When Megan Popps noticed her newborn son Leo had an unusually sweet smell, she initially thought little of it. The odour reminded her of pancakes and maple syrup, which she associated with food she had eaten after giving birth.

But further medical checks eventually led to a diagnosis of maple syrup urine disease (MSUD), a rare inherited metabolic disorder that can become life-threatening without prompt treatment.

The case highlights why unusual symptoms in newborns should not always be dismissed, particularly when accompanied by feeding difficulties, unusual sleepiness, or other concerning changes.

What Is Maple Syrup Urine Disease?

MSUD is a genetic condition in which the body cannot properly break down three branched-chain amino acids: leucine, isoleucine, and valine.

These amino acids are found in protein-containing foods. In people with MSUD, a deficiency in the enzyme complex responsible for breaking them down causes them and their by-products to build up in the blood and tissues.

The condition gets its name from the characteristic sweet smell that can develop in the urine, sweat, and earwax of affected babies. The odour has been described as resembling maple syrup or burnt sugar.

MSUD is rare. GeneReviews estimates that it affects about one in 185,000 births worldwide, although prevalence varies between populations.

Why Can It Be Dangerous?

Babies with the severe, or 'classic', form of MSUD can initially appear healthy. However, toxic levels of amino acids can build up rapidly during the first days of life.

According to GeneReviews, babies may develop poor feeding, irritability, excessive sleepiness, and loss of appetite within the first few days.

Without treatment, the condition can progress to neurological complications, including brain swelling, coma, and respiratory failure.

The build-up of leucine is particularly concerning because high levels can damage the developing brain.

How Is MSUD Diagnosed?

Newborn screening can help identify babies at risk before serious symptoms develop. In the UK, MSUD is among the conditions assessed through the newborn blood spot screening programme.

A small blood sample is usually taken from a baby's heel around five days after birth and tested for markers associated with several serious conditions.

If scree ning suggests MSUD, further blood tests can measure amino acid levels and confirm the diagnosis. However, parents should not wait for screening results if their newborn appears unwell.

Persistent poor feeding, unusual lethargy, vomiting, abnormal movements, or difficulty waking should be assessed urgently.

How Is It Treated?

Treatment aims to prevent harmful amino acids from accumulating while ensuring the baby receives adequate nutrition.

People with MSUD generally require a carefully controlled diet that restricts leucine and other branched-chain amino acids. Special medical formulas can provide nutrients without excessive amounts of the amino acids the body cannot process.

Regular blood tests help doctors monitor amino acid levels and adjust treatment. Illness, fasting, and other forms of physical stress can trigger a metabolic crisis, so families are usually given specific emergency treatment plans.

Why Early Treatment Matters

Without treatment, classic MSUD can progress rapidly and become fatal. Early diagnosis allows dietary treatment to begin before severe neurological damage develops.

While the pancake-like smell can be a characteristic sign, it does not by itself mean a baby has MSUD. Many newborn symptoms have other, more common explanations.

For parents, however, an unusual sweet smell accompanied by poor feeding, marked sleepiness, vomiting, or changes in behaviour warrants prompt medical attention.

Early recognition and treatment can make a significant difference to children living with this rare condition.

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Frequently Asked Questions

  • What is Maple Syrup Urine Disease (MSUD)?
    MSUD is a genetic condition where the body cannot properly break down certain amino acids, leading to a build-up that can be life-threatening.
  • What are the symptoms of MSUD?
    Symptoms include a sweet, pancake-like smell in the urine, poor feeding, unusual sleepiness, irritability, and vomiting.
  • How is MSUD diagnosed?
    MSUD is diagnosed through newborn screening and confirmed with blood tests measuring amino acid levels.
  • How is MSUD treated?
    Treatment involves a controlled diet restricting certain amino acids, regular blood tests, and emergency plans for metabolic crises.
  • Why is early treatment important for MSUD?
    Early treatment prevents severe neurological damage and can be life-saving for children with MSUD.