What’s Phenylketonuria? Symptoms, Causes & Treatment

Last updated on May 6th, 2023 at 12:15 pm

10 minutes read.    

Phenylketonuria also called PKU, is a rare inherited genetic disorder that causes an essential amino acid called phenylalanine to build up in the blood.

 PKU is caused by a change in the phenylalanine hydroxylase (PAH) gene. The PAH gene helps create the enzyme needed to break down phenylalanine.

An individual with Phenylketonuria may show symptoms like eczema, skin or hair discoloration, and musty odor to their breath, urine, or skin.

It’s important you get informed about this rare disease perhaps to help yourself and loved ones, although it rarely occurs in life.

In this article:

• What’s Phenylketonuria?
• Types
• Symptoms
• Causes
• Diagnosis
• Treatment
• Pregnancy & Phenylketonuria
• Prevention
• FAQ
• Conclusion

What’s Phenylketonuria?

Phenylketonuria (PKU) is a rare metabolic disorder characterized by the impaired conversion of phenylalanine (Phe) to tyrosine.

Phenylalanine is highly found in food that is rich in protein and an artificial sweetener called aspartame.

An individual with Phenylketonuria lacks the enzyme phenylalanine hydroxylase (PAH) and thus cannot break down phenylalanine in proteinous food into tyrosine which your body needs to create vital compounds such as neurotransmitters such as epinephrine, norepinephrine, and dopamine.

This lead to a high accumulation of phenylalanine in the blood.

Phenylketonuria is caused by a defect in the gene that helps create phenylalanine hydroxylase (PAH).

The PAH gene gives your body instructions to make an enzyme phenylalanine hydroxylase that’s responsible for converting essential amino acids phenylalanine from the proteinous food you eat into tyrosine. When this enzyme is missing, your body can’t break down phenylalanine.

Babies in developed countries are normally screened for PKU shortly after birth. The condition is quite rare in life, only affecting about 1 in 10,000 to 15,000 newborns each year.

The severe signs and symptoms of PKU are rare as early screening allows treatment to begin soon after birth. Early diagnosis and treatment can help relieve symptoms of PKU and prevent brain damage that may arise later in life. (1,2,3)

Types

What are the types of phenylketonuria (PKU)?

There are different types of phenylketonuria (PKU) based on the severity of the diagnosis. Symptoms are worse among severe cases in those who are untreated.

Types of PKU include:

• Classic PKU (most severe).
• Moderate or mild PKU.
• Mild hyperphenylalaninemia (least severe).(3)

Symptoms

What are the possible Symptoms of phenylketonuria to know?

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:

• seizures
• tremors, or trembling and shaking
• stunted growth
• hyperactivity
• skin conditions such as eczema
• a musty odor of their breath, skin, or urine

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 children

A less severe form of PKU is called variant PKU or non-PKU hyperphenylalaninemia. This occurs when the baby has too much phenylalanine in the 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.

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.(2,4)

Causes

What Causes phenylketonuria?

Phenylketonuria (PKU) is an inherited condition caused by a defect in the phenylalanine hydroxylase (PAH) gene. The PAH gene signals your body to create phenylalanine hydroxylase, the enzyme responsible for breaking down phenylalanine. 

When phenylalanine from high-protein food doesn’t break down it causes a high accumulation of phenylalanine in the blood which is very toxic to our body.

For a child to inherit PKU disorder from parents, both parents must be carriers of this defective phenylalanine hydroxylase gene

 If just one of the parents is a carrier and passes on an altered gene to their child, the child won’t have any symptoms of PKU, but they’ll be a carrier of the gene, just as it is in those with alkaptonuria. (5,6,7,8,9)

Diagnosis

How is Phenylketonuria (PKU) diagnosed?

Since the 1960s, hospitals in developed countries have routinely screened newborns for PKU by taking a blood sample. A doctor uses a needle or lancet to take a few drops of blood from your baby’s heel to test for PKU and other genetic disorders.

The screening test is performed when the baby is one to two days old and still in the hospital. If you don’t deliver your baby to a hospital, you’ll need to schedule the screening test with your doctor.

Additional tests may be performed to confirm the initial results. These tests search 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 involves taking a sample of blood and analyzing it for the presence of the enzyme needed to break down phenylalanine.(5,6,2, 3)

Treatment

How can Phenylketonuria be treated?

People with PKU can relieve their symptoms and prevent complications by following a special diet and by taking medications.

Is there a diet plan for people with Phenylketonuria?

Diet

The main way to treat PKU is to eat a special diet that limits foods containing phenylalanine. Infants with PKU may be fed breast milk. They usually also need to consume a special formula known as Lofenalac. When your baby is old enough to eat solid foods, you need to avoid letting them eat foods high in protein.

Source of phenylalanine

What foods contain phenylalanine?

• eggs
• cheese
• nuts
• milk
• beans
• chicken
• beef
• pork
• fish

To make sure that they still receive an adequate amount of protein, children with PKU need to consume PKU formula.

It contains all the amino acids that the body needs, except for phenylalanine. There are also certain low-protein, PKU-friendly foods that can be found at specialty health stores.

Related: Is it ok take a Phenylalanine supplement?

