Bilirubin is the end product of heme degradation. From 70–90% of bilirubin is derived from degradation of the hemoglobin of senescent red blood cells. Bilirubin produced in the periphery is transported to the liver within the plasma, where, due to its insolubility in aqueous solutions, it is tightly bound to albumin.
Under normal circumstances, bilirubin is removed from the circulation rapidly and efficiently by hepatocytes.
Transfer of bilirubin from blood to bile involves four distinct but interrelated steps:
1) Hepatocellular uptake: Uptake of bilirubin by the hepatocyte has carrier-mediated kinetics.
2) Intracellular binding: Within the hepatocyte, bilirubin is kept in solution by binding as a non substrate ligand to several of the glutathione-S-transferases, formerly called ligandins.
3) Conjugation: Bilirubin is conjugated with one or two glucuronic acid moieties by a specific UDP-glucuronosyl transferase to form bilirubin mono- and diglucuronide, respectively. Conjugation disrupts the internal hydrogen bonding that limits aqueous solubility of bilirubin, and the resulting glucuronide conjugates are highly soluble in water. Conjugation is obligatory for excretion of bilirubin across the bile canalicular membrane into bile. The UDP-glucuronosyl transferases have been classified into gene families based on the degree of homology among the mRNAs for the various isoforms. Those that conjugate bilirubin and certain other substrates have been designated the UGT1 family. These are expressed from a single gene complex by alternative promoter usage.
4) Biliary excretion: Bilirubin mono- and diglucuronides are excreted across the canalicular plasma membrane into the bile canaliculi by an ATP-dependent transport process mediated by a canalicular membrane protein called multidrug resistance–associated protein 2 (MRP2). Mutations of MRP2 result in the Dubin-Johnson syndrome.
Extra hepatic Aspects of Bilirubin Disposition
Bilirubin in the Gut
Following secretion into bile, conjugated bilirubin reaches the duodenum and passes down the gastrointestinal tract without reabsorption by the intestinal mucosa. An appreciable fraction is converted by bacterial metabolism in the gut to the water-soluble colorless compound, urobilinogen. About 80–90% of these products are excreted in feces, either unchanged or oxidized to orange derivatives called urobilins. The remaining 10–20% of the urobilinogens are passively absorbed, enter the portal venous blood, and are reexcreted by the liver. A small fraction (usually <3 mg/dL) escapes hepatic uptake, filters across the renal glomerulus, and is excreted in urine.(See figure 1)
Figure-1 – showing formation and fate of bilirubin
Unconjugated bilirubin ordinarily does not reach the gut except in neonates or, by ill-defined alternative pathways, in the presence of severe Unconjugated hyperbilirubinemia [e.g., Crigler-Najjar syndrome, type I (CN-I)]. Unconjugated bilirubin that reaches the gut is partly reabsorbed, amplifying any underlying hyperbilirubinemia.
Renal Excretion of Bilirubin Conjugates
Unconjugated bilirubin is not excreted in urine as it is too tightly bound to albumin for effective glomerular filtration and there is no tubular mechanism for its renal secretion. In contrast, the bilirubin conjugates are readily filtered at the glomerulus and can appear in urine in disorders characterized by increased bilirubin conjugates in the circulation.
Jaundice is a yellow discoloration of the skin, mucous membranes, and sclera caused by increased amounts of bilirubin in the blood. Jaundice is a sign of an underlying disease process.
Jaundice results from the accumulation of bilirubin. Hyperbilirubinemia may be due to abnormalities in the formation, transport, metabolism, and excretion of bilirubin. Total serum bilirubin is normally 0.2–1.2 mg/dL (mean levels are higher in men than women and higher in whites and Hispanics than blacks), and jaundice may not be recognizable until levels are about 3 mg/dL.
