Gallbladder Agenesis in a Maltese Dog

Introduction

In people, gallbladder agenesis is a rare cause of elevated liver enzymes, but the condition has not been reported in the dog.¹ Alanine transferase (ALT) is found in the cytosol of hepatocytes, red blood cells, and muscle cells.² In dogs, the concentrations of ALT are highest in hepatocytes, with only minor concentrations found in red blood cells and muscle cells; therefore, marked elevations in the concentration of ALT generally develop from hepatocellular damage.² The most likely causes of hepatocellular damage in young, asymptomatic dogs include toxins, infectious/inflammatory liver diseases, hepatic anoxia, trauma, portosystemic shunt, and biliary obstruction.^2,3 This case report describes the findings and diagnostic evaluation of a dog with gallbladder agenesis that was presented for elevations in liver enzymes, especially ALT.

Case Report

A 7-month-old, female Maltese was presented to the referring veterinarian for elective ovariohysterectomy. Preoperative laboratory tests showed a significant increase in ALT of 855 U/L (reference range 8 to 75 U/L), although physical examination findings were unremarkable. A more extensive biochemical panel was rechecked 2 weeks later, and further elevation of ALT was noted (1450 U/L; reference range 12 to 118 U/L). Aspartate transferase (AST; 137 U/L, reference range 15 to 66 U/L) and gamma-glutamyl transpeptidases (GGT; 13 U/L, reference range 1 to 12 U/L) were mildly increased. The dog was referred to the Veterinary Teaching Hospital for further evaluation of liver function.

On presentation, the dog was bright, alert, and responsive. Physical examination findings were normal. A serum biochemical profile showed further elevations in ALT (1798 U/L; reference range 20 to 200 U/L), alkaline phosphatase (ALP; 108 U/L; reference range 4 to 95 U/L), and albumin (4.2 g/dL; reference range 2.6 to 3.5 g/dL), and a mild decrease in globulins (1.7 g/dL; reference range 2.6 to 5 g/dL). Blood ammonia concentrations were normal. Fasting bile acids were elevated (64.7 μmol/L; reference range 0.1 to 6.5 μmol/L), but postprandial (2 hours) bile acids were normal (1.6 μmol/L; reference range 0.6 to 11 μmol/L). No abnormalities were noted on urinalysis (specific gravity >1.050; negative protein). Activated partial thromboplastin time and prothrombin time were also normal.

Findings on abdominal radiography included decreased serosal detail and possible microhepatia characterized by cranial displacement of the stomach and cranial duodenal flexure on both the lateral and ventrodorsal projections. An abdominal ultrasound identified a potential portosystemic shunt from the portal vein to the caudal vena cava. Kidneys, spleen, adrenal glands, urinary bladder, gastrointestinal tract, and pancreas were normal in appearance and size. A gallbladder was not visualized, and the liver size was considered normal on ultrasound.

Based on the elevated ALT concentrations and possible microhepatia in a young toy-breed dog, a portosystemic shunt was suspected. Ultrasound findings were consistent but not definitive for a portosystemic shunt, and other findings (i.e., normal ammonia, blood urea nitrogen, postprandial bile acids) were also inconsistent with a portosystemic shunt. Because of these inconsistencies, mesenteric portography was performed. Under general anesthesia, a jejunal vein was isolated and cannulated through a 4-cm ventral midline incision. Contrast medium (diatrizoate meglumine and diatrizoate sodium, 2 mL/kg)^a was injected into a jejunal vein. Sequential radiographs indicated normal contrast filling of the main portal vein and intrahepatic portal veins, which ruled out a portosystemic shunt [Figure 1]. On the portography, the liver appeared normal in size.

