An update on gallbladder mucoceles in dogs
Before 2000, gallbladder mucoceles were rarely reported in dogs. However, they are now considered one of the more common causes of extrahepatic biliary disease. While gallbladder mucoceles were initially treated by cholecystectomy, case-based evidence indicates that some may resolve with medical therapy.1
In this article, we review gallbladder anatomy and physiology and discuss the pathophysiology of mucocele development. We also describe the clinical presentation, diagnosis, and management of dogs with gallbladder mucoceles.
GALLBLADDER ANATOMYThe gallbladder is an excretory organ found between the quadrate and right medial liver lobes. It is pear-shaped and composed of a fundus, body, and neck. From the gallbladder's neck, the hepatic ducts join the cystic duct to form the common bile duct, which leads to the duodenum.
The gallbladder wall has five layers: epithelium, mucosa, tunica muscularis externa, tunica serosa, and tunica adventitia.2
1. The epithelium is simple columnar and highly absorptive. It plays an important role in gallbladder function because it secretes mucin, immunoglobulins, and acid.
2. The mucosa is a combined layer consisting of the lamina propria and tunica submucosa.2 These layers of the mucosa are indistinguishable and contain a dense population of lymphocytes and plasma cells.
3. The tunica muscularis externa consists of scant, randomly organized smooth muscle fibers.
4. The tunica serosa is a membranous layer surrounding the gallbladder that faces away from the liver.
5. The tunica adventitia is the outermost gallbladder layer and faces the liver.
The gallbladder plays many important roles in digestive health and function, including storing and concentrating bile. Within the gallbladder, water, electrolytes, lipids, and proteins are absorbed from the bile. This absorption allows for a five- to 20-time increase in bile bilirubin and bile salt concentrations. The gallbladder also acidifies bile through epithelial acid secretions and adds mucin to bile by bile acid stimulation of the mucosa.
Bile is required for the successful digestion and absorption of nutrients. It aids in fat digestion and absorption by emulsifying large fat particles into smaller ones that are more susceptible to pancreatic lipase. Bile also enhances intestinal absorption of digested fats and aids in the excretion of waste products, including cholesterol and bilirubin.
Bile is produced by hepatocytes and collected in the hepatic canaliculi. From the canaliculi, bile drains into interlobular ducts. These ducts progressively merge to form hepatic ducts, which join the cystic duct to form the common bile duct.
The common bile duct enters the duodenal lumen at the major duodenal papilla; this junction is commonly referred to as the sphincter of Oddi. The pancreatic duct also empties into the small intestine at this point.
The pH of bile ranges from 5.9 to 7.8. Bile is made of water, bile acids, bilirubin, cholesterol, lecithin, and electrolytes.3 Although some bile flows directly into the small intestine from the liver, most of it is temporarily stored in the gallbladder. Substantial bile modifications occur within the gallbladder: sodium, chloride, and water are removed and hydrogen ions are added. After a meal, the gallbladder also secretes bicarbonate-rich mucin, which mixes with the stored bile. The gallbladder contents become inspissated (sludgelike) when more fluid is reabsorbed or more mucin is added.
Biliary sludge is a mobile mixture of precipitated cholesterol crystals, bile pigments, bile salts, and mucin. In people, inspissated bile is considered abnormal and is associated with cholestasis, choleliths, cholecystitis, biliary infection, fasting, and partial parenteral nutrition.4 In dogs, biliary sludge may be associated with disease but is also often seen in clinically normal geriatric dogs.5 At present, the significance of biliary sludge in dogs is unknown.