Phenylketonuria – Symptoms and Causes
Overview
Phenylketonuria (PKU) is a rare genetic disorder. It prevents the body from properly breaking down an amino acid called phenylalanine.
This happens because of a change in the phenylalanine hydroxylase (PAH) gene. This gene should create an enzyme to process this substance.
When someone has PKU, eating foods with protein or products containing aspartame causes phenylalanine to build up to harmful levels. This buildup can lead to serious health problems over time.
People with PKU must follow special diets that limit phenylalanine throughout their lives. These diets restrict many protein-containing foods. Some newer medications may allow certain PKU patients to eat more foods with phenylalanine or even have no restrictions.
Screening and Treatment:
- Newborn screening for PKU is standard in the United States and many countries
- Early detection is crucial for preventing complications
- While there’s no cure, proper management prevents intellectual disability
- Treatment must begin immediately after diagnosis
Most people with PKU need lifelong management of their condition. With proper care and diet control, individuals with PKU can lead healthy, normal lives despite having this inherited metabolic disorder.
Signs and Warning Signs
PKU symptoms vary in severity among people. Classic PKU is the most serious type, where the enzyme needed to break down phenylalanine is missing or barely working. This can lead to high phenylalanine levels that may cause severe brain damage.
In milder forms, the enzyme still works partially, so phenylalanine levels don’t rise as high. This means there’s less risk of serious brain damage.
No matter which type someone has, most people with PKU need to follow a special diet throughout their lives to prevent intellectual disabilities and other problems.
Expecting Mothers with PKU
Women with PKU who become pregnant face special concerns. If they don’t follow their PKU diet before and during pregnancy, high phenylalanine levels can harm their developing baby.
Even women with milder PKU can put their unborn children at risk by not following the diet. Babies born to mothers with high phenylalanine levels might have:
- Low birth weight
- Unusually small head
- Heart problems
- Delayed development
- Intellectual disabilities
- Behavior problems
These issues can happen even if the baby doesn’t inherit PKU.
When to Contact a Doctor
For Newborns
If screening tests show your baby might have PKU, your doctor will want to start treatment right away to prevent long-term problems.
For Women Planning Pregnancy
Women with PKU should see their doctor and follow the PKU diet before getting pregnant and during pregnancy to protect their babies.
For Adults with PKU
Ongoing care is important. Adults who stopped their PKU diet during their teen years should consider talking to their doctor. Returning to the diet may improve thinking and behavior and prevent further nervous system damage.
Causes
PKU happens when a genetic change affects how the body handles an amino acid called phenylalanine. Without the right enzyme, this substance builds up and can harm the body, especially the brain.
How PKU Is Passed Down
PKU follows what doctors call an autosomal recessive pattern. This means a child needs to get the changed gene from both parents to develop the condition.
When someone has just one copy of the changed gene, they’re called a carrier. Carriers don’t have PKU themselves.
If both parents are carriers:
- Each child has a 25% chance of having PKU
- Each child has a 50% chance of being a carrier
- Each child has a 25% chance of not having the gene change at all
Many parents who pass on PKU don’t know they carry the gene. They may have no family history of the condition.
When a person with PKU eats foods high in protein like:
- Dairy products
- Meats
- Nuts
- Bread and pasta
Their body can’t process the phenylalanine properly. Even artificial sweeteners like aspartame can cause problems.
The severity of PKU varies. Some people have mild forms, while others have moderate or severe cases, depending on how much enzyme their body can make.
Risk Factors
Several factors increase the chance of a person having PKU:
Genetic Inheritance: PKU occurs when a child receives altered genes from both parents. Each parent must pass down the mutated gene for the condition to develop.
Ethnic Background: While PKU affects people from many ethnic groups worldwide, in the United States it appears more frequently in people with European ancestry. The condition is less common among those with African ancestry.
Health Issues
Untreated PKU can cause serious health problems. When babies don’t get proper treatment, they can develop brain damage that cannot be fixed. This damage often starts in the first few months of life.
People with untreated PKU might have seizures or tremors. Children and adults may face behavioral and emotional difficulties that affect their social lives.
When pregnant women with PKU have high phenylalanine levels, their unborn babies can be harmed.
Common complications include:
- Permanent brain damage
- Significant intellectual disabilities
- Neurological problems
- Behavioral challenges
- Developmental delays
Prevention
Planning ahead can help reduce risks for women with PKU who wish to become pregnant.
Diet Management Before Pregnancy
Women with PKU should return to a low-phenylalanine diet before trying to conceive. This preventive step helps protect the developing baby from potential harm. Special nutritional supplements designed for people with PKU can provide adequate protein and nutrition during pregnancy.
Important steps for women with PKU planning pregnancy:
- Restart the low-phenylalanine diet before conception
- Consult with healthcare providers about pregnancy planning
- Use PKU-specific nutritional supplements as recommended
Genetic Counseling
Meeting with a genetic specialist before pregnancy can be valuable for a few groups of people. These include people with PKU, close relatives of someone with PKU, and parents who already have a child with PKU.
A geneticist can explain how PKU passes through families. They can also assess the likelihood of having a child with this condition. This information helps with informed family planning decisions and prepares prospective parents for possible outcomes.
