How does phototherapy work in treating jaundice?

Phototherapy Mechanism in Treating Neonatal Jaundice

Phototherapy works by photoisomerization of bilirubin to water-soluble forms that can be excreted without hepatic conjugation—statement 2 is correct. 1, 2

Mechanism of Action

Photoisomerization is the primary mechanism by which phototherapy converts unconjugated bilirubin into excretable forms. 1, 3 The process occurs as follows:

• Light absorption transforms native Z,Z-bilirubin in the skin microcirculation into multiple photoisomers through two distinct pathways 1, 3:

  • Conformational isomers: Z,E-bilirubin (4E,15Z photoisomer) and E,Z-bilirubin (4Z,15E photoisomer), which account for approximately 20% of conversion 1
  • Structural isomers: Lumirubin (E,Z-lumirubin and E,E-lumirubin), which accounts for approximately 5% of conversion 1

• The 4E,15Z photoisomer forms almost immediately, with peak formation occurring within 120 minutes of phototherapy initiation 1

• These photoisomers are water-soluble and polar, allowing excretion through bile and urine without requiring hepatic conjugation 3, 4

• Photoisomer formation begins within 15 minutes of starting phototherapy, with the 4Z,15E photoisomer reaching 20-25% of total serum bilirubin by 2 hours 5, 6

Why the Other Statements Are Incorrect

Statement 1 is backwards: Phototherapy does NOT convert lumirubin to bilirubin—it converts bilirubin TO lumirubin and other photoisomers. 1, 3

Statement 3 is completely wrong: Phototherapy is used for unconjugated hyperbilirubinemia (indirect bilirubin), NOT obstructive jaundice. 1, 2 In fact:

• Obstructive jaundice causes conjugated (direct) hyperbilirubinemia with cholestasis 1
• Phototherapy efficacy is decreased in cholestatic jaundice because phototherapy products are excreted in bile 1
• The bronze infant syndrome can occur when infants with cholestasis receive phototherapy 1

Statement 4 is incorrect: Not any type of light can be used effectively. 1, 2

• Blue-green light in the 460-490 nm wavelength range is required, with optimal peak at 478 nm 1, 2
• This specific wavelength range overlaps the in vivo bilirubin absorption spectrum 1
• LED light sources delivering narrow-band blue-green light are now preferred over fluorescent tubes, halogen lamps, or white light 2, 3
• Sunlight is not recommended despite containing appropriate wavelengths, due to practical difficulties and risk of sunburn 1

Clinical Pearls

The photoconversion occurs primarily in plasma within the skin microcirculation, not within tissues, and the rate is affected by hemoglobin concentration. 3 This explains why:

• Maximizing exposed body surface area (35-80%) enhances efficacy 2
• Phototherapy response should be evident within 4-6 hours with a decrease >2 mg/dL in serum bilirubin 2, 7
• The transformation results in rapid decrease of unconjugated bilirubin concentrations, reducing neurotoxicity risk 1