In the normal adult the rate of systemic bilirubin production is equal to the rates of hepatic uptake, conjugation, and biliary excretion. Jaundice occurs (bilirubin levels may reach 30-40 mg/dL in severe disease) when the equilibrium between bilirubin production and clearance is disturbed by one or more of the following mechanisms (1) excessive production of bilirubin, (2) reduced hepatic uptake, (3) impaired conjugation, (4) decreased hepatocellular excretion, and (5) impaired bile flow (both intra hepatic and extra hepatic). The first three mechanisms produce unconjugated hyperbilirubinemia, and the latter two produce predominantly conjugated hyperbilirubinemia. More than one mechanism may operate to produce jaundice, especially in hepatitis, which may produce Unconjugated and conjugated hyperbilirubinemia. In general, however, one mechanism predominates, so that knowledge of the predominant form of plasma bilirubin is of value in evaluating possible causes of hyperbilirubinemia
Classification of jaundice
1) Pre-hepatic (before bile is made in the liver)
Jaundice in these cases is caused by rapid increase in the breakdown and destruction of the red blood cells (hemolysis), overwhelming the liver’s ability to adequately remove the increased levels of bilirubin from the blood.
Examples of conditions with increased breakdown of red blood cells include:
-Sickle cell crisis,
-Glucose-6-phosphate dehydrogenase deficiency (G6PD),
-Drugs or other toxins, and
2) Hepatic (the problem arises within the liver)
Jaundice in these cases is caused by the liver’s inability to properly metabolize and excrete bilirubin.
–Viral – hepatitis A, B, or C, yellow fever, Bacterial sepsis, tuberculosis,
–Alcohol, Drugs e.g. estrogens, contraceptive pills and Pregnancy
–Carcinoma: metastases. Lymphoma. Adenocarcinoma of kidney (non-metastatic)
–Preoperative hypo perfusion/shock
-Chronic active hepatitis
3) Post-hepatic (after bile has been made in the liver)
Jaundice in these cases, also termed obstructive jaundice, is caused by conditions which interrupt the normal drainage of conjugated bilirubin in the form of bile from the liver into the intestines. The obstruction may be intrahepatic or extra hepatic
Causes of obstructive jaundice include:
a) Intra hepatic Obstruction
-Primary Biliary Cirrhosis
-Malignant infiltration of ducts
b) Extra hepatic obstruction
-Gallstones in the bile ducts,
-cancer (pancreatic and gallbladder/bile duct carcinoma),
-strictures of the bile ducts,
-Pressure on the common bile duct from enlarged lymph nodes,
Congenital conditions that may cause jaundice
1) Crigler-Najjar syndrome: an inherited condition that may lead to severe Unconjugated hyperbilirubinemia (high bilirubin concentrations); a gene mutation leads to a deficiency in an enzyme necessary for bilirubin conjugation.
2) Dubin-Johnson syndrome: an inherited disorder that causes the retention of conjugated bilirubin (and other compounds that turn the liver black) in liver cells; patients may have intermittent jaundice.
3) Rotor’s syndrome: an inherited conjugated hyperbilirubinemia that causes intermittent jaundice; similar to Dubin-Johnson without the retention of other compounds or a black liver.
4) Gilbert syndrome-There is compensatory hemolysis, impaired uptake and conjugation of bilirubin.
Figure-2- showing the causes of jaundice
Physiologic Neonatal Jaundice
Bilirubin produced by the fetus is cleared by the placenta and eliminated by the maternal liver. Immediately after birth, the neonatal liver must assume responsibility for bilirubin clearance and excretion. However, many hepatic physiologic processes are incompletely developed at birth. Levels of UDP –Glucuronyl Transferase levels are low, and alternative excretory pathways allow passage of unconjugated bilirubin into the gut. Since the intestinal flora that convert bilirubin to urobilinogen are also undeveloped, an enterohepatic circulation of Unconjugated bilirubin ensues. As a consequence, most neonates develop mild unconjugated hyperbilirubinemia between days 2 and 5 after birth. Peak levels are typically <85–170 mol/L (5–10 mg/dL) and decline to normal adult concentrations within 2 weeks, as mechanisms required for bilirubin disposition mature. Prematurity, with more profound immaturity of hepatic function, or hemolysis, results in higher levels of unconjugated hyperbilirubinemia.
