Asymptomatic 3-methylglutaconic aciduria type 1 detected by high C5-OH on newborn screening

3-Methylglutaconic aciduria type 1 (MGCA1) is an inborn error of leucine catabolism caused by pathogenic variants of the AUH gene. MGCA1 can be identified by newborn screening (NBS) with elevated C5-OH levels. We herein report a girl with MGCA1 detected by NBS. On day 5 after birth, NBS detected high C5-OH levels of 1.17 μmol/L (1.56 μmol/L [retest]). A urinary organic acid analysis revealed the abnormal excretion of 3-methylglutaconic, 3-methylglutaric, and 3-hydroxyisovaleric acids. Two novel heterozygous loss-of-function variants in the AUH gene were identified by genetic testing. We observed the patient without any treatment, such as a leucine-restricted diet. She had episodes of febrile illness several times in infancy but did not develop febrile convulsions or encephalopathy. She is now two years and six months old, and her physical growth and psychomotor development have progressed normally. In a review of the literature and our case, four children with MGCA1 identified during the neonatal period were asymptomatic or exhibited speech delay, regardless of whether or not they had been managed with specific treatments. Treatments such as dietary leucine restriction and carnitine supplementation may have little effect on MGCA1 in childhood; however, further investigation is warranted to evaluate the benefits of specific treatments to prevent potential long-term neurological complications.