Phototherapy for neonatal hyperbilirubinemia.

Abstract

New light has recently been shed on the way phototherapy reduces bilirubin concentration in icteric infants. The introduction of a high-performance liquid chromatography technique led to the discovery of new photoisomers of bilirubin, the configurational and structural isomers with high and low quantum yields, respectively, and to a renewed interest in the photochemical properties of bilirubin in vitro and in vivo. Circular dichroism and absorption spectroscopies have then shown that bilirubin behaves like a bichromophoric system, with the 2 halves of the molecule strongly interacting in the excited state. This coupling mechanism makes the quantum yields of bilirubin photochemistry wavelength-dependent, with marked effects in the long wavelength edge of the bilirubin absorption spectrum. The photochemistry of bilirubin is substantially similar in icteric rats and babies, and is consistent with what is observed in vitro. However, the metabolism of bilirubin photoproducts in rats sometimes differs quite significantly from that in babies. In particular, only the low quantum yield structural isomer, lumirubin, is efficiently excreted by babies. Although the relative role of the bilirubin photoprocesses in the therapy of hyperbilirubinemia is not yet known with certainty, the structural photoisomerization is generally assumed to represent the main route of bilirubin elimination. As a consequence, the determination of the spectral band that optimizes the process of formation of lumirubin in neonate may represent an important step in the improvement of the clinical protocol of phototherapy. Therefore, in addition to reviewing the most recent data on bilirubin photochemistry and the metabolism of bilirubin products, this article presents a computation of the optimal light for lumirubin formation. The combined effects of long-wavelength photochemistry of bilirubin and skin attenuation show that the optimal spectral range should be between 480 and 510 nm.