A rapidly rising Unconjugated bilirubin concentration, or absolute levels >340 mol/L (20 mg/dL), puts the infant at risk for bilirubin encephalopathy, or kernicterus. Under these circumstances, bilirubin crosses an immature blood-brain barrier and precipitates in the basal ganglia and other areas of the brain. The consequences range from appreciable neurologic deficits to death. Treatment options include phototherapy, which converts bilirubin into water-soluble photo isomers that are excreted directly into bile, Administration of inducing agents(Phenobarbitone), Albumin infusion and exchange transfusion. The canalicular mechanisms responsible for bilirubin excretion are also immature at birth, and their maturation may lag behind that of UGT, this can lead to transient conjugated neonatal hyperbilirubinemia, especially in infants with hemolysis.
Maternal-fetal blood group incompatibility (Rh, ABO)
This form of jaundice occurs when there is incompatibility between the blood types of the mother and the fetus. This leads to increased bilirubin levels from the breakdown of the fetus’ red blood cells (hemolysis).
Breast milk jaundice
This form of jaundice occurs in breastfed newborns and usually appears at the end of the first week of life. Certain chemicals in breast milk are thought to be responsible for inhibition of UDP Glucuronyl transferase. It is usually a harmless condition that resolves spontaneously.
1) Unconjugated Hyperbilirubinemia (Pre hepatic jaundice)Stool and urine color are darker than normal, and there is mild jaundice and indirect (Unconjugated) hyperbilirubinemia with no bilirubin in the urine. Splenomegaly occurs in hemolytic disorders except in sickle cell anemia.
2) Hepatocellular disease (Hepatic jaundice)
Malaise, anorexia, low-grade fever, and right upper quadrant discomfort are frequent. Dark urine, jaundice, and, in women, amenorrhea occur. An enlarged tender liver, depending on the cause, severity, and chronicity of liver dysfunction is observed
3) Conjugated Hyperbilirubinemia (Post hepatic jaundice)
There may be right upper quadrant pain, weight loss (suggesting carcinoma), jaundice, dark urine, and light-colored stools. Symptoms and signs may be intermittent if caused by stone, carcinoma of the ampulla, or cholangiocarcinoma. Pain may be absent early in pancreatic cancer. Occult blood in the stools suggests cancer of the ampulla. Hepatomegaly and a palpable gallbladder (Courvoisier’s sign) are characteristic, but neither specific nor sensitive, of a pancreatic head tumor. Fever and chills are more common in benign obstruction with associated cholangitis.
The goal of testing is to determine the cause of the jaundice and to evaluate the severity of the underlying condition. The initial step is to obtain appropriate blood tests to determine if the patient has an isolated elevation of serum bilirubin. If so, is the bilirubin elevation due to an increased Unconjugated or conjugated fraction? If the hyperbilirubinemia is accompanied by other liver test abnormalities, is the disorder hepatocellular or cholestatic? If cholestatic, is it intra- or extra hepatic? All of these questions can be answered with a thoughtful history, physical examination, and interpretation of laboratory and radiologic tests and procedures.
Measurement of Serum Bilirubin
The terms direct- and indirect-reacting bilirubin are based on the original van den Bergh reaction. This assay, or a variation of it, is still used in most clinical chemistry laboratories to determine the serum bilirubin level. In this assay, bilirubin is exposed to diazotized sulfanilic acid, splitting into two relatively stable dipyrrylmethene azopigments that absorb maximally at 540 nm, allowing for photometric analysis.
The direct fraction is that which reacts with diazotized sulfanilic acid in the absence of an accelerator substance such as alcohol. The direct fraction provides an approximate determination of the conjugated bilirubin in serum. The total serum bilirubin is the amount that reacts after the addition of alcohol. The indirect fraction is the difference between the total and the direct bilirubin and provides an estimate of the Unconjugated bilirubin in serum.
