abnormal amounts of phenylalanine. The normal phenylalanine level is less than two milligrams per deciliter (mg/dl). Those with phenylalanine levels of 20.0 mg/dl or higher are considered likely to have “classical” PKU. Infants with these high levels are further tested to confirm the diagnosis before treatment is started.
Some infants with slightly higher levels of phenylalanine have “mild hyperphenylalanemia.” Today many clinicians believe that any child with a phenylalanine level greater than six or eight mg/dl should be treated with a modified phenylalanine restricted diet.
Although PKU is not preventable, its symptoms can often be treated successfully through the use of a carefully regimented diet with a restricted phenylalanine content. Babies are given a special formula that contains very low phenylalanine levels; then they gradually progress to eating certain vegetables and other foods that are low in phenylalanine. Affected children must have their blood tested regularly to ensure the presence of the correct level of phenylalanine. Foods recommended for those affected by PKU contain small amounts of protein, such as fruits and vegetables, limited amounts of cereal and grain products, and special low-protein products.
High-protein foods should be avoided, including meat, fish, eggs, poultry, dairy products, nuts, peanut butter, legumes, and soy products. Artificial sugar products containing Nutrasweet also should be avoided. The food program used to treat those with PKU is quite expensive, typically costing up to $10,000 a year or more. Although health departments may pay for the formula in some states and mandated insurance coverage may cover the cost in other states, most insurance companies do not cover the cost of treatment for those with PKU because it is considered nutritional rather than medical therapy.
While phenylalanine-restricted diets have proven to be highly effective in preventing mental retardation, it is now recognized that there may still be subtle cognitive deficits. Usually the individual has a normal IQ, but the incidence of ATTENTION DEFICIT HYPERACTIVITY DISORDER (ADHD) and learning disabilities is higher compared to those children who do not have PKU.
Recent studies have found that children with PKU who stopped the diet in early childhood did not develop as rapidly as children who remained on the diet, and they also had more learning disabilities, behavioral problems, and other neurological problems. Thus, until research provides alternative treatments, everyone with PKU should remain on a restricted diet indefinitely in order to maintain a safe level of phenylalanine (believed to be in the range of two to six mg/dl).
Pregnancy and PKU
Pregnant women with PKU who are not receiving dietary therapy have high levels of phenylalanine that can damage their unborn child, causing mental retardation and other congenital defects. High levels of phenylalanine are extremely toxic to the brain of a fetus. Although the child does not have PKU, there will be brain damage from the toxic effects of phenylalanine in the womb. This is known as maternal PKU.
More than 90 percent of infants born to women with PKU who are not on a specialized diet will have mental retardation and may also have small head size, heart defects, and low birth weight. These infants cannot be treated with a special diet since they do not have PKU. Therefore, women who have PKU should be on a phenylalanine restricted diet at least one year before pregnancy and should stay on the diet while breast-feeding to increase the chance of having a healthy child.