Neonatal jaundice pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Ahmed Elsaiey, MBBCH [2]

Pathophysiology

Bilirubin formation and metabolism

• Bilirubin is the final catabolic product of the heme. The heme is a component of various body substances and enzymes but it is mainly incorporated in the hemoglobin which is the main component of the red blood cells.
• Bilirubin is formed mainly in the liver and spleen through two steps which include the following:
  • Heme oxygenase enzyme dysregulates the porphyrin ring of the heme and breaks it down. A green substance called biliverdin is then formed as a result of the previous reaction. Carbon monoxide is a result of the reaction as well
  • Biliverdin reductase enzyme catalyzes the formation of bilirubin from biliverdin.
• Bilirubin is a toxic metabolite so, the body has physiologic processes in order to eliminate the bilirubin. Bilirubin elimination includes the following process:
  • Hepatic uptake:
    • After the formation of the bilirubin and its secretion into the bloodstream, bilirubin becomes bound to the albumin to facilitate its transportion to the liver.
    • The hepatocytes then reuptake the bilirubin and prepare it for excretion.
  • Conjugation:
    • Bilirubin is then conjugated with glucuronic acid producing bilirubin diglucuronide which is water soluble.
    • Being water soluble, hence, the conjugated bilirubin can be excreted into bile.
    • The conjugation process occurs by the glucuronosyltransferase enzyme in the liver cells.
  • Clearance and excretion:
    • After conjugation of the bilirubin in the liver, it is secreted into the bile then into the gastrointestinal tract.
    • In the GIT, the conjugated bilirubin is metabolized by the gut enzymes into urobilinogen which is oxidized into urobilin.
    • Metabolism of the conjugated bilirubin occurs properly in the adults. However, the newborns have sterile gastrointestinal canal which impedes the catalyzation of the conjugated bilirubin.
    • The sterile tract will end up with a small amount of excreted bile.
    • The remaining conjugated bilirubin will be unconjugated by the beta-glucuronidase enzyme in the neonatal intestine.
    • The unconjugated bilirubin can be reabsorbed back into the blood and to the liver through the enterohepatic circulation of bilirubin.
    • A small amount of bilirubin is cleared into the urine as urobilinogen.

Pathogenesis

• Neonatal jaundice may be a result of physiologic or pathological mechanisms. The different mechanisms of developing jaundice are concluded into either an increase in the bilirubin production, or increase the enterohepatic circulation, or decrease bilirubin elimination.
• Physiological jaundice:
  • The child has red blood cells twice or more than what the adults have and with shorter life span.
  • Increase rate of the red blood cells destruction produce more levels of bilirubin which end up with jaundice.
  • The newborn gastrointestinal gut is considered sterile so, a little amount of the unconjugated is converted to conjugated and excreted. Most of the unconjugated is recirculated through the enterohepatic circulation.
  • Unconjugated hyperbilirubinemia is the predominent form of physiological jaundice.
  • Physiologic jaundice is a benign case and resolves within a 10 to 14 days of life.
• Pathological jaundice:
  • The majority of neonatal jaundice are due to pathological conditions. Pathological neonatal jaundice is due to acquired or inhereted conditions.
  • Pathological jaundice is the result of an increase in the level of unconjugated bilirubin which is named as "Indirect hyperbilirubinemia".
  • It includes some features like the appearance of jaundice within the first day of life, persistent jaundice manifestations more than two weeks, and dark urine.
  • Acquired pahthological neonatal jaundice develops mainly due to hemolysis of the red blood cells via three main diseases:
    • Rhesus (Rh) hemolytic disease
    • ABO blood group incompatibility
    • Glucose 6 phosphate dehydrogenase enzyme deficiency (G6PD deficiency)
  • Inhereted pathological neonatal jaundice occurs due to a defect in the bilirubin metabolism process and it includes the following:
    • Defective hepatic uptake and storage of the bilirubin
    • Defective bilirubin conjugation to glucuronic acid and it includes the following syndromes:
      • Gilbert syndrome
      • Crigler-Najjar syndrome
      • Lucey-Driscoll syndrome
      • Breast milk jaundice
    • Defective excretion of bilirubin into the bile and this syndrome called Dubin-Johnson syndrome
    • Defective reuptake of the conjugated bilirubin through the enterohepatic ciruclation. This syndrome called Rottor syndrome.

