Icterus is characterized by hyperbilirubinemia and deposition of bile pigments in tissues, including the blood, skin, and mucous membranes, resulting in a yellowish discoloration of these tissues.

Approximately 70% of bilirubin is derived from the death of senescent erythrocytes in reticuloendothelial cells, which occurs mainly in the spleen and liver; another 10% is derived from ineffective erythropoiesis in the bone marrow. Most of the remaining 20% of bilirubin is derived from hepatic cytochromes, catalase, peroxidase, and myoglobin. Bilirubin is the only major breakdown product of heme that requires excretion. The protein and iron components of the heme molecule enter the body pool and are then reused.

Newly formed unconjugated bilirubin is insoluble in water and bound to circulating albumin. In addition to allowing transport of bilirubin via the blood to the liver, binding prevents diffusion of the large bilirubin–albumin complex across cell membranes and thus tends to retain bilirubin in the vascular space. Beyond this capacity, or if binding capacity is reduced, unbound bilirubin may cross cell membranes and enter tissues. Central nervous system toxicity termed kernicterus (rare in animals) occurs in human neonates whenever unconjugated bilirubin crosses the blood–brain barrier, but the toxic effect in older patients is minimal. Bilirubin-binding capacity of albumin may be reduced by (1) decreased plasma albumin, (2) decreased binding sites on the albumin molecule caused by competition for sites by drugs (e.g., sulfonamides or salicylates), and (3) decreased affinity of albumin for bilirubin, caused by acidosis (Center 1996).

Bilirubin dissociates from albumin before entering the liver cell. The flow of bilirubin between plasma and liver is bidirectional, with approximately one-third of the unconjugated bilirubin that enters the liver cell ultimately returning unaltered to the plasma. Hepatic uptake of unconjugated bilirubin is apparently unaffected by severe impairment of hepatic conjugation or excretion of bilirubin. Hepatic uptake of bilirubin is not affected by bile acids. Once inside the hepatocyte, specific proteins termed Y (or ligandin) and Z bind bilirubin. These proteins also bind other anions, such as drugs and steroids (Center 1996).

Hepatic conjugation of bilirubin with glucuronic acid to form bilirubin monoglucuronide and diglucuronide is catalyzed by the glucuronyl transferase enzyme. The importance of conjugation is that bilirubin is transformed from a lipid-soluble to a water-soluble compound. Water solubility is mandatory for excretion of bilirubin in the urine. Lipid insolubility limits back-diffusion of bilirubin into hepatocytes and reabsorption from the intestine, causing it to be excreted through bile into the feces.

Conjugated bilirubin is excreted from the liver cell into the bile canaliculus by a system that appears to have the properties of active, carrier-mediated transport. The transport mechanism involved is different from that for hepatocellular secretion of bile salts. Hepatic excretion of conjugated bilirubin is the rate-limiting step in the overall capacity of the liver to move bilirubin from blood to bile. In contrast to uptake and conjugation, which are well preserved in the presence of hepatocellular injury, the excretory transport system is very sensitive to various types of liver damage. Hence, an increase in plasma-conjugated bilirubin occurs early in the course of hepatocellular injury. However, total plasma bilirubin concentration may not increase until later in dogs with hepatocellular injury, because dogs have a low renal threshold for conjugated bilirubin, and significant bilirubinuria occurs soon after hepatocellular injury. Trace to 1+ amounts of conjugated bilirubin may also appear in the urine of normal dogs, especially in concentrated urine samples. In cats, the renal threshold for bilirubin is slightly higher. Bilirubinuria in cats is nearly always pathologic, indicating either cholestasis or hepatocellular damage (Center 1996).

Bacteria in the lower intestinal tract reduce most of the conjugated bilirubin to a group of chromagens termed urobilinogen. Much of the urobilinogen is excreted in feces, but some is reabsorbed from the colon and carried via portal blood back to the liver. Most urobilinogen is subsequently re-excreted into bile, but a small amount may gain entrance to systemic circulation and be excreted in the urine.

Hyperbilirubinemia and Cholestasis

Bilirubin uptake, conjugation, and excretion are controlled by hepatocellular mechanisms that are separate from the conjugation and excretion of bile acids. Disturbances in bilirubin transport are recognized by hyperbilirubinemia and icterus. Cholestatic syndromes (reduced bile flow) are characterized by marked bile acidemia, usually with normal to slightly elevated bilirubin levels. Severe cholestasis can, however, cause icterus. Measurement of serum bile acids is a reliable procedure for the detection of cholestasis and is also a sensitive test of hepatic function. Once a patient is icteric though, the bile acid measurement yields little information regarding hepatic function since they will reach high levels simply from cholestasis even if hepatocellular function is still normal. Increased serum alkaline phosphatase and gamma glutamyl transpeptidase (GGT) activities also occur with cholestasis (see Chapter 55).

Hyperbilirubinemia may be caused by increases in unconjugated or conjugated bilirubin in plasma. The van den Bergh test has classically been used to fractionate plasma bilirubin into indirect (unconjugated) and direct (conjugated) reacting bilirubin, but this information is usually of little value in determining the etiologic cause of icterus. Since the dog and cat can conjugate bilirubin quite effectively, even in the presence of liver disease, there is too much overlap in the levels of conjugated versus unconjugated bilirubin for this test to be of much value in differentiating causes of icterus.

