These single cysts are often limited to one liver lobe, usually cause no substantial compressive injury, and are occasionally discovered serendipitously during ultrasonographic examinations for other disorders, at surgery, or at necropsy. They do not expand to damage adjacent tissues, are not associated with increased liver enzyme activity, and are considered inconsequential. However, they may be problematic if they enlarge or interfere with bile flow at the porta hepatis.
Hepatic fibropolycystic disorders are congenital disorders that have been identified in dogs and cats and reflect embryologic malformations involving the ductal plate (portal triad). Ductal plate malformations (DPMs) reflect embryogenic malformations due to dysfunction of primary cilia causing defective tubulogenesis, affecting formation of bile ducts and renal tubules. Either or both organs may be affected. Fibropolycystic syndromes are divided into six categories in people, and this divisional classification also appears relevant to animals: 1) diffuse DPMs associated with increased deposition of extracellular matrix (congenital hepatic fibrosis) causing intrahepatic presinusoidal portal hypertension, 2) Caroli malformation causing sacculation of large intrahepatic or interlobular bile duct (may be associated with increased deposition of extracellular matrix), 3) von Meyenburg complexes representing microscopic isolated DPMs, 4) simple hepatic cysts, 5) choledochal cysts representing an appendix-like diverticulum from the extrahepatic bile duct that can act as an infection sump, and 6) biliary cystadenomas that likely represent expanding cystic malformations involving large biliary ducts usually located adjacent to the gallbladder and cystic duct. Different malformations can predispose to cholangitis, lead to prehepatic portal hypertension, or evolve into space-occupying cystic lesions. A single genetic mutation in cats (autosomal onset dominant polycystic kidney disease) can lead to biliary ductule malformations and hepatic fibrosis, which are commonly misidentified as advanced biliary cirrhosis subsequent to the feline cholangitis/cholangiohepatitis syndrome. Most cats with this mutation demonstrate polycystic renal malformations rather than biliary malformations, although a small subset of cats develop diffuse fibropolycystic liver lesions. Multiple mutations have been identified to cause fibropolycystic malformations in people, and it is suspected that a similar spectrum of gene mutations also may underlie similar syndromes in dogs and cats. In some cats, many large hepatic cysts cause profound hepatomegaly, requiring repeated drainage, fenestration, marsupialization, or surgical resection. Uncommonly, cystic structures may become mineralized or complicated by formation of mineralized choleliths.
Cats with diffuse DPMs have islands of normal hepatic parenchyma segregated by bridging portal tracts that represent proliferative malformed bile ductules embedded in a complex of extracellular matrix (dense fibrillar collagen). Extensive connective tissue causes intrahepatic portal hypertension, a firm large liver, development of APSSs, signs of HE, and ascites. Affected cats may present with increased liver enzyme activity (ALT, AST, ALP) due to low-grade inflammation or development of septic cholangitis or cholelithiasis. Most are not hyperbilirubinemic or jaundiced. Liver enzymes may be within the normal range, with diagnostic biopsy initiated by ultrasonographic findings.
Biliary dysplastic syndromes rarely occur in dogs concurrent with renal cystic malformations. However, DPMs are as commonly identified in dogs as in cats. Affected dogs may develop increased ALP activity and increased TSBA concentrations. As in cats, extensive connective tissue deposition in the portal tract can cause portal-to-portal bridging fibrosis, intrahepatic presinusoidal portal hypertension, APSSs, HE, and ascites.
The only treatment for DPM associated with bridging portal fibrosis is to palliate HE with protein-modified diets and efforts to alter the enteric microbial flora and pH (lactulose, milk, or low-dose metronidazole). Diuretics and dietary sodium restriction are used to control ascites. Similar syndromes in people are managed by liver transplantation.
Congenital, appendix-like cystic dilation of the extrahepatic bile duct is recognized uncommonly in cats. Clinical signs include fever, abdominal pain, and jaundice, associated with cyst infection, with the amotile cyst functioning as an infected sump. Surgical exploration is usually required for definitive diagnosis. Extirpation of the cystic malformation or its marsupialization into the common bile duct has been a successful treatment. Postoperative longterm antimicrobial therapy is guided by bile culture, cytology, and repeated ultrasonographic imaging.
These lesions, also termed cystadenomas, bile-duct adenomas, cholangiocellular adenomas, cystic cholangiomas, and hepatobiliary cystadenomas, are benign lesions most commonly encountered in older cats. Well-demarcated, single lesions often have a complex internal structure and may invade or compress adjacent hepatic parenchyma, causing compressive atrophy. Cyst contents range from clear, watery fluid to viscous or solid material; typically there are no bile acids or bilirubin. Cyst sizes vary, ranging from 1 mm to 8 cm, with masses ranging from 5 mm to 12.5 cm. Imaging studies (ultrasonography or CT) are key to diagnosis.
Although surgical excision is the treatment of choice, this may not be possible if the structure integrates into the porta hepatis. However, because these lesions are often discovered serendipitously, surgical excision is commonly not indicated. Prognosis after complete cystadenoma excision is good; however, if surgical intervention is necessary and complete excision is not possible, partial resection may delay complications from mechanical invasion of normal tissue. Repeated aspiration, catheter drainage, marsupialization, and partial excision have been used for palliative management but carry risk of infections. Neoplastic transformation is also possible. Cystadenomas are usually difficult to aspirate owing to their complex internal structure.