Maple syrup urine disease type 1B

This is a disease that affects the metabolism of the amino acids leucine, isoleucine and valine. This condition causes neurological and developmental delay, encephalopathy and a characteristic urine odor.

Maple syrup urine disease (MSUD), also known as branched-chain ketoaciduria, is an inherited disorder of branched-chain amino acid (leucine, isoleucine, and valine) metabolism. The name of this disease is due to the fact that the urine has a peculiar maple syrup odor.

The disease is caused by a deficiency in a group of enzymes that form the branched-chain alpha-ketoacid dehydrogenase complex (BCKDC) involved in the breakdown of the amino acids leucine, isoleucine, and valine.

It is estimated to affect 1 in 185,000 children worldwide, however, it has a higher incidence in certain populations such as Mennonites, where 1 in 380 births is observed, and in the Ashkenazi Jewish population, with an incidence of approximately 1 in 26,000 newborns.

It is a pathology that is usually included in neonatal screening programs.


MSUD is characterized by three main symptoms: psychomotor retardation, feeding problems, and a maple syrup odor of the urine. It is detected at birth and, if left untreated, can lead to progressive encephalopathy and respiratory failure. Long-term complications include increased risks of cerebral edema, attention deficit/hyperactivity disorder, depression and anxiety.

There are five clinical forms: classical (severe form), intermittent, intermediate, thiamine-sensitive and E3-deficient. Classification is based on age of onset, severity of symptoms, and response to thiamine treatment.

Disease management

Dietary restriction of branched-chain amino acids (leucine, isoleucine, and valine) is usually applied, and with early treatment, patients usually have a favorable outcome. In addition, expert monitoring is important to ensure that patients receive the necessary nutrients, and supplements containing amino acids that are free of leucine, isoleucine, and valine are available.

Genes analyzed



Blackburn PR, Gass JM, Vairo FPE, et al . Maple syrup urine disease: mechanisms and management. Appl Clin Genet. 2017 Sep 6;10:57-66.

Strauss KA, Puffenberger EG, Carson VJ. Maple Syrup Urine Disease. 2006 Jan 30 [updated 2020 Apr 23]. In: Adam MP, Mirzaa GM, Pagon RA, Wallace SE, Bean LJH, Gripp KW, Amemiya A, editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2022.

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