What is PSS? Portosystemic shunts (aka portosystemic vascular anomalies, portocaval shunts) are a group of vascular anomalies characterized by a communication between the portal vein and the systemic venous circulation (usually the caudal vena cava). These communications result in portal blood bypassing the sinusoids and hepatic parenchyma. PSS can be intrahepatic (usually a communication between the portal vein and caudal vena cava, as a remnant of the ductus venosus), or extrahepatic. They can be primary anomalies or arise secondarily to obstructed portal venous return to the liver.

What is MVD? Microvascular dysplasia (aka primary hypoplasia of the portal vein without portal hypertension) is a microscopic malformation of the hepatic microvasculature. Intrahepatic portal vessels are small, with endothelial hyperplasia, portal vein dilation and central venous hypertrophy and fibrosis. Arterioles can develop connections with more than one single liver acinus and their branches can directly drain into sinusoids as well as terminal hepatic venules. The latter communications establish conduits of intrahepatic shunting. Thus, these lesions can result in direct shunting of blood from the portal circulation to the systemic circulation at a microscopic level.

Which condition is more common – PSS or MVD? MVD appears to be far more common than PSSs (Center 2008). It can occur as a single entity or can coexist with a PSS. Unfortunately, microscopic changes that occur in the liver with a PSS are indistinguishable from those associated with MVD. Unlike most patients with a PSS, dogs with MVD are typically subclinical and their hepatic vascular abnormalities are non-progressive. The majority of dogs do NOT need medical therapy and have a NORMAL life expectancy. Most dogs affected with MVD are discovered upon blood screening tests at the time of a non-related illness or during pre-anesthetic blood chemistry evaluation, as there can be a mild increase in hepatic enzyme concentrations.

Are breed predispositions of diagnostic value? Congenital extrahepatic PSS are the most commonly seen and occur predominantly in small toy breed dogs: Yorkshire Terrier, Havanese, Maltese, Dandie Dinmont Terrier, Pug, and Miniature Schnauzer. Intrahepatic PSS are overrepresented in larger breed dogs with a prevalence in the Irish Wolfhound, Retrievers (Labrador and Golden), Australian Cattle Dogs, and Australian Shepherds. However, many of these same breeds are anecdotally thought to be predisposed to MVD. Therefore, breed/signalment is of no discriminating value. Shunts also occur in cats with extrahepatic PSS occurring more frequently than intrahepatic PSS. They are most commonly reported in the domestic shorthair, Persian, Siamese, Himalayan, and Burmese.

Are clinical signs helpful in discriminating between PSS and MVD? Most dogs with MVD are subclinical. Many, but not all, dogs with PSS have some clinical signs or develop clinical signs with age. As a result of blood bypassing the liver, trophic factors (insulin and glucagon) do not stimulate hepatic growth. This results in poor hepatic development, deficient protein production, reticuloendothelial dysfunction, altered fat and protein metabolism, hepatic atrophy, and eventually liver failure. Most pets present with signs of chronic or acute illness before 1-2 years of age. The three most commonly affected systems are the CNS, GI, and urinary systems. The history is suggestive when a patient “fails to thrive,” is smaller in stature, has weight loss, does not tolerate anesthesia well, is dull or lethargic at times, and may display dysuria or PU/PD. Clinical signs of hepatic encephalopathy (HE) will develop when more than 70% of the liver function is lost. Signs of HE can be subtle or quite obvious. Common signs include ataxia, unresponsiveness, pacing, circling, blindness, seizures, and coma. GI signs include vomiting, diarrhea, anorexia, pica, and/or GI bleeding/melena/hematemesis. Lower urinary tract signs include hematuria, stranguria, pollakiuria, or urinary obstruction.

