Breast milk jaundice refers to an unconjugated hyperbi-lirubinemia associated with breastfeeding that develops after 4-7 days of life in an otherwise-healthy neonate and has no other identifiable cause. It is distinguished from breastfeeding jaundice, which occurs in the first week of life and is the result of insufficient intake or production of breast milk. The cause of breast milk jaundice has been the subject of numerous investigations based on the assumption that either a factor in the breast milk itself or in the neonate could contribute to the observed clinical presentation. Bilirubin conjugation and subsequent elimination is catalyzed by bilirubin uridine 5'-diphospho-glucuronosyltrans-ferase 1 (UGT1A1). Defects in UGT1A1 cause hereditary unconjugated hyperbilirubinemias-Crigler-Najjar syndrome type I (MIM #218800), in which there is complete deficiency of the enzyme; Crigler-Najjar syndrome type II (MIM #606785), where there is some limited activity of the enzyme; and Gilbert syndrome (MIM #43500), in which there is more enzyme activity but less than in normal individuals.
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