Canine Gallbladder Mucocelesvetfolio-vetstreet.s3. · PDF filegallbladder mucocele is an...

Click here to load reader

  • date post

    12-Feb-2018
  • Category

    Documents

  • view

    212
  • download

    0

Embed Size (px)

Transcript of Canine Gallbladder Mucocelesvetfolio-vetstreet.s3. · PDF filegallbladder mucocele is an...

  • gallbladder mucocele is an enlargedgallbladder containing an excessiveamount of mucus. The word cele is

    derived from the Greek word kele for tumor. Amucocele therefore describes a solid accumula-tion of mucus that may resemble a tumor ormass. For many years, pathologists have notedgross accumulation of excess mucus in the gall-bladder of many older dogs during necropsyand have considered it an age-related inci-dental lesion.1 Recently, a series of reports27

    have shown that not all gallbladder mucocelesare clinically silent and that they can be associ-ated with extrahepatic bile duct obstruction(EHBDO), cholecystitis, and gallbladder wallrupture. This article reviews normal gallbladderstructure and function and discusses what iscurrently known about the pathogenesis, diag-nosis, and treatment of gallbladder mucoceles.

    NORMAL ANATOMYThe gallbladder is a pear-

    shaped sac that lies on thevisceral surface of the liverbetween the quadrate lobemedially and the right medial

    lobe laterally. The cystic duct extends fromthe neck of the gallbladder until it emptiesinto the common bile duct (Figure 1). Thelast part of the common bile duct lies embed-ded in the duodenal musculature as it coursesto its exit site at the major duodenal papilla.This intramural part behaves like a sphincterand is therefore referred to as the sphincter ofOddi in humans. In dogs, the pancreatic ductalso exits at or near the major papilla. Theaccessory pancreatic duct, which is the largerof the two pancreatic ducts in dogs, opensabout 2.5 cm caudal to this at the minor duo-denal papilla.8,9

    The gallbladder is a thin-walled, muscularsac that can distend to accommodate bile forstorage during interdigestive periods. Histolog-ically, the gallbladder wall is composed of aninnermost mucosal epithelial cell layer, a sub-mucosal layer containing the lamina propria, amiddle smooth muscle layer, a layer of connec-tive tissue, and an outermost serosa10 (Figure2). The lamina propria contains the vascularsupply, which originates from the cystic artery,a branch of the left branch of the hepaticartery.11

    COMPENDIUM 912 December 2005

    Canine Gallbladder MucocelesLilian Cornejo, DVMCynthia R. L. Webster, DVM, DACVIMCummings School of Veterinary Medicine at Tufts University

    ABSTRACT: Gallbladder mucoceles occur when the gallbladder distends abnormally from excessive

    accumulation of mucus within the lumen. Mucoceles have become an important cause of

    extrahepatic biliary tract disease in dogs.The clinical presentation is variable, and signs may even be

    absent. Gallbladder rupture can occur and lead to significant morbidity in affected dogs. Cystic

    mucinous hyperplasia of the gallbladder epithelial cells represents the key histologic feature in this

    disease.The pathogenesis of mucocele formation is unknown. Surgical cholecystectomy is an

    effective treatment of affected patients.The long-term prognosis is excellent in patients that survive

    the postoperative hospitalization period.

    Article #2CE

    Send comments/questions via emaileditor@CompendiumVet.comor fax 800-556-3288.

    Visit CompendiumVet.com for full-text articles, CE testing, and CE test answers.

    A

  • December 2005 COMPENDIUM

    Canine Gallbladder Mucoceles 913CE

    NORMAL GALLBLADDER FUNCTIONBile is the secretory product of hepatocytes and prima-

    rily consists of water, bile acids, bile pigments, choles-terol, and other inorganic salts.1214 Certain drugs andantimicrobials may also enter bile. Bile is secreted contin-uously by the liver.15 It eventually enters the gallbladder,where it is stored and modified. After a meal, the gall-bladder contracts and discharges bile through the cysticduct and common bile duct into the duodenum. At thesame time, the sphincter of Oddi opens to facilitate bileflow through the common bile duct into the duodenum.

    Between meals, most bile is diverted into the gallblad-der, where bile can be stored, acidified, and concentrated.The primary changes that occur in bile compositioninclude transport of sodium, chloride, water, and othersmall electrolytes across the mucosal lining and out of thegallbladder lumen.10,1618 Secretion of hydrogen ions by thegallbladder mucosal epithelial cells acidifies the gallblad-der contents. At mealtime, the gallbladder mucosa secretesa mucinous, bicarbonate-rich fluid into its lumen thatmixes with the modified stored hepatic bile. Secretion ofthis fluid is facilitated by the action of the gastrointestinalpeptides secretin and vasoactive intestinal polypeptide ongallbladder epithelial cells.17,18 Following secretion of thisfluid, the gallbladder contracts and discharges its contentsinto the cystic duct and common bile duct.

