Portosystemic shunt

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Characteristic feature of a distended gastrosplenic vein in a 3 year-old Maltese[1]
Same dog following use of an ameroid constrictor[1]

Portosystemic shunt (PSS) is a relatively common congenital disease of small-breed dogs characterised by vascular shunting of the portal vein away from the liver and directly into the vena cava[2].

Portosystemic shunts are defined as a connection between the portal vessels and systemic circulation that diverts blood flow, in varying degrees, from the liver. They are categorized as either intrahepatic or extrahepatic shunts, and invariably result in poor hepatic hemoperfusion, with secondary fatty infiltration and iron accumulation within hepatocytes (lipogranulomas)[3][4].

Decreased blood flow results in liver atrophy and subsequent dysfunction, decreasing liver metabolism of neurotoxins.

Extrahepatic shunts are the most frequent form of PSS and may arise from any part of the portal system. Most commonly, they originate from the main portal vein trunk, gastro-splenic or gastro-duodenal branches. Extrahepatic shunts usually drain into the posterior caval vein or the hemiazygos vein, but they can also empty into the hepatic veins, renal vein, phrenicoabdominal vein, internal thoracic vein or the thoracic posterior caval vein[5][6].

Although most extrahepatic congenital PSS are single vascular anomalies, multiple (mostly double) CPSS are occasionally diagnosed[7].

Portosystemic shunts can also be an acquired condition as a result of portal hypertension due to chronic hepatitis, congenital hepatic fibrosis, hepatic arteriovenous fistulas, hepatoportal vascular hypoplasia veno-occlusive disease in Cocker Spaniels or portal vascular atresia.

A number of breeds are predisposed, such as the Cairn Terrier, Miniature Schnauzer, Yorkshire Terrier[8], Maltese, Scottish Terrier, Pug, Irish Wolfhound[9], Golden Retriever, Labrador Retriever, German Shepherd and Poodle.

In the Irish Wolfhound, portosystemic shunts commonly occur due to delayed closure of the ductus venosus[10], resulting in an intrahepatic portosystemic shunt. The incidence of this condition in this breed is approximately 2.1 –3.4% and is due to a polygenic mode of inheritance[11].

Clinical signs of hepatic encephalopathy are frequently noted in most cases due to hyperammonemia and can be most severe postprandially, especially after a high-protein meal[12].

Clinically-affected dogs are usually smaller than litter mates and may have concurrent abnormalities such as cryptorchidism. Symptoms first appear at about 6 months of age with mental depression due to hepatic encephalopathy, vomiting, diarrhea, blindness, pica, cystitis, polyuria and polydipsia.

A tentative diagnosis can be established on presenting clinical signs, and radiographic evidence of reduced liver size. Ultrasonographic[13], CT, MRI[14][15] and transplenic portal scintigraphy imaging[16] will assist a stronger suspicion of primary congenital liver disease.

Blood tests usually reveal a microcytic, nonregenerative anemia, poikilocytosis, target cells, hypoproteinemia, hypoalbuminemia, hypoglycemia (especially toy-breed dogs), low BUN, hypocholesterolemia, normal to mildly increased enzymes (ALT, AST, and AP), elevated c-reactive protein[17], reduced gastrin[18] and normal bilirubin. After a prolonged fast, bile acids may rise from normal range of 15 - 30 to as high as 180 μmol/L[19].

Urinalysis usually reveals hematuria, pollakiuria, stranguria or urolithiasis due to ammonia biurate urolithiasis.

Microhepatica and renomegaly are usually noted on abdominal radiographs. Ultrasonography is a useful noninvasive tool for identifying the shunt, determining if the shunt is intrahepatic or extrahepatic, and identifying radiolucent uroliths in the kidneys or bladder. Contrast portography is a more invasive method to identify the shunt but is the best way to evaluate portal vessel anatomy. Rectal portal scintigraphy is noninvasive but is not widely available and cannot differentiate between intrahepatic and extrahepatic shunts or determine location of the shunt. Liver biopsy is indicated in shunt repair or if multiple shunts are noted to determine the primary underlying disease.

Treatment in most cases begins with medical therapy, with nutritional support (low-protein diet) and drugs such as lactulose (to acidify the gastrointestinal tract ), neomycin (to minimize gastrointestinal microflora production of ammonia) and S-Adenosylmethionine. Although surgical intervention is associated with a better chance of long-term survival, medical management provides an acceptable first-line option[20].

In case where the disease progresses, surgical attenuation or ligation of the shunt with an ameroid constrictor is required. Portacaval shunts are usually occluded at their termination at the caudal vena cava and portoazygous shunts can be occluded at the abdominal side of the diaphragm. An intravascular Amplatzer vascular plug has also shown promise as a noninvasive alternative procedure in dogs[21].

Correction of the shunt results in increased liver mass, development of intrahepatic portal vasculature and improved liver function[22]. Routine postoperative management consists of systemic antibiotics and fluid therapy. Oral lactulose and neomycin (or metronidazole) and a protein-restricted diet are usually continued for at least 4 to 8 weeks or longer, depending on the individual patient's clinical response.

