If the first time you heard the term phenylketonuria (PKU) was when your child got a positive screening result after birth, you're not alone. PKU is rare. Only 1 in 25,000 newborns in the U.S. get a PKU diagnosis.
Even though PKU is uncommon, experts have a good understanding of the disorder. It has been included in newborn screening programs for over 50 years, in part because doctors know that with early and lifelong treatment, children can thrive.
"There's nothing about this diagnosis or the treatment for the diagnosis that limits what your kid can achieve in the future," says Michael Finkel, DO, a clinical geneticist at Children's Wisconsin. "They can be an athlete, they can be a physician, an astronaut, a lawyer, a writer. They can do anything they want in the future."
What Is PKU?
PKU is a metabolic disorder, which means it disrupts the way your child processes certain nutrients. When your child has PKU, their body doesn't make enough of an enzyme called phenylalanine hydroxylase (PAH). Without this enzyme, they can't break down the amino acid phenylalanine (Phe). Normally, PAH breaks Phe down into another amino acid called tyrosine. Tyrosine helps make hormones, neurotransmitters, and melanin, the pigment that gives skin, hair, and eyes their color.
Phe is in many foods high in protein, such as meat, fish, eggs, dairy products, nuts, and beans. Without treatment that helps limit Phe levels, Phe builds up in the blood and can damage the brain and nervous system.
PKU is a genetic condition. Your body makes PAH using instructions from the PAH gene. When a child inherits mutated (changed) versions of this gene from both parents, PAH can't work the way it's supposed to in their body. You have to get a mutated PAH gene from both parents to have PKU. If only one parent passes it down, you're a carrier for PKU and don't have the condition.
How severe PKU is can vary, depending on your child's blood levels of Phe at their newborn screening:
- Benign hyperphenylalaninemia: The lowest level of Phe (just above normal)
- Mild PKU: Slightly elevated Phe levels
- Moderate or variant: Phe levels that aren't low but aren't very high
- Classic PKU: This is the most severe form, where Phe levels are high.
Genetic testing can help doctors understand how severe your child's PKU is and which treatments they may need to manage it.
Starting treatment right away at birth is vital. If Phe levels continue to be high during your child's infancy, it can lead to problems such as:
- Severe intellectual disability
- Developmental delays
- Behavioral problems
- Seizures
- Microcephaly (an abnormally small head size)
- Learning difficulties
These outcomes are very preventable with treatment through diet and medication.
Treating PKU
The primary treatment for PKU is a carefully controlled diet that limits Phe. Many children also need medications that help lower Phe in the body. The combination of the two helps your child's Phe levels stay in a healthy range.
This starts right away, even in babies who haven't yet started solid foods. Your baby may be able to breastfeed as part of their nutrition, but they'll also need to take a special metabolic formula that has all the essential amino acids found in protein except for Phe.
"Everybody needs amino acids, and they also need phenylalanine as one of their amino acids, so we don't totally eliminate it out of the diet," says Kimberly Chapman, MD, PhD, a medical geneticist at Children's Hospital Los Angeles. "We only decrease it to a certain amount, to what someone's tolerance is."
Your doctor figures out your child's tolerance based on blood tests that tell them their Phe levels. This helps them know if your child needs medication, along with a limited diet, and what kind may work best.
"We watch the labs and figure out what your child's protein allowance is," says Chapman. "And then as soon as the phenylalanine sits in a level that it's relatively stable, then we try an oral medicine to see if the phenylalanine level drops and the tyrosine level climbs a little bit, without changing the diet."
If your child responds well, she says, it may be possible to increase the amount of natural protein in their diet and reduce the amount of metabolic formula. In some cases, your baby may be able to have more breast milk and stay at safe Phe levels.
PKU diet
You'll work closely with a metabolic dietitian to create a feeding plan and make adjustments as your child grows. Once your child is old enough to start solid foods, you'll learn what's easiest for them to eat and what you'll need to limit.
Many fruits and vegetables are naturally low in Phe. Foods you'll need to limit or avoid include:
- Meat
- Poultry
- Fish
- Eggs
- Dairy products
- Nuts
- Many beans
- Aspartame (an artificial sweetener in diet drinks, gum, and some foods)
As your child grows, you (and they) will learn how to count protein or Phe and make food choices that keep blood levels within the target range.