Abdominal exploration was performed to evaluate the liver and obtain a liver biopsy. Examination of the gallbladder fossa at the level of the hepatic ducts between the quadrate and right medial liver lobes confirmed the absence of a normally positioned gallbladder. Each liver lobe was examined, and an ectopic gallbladder was not identified. The biliary duct and hepatic ducts appeared intact. Liver size appeared normal, and no other abnormalities were noted on abdominal exploration. A liver biopsy was obtained using a guillotine suture technique (4-0 polydioxanone).^b The biopsy was divided and submitted for histopathology [Figures 2, 3] and bacterial culture. Histopathological findings were unremarkable except for mild perivascular inflammation around the central vein. Both aerobic and anaerobic bacterial cultures of the liver tissue were negative for growth at 7 days.

The dog recovered from anesthesia without incident. Postoperatively, buprenorphine^c (0.02 mg/kg intravenously q 6 hours) followed by meloxicam^d (0.1 mg/kg per os [PO] q 24 hours for 7 days) were given for analgesia. The dog was discharged on a low-fat diet. A trial of S-adenosylme-thionine^e (7.5 mg/kg PO q 24 hours) was instituted; however, ALT (994 U/L, reference range 12 to 118 U/L), GGT (16 U/L, reference range 1 to 12 U/L), and AST (85 U/L, reference range 15 to 66 U/L) concentrations remained elevated. The S-adenosylmethionine was discontinued, and the dog has remained asymptomatic for 13 months after referral. The owners declined additional diagnostic tests.

Discussion

Gallbladder agenesis has been reported in humans as a rare anomaly, with an incidence of <1 in 6000 live births.^1,4–9 In humans, agenesis of the gallbladder is thought to arise from a lack of vacuolation of the epithelium of the hepatic bud during embryogenesis.^1,9 Three categories of gallbladder agenesis have been described in humans. The first category includes gallbladder agenesis identified early in life.^10,11 It is often associated with other congenital abnormalities.^10,11 The second category includes cases of gallbladder agenesis found as an incidental finding on postmortem examination.¹² These patients have been asymptomatic, and it is estimated that 77% of humans with gallbladder agenesis fall into this category.¹² A final category is composed of adults diagnosed with gallbladder agenesis after presenting with right upper abdominal pain, nausea, and fatty food intolerance.⁵ Other symptoms have included dyspepsia and jaundice.¹³ Reports of gallbladder agenesis have identified elevated liver function tests, especially total bilirubin, as common laboratory abnormalities.¹ The proposed mechanisms for these laboratory abnormalities are bile duct or common bile duct stones, biliary dyskinesia, or regurgitation of duodenal or pancreatic contents.^12,14 In most published reports of gallbladder agenesis in humans, histological findings were not discussed.^{1,5–8,13–15}

A search of the veterinary literature did not produce any reports of gallbladder agenesis in small animals. A few cases of biliary atresia have been reported, and affected animals have been presented with jaundice.^16–21 In the case reported here, ALT was consistently elevated on serial biochemical panels. Alkaline phosphatase, which is often elevated in young animals, was also mildly elevated.² Total bilirubin remained within normal limits (0.2 mg/dL; reference range 0.1 to 0.3 mg/dL). The fasting bile acid elevation was not considered clinically significant, because the postprandial concentrations were normal. Also, bile acid concentrations may be elevated in Maltese dogs with normally functioning livers.²² It is not possible to compare these laboratory results to asymptomatic humans, because most affected people have been diagnosed postmortem.¹² Of 63 symptomatic humans with gallbladder agenesis, 35% had elevated ALP, 20% had elevated AST, 9% had elevated ALT, and 38% had elevated total bilirubin values.¹²