Postoperative complications are common, including portal hypertension, postligation seizures[23], heart failure, bacterial hepatitis and pyelonephritis[24]. Seizures are commonly controlled with use of phenobarbital or levetiracetam[25].

The prognosis is good if complete correction can be achieved before 1 year of age and the prognosis is less favorable with partial correction, multiple shunts, and intrahepatic shunts.

References

  1. 1.0 1.1 Pet MD
  2. Nelson NC & Nelson LL (2011) Anatomy of extrahepatic portosystemic shunts in dogs as determined by computed tomography angiography. Vet Radiol Ultrasound 52(5):498-506
  3. Isobe K et al (2008) Histopathological characteristics of hepatic lipogranulomas with portosystemic shunt in dogs. J Vet Med Sci 70(2):133-138
  4. Lee KC et al (2011) Association between hepatic histopathologic lesions and clinical findings in dogs undergoing surgical attenuation of a congenital portosystemic shunt: 38 cases (2000-2004). J Am Vet Med Assoc 239(5):638-645
  5. Gómez-Ochoa P et al (2011) Use of transsplenic injection of agitated saline and heparinized blood for the ultrasonographic diagnosis of macroscopic portosystemic shunts in dogs. Vet Radiol Ultrasound 52(1):103-106
  6. Santilli RA & Gerboni G (2003) Diagnostic imaging of congenital porto-systemic shunts in dogs and cats: a review. The Veterinary Journal 166:7-18
  7. Winkler JT et al (2003) Portosystemic shunts: diagnosis, prognosis and treatment of 64 cases (1993-2001). J Am An Hosp Assoc 39:169-185
  8. Torisu S et al (2008) Measurement of brain trace elements in a dog with a portosystemic shunt: relation between hyperintensity on T1-weighted magnetic resonance images in lentiform nuclei and brain trace elements. J Vet Med Sci 70(12):1391-1393
  9. van Steenbeek FG et al (2009) Evidence of inheritance of intrahepatic portosystemic shunts in Irish Wolfhounds. J Vet Intern Med 23(4):950-952
  10. van Steenbeek FG et al (2013) Altered subcellular localization of heat shock protein 90 is associated with impaired expression of the aryl hydrocarbon receptor pathway in dogs. PLoS One 8(3):e57973
  11. Meyer HP et al (1995) Increasing incidence of hereditary intrahepatic portosystemic shunts in irish wolfhounds in the netherlands (1984 to 1992). Vet Rec 136:13–16
  12. Merck Veterinary Manual
  13. Saponaro V et al (2012) Intraoperative contrast echocardiography to verify the surgical occlusion of a single extrahepatic portosystemic shunt in a dog. Res Vet Sci 93(1):463-465
  14. Mai W & Weisse C (2011) Contrast-enhanced portal magnetic resonance angiography in dogs with suspected congenital portal vascular anomalies. Vet Radiol Ultrasound 52(3):284-288
  15. Moon SJ et al (2012) Magnetic resonance imaging findings of hepatic encephalopathy in a dog with a portosystemic shunt. J Vet Med Sci 74(3):361-366
  16. Morandi F et al (2010) Characterization of multiple acquired portosystemic shunts using transplenic portal scintigraphy. Vet Radiol Ultrasound 51(4):466-471
  17. Gow AG et al (2012) Dogs with congenital porto-systemic shunting (cPSS) and hepatic encephalopathy have higher serum concentrations of C-reactive protein than asymptomatic dogs with cPSS. Metab Brain Dis 27(2):227-229
  18. Mazaki-Tovi M et al (2012) Serum gastrin concentrations in dogs with liver disorders. Vet Rec 171(1):19
  19. Yoon H et al (2011) Contrast-enhanced computed tomography angiography and volume-rendered imaging for evaluation of cellophane banding in a dog with extrahepatic portosystemic shunt. J S Afr Vet Assoc 82(2):125-128
  20. Greenhalgh SN et al (2010) Comparison of survival after surgical or medical treatment in dogs with a congenital portosystemic shunt. J Am Vet Med Assoc 236(11):1215-1220
  21. Hogan DF et al (2010) Intravascular occlusion for the correction of extrahepatic portosystemic shunts in dogs. J Vet Intern Med 24(5):1048-1054
  22. Tivers MS et al (2012) Vascular endothelial growth factor (VEGF) and VEGF receptor expression in biopsy samples of liver from dogs with congenital portosystemic shunts. J Comp Pathol 147(1):55-61
  23. Fryer KJ et al (2011) Incidence of postoperative seizures with and without levetiracetam pretreatment in dogs undergoing portosystemic shunt attenuation. J Vet Intern Med 25(6):1379-1384
  24. Worley DR & Holt DE (2008) Clinical outcome of congenital extrahepatic portosystemic shunt attenuation in dogs aged five years and older: 17 cases (1992-2005). J Am Vet Med Assoc 232(5):722-727
  25. Gommeren K et al (2010) Outcome from status epilepticus after portosystemic shunt attenuation in 3 dogs treated with propofol and phenobarbital. J Vet Emerg Crit Care (San Antonio) 20(3):346-351
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