Are bile-acid and hepatic function tests discriminating? Bile acids are discussed in more detail in the Bile Acids FAQ. In general, bile acid quantification should be done with paired samples (pre-meal and 2-hours post-meal). Some clinicians no longer fast dogs for the pre-meal samples arguing that the fasting serum concentration can be higher in 15-20% of dogs due to physiologic variables influencing the test. Production, excretion, and enterohepatic recirculation are all evaluated when performing a bile acids test. The results of this test can therefore be affected by: 1. the timing of gallbladder contraction; 2. intestinal transport rate; 3. degree of bile acid deconjugation in the small intestine; 4. rate and efficiency of absorption in the ileum; 5. portal blood flow; or 6. efficiency of hepatocyte uptake and canalicular transport. A few patients will have elevated fasting and normal postprandial concentrations, while some will have normal fasting and elevated postprandial concentrations. Most will have elevated fasting and postprandial concentrations. Some Maltese dogs have been identified to have elevated bile acids without evidence of hepatocellular dysfunction (Tisdall et al 1994). Additionally, false positive and false negative test results can occur (Bauer et al 2006, Center et al 1995, Center 1996, Willard & Twedt 1999). Causes of false positive results include: 1. inappropriate sample timing; 2. other hepatobiliary diseases/cholestasis; 3. glucocorticoid or anticonvulsant therapy; 4. tracheal collapse; 5. seizures; 6. GI disease. False negative results can occur with: 1. delayed intestinal absorption from prolonged transport time; 2. lack of gallbladder contraction; 3. inadequate food intake/delayed gastric emptying; 4. malabsorption or maldigestion (malassimilation). Anecdotally, the higher the concentration(s) of the bile acids, the more probable the presence of a PSS. The author believes that most dogs with bile acid concentrations of 100 µmol/L or higher would be strongly considered to have some form of a PSS. However, this is not always true. However, mild or moderate increases in BA do not rule out PSS or increase the likelihood of a diagnosis of MVD. For example, Winkler et al evaluated 64 cases of PSS from 1993-2001 and several patients had only mild elevations in their bile acids, which would be considered by many clinicians to be more consistent with a diagnosis of MVD (Winkler et al 2003). The lesson here is that an elevation in the bile acid concentrations should be interpreted in light of the breed, age, and clinical signs of the patient, and appropriate screening for a PSS should be pursued despite the concentration of the bile acids if the clinician suspects the presence of a shunt.

Can Protein C concentrations be helpful? Protein C is an anticoagulant protein that is important for maintenance of hemostatic balance and to protect against thromboembolism. It is a vitamin-K dependent anticoagulant protein that is synthesized in the liver and circulates as a plasma zymogen and is activated on the luminal surface of endothelial cells by interaction with thrombin. It then combines with protein S and this complex degrades factors Va and VIIIa. Toulza et al suggested that protein C activity appeared to function as a biomarker of hepatic function and hepatoportal perfusion. Their results indicated that protein C activity is significantly lower in dogs with congenital or acquired PSS, compared with dogs without PSS, affirming that protein C activity reflects the adequacy of hepatic portal perfusion in dogs as suggested in humans. In this study, protein C activity was greater than or equal to 70% in patients with MVD in 95% of dogs. A protein C of less than 70% was found in 88% of dogs with portosystemic vascular anomalies. The conclusion was that protein C activity can help prioritize tests used to distinguish PSS from MVD. Therefore, a patient with high serum bile acids and a low protein C concentration (<70%) would be more likely to have a PSS than MVD and may indicate more extensive evaluation (i.e. diagnostic imaging) in confirming the presence of a PSS is required.

