Maple Syrup Urine Disease

What else is it called?

  • BCKD Deficiency
  • Branched Chain Alpha-Ketoacid Dehydrogenase Deficiency
  • Maple Syrup Urine Disease
  • MSUD

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What causes it?

MSUD is a rare inherited metabolic disorder.

In MSUD there is too much of the amino acid leucine in the blood and too many ketoacids in the urine.

MSUD affects the way your baby breaks down protein. Many foods contain protein. The body needs protein for growth and repair.

Protein is broken down into amino acids (building blocks of protein) by enzymes (which are like chemical scissors). Enzymes then further break the amino acids into smaller parts.

Metabolism refers to the chemical processes that occur inside the cells of the body. In MSUD, the body lacks an enzyme called branched chain ketoacid dehydrogenase. This means that the body is unable to break down three amino acids called leucine, isoleucine and valine. As a result, levels of these amino acids build up in the blood.

The build-up of harmful chemicals can cause damage to the brain. It may cause delays to normal development like walking and talking.

How common is it?

MSUD is thought to affect approximately 1 person in every 100,000 in the UK and 1 person in every 185,000 worldwide.

What are the signs and symptoms?

Some babies with MSUD become ill in the first few days of life before the newborn screening result is available.

Symptoms include:

  • poor feeding
  • vomiting
  • dehydration (lack of body fluids)
  • floppy baby
  • excessively sleepy
  • seizures
  • rapid breathing
  • sweet-smelling urine (like maple syrup)

The effects of MSUD quickly become life-threatening if left untreated.

Some children may develop symptoms at a later stage. They may present with developmental delay or with an acute childhood illness such as vomiting or diarrhoea.

A liver transplant can sometimes an option to treat MSUD. If a person with MSUD has a liver transplant, they’ll no longer be at risk of a metabolic crisis and can follow a normal diet. However, this is a major procedure and carries risk and should therefore be very carefully considered. Your doctor will be able to discuss whether this is an advisable option.

How is it diagnosed?

MSUD is usually diagnosed by newborn screening. High levels of leucine are found in the blood.

Can it be treated?

MSUD is managed with the following special diet:

  • Limited high protein foods
  • Measured amounts of leucine (protein) containing foods
  • A protein substitute
  • Low protein foods
  • Isoleucine and valine supplements

These foods are high in leucine (protein) and must be avoided: meat, fish, eggs, cheese, bread, pasta, nuts, seeds, soya, Quorn and tofu.

In babies, a restricted amount of leucine (protein) is given from breast milk or measured amounts of infant formula.

The amount given will be monitored regularly by your specialist metabolic dietitian.

Protein substitute is essential for metabolic control. It will help to meet your baby’s protein, energy, vitamin and mineral requirements. It is available on prescription.

There are many low protein foods. These include fruit, many vegetables and prescribable low protein foods such as bread and pasta.

They provide:

  • a source of energy
  • variety in the diet

A ‘metabolic crisis’ causes a build-up of leucine and other toxic chemicals. It is usually triggered by childhood illnesses e.g. vomiting and diarrhoea. It is important to manage a metabolic crisis quickly and properly.

During any childhood illness, an emergency regimen is given. Illnesses can cause catabolism or protein breakdown. This will lead to a rapid build-up of leucine which can cause a metabolic crisis.

During any illness:

  • Stop all protein in food & drink. Start the emergency regimen. This is made up of protein substitute and glucose polymer
  • Continue isoleucine and valine supplements
  • Do a blood test and phone your metabolic team
  • Always take full amounts of emergency feeds as prescribed
  • If symptoms continue and/or you are worried, go immediately to the hospital
  • Regularly update your metabolic team

It is imperative that emergency feeds are started promptly and there are no delays in management.

Frequent blood tests to check leucine, isoleucine and valine levels.

  • Height and weight
  • Developmental checks
  • Diet is adjusted according to age, weight and blood tests

MSUD is a serious inherited metabolic disorder that can lead to severe brain damage. Damage can be prevented with a protein restricted diet, a protein substitute and appropriate illness management. During illness, it is imperative that emergency feeds are started promptly, followed strictly and there are no delays in management. Regular blood tests are essential to monitor the management of MSUD

Helpful hints:

  • Always ensure you have a good supply of your dietary products and that they are in date.
  • Your dietary products are prescribed by your GP.
  • These are obtained via a pharmacy or home delivery.
  • Always ensure you have your emergency feed products and a written emergency plan.
  • Always ensure you have sufficient blood testing equipment and send samples on a regular basis.
  • Medications to control fever should be given as normally recommended – always keep supplies available.

Do my family need to be tested?

Humans have chromosomes composed of DNA. Genes are pieces of DNA that carry the genetic instruction. Each chromosome may have several thousand genes.

The word mutation means a change or error in the genetic instruction.

We inherit particular chromosomes from the egg of the mother and sperm of the father.

The genes on those chromosomes carry the instruction that determines characteristics, which are a combination of the parents.

MSUD is an inherited condition. There is nothing that could have been done to prevent your baby from having MSUD.

Everyone has a pair of genes that make the branched chain ketoacid dehydrogenase enzyme. In children with MSUD, neither of these genes works correctly. These children inherit one non-working MSUD gene from each parent.

Parents of children with MSUD are carriers of the condition. Carriers do not have MSUD because the other gene of this pair is working correctly.

When both parents are carriers, in each pregnancy the risk to the baby is as follows:

  • 25% chance (1 in 4) of MSUD
  • 50% chance (1 in 2) for the baby to be a carrier of MSUD
  • 25% chance (1 in 4) for the baby to have two working genes and neither have MSUD or be a carrier

Relevant Organisations


This information is from the TEMPLE Guides which have been adapted by the Dietitians Group of the British Inherited Metabolic Disease Group (BIMDG) and is based on the original TEMPLE written by Burgard and Wendel. The TEMPLE Guides are supported by Nutricia. Dated February 2017.

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