Related: Scurvy diseases: what are the causes and how do you prevent it?

People with PKU will have to follow these dietary restrictions and consume PKU formulas throughout their lives to manage their symptoms.

It’s important to note that PKU meal plans vary from person to person. People with PKU need to work closely with a doctor or dietitian to maintain a proper balance of nutrients while limiting their intake of phenylalanine.

They also have to monitor their phenylalanine levels by keeping records of the amount of phenylalanine in the foods they eat throughout the day.

In developed countries, 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 the PKU formula.

I hope you’re getting value.

Medication

The United States Food and Drug Administration (FDA) recently approved sapropterin (Kuvan) for the treatment of PKU. This may be obtainable in other countries. Check for your country to know if such exist.

 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. (2,3,10,11,12,13)

Pregnancy & Phenylketonuria

How does Phenylketonuria affect Women?

Women with PKU may be at risk of complications, including miscarriage if they don’t follow a PKU meal plan during their childbearing years.

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, including:

• 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 medical concerns your child may have.

What is the long-term outlook for people with Phenylketonuria?

The long-term outlook for people with PKU is very good if they follow a PKU meal plan closely and shortly after birth. When diagnosis and treatment are delayed, brain damage may occur. This can lead to intellectual disabilities in the child’s first year of life. Untreated PKU can also eventually cause:

• delayed development
• behavioral and emotional problems
• neurological problems, such as tremors and seizures

People with Phenylketonuria mostly benefit from working closely with a dietician who can advise on the best foods to eat, what to avoid, and how to get all the nutrients you need to stay healthy. (2,3,14,15,16,17)

One 2020 study in China showed that it’s possible for a pregnant woman that has Phenylketonuria to deliver a healthy child.

According to the research, the huge success was related to maternal blood Phenylalanine (Phe) management prior to and during pregnancy.

Thus in maternal PKU patients with poor compliance to dietary treatment, sapropterin dihydrochloride (6R-BH4) medication may be an option to improve the management of blood Phe levels. (18)

Prevention

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.

Frequently Asked Questions (FAQ)

Who does phenylketonuria (PKU) affect?

Phenylketonuria (PKU) can affect anyone who has mutations in both copies of the PAH gene. Studies suggest that there’s a higher risk among people of Native American or European descent. More studies are needed to find out for other countries.

If a person with uncontrolled PKU has high levels of phenylalanine during pregnancy, this can cause intellectual disability, birth defects, and other problems in their baby, even if the baby doesn’t have PKU.

Is phenylketonuria (PKU) dominant or recessive?

Phenylketonuria (PKU) is a genetic condition that passes to children from their parents in an autosomal recessive pattern. This means that babies receive one copy of the mutated gene that causes PKU from each parent during conception. In most cases, parents are carriers of the gene but don’t have symptoms of the condition.

How do I take care of myself?

Your phenylketonuria (PKU) diagnosis is lifelong. So if you are on a restricted diet, you still need to eat foods and take vitamins to make up for the lack of protein in your diet.

Make sure you follow your diet for your entire life to prevent symptoms that could be dangerous to your health.

If you plan on becoming pregnant, talk to your provider about genetic testing to understand your risk of having a child with a genetic condition.

When should I see my healthcare provider?

If you or your child develops symptoms of phenylketonuria, contact your healthcare provider for a thorough evaluation.

Before becoming pregnant or during your first prenatal examination, you can ask your doctor about testing (called carrier screening) to determine if you and your partner are at risk for having a child with PKU.

What questions should I ask my doctor?

• How do I make sure my baby gets enough nutrients to stay healthy?
• Do I need to take any vitamins or supplements?
• Is a low-protein diet enough to reduce my phenylalanine levels or do I need medication?

Conclusion

Phenylketonuria also called PKU, is a rare inherited genetic disorder that causes an essential amino acid called phenylalanine to build up in the blood. Food that contains phenylalanine includes all high proteinous food

PKU is caused by a change in the phenylalanine hydroxylase (PAH) gene. The PAH gene helps create the enzyme needed to break down phenylalanine

PKU symptoms can range from mild to severe. The most severe form of this disorder is known as classic PKU.

An individual with Phenylketonuria may show symptoms like eczema, skin or hair discoloration, and musty odor to their breath, urine, or skin. Early diagnosis is important the child born with phenylketonuria may seem normal at first.

It’s important to note that PKU meal plans vary from person to person. People with PKU need to work closely with a doctor or dietitian to maintain a proper balance of nutrients while limiting their intake of phenylalanine.

The United States Food and Drug Administration (FDA) recently approved sapropterin (Kuvan) for the treatment of PKU. This may be obtainable in other countries. Check for your country to know if such exist.

 Sapropterin helps lower phenylalanine levels. This medication must be used in combination with a special PKU meal plan.

Women with PKU may be at risk of complications, including miscarriage if they don’t follow a PKU meal plan during their childbearing years.

PKU is a genetic condition, so it can’t be prevented. Your phenylketonuria (PKU) diagnosis is lifelong.  Make sure you follow your diet for your entire life to prevent symptoms that could be dangerous to your health.

If you plan on becoming pregnant, talk to your provider about genetic testing to understand your risk of having a child with a genetic condition.

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