The significance of and mechanism of action for the increased ALT concentration in this dog were unknown. The elevated concentration of GGT supported a possible hepatic origin for the ALT elevation, as did a normal creatine kinase and complete blood count.² Increased concentrations of ALT are not a distinct anomaly of the Maltese breed.²² In normal dogs, bile is secreted by hepatocytes into the bile canaliculi that empty into the bile ducts. The bile ducts then empty into the hepatic duct, and bile either flows into the cystic duct for storage in the gallbladder or empties into the duodenum.²³ Stimulation by cholecystokinin and nerve fiber from the vagus and intestinal enteric system cause gallbladder contraction and emptying.²³ For bile to empty into the duodenum from the common bile duct, the sphincter of Oddi must relax in response to cholecystokinin.²³ Agenesis of the gallbladder may allow a constant flow of bile toward the duodenum without relaxation of the sphincter of Oddi, resulting in biliary dyskinesia.^14,23 Dyskinesia in turn may cause back pressure on the hepatocytes, resulting in inflammation.¹⁴ Alternatively, the sphincter of Oddi may also be dysfunctional and allow regurgitation of duodenal or pancreatic contents into the biliary tract, causing hepatic inflammation.^12,14 The resulting biliary inflammation and hepatitis might increase ALT and AST concentrations.²⁴ However, such reflux might also cause increased bilirubinemia and neutrophilia, which were not present in this case.^12,24

Mild perivascular inflammation was reported on the hepatic histopathology. The significance of this inflammation was unknown. If the inflammation noted was secondary to ductal inflammation and hepatitis from biliary dyskinesia, there should have been inflammatory cells around the bile ducts, not around the central vein. The inflammation seen may have been unrelated to the gallbladder agenesis and may have accounted for the increased ALT values. Explanations (other than gallbladder agenesis) for the increased ALT and perivascular inflammation included unknown hepatotoxins.

In the evaluation of this case, gallbladder agenesis was diagnosed on both abdominal ultrasonography and surgical exploration. This was consistent with the practice in humans of requiring meticulous hepatic exploration at the time of laparotomy and supportive findings on ultrasonographic evaluations, in order to diagnose true agenesis and avoid the misdiagnosis of an ectopic gallbladder.¹⁴

During the Doppler ultrasound evaluation of this dog, a portosystemic shunt was misidentified. This may have occurred from a motion artifact that mimicked blood flow or from a misidentification of the hepatic vein.²⁵ Motion artifact can be minimized and visualization optimized by using positive-pressure ventilation under anesthesia, but this is not standard practice for conducting most ultrasound examinations.²⁶ An alternative method of using multiple planes during ultrasonography has recently been described, and this technique may prove clinically useful for avoiding misdiagnosis of portosystemic shunts and for correctly identifying existing portosystemic shunts.²⁷ The initial radiographic diagnosis of microhepatia supported the possibility of a portosystemic shunt; however, the portogram definitively ruled out a single portosystemic shunt, and histopathology ruled out microvascular dysplasia.

As a precautionary measure, this dog was started on a low-fat diet. Medications for biliary stasis were not initiated, because there was no evidence of biliary stasis on biochemical survey profile or histopathology. The indication for using neutraceuticals such as S-adenosylmethionine was unclear in this case. This dog was asymptomatic; however, the serum concentrations of ALT indicated persistent increased permeability of hepatocellular membranes.² Since S-adenosylmethionine has been shown to have hepatoprotection effects, a trial administration of S-adenosylmethionine was initiated to see whether it would decrease the ALT values.²⁸ When no effect was determined, the S-adenosylmethionine was discontinued.

Although the prognosis for dogs with gallbladder agenesis is uncertain, it may be better than the prognosis for humans with this anomaly. Choleliths have been responsible for the development of symptoms in 66% of humans with gallbladder agenesis that presented with jaundice.¹³ Dogs may be less likely to develop choleliths than people, possibly because of decreased cholesterol in their bile; therefore, dogs with gallbladder agenesis may be less likely to become symptomatic.²⁴

Clinical markers for gallbladder agenesis have yet to be defined, but they should include absence of a gallbladder on ultrasonography. Elevations in hepatic enzymes may or may not be related to this anomaly.

Conclusion

Gallbladder agenesis was diagnosed in a young, asymptomatic Maltese with elevated liver enzymes. Definitive diagnosis was based on ultrasonographic findings and meticulous surgical exploration. While the long-term prognosis for dogs with gallbladder agenesis is unknown, this case suggests that the condition may not cause clinical signs unless it is accompanied by other abnormalities.