Acquired pathological neonatal jaundice

• The following table contains the different hemolytic mechanisms which lead to neonatal jaundice:

Hemolytic disease	Pathogenesis
Rhesus factor (Rh) hemolytic disease	It is known as the Rh hemolytic disease of the newborns (RHDN). RHDN is the result of alloimmunization of the maternal red blood cells when the mother is pregnant with a Rh-positive fetus. In the first pregnancy, if the fetus is a Rh-positive, some of the fetal blood is mixed with the maternal blood during birth. The maternal body will form antibodies (IgG) against the fetal Rh antigen and the first birth is not affected. In the second birth, if the fetus is a Rh-positive, the formed maternal anti-Rh antibodies will cause hemolysis to the fetal blood. This condition may be mild or severe hemolytic anemia and may end up with hydrops fetalis.
ABO blood group incompatibility	ABO blood group incompatibility is another form of the alloimmunization of the maternal blood cells against the fetal erythrocytes. ABO incompatibility occurs when the mother has O group of the blood and pregnant in a fetus with A or B blood group. The maternal blood cells will form eitantibodieA antibodies or anti-B antibodies (IgM) which can cross the placenta and causes hemolysis of the fetal erythrocytes causing increase the unconjugated bilirubin and jaundice. This condition, unlike RHDN, develops in the first newborn.
G6PD deficiency	G6PD is an important enzyme found in the red blood cells and incorporated in the hexose monophosphate pathway. G6PD collaborates in the production of NADPH and reduction of glutathione thus, helping in decrease the oxidative stress around the RBCs. A deficiency in the G6PD occurs due to a genetic defect which will lead to increas the oxidative stress on the RBCs and the hemolysis of the fetal blood cells.

Inherited pathological neonatal jaundice

• The following table includes the different causes of inherited neonatal jaundice:

Defective mechanism	Pathogenesis
Defective bilirubin hepatic reuptake and storage	Defective of bilirubin hepatic uptake and storage is not well understood. The recently held studies revealed the correlation between mutations in the GST gene and neonatal jaundice. The gene deletion in GST-M gene class is believed it leads to dysfunction of the GSTM1 enzyme and defective hepatic uptake of bilirubin
Disorder of bilirubin conjugation	Gilbert syndrome: Gilbert syndrome, the most common inherited neonatal jaundice syndrome, is an autosomal recessive disease which is one of the causes of neonatal jaundice due to a defect (not total absence) in the Uridine diphosphate Glucuronsyl Transferase (UGT) enzyme. It is accompanied by several gene mutations (about 100 different mutations). The most common gene mutation occurs in the TA sequence of the TATAA box of the promoter region of UGT1A1 gene.
	Crigler-Najjar syndrome type I: Crigler Najjar syndrome type I is characterized by a total absence of the UGT1A1 enzyme, unlike Gilbert syndrome. Gene mutation of the UGT1A1 enzyme occurs due to deletion of the amino acid sequences of the exons of the UGT1A1 enzyme. Genetic mutations in the introns also can lead to frameshift of the amino acid sequences or create premature stop codons which result in cessation of the enzyme formation.
	Crigler-Najjar syndrome type II (Arias syndrome): Crigler Najjar syndrome type II has a reduced activity of the UGT1A1 enzyme (not completely inactive). The gene mutation in the UGT1A1 gene is point mutation which results in amino acid substitution not stop codon. Hereby, a decrease in the UGT enzyme occurs.
	Lucey-Driscoll syndrome: Also known as the transient familial neonatal hyperbilirubinemia as it is a rare familial disease which results in severe hyperbilirubinemia in the first 24 hours of life. It is believed that Lucey-Driscoll syndrome is associated with an inhibitor of the UGT1A1 enzyme and this inhibitor is unidentified until the moment.
	Breast milk jaundice: Breast milk jaundice is one of the benign causes of neonatal jaundice with no specific pathogenesis process. It is considered as the continuation of physiologic jaundice beyond one week. l It is believed to be a combination of genetic mutation and environmental (breast milk components) that leads to the jaundice development.
Disorders of excretion into Bile	Dubin-Johnson syndrome:
Disorders of reuptake	Rotor syndrome (RS)