Unconjugated Hyperbilirubinemia. Increased plasma unconjugated bilirubin can result from increased pigment production (hemolysis or ineffective erythropoiesis) or impaired hepatic uptake or conjugation of bilirubin. Hemolysis accounts for most instances of unconjugated hyperbilirubinemia in icteric dogs and cats and is termed prehepatic icterus. The liver normally has a large reserve capacity for uptake, conjugation, and excretion of bilirubin (30–60 times basal rate). Because of this fact, even with prehepatic causes of icterus (unless it is extremely early in the process), the direct bilirubin levels will almost always be higher than the indirect levels. In addition, it is usually very easy to make a diagnosis of prehepatic icterus without the need for distinction of the type of bilirubin present. In hemolytic icterus, hyperbilirubinemia is initially characterized by a preponderance of unconjugated bilirubin, but dogs and cats have a great capacity to quickly conjugate the bilirubin so that the relative concentrations of conjugated and unconjugated bilirubin may overlap those observed in hepatocellular disease and biliary obstruction. Although the process is incompletely understood, it has been suggested that increases in conjugated bilirubin later in hemolytic icterus may be due to compromise of the excretory system of a previously normal liver injured by anemic anoxia (Center 1996). In this situation, it is assumed that hepatocellular uptake and conjugation of bilirubin remain relatively normal; the lack of excretion would lead to regurgitation from the hepatocyte into blood. Another possible explanation is that excessive hepatocellular production of conjugated bilirubin causes bile inspissation; this would result in intrahepatic biliary obstruction and regurgitation of conjugated bilirubin into blood (Center 1996). It is doubtful that serum alkaline phosphatase activity would help differentiate these two possible mechanisms, since it takes 7–8 days of total biliary obstruction to produce maximum increases of serum alkaline phosphatase. Patients with hemolytic icterus have anemia characterized by signs of regeneration (reticulocytosis, anisocytosis, or poikilocytosis), provided that the bone marrow is functional and 3–5 days have elapsed since the hemolytic crisis (see Chapter 54). If there is intravascular hemolysis rather than extravascular in the reticuloendothelial cells and it is sufficiently rapid, the plasma may be red because of the presence of increased amount of free hemoglobin prior to development of icterus. Normally, a plasma protein referred to as haptoglobin, binds the free circulating hemoglobin and prevents it from entering the urine. With rapid, massive hemolysis, all haptoglobin may be saturated with hemoglobin; the unbound hemoglobin is then filtered by the glomerulus and appears in the urine resulting in red urine (hemoglobinuria). In this instance, there are few if any intact erythrocytes present in the urine to account for the red color or the strongly positive occult blood test results. In addition, one would not expect significant increases in serum levels of leaked enzymes (alkaline phosphatase, GGT) in hemolytic disease. However, anemic anoxia can cause hepatocellular membrane damage and increases in serum ALT. It is possible that biliary obstruction due to bile inspissation, as previously mentioned, could eventually cause increases in serum alkaline phosphatase.

Conjugated Hyperbilirubinemia. Hepatocellular disease and biliary obstruction (intrahepatic or posthepatic) may result in an increase in plasma conjugated and unconjugated bilirubin, with the former predominating. It is difficult to distinguish icterus caused by hepatic disease from icterus due to posthepatic biliary obstruction on the basis of serum chemistry profile data alone. Ongoing hepatocellular disease is often associated with marked increases in serum ALT. Other laboratory abnormalities that may be present with severe hepatic dysfunction include decreased serum concentration of urea nitrogen, albumin, and glucose and increased serum bile acid and plasma ammonia levels. Conjugated bilirubin appears in the urine in increased amounts.

With chronic cholestatic icterus (hepatic or posthepatic causes) the existence of a conjugated bilirubin fraction, which is tightly bound (covalent bonding) to albumin (“biliprotein” complexes) results in an albumin-bound conjugated bilirubin fraction too large to be excreted in urine, its elimination from the body is determined by the half-life of albumin (12–14 days). This results in a delay in clearance of circulating bilirubin and may explain why icterus persists in some patients even after improvement in hepatic function and a disappearance of bilirubin from the urine. In addition, the fat tissues that are saturated with bilirubin in the icteric patient take much longer to clear than the serum. This is why the serum clearance of bilirubin and the yellow discoloration of the serum always precede the clearance of the tissue icterus. Therefore, a patient may be improving clinically in relationship to the disease causing the icterus while maintaining some degree of “tissue icterus.”

Biliary tract obstruction, whether intrahepatic or extrahepatic, is characterized by a marked increase in serum ALP and GGT activities as a result of induced synthesis of these enzymes by hepatocytes and biliary endothelial cells.

Hepatocellular disease and partial biliary obstruction (usually intrahepatic) frequently coexist in patients with liver disease. Bacterial sepsis and endotoxemia may cause impaired hepatic secretion of conjugated bilirubin and decrease bile flow, and therefore increased serum ALP activity and conjugated hyperbilirubinemia may result (Taboada and Meyer 1989).

Cholestasis. Cholestasis refers to decreased bile flow and is characterized by increased serum bile acid levels, ALP and GGT activities, and in more severe cases, by hyperbilirubinemia (Center 1996). Causes of cholestasis include partial or complete biliary obstruction and drug administration. Drugs thought to cause cholestasis in dogs include corticosteroids, phenobarbital, and primidone. Their mechanisms have not been completely worked out, although it is known that long-term use of corticosteroids induces a characteristic hepatopathy in many dogs, characterized by hepatocellular swelling due to accumulation of glycogen and water. Cholestasis probably results from intrahepatic partial biliary obstruction caused by diffuse swelling of hepatocytes.

History and Physical Examination