Mild elevations in serum bilirubin levels may not be appreciated clinically and identification will be noted on blood tests.
fig. 1 - Jaundice may be readily apparent on clinical examination
However, once bilirubin levels are moderately elevated, identification is a relatively straight forward aspect of the routine clinical examination (figure 1). Following the identification of jaundice, a logical and staged approach can be applied to determine the underlying cause. The first and most important question to be answered is whether jaundice has resulted from pre-hepatic, hepatic or post-hepatic pathology:
Pre-Hepatic Jaundice
The first rule out in any investigation of jaundice is to look for evidence of haemolytic anaemia. Routine haematology (+/- manual PCV) will quickly identify the presence of anaemia and examination of a fresh blood smear can then be used for look for evidence of haemolysis. The smear should be scanned observed for polychromasia and aniscocytosis to confirm the presence of regeneration and an aggregate reticulocyte count may also be
fig. 2 — Blood smear illustrating regenerative anaemia with polychromasia (variation in erythrocyte staining characteristics) and anisocytosis (variation in erythrocyte size).
useful to further quantify the degree of regeneration (fig. 2). However, it is important to note that regeneration may be absent or weak early in the course of the disease (first 3-4 days). Normal feline erythrocytes are small compared to those of dogs, which can make the identification of spherocytes (a typical feature of haemolysis) difficult. However, identification of micro-agglutination is relatively straight forward. A slide agglutination test is a quick and easy way to identify macroscopic agglutination and, if positive, negates the need for a confirmatory Coombs test. Coombs testing can be a useful tool but the possibility of false positive and negative results must be considered. If haemolysis is confirmed then a thorough search for infectious (eg. FeLV, Mycoplasma haemofelis), inflammatory and neoplastic disease must be undertaken via further blood testing and diagnostic imaging. Only when no underlying cause is identified despite a comprehensive search can an exclusion diagnosis of primary autoimmune haemolytic anaemia be made.
Post-Hepatic Jaundice
Once haemolysis has been excluded, the possibility of post-hepatic jaundice should be investigated. Elevations in hepatic enzymes (ALP>ALT) are often noted but ultrasonography is invariably the most useful diagnostic tool. While the intra-hepatic biliary tree is not seen in normal patients, the common bile duct is easily identified in normal cats and should be less than 4mm in diameter. Bile duct obstruction results in dilation of the bile duct and biliary tree which will then appear prominent during the ultrasound examination. The gall bladder itself may or may not be dilated. A thorough search should be made for mass lesions as a cause for biliary obstruction. These are most commonly pancreatic (eg. pancreatitis, pancreatic carcinoma) or hepatic (eg. primary neoplasia, hepatic metastases, hepatic cysts) in origin. Primary cholangitis can also result in jaundice but more commonly this is part of a cholangiohepatitis complex.
Hepatic Jaundice
If jaundice is present in the absence of haemolysis and biliary distension then a primary hepatic origin should be suspected. Hepatic disease does not always result in jaundice and, when it does, it usually causes only mild to moderate increases in bilirubin levels rather than marked elevations. This is generally in combination with elevated liver enzymes. An elevated bilirubin level resulting from liver disease confirms the presence of hepatic dysfunction. Consequently, bile acid assay is rendered unnecessary as it will add no further information. A detailed clinical history is required to rule out the possibility of toxin or drug exposure and further serology can be useful to investigate the possibility of infectious causes (FIP, toxoplasmosis, bartonellosis). Radiography is helpful, particularly to ascertain liver size (hepatomegally being consistent with acute inflammatory conditions or neoplasia, while microhepatica is most suggestive of chronic cirrhosis). Ultrasonography is then used to further assess parenchymal character. Fine needle aspiration can be useful, particularly for the diagnosis of lymphoma and hepatic lipidosis. However, in most other cases, ultrasound guided or surgical biopsy is required for the definitive diagnosis of inflammatory hepatobiliary disease (acute neutrophilic, chronic neutrophilic, lymphocytic) or hepatic neoplasia (primary, metastatic). Since bleeding disorders may accompany hepatic dysfunction a coagulation profile is indicated prior to hepatic biopsy.
In summary, the underlying causes for jaundice are varied and the therapeutic plan will alter dramatically dependent on cause. Employing a staged investigative approach with systematic elimination of possible aetiologies will help to achieve a definitive diagnosis, thus facilitating the instigation of specific treatment modalities in a timely fashion.
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