Maple Syrup Urine Disease: Understanding Causes, Symptoms & Treatment


The uncommon metabolic illness known as maple syrup urine disease (MSUD) is inherited. Your body cannot break down some amino acids due to the illness.

As your body digests the protein in food, only amino acids are left. Amino acids are transformed by certain enzymes so that they may be utilised to support every bodily function. The amino acids and their waste products referred to as keto acids, build up in your body if any of the required enzymes are deficient or absent. As the concentrations of these compounds rise, it may cause:

  • Psychological harm
  • Coma
  • Potentially fatal situations

An enzyme called BCKDC is missing from the body in MSUD (branched-chain alpha-keto acid dehydrogenase complex). Leucine, isoleucine, and valine, generally known as BCAAs, are three significant amino acids that are processed by the BCKDC enzyme (branched-chain amino acids).

Protein-rich meals including meat, eggs, and milk include BCAAs.

Untreated MSUD can result in serious neurological and physical issues. Dietary restrictions can be used to manage MSUD. Blood tests can be used to track the effectiveness of this approach. Early detection and treatment increase the likelihood of long-term success.

Different MSUDs

There are four MSUD subtypes. All have genetic roots and are inherited. The degree of enzyme activity, severity, and age at which the illness manifests are different.

Typical MSUD

This is the condition's most prevalent and severe manifestation. The enzyme activity of a person with this type is minimal if any at all. After a few days of birth, babies exhibit symptoms. The body of the newborn normally starts processing protein from feedings at the time of onset.

Intermediate MSUD

This MSUD variant is uncommon. The age of onset and symptoms varies widely. Around 3 to 8% more enzyme activity than in people with traditional MSUD is present in those with this kind of MSUD.

Intermittent MSUD

Normal physical and intellectual growth and development are unaffected by this type. The typical onset of symptoms is between the ages of 1 and 2 years. This is a more moderate variation of the standard MSUD. Around 8 to 15 percent of normal enzyme activity is present in individuals. The first response of the disease frequently develops when a youngster is under stress, is unwell, or has an unusually high protein intake.

MSUD responsive to thiamine

With high doses of thiamine or vitamin B-1, this uncommon variant of the illness frequently gets well. Symptoms typically start after childhood. Notwithstanding the potential benefits of thiamine, dietary limitations are also required.


A recessive genetic condition is MSUD. Your parents are the source of all illness variants. The four types of MSUD are brought on by alterations, or mutations, in the BCKDC enzyme-related genes. The BCKDC enzymes are either not produced or function improperly when those genes are faulty. The chromosomes you obtain from your parents have these gene alterations.

Parents of children with MSUD often carry one mutant gene and one normal gene for the condition, but they do not themselves suffer the illness. Despite possessing the faulty recessive gene, they are unaffected by it. If you have MSUD, one defective BCKDC gene comes from each parent.

Risk Factors

Whether or not your parents are MSUD carriers affects your likelihood of developing the condition. When both parents carry the gene, their offspring have a:

  • 25 percent likelihood of developing MSUD and two mutant genes
  • 50% probability of becoming a carrier and having just one faulty gene
  • 25% likelihood that each parent will contribute one normal gene.
  • You cannot convey BCKDC to your offspring if you have two normal BCKDC genes.

One of their offspring may get the condition if two parents have the recessive BCKDC gene, while other children may not. Their children have a 50% probability of being carriers, nevertheless. They could also run the chance of conceiving a kid with MSUD later in life.


The following are some first signs and symptoms of the typical MSUD:

  • Lethargy
  • Lack of appetite
  • Slim down
  • Irritation and poor sucking ability
  • Characteristic ear wax, perspiration, and urine smell of maple sugar
  • inconsistent sleeping habits
  • Hypertonia (muscle stiffness) and hypotonia occur in alternating bouts (muscle limpness)
  • Cry that is loud

The following are symptoms of intermediate and thiamine-responsive MSUD:

  • Seizures
  • Deficits in the nervous system
  • Developing slowly
  • Feeding issues
  • Bad growth
  • Characteristic ear wax, perspiration, and urine smell of maple sugar


There is no way to avoid MSUD because it is a hereditary condition. Your risk of having a child with MSUD can be evaluated with the assistance of a genetic counselor. You can find out if you or your spouse is a carrier of the illness through genetic testing. Before birth, a fetus with the condition can be detected by DNA testing.


If your baby is identified as having MSUD, seeking immediate medical attention can help prevent significant health issues and intellectual disabilities. The first course of therapy entails lowering the blood levels of BCAAs in your infant.

This typically entails the intravenous (IV) delivery of amino acids devoid of BCAAs together with glucose for increased caloric intake. Due to the treatment, the body will utilize leucine, isoleucine, and valine more efficiently. It will simultaneously lower the BCAA level and supply the required protein.

Along with a metabolic expert and a dietician, your doctor will develop a long-term treatment plan for your child with MSUD. Your child will receive all the protein and nutrients required for healthy growth and development as part of the treatment plan. The scheme will also prevent their blood from accumulating an excessive amount of BCAAS.

Complications of Maple Syrup Urine Disease

Undiagnosed and untreated MSUD complications can be serious and even deadly. Metabolic crises, instances of acute illness that can affect even infants undergoing therapy, are possible.

When the body's level of BCAAs suddenly and dramatically increases, metabolic crises result. The condition can cause catastrophic physical and neurological harm if left untreated. A metabolic crisis often manifests as:

  • Excessive sluggishness or tiredness
  • Lack of attention
  • Irritability
  • Vomiting

Severe problems like the ones listed below can develop when MSUD is not recognised or metabolic crises are not treated:

  • Seizures brain enlargement
  • Brain's absence of blood flow
  • Metabolic acidosis: coma caused by an excessive quantity of acid in the blood

When these circumstances arise, they may lead to:

  • Significant brain injury
  • Intellectual disability
  • Blindness
  • Spasticity, or excessive muscular tension
  • If untreated, life-threatening complications may eventually arise and result in death.


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Page last reviewed: Mar 22, 2023

Next review due: Mar 22, 2025

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