Why does physiological jaundice occur?
The accumulation of pigment in newborns is a consequence of the transient adaptation of metabolism. The main biochemical and physiological causes include:
- Massive hemolysis: in infants, the volume of red blood cells relative to body weight is significantly higher than in adults. At the same time, an active replacement of fetal hemoglobin (HbF) with adult-type hemoglobin (HbA) occurs, accompanied by the physiological destruction of a huge number of erythrocytes.
- Liver immaturity: the expression ("turning on") of the gene responsible for the synthesis of the key enzyme, UDP-glucuronosyltransferase, is delayed. Hepatocytes have insufficient capacity to uptake and excrete the avalanche-like amount of pigment being formed.
- 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 into stercobilin. As a result, its reabsorption into the systemic circulation (enterohepatic circulation) increases.
Laboratory changes and brain toxicity
In a standard course, a pronounced increase in indirect (unconjugated) bilirubin is recorded in the child's blood. Since this pigment cannot pass through the renal filter, it is absent in the urine, and urobilin is normally absent as well. The concentration of stercobilin in the feces decreases, which causes the stool to become hypocholic in case of impaired excretion.
If the concentration of the unconjugated fraction exceeds acceptable thresholds, bilirubin encephalopathy (kernicterus) develops. Lipophilic unconjugated bilirubin crosses the blood-brain barrier and accumulates in the nuclei of the brain. In 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.
Medical and non-medical treatment
Modern medicine uses two main biochemical mechanisms to reduce the level of the toxic pigment:
- Phototherapy. The infant is irradiated with blue-green spectrum light with a wavelength of 620 nm. Under the influence of light quanta, photoisomerization occurs right in the skin. 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 the stage of conjugation in the liver.
- Barbiturates (phenobarbital). They act as systemic inducers of microsomal liver enzymes. They force the organ to synthesize UDP-glucuronosyltransferase more actively.
The biochemically induced process looks as follows: a bilirubin molecule interacts with two molecules of UDP-glucuronate in the presence of UDP-glucuronosyltransferase. As a result, non-toxic water-soluble bilirubin diglucuronide is formed, and two UDP molecules are released.
Pathological forms of neonatal jaundice
Skin yellowing is far from always a normal variant. Pathological hyperbilirubinemia in newborns can develop against the background of:
- Hemolytic disease (immune conflict due to Rh factor or ABO system).
- 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 genetically absent in the body. In such a severe case, treatment with phenobarbital has absolutely no effect, since there is no substrate for enzymatic induction, and the patient inevitably suffers from bilirubin encephalopathy unless radical measures are taken.
Briefly: answers to topical questions
Which other hereditary syndromes are accompanied by impaired bilirubin metabolism, apart from Gilbert and Crigler-Najjar syndromes?
Other hereditary syndromes accompanied by impaired bilirubin metabolism (in particular, impaired excretion) include Dubin-Johnson syndrome and Rotor syndrome.
- Dubin-Johnson syndrome is caused by impaired excretion of already conjugated bilirubin by the hepatocyte.
- Rotor syndrome is also associated with a disorder of the 3rd stage of metabolism (excretion) of conjugated bilirubin.
The laboratory marker for both syndromes is an increase in the level of conjugated bilirubin in the blood.
What are the differential diagnostic criteria for pathological and physiological neonatal jaundice?
Differential diagnostic criteria between physiological and pathological jaundice are based on clinical and laboratory indicators.
| Criterion | Physiological jaundice | Pathological jaundice |
|---|---|---|
| Time of onset | 2nd–3rd day of life | - |
| Duration | - | Persistence of jaundice for more than 2 weeks |
| Stool | Normal color (no acholia) | Hypocholic stool in case of impaired excretion |
| Urine | Light, bile pigments absent, no bilirubin | Possible appearance of bilirubin (in severe pathology) |
| Complete blood count | Within age norm (no signs of hemolysis) | - |
Furthermore, physiological jaundice manifests when the concentration of unconjugated bilirubin reaches 80–90 µmol/L in full-term infants and more than 120 µmol/L in preterm infants.
To remember why there is no bilirubin in the urine during physiological jaundice, keep in mind: unconjugated bilirubin "fears" water (hydrophobic) and "loves" fats, so it does not pass through the renal filter, yet easily slips through the lipid blood-brain barrier straight into the brain.
Class and exam questions
Why is phenobarbital ineffective in Crigler-Najjar syndrome type I?
In this genetic disease, the patient completely lacks the enzyme UDP-glucuronosyltransferase. Phenobarbital is an inducer of microsomal liver enzymes, but it is physically incapable of stimulating the work of a protein whose gene is completely "turned off" or damaged.
How does phototherapy help excrete bilirubin bypassing the liver?
Under the influence of blue-green light with a wavelength of 620 nm, unconjugated bilirubin in the skin undergoes photoisomerization. It rearranges into a hydrophilic isomer — lumirubin, which dissolves remarkably well in water and is excreted by the kidneys without the participation of liver enzymes.
What is the mechanism of the neurotoxic action of bilirubin?
Crossing the blood-brain barrier, unconjugated bilirubin acts as an uncoupler of oxidative phosphorylation. This sharply reduces ATP production in neurons, causing their energy starvation, a pyrogenic effect, and irreversible cell degeneration.
Why do newborns experience massive erythrocyte breakdown?
Immediately after birth, the child no longer needs fetal hemoglobin (HbF), which helped extract oxygen from the mother's bloodstream. A massive destruction of old erythrocytes occurs to replace HbF with classic adult hemoglobin — HbA.
What else to study on this topic
- Hemolytic disease of the newborn: pathogenesis of Rh conflict and ABO system conflict
- Genetic enzymopathies: differences between Crigler-Najjar syndrome and Gilbert syndrome
- Obstructive jaundice in infants: biliary atresia
- The role of enterohepatic circulation and intestinal microflora in pigment metabolism
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