Despite the name, metabolic formula is not only for babies. It's often a lifelong part of treatment for many people with PKU. Drinking it helps prevent nutritional problems, such as iron deficiency and vitamin D deficiency, your child may have because of dietary restrictions.
"We have adults on these formulas because we're restricting protein from the natural foods that we eat," says Finkel.
PKU medications
For many years, the only way to treat PKU was through diet changes. These days, there are several medical therapies to help manage PKU more effectively. Not all medicines work in everyone with PKU, but some people respond so well that they can eat an almost typical diet.
Sapropterin (Kuvan). This pill you take by mouth helps PAH work better in certain people with PKU. You can take this medication at any age.
Pegvaliase (Palynziq). This treatment is a shot that works like the PAH enzyme for some people with PKU. Palynziq is approved for kids 12 and older.
Large neutral amino acids (LNAAs). Doctors call this treatment a "medical food product." It comes in powder or pill form and has the same essential amino acids you get in medical formula but in more concentrated amounts. It is typically for adults who are having a hard time keeping Phe levels in check.
Researchers are also studying gene therapies and other treatments, such as liver transplant, to help control PKU in the future.
PKU: A Lifelong Condition
PKU doesn't have a cure. Stopping treatment of the condition will affect the brain at any age. Adults who stop managing their PKU often get symptoms such as:
- Brain fog
- Trouble with concentration
- Anxiety
- Depression
- Crankiness
- Poor executive functioning
- Tremors
"There's no immediate life-threatening complications from a too high or too low phenylalanine level, it's more of a build," says Finkel. "The hints can be subtle, but many people who have become accustomed to a certain Phe level will be able to tell you when their phenylalanine level is high because they get that feeling of grogginess like they just woke up, or they feel irritable."
Your child will eventually need to be able to manage their condition on their own, so it's important to start teaching them about their condition and including them in their PKU management. Every child is different, but Finkel recommends starting conversations around age 9 that start with basic concepts.
"You can ask: 'How would you say what PKU is to your friend at lunch?'" says Finkel. "Then choosing one thing that they can start to help Mom or Dad out with as part of their PKU management."
You can start small with steps like:
- Learning how to read nutrition labels
- Helping plan family meals
- Learning how to calculate protein levels in food
- Helping with blood draws, cleaning their finger, or even doing the finger prick themselves
"Try to give them ownership over parts of their care, and then build on that," says Finkel.
Chapman encourages parents to remember that teaching self-management is part of preparing children for adulthood. Over time, they should learn how to mix their own formula, monitor blood levels, make food choices, and advocate for themselves.
PKU and Pregnancy
Although it may not yet be on your radar, with a newborn who has PKU, it's important to understand how having the condition could affect a pregnancy for them later in life.
High Phe levels during pregnancy can be toxic to a growing fetus, even if the baby doesn't have PKU. Uncontrolled maternal PKU can increase the risk of:
- Heart defects
- Microcephaly
- Growth problems
- Intellectual disability
- Other birth defects
Keeping PKU well-controlled before and during pregnancy greatly lowers these risks.
Living With PKU
If your child has just been diagnosed with PKU, it's normal to feel overwhelmed. Although there is a lot to learn, doctors have been able to successfully treat PKU for decades. They understand the condition better than ever before, and new therapies are expanding treatment options.
Other families who are living with PKU can be a valuable resource as you navigate this new diagnosis. Check out:
"You're not alone," says Chapman. "Talk to your specialist. They'll be able to walk you through. There is no stupid question. There's only the question you don't ask."
Show Sources
Photo Credit: Science Photo Library/Getty Images
SOURCES:
Michael Finkel, DO, clinical geneticist, Children's Wisconsin.
Kimberly Chapman, MD, PhD, medical geneticist, Children's Hospital Los Angeles.
MedlinePlus: "Phenylketonuria."
Pediatrics: "The Political History of PKU: Reflections on 50 Years of Newborn Screening."
StatPearls: "Phenylketonuria (PKU)."
National PKU Alliance: "A PKU Guide for New Parents."
Children's Hospital of Pittsburgh: "What Is Phenylketonuria (PKU)?"
National Organization for Rare Disorders: "Phenylketonuria."
FDA: "Palynziq."
Boston Children's Hospital: "Phenylketonuria (PKU)."
National Health Service (U.K.), Cambridge University Hospitals: "Information for individuals with phenylketonuria (PKU) on planning a pregnancy."