    The strongest stimulus for postprandial gallbladdercontraction is the hormone cholecystokinin (CCK),which is released by enterocytes when fat from a mealenters the duodenum. CCK also causes relaxation of thesphincter of Oddi, which facilitates bile flow into theduodenum.19,20 Parasympathetic stimulation (vagus) and

    sympathetic inhibition (greater splanchnic nerve) havemilder effects on gallbladder contraction.20,21 Other gas-trointestinal hormones may enhance contraction (e.g.,neurotensin, substance P, neuropeptide Y) or gallbladderrelaxation (e.g., somatostatin, vasoactive intestinalpolypeptide, nitric oxide, pancreatic polypeptide) butgenerally have only mild effects.2022

    GALLBLADDER MUCOCELESeveral synonyms for canine gallbladder mucoceles,

    including cystic mucinous hyperplasia, cystic mucinous

    hypertrophy, and mucinous cholecystitis, have appeared inthe veterinary literature.14,23 The principal gross abnor-mality associated with gallbladder mucoceles is gallblad-der enlargement secondary to buildup of excessiveamounts of mucus within the lumen. The predominant

    histologic feature of gallbladder mucoceles is cysticmucinous hyperplasia. Mucocele formation has beenreported as an incidental lesion on necropsy1 but can beassociated with extrahepatic biliary tract disease.27

    PathogenesisLittle is known about the pathogenesis and potential

    causative factors that can lead to gallbladder mucoceleformation in animals. The pathogenesis and cause ofmucoceles in humans are equally uncertain. Mucocelesare an infrequent cause of human noninflammatory

    The appearance of a characteristic stellate or finely striated immobile gallbladderbile pattern on an ultrasonogram accurately identifies a gallbladder mucocele.

    Figure 1. Schematic drawing of the anatomy of the intra-and extrahepatic biliary tree showing the gallbladderfundus (f), body (b), and neck (n). (Illustration by Felecia Paras)

  • COMPENDIUM December 2005

    Canine Gallbladder Mucoceles914 CE

    gallbladder disease but can occur as a result of obstruc-tion of the neck of the gallbladder (i.e., usually bystones, and occasionally by neoplasia). Mucoceles canoccur with or without concurrent cholecystitis.

    The factors that promote mucocele development indogs remain poorly understood. Extrahepatic bile ductobstruction may accompany this disorder, and someauthors had previously considered obstruction of thecommon bile duct or cystic duct to be a potential incit-ing factor in mucocele development.4,7 In studies withdogs,1,24 experimental ligation of either duct failed toconsistently lead to gallbladder mucocele developmenteven after weeks of duct ligation. Although experimen-tal ligation of the cystic duct in dogs resulted inincreased amounts of mucus within the gallbladder inone study,24 cystic mucinous hyperplasia did not developin any of these dogs and intraluminal gallbladder pres-sure did not change significantly during the experiment.In a separate study25 in which the cystic duct of dogswas obliterated using electrocoagulation, gallbladdermucocele formation did result in dogs not immediatelytreated with tetracycline. In two large retrospective

    studies of dogs with mucoceles,EHBDO was not present in all the sub-jects. EHBDO (as defined by nonex-pressible bile from the gallbladder orcystic duct) was present in only 17 of 33cases at surgery.2,3 This suggests thatEHBDO may be a secondary event tomucocele development.

    Choleliths have also been implicated inthe pathogenesis of canine mucocele for-mation.7,24 However, choleliths have beenreported in only one dog with a gallblad-der mucocele.7

    In one case report,23 a gallbladdermucocele occurred in a dog during prog-estational therapy, which led the authorsto speculate on a relationship betweenprogestational compounds and gallblad-der mucocele formation. However, gall-bladder abnormalities were notreproducible in a subsequent study inwhich dogs received a similar progesta-tional compound.26 In addition, previoususe of a progestational drug did not occurin any of the gallbladder mucocele casesreported in the veterinary literature.

    Interestingly, in one study,2 glucocorti-coid excess (endogenous and exogenous) accompaniedgallbladder mucocele development in nine of 30 dogs. Itremains to be established whether corticosteroids playeda causative or perpetuating role in the gallbladderpathology of these patients.

    Cystic mucinous hyperplasia has concurrently beenidentified with other gallbladder diseases, such as chole-cystitis3,5 and, in one dog, a porcelain gallbladder.27

    Whether cholecystitis precipitated mucocele formationin some patients or inflammatory changes developedsecondarily to the presence of mucoceles remains unan-swered. Because mucoceles can occur in the absence ofinflammatory lesions, the presence of inflammation doesnot appear to be a prerequisite for abnormal mucusbuildup and gallbladder distention.

    The very limited information available to date in theveterinary literature suggests that mucocele develop-ment may result from an abnormality of the mucosalcells within