Phenylketonuria: Symptoms, Causes, Treatment and Prevention


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Phenylketonuria is a rare inherited disease that is caused by the buildup of a certain amino acid in the body called phenylalanine. Phenylketonuria which is also known as PKU occurs when phenylalanine builds up in the body to an unusual degree. Generally, the body needs proteins and proteins are made up of amino acids and phenylalanine is one of the 20 amino acids that make up proteins.


Before phenylalanine which is found in most sweeteners can be broken down, it needs an enzyme which is known as phenylalanine hydroxylase. This enzyme helps your body to break down phenylalanine and convert it to a hormone which is known as tyrosine. Tyrosine then helps your body to create neurotransmitters such as adrenaline, noradrenaline, and dopamine which controls a lot of processes in the body.

Once there is a problem with the distribution of phenylalanine in the body, it will cause a host of problems which are all classified as phenylketonuria. Research has shown that phenylketonuria is an extremely rare disease that affects 1 in 10,000 or 1 in 15,000 newborns every year.

In the United States, it has been observed that doctors screen their newborns for phenylketonuria and as such are able to detect this disease on time. Early detection of this disease can help to prevent severe health problems and help the individual to live a normal healthy life.


Phenylketonuria is caused by a defective gene from an abnormal gene mutation. This gene helps in the production of the enzyme that causes the breakdown of phenylalanine to tyrosine and if this gene is absent or defective, then the patient will suffer from a buildup of phenylalanine hence leading to phenylketonuria.

Phenylketonuria is an inherited genetic disorder that is caused by a malformation of the PAH gene. The PAH gene is responsible for the production of the enzyme phenylalanine hydroxylase which is turn is responsible for the breakdown of phenylalanine.

Once this disease goes undetected and the patient takes in a high protein food which includes meat, eggs, fish, etc, the symptoms of phenylketonuria begins to occur. Once the person has a buildup of phenylalanine, the person can begin to experience nerve cell damage in the brain and spinal cord.

For a baby to develop a defective PAH gene, both the father and the mother of the baby must have the defective gene as well and as such transfer it to the child. This kind of inheritance is known as the autosomal recessive inheritance of genes.

It is also very possible for a parent to be a carrier of this disease and not be infected with it. It is possible that the parents who are carriers of this disease not to experience any symptom of it.

For the child to develop phenylketonuria, both parents must pass on the defective gene to the child. If only one parent has the defective gene and the other parent doesn’t, then the child only becomes a carrier and will not experience the symptoms of this disease.

Hence, this disease is often caused by two parents who are carriers of this defective gene without even knowing it.


At the initial stage of phenylketonuria, there are no symptoms yet and this is because the baby has not yet been exposed to a high diet of protein. However, as the baby grows and this disease goes undetected, within a few months, the child will begin to experience symptoms of this disease.

Depending on the severity of the disease, the symptoms of phenylketonuria can be divided into two stages:

Mild stage

This stage is also known as variant phenylketonuria or non-PKU hyperphenylalaninemia. This occurs when the infant has too much phenylalanine within their body system. When the disease is still at its early stages, the baby will experience the following symptoms

  1. Musty Odour: The child will have a musty-like odour which is perceived on either the breath of the baby, the skin and/or in the urine of the infant.
  2. Hyperactivity
  3. Skin rashes
  4. Blue eyes and fair skin which is as a result of the fact that phenylalanine is not broken down to melanin which forms the dark pigment of the skin.

Severe stage

This stage is also known as the classic PKU. It is the most severe form of phenylketonuria and the symptoms associated with this stage includes

  1. Microcephaly which is known as abnormally small head.
  2. Delayed development
  3. Intellectual disabilities
  4. Neurological disorders that can include seizures
  5. Behavioural, emotional and social problems
  6. Eventual psychiatric disorders.

Phenylketonuria and pregnancy

A woman who has phenylketonuria and become pregnant is at the risk of developing a condition known as maternal PKU. Whenever these women develop this condition, they are always advised to follow a special diet before and during pregnancy and failure to do so can lead to serious conditions such as miscarriage or malformation of the baby.

This is because the buildup of phenylalanine in the blood can pose as a threat to the fetus and as such cause malformation or miscarriage. Babies who are born to mothers who have a high level of phenylalanine in their blood mostly don’t develop phenylketonuria. However, they can develop serious complications during birth. These complications may include:

  1. A decline in birth weight
  2. Abnormalities of the face
  3. Delayed development
  4. Cardiac problems such as heart defects etc
  5. Mental disabilities
  6. Behavioural problems

When to see a doctor?

You must see a doctor when the following occurs

1. When you give birth to a newborn:

After having your baby, it is important that your baby sees a doctor to undergo the PKU test. Routine medical checkups such as this help to detect if your baby is at the risk of developing phenylketonuria.

Once PKU has been detected, your doctor will start your baby on a diet which will prevent the baby from having long term severe problems.

2. Women with a history of phenylketonuria:

Women who have a history of phenylketonuria and are also of childbearing age must see a doctor and also follow the PKU diet before they become pregnant.

They must also ensure that even during pregnancy, they must continue to take the diet else they run the risk of having high levels of phenylalanine in their blood and this can also affect their babies.

3. Adults with a history of phenylketonuria:

Adults who have a history of PKU must continue to see their doctor in order to continue receiving treatments as well as medical tests and screening.

Adults who stopped their treatments should continue their treatments as this will help improve their mental health, their cognitive functions, behavioural patterns and so on.


When phenylketonuria goes u detected and untreated for a long period, complications can arise. These complications can range from mild to severe.

Some complications which may occur include:

  1. Brain malfunction which can lead to brain damage. This is characterized by intellectual disability which is seen during the first few years of life.
  2. Neurological problems such as tremors and seizures
  3. Major health problems such as heart failures etc
  4. Behavioural and social problems


If you have phenylketonuria or you have a family that has this disease, then it is important that you go for genetic screening and also maintain a low phenylalanine diet before considering the possibility of either getting pregnant or not.



This article is for informational/educational purposes only. Healthtian does not provide medical advice, diagnosis, or treatment, read more.

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