Maple syrup urine disease (MSUD) is a rare but serious inherited condition.

It means the body can’t process certain amino acids (the “building blocks” of protein), causing a harmful build-up of substances in the blood and urine.

Normally, our bodies break down protein foods such as meat and fish into amino acids. Any amino acids that aren’t needed are usually broken down and removed from the body.

Babies with MSUD are unable to break down the amino acids leucine, isoleucine and valine. Very high levels of these amino acids are harmful.

One of the characteristic symptoms of MSUD is sweet-smelling urine, which gives the condition its name.

Diagnosing MSUD

At around five days old, babies are offered newborn blood spot screening to check if they have MSUD. This involves pricking your baby’s heel to collect drops of blood to test.

If MSUD is diagnosed, treatment can be given straight away to reduce the risk of serious complications. With early diagnosis and the correct treatment, the outcome can be greatly improved. However, treatment for MSUD must be continued for life.

Without treatment, severe and life-threatening symptoms can develop, including seizures (fits) or falling into a coma. Some children with untreated MSUD are also at risk of brain damage and developmental delay.

Around 1 in 185,000 children worldwide are thought to be affected by the condition.

Symptoms

Symptoms of MSUD usually appear within the first few days or weeks after birth. More general symptoms include:

sweet-smelling urine and sweat

poor feeding or loss of appetite

weight loss

Babies with MSUD may also have episodes known as a metabolic crisis, sometimes early in their life. Symptoms of a metabolic crisis include:

lack of energy

vomiting

irritability

breathing difficulties

It’s important to get medical help immediately if your baby develops symptoms of a metabolic crisis. Your doctor will give you advice to help you recognise the signs.

In some cases, a metabolic crisis may be triggered later in childhood by an infection or illness. The hospital will provide you with emergency treatment instructions to follow if your child is ill, which helps prevent these symptoms developing.

Treating MSUD

Diet

Children diagnosed with MSUD are first referred to a specialist metabolic dietitian and given a low-protein diet. This is tailored to reduce the amount of amino acids your baby receives, especially leucine, valine and isoleucine.

High-protein foods need to be limited, including:

meat

fish

cheese

eggs

pulses

nuts

Your dietitian will provide detailed advice and guidance, as your baby still needs some of these foods for healthy growth and development.

Some children need to take supplements of isoleucine and valine alongside the prescribed diet. This is to help maintain a healthy level of these amino acids in the blood without causing harm. Blood tests are needed to monitor these levels.

Breastfeeding and baby milk also need to be monitored and measured, as advised by your dietitian. Regular baby milk contains the amino acids that need to be restricted, so a special formula is used instead. This contains all the vitamins, minerals and other amino acids your baby needs.

People with MSUD need to follow a low-protein diet for the rest of their life to reduce the risk of a metabolic crisis. As your child gets older they’ll eventually need to learn how to control their diet and will stay in contact with a dietitian for advice and monitoring.

Emergency treatment

If your baby develops an infection, such as a high temperature or cold, their risk of having a metabolic crisis increases. It’s possible to reduce the risk by changing to an emergency diet while they’re ill.

Your dietitian will provide detailed instructions, but the aim is to replace milk and foods containing protein with special high-sugar drinks and amino acid supplements.

If your baby can’t keep down their emergency feeds or has repeated diarrhoea, contact the metabolic team at the hospital to let them know you’re heading straight to the accident and emergency (A&E) department.

You should also be given a leaflet to bring with you in the event of an emergency in case the doctors haven’t seen MSUD before.

Once in hospital, your baby can be monitored and treated with fluids given directly into a vein (intravenous fluids).

You should also take your baby to hospital if they develop the symptoms of a metabolic crisis, such as irritability, loss of energy or breathing difficulties.

Liver transplant

A liver transplant is sometimes an option to treat MSUD. If a person with MSUD receives a donated liver, they’ll no longer be at risk of a metabolic crisis and can have a normal diet.

However, a liver transplant is a major procedure with its own risks. You will have to take medicine to suppress the immune system (immunosuppressant medication) for the rest of your life to stop your body rejecting the new liver.

It’s important to consider all the pros and cons before deciding whether or not to have a liver transplant. Your doctor will be able to discuss whether this is a suitable option.

How MSUD is inherited

The genetic change (mutation) responsible for MSUD is passed on by the parents, who usually don’t have any symptoms of the condition. This is known as autosomal recessive inheritance.

This means a baby needs to receive two copies of the altered genes to develop the condition – one from their mother and one from their father. If the baby only receives one mutated gene, they’ll just be a carrier of MSUD.

If you’re a carrier of the affected genes and you have a baby with a partner who’s also a carrier, your baby has:

a 25% chance of developing the condition

a 50% chance of being a carrier of MSUD

a 25% chance of receiving a pair of normal genes

Although it’s not possible to prevent MSUD, it’s important to let your midwife and doctor know if you have a family history of the condition. Any further children you have can be tested for the condition as soon as possible and given appropriate treatment.

You may also wish to consider genetic counselling for support, information and advice about genetic conditions.

Information about your child

If your child has MSUD, your clinical team will pass information about him or her on to the National Congenital Anomaly and Rare Diseases Registration Service (NCARDRS).

This helps scientists look for better ways to prevent and treat this condition. You can opt out of the register at any time.