With the van den Bergh method, the normal serum bilirubin concentration usually is 17 mol/L (<1 mg/dL). Up to 30%, or 5.1 mol/L (0.3 mg/dL), of the total may be direct-reacting (conjugated) bilirubin. Total serum bilirubin concentrations are between 3.4 and 15.4 mol/L (0.2 and 0.9 mg/dL) in 95% of a normal population.
Measurement of Urine Bilirubin
Unconjugated bilirubin is always bound to albumin in the serum, is not filtered by the kidney, and is not found in the urine. Conjugated bilirubin is filtered at the glomerulus and the majority is reabsorbed by the proximal tubules; a small fraction is excreted in the urine. Any bilirubin found in the urine is conjugated bilirubin. The presence of bilirubinuria implies the presence of liver disease. A urine dipstick test (Ictotest) gives the same information as fractionation of the serum bilirubin. This test is very accurate. A false-negative test is possible in patients with prolonged cholestasis due to the predominance of conjugated bilirubin covalently bound to albumin.
Liver biochemical tests: changes in three types of jaundice.
|Tests||Prehepatic jaundice||Hepatocellular Jaundice||Uncomplicated Obstructive Jaundice|
|Urine bilirubinUrine Urobilinogen||NoneIncreased||IncreasedIncreased||IncreasedAbsent|
|Serum albumin/total protein||Normal||Albumin decreasedTotal protein, 6.5–8.4 g/dL||Unchanged|
|Alkaline phosphatase||Normal||Increased (+)||Increased (++++)|
|Prothrombin time||Normal||Prolonged if damage severe and does not respond to parenteral vitamin K||Prolonged if obstruction marked, but responds to parenteral vitamin K|
|ALT, AST||Normal||Increased in hepatocellular damage, viral hepatitis||Minimally increased|
ALT, alanine aminotransferase; AST, aspartate aminotransferase.
Serum alanine and aspartate aminotransferase (ALT and AST) levels vary with age and correlate with body mass index. Elevated alkaline phosphatase levels are seen in cholestasis or infiltrative liver disease (such as tumor or granuloma). Alkaline phosphatase elevations of hepatic rather than bone, intestinal, or placental origin are confirmed by concomitant elevation of γ-glutamyl transpeptidase or 5′-nucleotidase levels. The differential diagnosis of any liver test elevation includes toxicity caused by drugs, herbal remedies, and toxins.
Percutaneous liver biopsy is the definitive study for determining the cause and histological severity of hepatocellular dysfunction or infiltrative liver disease.
Demonstration of dilated bile ducts by ultrasonography or CT scan indicates biliary obstruction (90–95% sensitivity). Ultrasonography, CT scan, and MRI may also demonstrate hepatomegaly,intrahepatic tumors, and portal hypertension. Magnetic resonance cholangiopancreatography (MRCP) is a sensitive, noninvasive method of detecting bile duct stones, strictures, and dilation; however, it is less reliable than endoscopic retrograde cholangiopancreatography (ERCP) for distinguishing malignant from benign strictures.
The treatment is entirely dependent on the cause of the jaundice. In some cases it will be curative e.g. surgical removal of a gallstone blocking the common bile duct. Viral infections e.g. hepatitis A, may recover spontaneously. In other cases treatment will modify the disease process and the jaundice will improve or disappear. Treatment may be purely symptomatic e.g., drugs to relieve itching or palliative as in malignant disease. Some types of advanced liver disease-causing jaundice may be treated by liver transplantation.
Phototherapy for Neonatal Jaundice
The goal of therapy is to lower the concentration of circulating bilirubin or keep it from increasing. Photo therapy achieves this by using light energy to change the shape and structure of bilirubin, converting it to molecules that can be excreted even when normal conjugation is deficient
Phototherapy converts bilirubin to yellow photo isomers and colorless oxidation products that are less lipophilic than bilirubin and do not require hepatic conjugation for excretion. Photo isomers are excreted mainly in bile, and oxidation products predominantly in urine.
Prognosis– is fairly good in the treated cases, except in cases of malignant obstruction of common bile duct.