Amino Acids, Maple Syrup Urine Disease Panel, Plasma
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Maple syrup urine disease (MSUD) is an inborn error of metabolism caused by the deficiency of the branched-chain ketoacid dehydrogenase (BCKDH) complex. The BCKDH complex is involved in the metabolism of the branched-chain amino acids (BCAA): isoleucine (Ile), leucine (Leu), and valine (Val). MSUD patients can be divided into 5 phenotypes: classic, intermediate, intermittent, thiamine-responsive, and dihydrolipoyl dehydrogenase (E3)-deficient depending on the clinical presentation and response to thiamine administration. Classic MSUD which is the most common and most severe form, presents in the neonate with feeding intolerance, failure to thrive, vomiting, lethargy, and maple syrup odor to urine and cerumen. If untreated, it progresses to irreversible mental retardation, hyperactivity, failure to thrive, seizures, coma, cerebral edema, and possibly death.
Age of onset for individuals with variant forms of MSUD is variable and some have initial symptoms as early as 2 years of age. Symptoms include poor growth and feeding, irritability, and developmental delays. These patients can also experience severe metabolic intoxication and encephalopathy during periods of sufficient catabolic stress.
MSUD is a panethnic condition, but is most prevalent in the Old Order Mennonite community in Lancaster, Pennsylvania with an incidence there of 1:760 live births. The incidence of MSUD is approximately 1:185,000 live births in the general population.
Treatment of MSUD aims to normalize the concentration of BCAA by dietary restriction of these amino acids. Because BCAA belong to the essential amino acids, the dietary treatment requires frequent adjustment, which is accomplished by regular determination of BCAA and allo-isoleucine concentrations. Orthotopic liver transplantation has been used with success and is an effective therapy for MSUD.
Follow-up of patients with maple syrup urine disease
Monitoring of dietary compliance for patients with maple syrup urine disease
The quantitative results of isoleucine, leucine, valine, and allo-isoleucine with age-dependent reference values are reported without added interpretation. When applicable, reports of abnormal results may contain an interpretation based on available clinical interpretation.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Reference values are for fasting patients.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
< or =23 months: 31-105 nmol/mL
2-17 years: 30-111 nmol/mL
> or =18 years: 36-107 nmol/mL
< or =23 months: 48-175 nmol/mL
2-17 years: 51-196 nmol/mL
> or =18 years: 68-183 nmol/mL
< or =23 months: 83-300 nmol/mL
2-17 years: 106-320 nmol/mL
> or =18 years: 136-309 nmol/mL
< or =23 months: <2 nmol/mL
2-17 years: <3 nmol/mL
> or =18 years: <5 nmol/mL
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Scriver's The Online Metabolic and Molecular Bases of Inherited Disease (OMMBID). Part 8 Amino Acids. Accessed July 6, 2016. Available at: http://ommbid.mhmedical.com/book.aspx?bookid=971
2. Chuang DT, Shih VE, Max Wynn RR: In Maple Syrup Urine Disease (Branched-Chain Ketoaciduria). Edited by D Valle, AL Beaudet, B Vogelstein, et al. New York, NY, McGraw-Hill, 2014. Accessed July 6, 2016. Available at http://ommbid.mhmedical.com/content.aspx?bookid=971&Sectionid=62675436.
3. Strauss KA, Puffenberger EG, Morton DH:.In Maple Syrup Urine Disease. 1993-2016. Edited by RA Pagon, MP Adam, HH Ardinger. GeneReviews. University of Washington, Seattle, WA. Updated 2013 May 9. Available at: http://www.ncbi.nlm.nih.gov/books/NBK1319