Neonatal jaundice is one of the first and perhaps the most common problem encountered by the practicing pediatrician. It is a natural phenomenon occurring in the majority of full-term infants and virtually in all preterm infants.1,2 Neonatal jaundice reflects the presence of pigment in the skin and sclera, although little is known about the exact location of the pigment and to what it might be bound in those locations.3 Nonetheless, it is related to hyperbilirubinemia in the transition after birth, which occurs in all babies, except those lacking albumin, which is an extremely rare condition. This transitional phenomenon is usually benign and may have a physiological role in development, but under some conditions bilirubin outside the circulation can be dangerous, such as its accumulation in the brain, contributing to neurologic dysfunction and, sometimes, permanent injury.1,4
The syndrome of neonatal jaundice results from an imbalance between bilirubin production and bilirubin elimination,1,2,5 which is temporarily exacerbated during the transition after birth. This imbalance can be understood by analogy to a sink where the turned on spigot represents the process of bilirubin production and the drain represents the process of bilirubin elimination (Figure 2-1). If the rate at which bilirubin is produced in the body exceeds the rate at which bilirubin is eliminated, then the level in the body increases. In the analogy, the size of the sink represents the capacity of the circulation to contain bilirubin, and this is dependent mainly on the albumin concentration and the affinity of albumin to bind bilirubin. In the newborn, the capacity of the sink is decreased, and thus the likelihood that bilirubin will escape the circulation and move into tissues such as the brain is increased. The situation is worse in this regard in the preterm infant where the capacity is even lower because of a decreased albumin concentration and lower affinity for binding bilirubin, especially in the first days after birth and further compromised by any illness reflected in physiological instability.6 A more general discussion of the physiology of neonatal unconjugated hyperbilirubinemia and the epidemiology of neonatal jaundice is contained in other chapters. However, the biochemistry of bilirubin production is fundamental to the problem of neonatal jaundice, which cannot occur without the existence of the pigment.7
Diagram of bilirubin production and elimination. (Modified from Stevenson DK, Dennery PA, Hintz SR. Understanding newborn jaundice. J Perinatol. 2001;21:S22. Modified by permission from Macmillan Publishers Ltd., copyright 2001.)
There is a single biochemical source of bilirubin in the body, which is the enzymatic two-step process of heme catabolism.8 The reaction is ubiquitous, occurring in all nucleated cells, and thus in all tissues including the nucleated cells in blood. The substrate for the reaction, heme, is a part of many important proteins, but is ...