What Is Phenylketonuria?
Phenylketonuria (PKU) is a rare genetic condition that causes an
amino acid called phenylalanine
to build up in the body. Amino acids are the building blocks of protein.
Phenylalanine is found in all proteins and some artificial sweeteners. Your
body uses an enzyme called phenylalanine hydroxylase to convert phenylalanine into
tyrosine, a nonessential amino acid. Your body needs tyrosine to create neurotransmitters,
such as epinephrine, norepinephrine, and dopamine.
PKU is caused by a defect in the gene that helps create phenylalanine
hydroxylase. When this enzyme is missing, the body is unable to break down phenylalanine. This causes a buildup of
phenylalanine in the body. Early diagnosis and treatment can help
relieve symptoms of PKU and prevent brain damage.
Babies in the United States are screened for PKU shortly after
birth. As a result, the condition is rare in this country, only affecting about
in 10,000 to 15,000 newborns each year.
What Are the Symptoms of Phenylketonuria?
PKU symptoms can range from mild to severe. The most severe form
of this disorder is known as classic
PKU. An infant with classic PKU may appear normal for the first few
months of their life. If the baby isn’t treated for PKU during this time, they’ll
start to develop the following symptoms:
- tremors, or trembling and shaking
- stunted growth
- skin conditions, such as eczema
- a musty odor of their breath, skin, or urine
A less severe form of PKU is called variant PKU or non-PKU hyperphenylalaninemia. This occurs when the
baby has too much phenylalanine in their body. Infants with this form of the
disorder may have only mild symptoms, but they’ll need to follow a special diet
to prevent intellectual disabilities.
If PKU isn’t diagnosed at birth and treatment isn’t started
quickly, the disorder can cause:
- irreversible brain damage and intellectual
disabilities within the first few months of life
- behavioral problems and seizures in older
Once a specific diet and other necessary treatments are started,
symptoms start to diminish. People with PKU who properly manage their diet usually
don’t show any symptoms.
What Causes Phenylketonuria?
PKU is an inherited condition caused by a defect in the PAH gene.
The PAH gene helps create phenylalanine hydroxylase, the enzyme responsible for
breaking down phenylalanine. A
dangerous buildup of phenylalanine can occur when someone eats high-protein
foods, such as eggs and meat. Both parents must pass on a defective version of
the PAH gene for their child to inherit the disorder. If just one parent passes
on an altered gene, the child won’t have any symptoms, but they’ll be a carrier
of the gene.
How Is Phenylketonuria Diagnosed?
Since the 1960s, hospitals in the United States have been routinely
screening newborns for PKU by taking a blood sample. A doctor will use a needle
or lancet (surgical instrument) to take a few drops of blood from your baby’s
heel to test for PKU and other genetic disorders.
The screening test will be performed when the baby is one to two
days old and still in the hospital. If you don’t deliver your baby in a
hospital, you’ll need to schedule the screening test with your doctor.
Additional tests may be performed to confirm the initial results.
They’ll test for the presence of the PAH gene mutation that causes PKU. These
tests are often done within six weeks after birth.
If a child or adult shows symptoms of PKU, such as developmental
delays, the doctor will order a blood test to confirm the diagnosis. This test
will involve taking a sample of blood and analyzing it for the presence of the
enzyme needed to break down phenylalanine.
How Is Phenylketonuria Treated?
People with PKU can relieve their symptoms and prevent
complications by following a special diet and taking medications.
The main way to treat PKU is to eat a special diet that limits foods
containing phenylalanine. Infants with PKU can’t be fed breast milk and must
consume a special formula that’s often known as Lofenalac. When your baby is
old enough to eat solid foods, you’ll need to avoid letting them eat foods high
in protein. These include:
To make sure that their body still receives an adequate amount of
protein, children with PKU need to consume PKU formula, which contains all the
amino acids that the body needs, except phenylalanine. There are also certain
low-protein, PKU-friendly foods that can be found at specialty health stores. Children
will have to follow the dietary restrictions and consume PKU formula throughout
their life to manage their symptoms.
It’s important to note that PKU meal plans vary from
person-to-person. People with PKU will need to work closely with a doctor or
dietitian to maintain a proper balance of nutrients while limiting their intake
of phenylalanine. They’ll also have to monitor their phenylalanine levels by
keeping records of the amount of phenylalanine in the foods they eat throughout
Some state legislatures have enacted bills that provide some
insurance coverage for the foods and formulas necessary to treat PKU. Check
with your state legislature and medical insurance company to find out if this
coverage is available for you. If you don’t have medical insurance, you can
check with your local health departments to see what options are available to
help you afford PKU formula.
The United States Food and Drug Administration (FDA) recently
(Kuvan) for the treatment of PKU. Sapropterin helps lower phenylalanine
levels. This medication must be used in combination with a special PKU meal
plan. However, it doesn’t work for everyone with PKU. It’s most effective in
children with mild cases of PKU.
Pregnancy and Phenylketonuria
Woman with PKU may be at risk for complications if they don’t
follow a PKU meal plan during their childbearing years. This includes possible
miscarriage. There’s also a chance that the unborn baby will be exposed to high
levels of phenylalanine. This can lead to various problems in the baby,
- intellectual disabilities
- heart defects
- delayed growth
- low birth weight
- an abnormally small head
These signs aren’t immediately noticeable in a newborn, but a
doctor will perform tests to check for signs of any abnormalities or medical
conditions your child may have.
What Is the Long-Term Outlook for People
The long-term outlook for people with PKU is very good if a PKU
meal plan is followed closely and started shortly after birth. When diagnosis
and treatment are delayed, brain damage may occur. This can lead to intellectual
disabilities by the child’s first year of life. Untreated PKU can also
- delayed development
- behavioral and emotional problems
- neurological problems, such as tremors and
Can Phenylketonuria Be Prevented?
PKU is a genetic condition, so it can’t be prevented. However, an
enzyme assay can be done for people who plan on having children. An enzyme
assay is a blood test that can determine whether someone carries the defective
gene that causes PKU. The test may also be done during pregnancy to screen
unborn babies for PKU.
If you have PKU, you can prevent symptoms by following your PKU
meal plan throughout your life.