What is maple syrup urine disease (MSUD)?

In countries where newborn screening is available MSUD can be diagnosed in the first few days of life, MSUD is an inherited genetic disorder caused by a malfunction of three essential enzymes that break down certain food-based proteins called amino acids, in particular, leucine, isoleucine, and valine otherwise known as the branched-chain amino acids (BCAAs).

In MSUD, without these enzymes functioning properly, the liver cannot effectively process these BCAAs, especially leucine, allowing them to build-up to toxic levels in the body, which causes severe health problems if left untreated.  

The name maple syrup urine disease (MSUD) comes from a core symptom of MSUD which is a sweet smelling urine odour in untreated children and adults. 

Upon diagnosis, people should be referred to a metabolic specialist for ongoing supervision of their treatment.

Incidence, Genetics and how to test for MSUD

MSUD is a rare genetic disorder that occurs in about 1 in every 185,000 births. MSUD is inherited as a "recessive genetic disorder", in which both parents are carriers of the disordered gene, although they do not display symptoms of the disease. With each pregnancy in affected couples, there is a 1 in 4 (25%) chance that the infant will carry the disorder. 

A specialist in genetics can help explain the details of the inheritance of MSUD and the risks to future infants if both parents are carriers of the gene. MSUD can be screened at birth through a simple heel-prick blood spot test (MSUD test). 

Symptoms of MSUD

MSUD symptoms typically show up shortly after a baby is born, often within the first few days or weeks. The general symptoms are:

• Sweet smell urine
• Loss in appetite
• Loss in weight

Dietary management of maple syrup urine disease

MSUD is effectively managed by lifelong adherence to a strict low-protein diet consisting of: Foods naturally low in protein, MSUD protein substitutes, specially manufactured low protein products (e.g. rice replacer and milk replacer) and BCAA amino acid supplements.

When these measures are combined, it will ensure the body receives the essential nutrition, calories, vitamins, and optimal amino acid intake, required to ensure optimal growth, development and physical health to enable the most normal lifestyle possible. 

• The goal of adhering to a low-protein MSUD diet is to provide the optimal intake of these essential amino acids (leucine, isoleucine, and valine) to ensure proper growth but avoid excess intake of these amino acids.
• Protein substitutes are Foods for Special Medical Purposes prescribed by Healthcare professionals to ensure daily requirements of the other essential amino acids are met WITHOUT the potentially harmful BCAAs.
• In addition to protein substitutes, specially manufactured foods, low in protein, can provide an important source of energy and increase variety and flavour in low-protein diets.
• To monitor and maintain the correct amino acid intake, your medical care team will regularly assess blood samples and recommend dietary adjustments as the patient matures.