Types of Hyperbilirubinemia
Hyperbilirubinemia is more commonly known as jaundice. Jaundice is a condition marked by yellow tint to the skin and the sclera (white part of the eyes). It is caused by an increase in bilirubin which is an orange-yellow pigment produced in the liver and found in the bile. There are two major types of hyperbilirubinemia, each caused by a variety of illnesses.
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Bilirubin Production
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Bilirubin is a waste product from the metabolism of hemoglobin or red blood cells. Red blood cells survive an average of 120 days; therefore, blood is constantly being renewed each day. Red blood cells degrade, usually in the spleen, in three distinct steps. First, hemoglobin is degraded into the globulin fraction and heme. Second, heme oxygenase (an enzyme that aids in the transfer of oxygen) converts the heme to biliverdin. Third, a reductase (an enzyme that catalyzes a reduction reaction) converts biliverdin to bilirubin.
Unconjugated Hyperbilirubinemia
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Unconjugated bilirubin, also called "free" bilirubin, is the lipid (fat) soluble waste product that results from the above reaction. Since bilirubin is a waste product, it must be excreted from the body. To do this, it must become water-soluble. The unconjugated bilirubin attaches to albumin (a protein abundantly found in the blood) to travel to the liver where it reacts with an enzyme (glucuronyl transferase) to become water-soluble, conjugated bilirubin. Unconjugated hyperbilirubinemia is the condition that results from a high level of this unconjugated form of bilirubin in the blood.
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Conditions Involving Unconjugated Hyperbilirubinemia
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Unconjugated hyperbilirubinemia can be caused by several factors. It can be caused by an increased production in bilirubin as a result of accelerated breakdown of red blood cells. It can also be caused by a decrease in the conjugation which is a result of an absence or decrease in glucuronyl transferase.
Conditions causing unconjugated hyperbilirubinemia include excessive hemolysis, Gilbert's syndrome (a mild metabolic disorder), Crigler-Najjar syndrome (congenital disorder marked by part or total deficiency in glucuronyl transferase and is usually fatal) and neonatal jaundice. Many babies, especially those born pre-term, suffer from jaundice. It is usually a result of an immature liver, not efficiently conjugating the bilirubin.
Conjugated Hyperbilirubinemia
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Conjugated bilirubin, now water-soluble, moves through the bile duct to the intestines as bile salts. Bacteria in the intestines convert the bilirubin to urobilinogens which are excreted from the body in the feces and urine. Conjugated hyperbilirubinemia is a high level of conjugated bilirubin and is usually caused by an inefficiency in the excretion process.
Conditions Involving Conjugated Hyperbilirubinemia
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Conjugated hyperbilirubinemia is a result of a defect in the excretion, either within the liver or after the liver. Hepatocellular disease, diseases of the liver including cirrhosis and hepatitis, can result in conjugated hyperbilirubinemia. Drugs can also affect the excretion of bilirubin from the liver, including antibiotics, steroids, oral contraceptives and barbiturates. Obstruction of the bile ducts, possibly due to gallstones, also results in conjugated hyperbilirubinemia. Any obstruction will usually require surgery to correct it.
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