Why Does Physiological Jaundice Occur?
Pigment accumulation in newborns is a consequence of transient metabolic adaptation. The main biochemical and physiological causes include:
- Massive hemolysis: In infants, the red blood cell mass relative to body weight is significantly higher than in adults. Active replacement of fetal hemoglobin (HbF) with adult-type hemoglobin (HbA) occurs, accompanied by the physiological breakdown of a huge number of erythrocytes.
- Liver immaturity: Expression ("turning on") of the gene responsible for synthesizing the key enzyme UDP-glucuronosyltransferase is delayed. Hepatocytes have insufficient capacity to uptake and excrete the avalanche-like accumulation of pigment.
- Carrier protein deficiency: Reduced plasma albumin levels (hypoalbuminemia) limit the transport of the unconjugated fraction to liver cells.
- Intestinal factors: Due to the absence of normal bacterial microflora in the gut, the pigment is less efficiently converted to stercobilin. As a result, its reabsorption into the systemic circulation (enterohepatic circulation) increases.
Laboratory Changes and Brain Toxicity
In a standard clinical course, a marked increase in indirect (unconjugated) bilirubin is detected in the infant's blood. Since this pigment cannot pass through the renal filter, it is absent in the urine, and urobilin is normally absent as well. Fecal stercobilin concentration decreases, which may cause hypocholic stools if excretion is impaired.
If the concentration of the unconjugated fraction exceeds safe thresholds, bilirubin encephalopathy (kernicterus) develops. Lipophilic unconjugated bilirubin crosses the blood-brain barrier and accumulates in the basal ganglia and brainstem nuclei. Within neurons, it acts as a potent uncoupler of tissue respiration and oxidative phosphorylation. This leads to a critical drop in ATP production, causes a pyrogenic effect, and triggers severe degenerative changes in nerve cells.
Pharmacological and Non-Pharmacological Treatment
Modern medicine utilizes two main biochemical mechanisms to lower toxic pigment levels:
- Phototherapy. The infant is exposed to blue-green spectrum light with a wavelength of 620 nm. Under the influence of light quanta directly within the skin, a photoisomerization reaction occurs. Toxic bilirubin changes its spatial structure and turns into hydrophilic isomers (lumirubin). This form is water-soluble and easily leaves the body via urine and bile, completely bypassing hepatic conjugation.
- Barbiturates (phenobarbital). These act as systemic inducers of microsomal liver enzymes, prompting the organ to actively synthesize UDP-glucuronosyltransferase.
Biochemically, the conjugation process involves a bilirubin molecule reacting with two molecules of UDP-glucuronate in the presence of UDP-glucuronosyltransferase. This yields non-toxic, water-soluble bilirubin diglucuronide and releases two UDP molecules.
Pathological Forms of Neonatal Jaundice
Yellowing of the skin is not always a normal variant. Pathological hyperbilirubinemia in newborns can develop against the background of:
- Hemolytic disease (Rh incompatibility or ABO blood group incompatibility).
- Hereditary enzyme system defects (Gilbert syndrome, Crigler-Najjar syndrome).
- Mechanical obstruction of the biliary tract (atresia).
- Intrauterine infections affecting liver tissue (hepatitis).
For example, in Crigler-Najjar syndrome type I, UDP-glucuronosyltransferase is completely absent genetically. In such severe cases, phenobarbital treatment is entirely ineffective because there is no substrate for enzymatic induction, and the patient inevitably suffers from bilirubin encephalopathy unless radical measures are applied.