Ultrasound in Feline Hepatopathies: An Essential Tool in Veterinary Practice

Normal Ultrasonographic Evaluation of the Feline Liver and Biliary System

When performing an ultrasonographic examination of the liver in cats, it is crucial to familiarize yourself with the normal appearance. We must evaluate:

· Size and Shape: The assessment of liver size in cats is quite subjective, without defined values or ratios. An enlarged liver (hepatomegaly) may present with rounded and bulging lobes. On a lateral view, a large liver may more completely surround the right kidney. Normal liver margins are sharp and triangular or linear.

· Echogenicity: The echogenicity of the liver in cats can be complex. Normally, the liver and the renal cortex have similar echogenicity. However, obese cats can show increased hepatic echogenicity due to non-pathologic fat accumulation. It is common for feline kidneys to have a more hyperechoic cortex than the liver, even in non-obese animals, due to fat deposition in the renal tubules. Comparison with the spleen is also helpful; the spleen is typically hyperechoic to the liver, except in obese cats. A useful finding is comparing the echogenicity of the hepatic parenchyma to the adjacent falciform fat. In normal cats, the hepatic parenchyma is typically hypoechoic to the falciform fat.

· Gallbladder: Located to the right of the midline, the normal gallbladder is oval to pear-shaped, with anechoic contents (bile) and thin walls, generally non-prominent and less than 1 mm thick. Artifacts such as reverberation and side lobe artifact can be observed. A common developmental anomaly in cats is a bilobed gallbladder.

· Biliary Tract: The common bile duct is usually visible ventral to the portal vein. Its normal diameter in cats is 4 mm or less, measured intraluminally. The vena cava, portal vein, and bile duct can be identified sequentially in a sagittal view, from right to left in the cat. The portal vein branches at the porta hepatis, while the bile duct continues caudally towards the duodenal papilla. The cystic duct, often tortuous, connects the gallbladder to the common bile duct. The pancreatic duct joins the bile duct before emptying into the duodenal papilla. The use of color Doppler can aid in identifying blood vessels adjacent to the bile duct.

Ultrasonographic Changes in Diffuse Hepatopathies

Diffuse liver diseases can cause changes in the size, shape, and echogenicity of the liver. However, these changes are subjective and often non-specific. It is important to note that the absence of ultrasonographic changes does not rule out diffuse liver disease.

· Hyperechoic Liver: May be suggestive of hepatic lipidosis, where the hepatic parenchyma becomes hyperechoic relative to the falciform fat. However, a normal obese cat can also exhibit this characteristic. Hyperechoic livers tend to attenuate sound more rapidly, making it difficult to visualize deeper portions and portal vessels.

· Hypoechoic Liver: Decreased hepatic echogenicity is a subjective finding but can occur in cases of acute inflammation (hepatitis), lymphoma, or other infiltrative diseases. In a hypoechoic liver, portal vein markings may appear more prominent. Lymphoma can present in various ways, including a normal liver, hepatomegaly, hypoechoic parenchyma, or nodules.

Ultrasonographic Changes in Focal Hepatopathies

Focal lesions in the liver, such as masses or nodules, can be identified and characterized ultrasonographically. Differential diagnoses include abscesses, hematomas (rare in cats), cysts, nodular hyperplasia, and neoplasia (primary or metastatic).

· Cysts: Generally appear as anechoic, well-defined structures with posterior acoustic enhancement. In Persian cats, the presence of liver cysts may be associated with polycystic kidney disease.

· Biliary Cystadenoma: This is the most common liver tumor in cats and is characterized by focal cystic regions, single or multiple. Although they can grow, they are generally considered incidental and benign.

· Malignant Neoplasia: The most common malignant tumors are cholangiocarcinoma and hepatocellular carcinoma. The ultrasonographic appearance is variable, without a specific pattern to differentiate tumor type or whether it is primary or metastatic. Identifying multiple lesions or the involvement of multiple lobes can aid in clinical decisions.

Ultrasonographic Evaluation of Biliary Diseases

Ultrasonography is particularly useful in evaluating biliary diseases in cats.

· Gallbladder Wall Thickening: A wall thickness greater than 1 mm is considered significant. Thickening can be diffuse (associated with chronic cholangitis) or edematous (with a hypoechoic double-layer pattern between two hyperechoic layers), which can occur in cases of acute cholecystitis but also in other non-biliary conditions such as right-sided congestive heart failure, hypoproteinemia, portal hypertension, post-blood transfusion reactions, anaphylaxis, and dexmedetomidine use. Adjacent inflammation or diffuse neoplasia can also cause gallbladder wall thickening.

· Cholangitis: Inflammation of the bile ducts (intra- and extrahepatic) is frequently associated with gallbladder wall thickening (cholecystitis). Suggestive ultrasonographic findings include a thickened and irregular gallbladder wall, the presence of gallstones (cholelithiasis) and/or biliary sludge in the gallbladder, thickened and tortuous bile duct, and possible biliary dilation. The hepatic parenchyma may show variable echogenicity. Concurrent pancreatitis and inflammatory bowel disease (feline triaditis) are common.

· Cholelithiasis and Choledocholithiasis: Gallstones in the gallbladder (cholelithiasis) or bile ducts (choledocholithiasis) can be identified as hyperechoic structures with acoustic shadowing. Obstructive calculi are usually found at the duodenal papilla. In cats, the presence of gallstones is more concerning than in dogs, being associated with a higher prevalence of bacterial infection and pancreatitis.

· Biliary Obstruction: Can be caused by inflammation (mucous plugs of bile), neoplasia (in the pancreas, duodenum, or bile ducts), calculi, or rarely pancreatitis. Dilation of the bile duct (diameter greater than 5 mm) is an important sign. However, the absence of gallbladder distension does not rule out obstruction. Dilation of intrahepatic bile ducts, visualized as multiple anechoic tubular structures within the hepatic parenchyma (best identified with color Doppler to differentiate from vessels), is a reliable sign of complete obstruction, taking about five days to develop. It is crucial to attempt to identify the cause of the obstruction.

Final Considerations

Ultrasonography is a sensitive tool for the evaluation of hepatopathies in cats, allowing the identification of various alterations in the liver and biliary system. However, it is important to remember that ultrasonographic findings should always be interpreted within the context of the patient's clinical history, physical examination, and laboratory results. Often, diagnostic confirmation requires additional procedures, such as fine-needle aspiration or liver and/or biliary biopsy. Gallbladder aspiration (cholecystocentesis) may be necessary in some cases of cholangitis.

By incorporating ultrasonography into our clinical routine, we significantly enhance our ability to diagnose and manage hepatopathies in our feline patients.