What imaging studies are useful for discriminating between PSS and MVD? Abdominal ultrasound (US) is the most widely used imaging modality for diagnosing PSS but results are extremely operator and experience dependent. US is noninvasive, does not require general anesthesia and does not require special licensing or handling as does scintigraphy. Decreased numbers of hepatic and portal veins, a subjectively small liver, and an anomalous vessel are most often seen with a congenital PSS. Considerable variation exists in the reported accuracy of US for detection of shunts with sensitivities ranging 74%-95% and specificity from 67%-100% (Lamb 1996, Holt et al 1995, Tiemessen et al 1995, d’Anjou et al 2004). Transcolonic scintigraphy uses radioactive technetium pertechnetate to non-invasively detect a PSS. The technetium is infused into the colon, per rectum, and the pet is imaged with a gamma camera. Normally, the isotope is absorbed and drained through the colonic veins and then caudal mesenteric vein, portal vein, liver, and heart. In a PSS, the isotope reaches the heart, bypassing the liver, and returns to the liver via the arterial circulation. If a shunt is present, a shunt fraction can be calculated which gives an estimate of the percentage of portal blood bypassing the liver. A fraction of <15% is normal with most shunt dogs having fractions >60% to 80%. Scintigraphy does not provide morphologic information regarding shunt type, location, or number, and cannot distinguish intrahepatic from extrahepatic PSS. In a dog with MVD, transcolonic scintigraphy would be unremarkable. Computed tomographic angiography (CTA) is the gold standard system used in people to evaluate the portal venous system. It is noninvasive, fast, and images all portal tributaries and branches from a single peripheral venous contrast injection. It is most valuable in animals with suspected intrahepatic PSS or for which US is not diagnostic and more invasive imaging such as portography is not desired. It is helpful in preprocedural planning for both surgical and interventional radiologic approaches to IHPSS. Magnetic resonance angiography (MRA) also can provide three-dimensional preoperative shunt images but images from dual-phased CTA are less costly than MRA, provide superior detail and are easy to interpret. Portovenography (portography) is performed infrequently due to availability of other less invasive imaging modalities (US, scintigraphy, CTA). Surgical mesenteric portography, percutaneous ultrasound-guided splenic venography, and injection of contrast material into the mesenteric artery (also known as a cranial mesenteric angiogram) via access of the femoral artery are three options available to perform portovenography (Morandi et al 2010). [Top] REFERENCES Journal Articles 1. Allen L, Stobie D, Mauldin GN, Baer KE. Clinicopathologic features of dogs with hepatic microvascular dysplasia with and without portosystemic shunts: 42 cases (1991-1996). J Am Vet Med Assoc. January 1999;214(2):218-20 2. Bauer NB, Scheneider MA, Neiger R, Moritz A. Liver disease in dogs with tracheal collapse. J Vet Intern Med. 2006; 20:845. 3. Center SA, Erb HN, Joseph SA. Measurement of serum bile acids concentrations for diagnosis of hepatobiliary disease in cats. J Am Vet Med Assoc. 1995; 207:1048. 4. Christiansen JS, Hottinger HA, Allen L, Phillips L, Aronson LR. Hepatic microvascular dysplasia in dogs: a retrospective study of 24 cases (1987-1995). J Am Anim Hosp Assoc. 2000 Sep-Oct;36(5):385-9 5. d'Anjou M-A, Penninck D, Cornejo L, Pibarot P. Ultrasonographic diagnosis of portosystemic shunting in dogs and cats. Vet Radiol Ultrasound. 2004 Sep-Oct;45(5):424-37 6. Daniel GB. Scintigraphic diagnosis of portosystemic shunts. Vet Clin North Am Small Anim Pract. July 2009;39(4):793-810 7. Holt DE, Schelling CG, Saunders HM, Orsher RJ. Correlation of ultrasonographic findings with surgical, portographic, and necropsy findings in dogs and cats with portosystemic shunts: 63 cases (1987-1993). J Am Vet Med Assoc. November 1995;207(9):1190-3 8. Hunt GB. Effect of breed on anatomy of portosystemic shunts resulting from congenital diseases in dogs and cats: a review of 242 cases. Aust Vet J. December 2004;82(12):746-9. 9. Lamb CR. Ultrasonographic Diagnosis Of Congenital Portosystemic Shunts In Dogs: Results Of A Prospective Study. Vet Radiol Ultrasound. 1996 ;37(4):281-288 10. Morandi F, Sura PA, Sharp D, Daniel GB. Characterization of multiple acquired portosystemic shunts using transplenic portal scintigraphy. Vet Radiol Ultrasound. 2010 Jul-Aug;51(4):466-71 11. Phillips L, Tappe J, Lyman R, Dubois J, Jarboe J. Hepatic Microvascular Dysplasia In Dogs. Prog Vet Neurol. 1996 ;7(3):88-96 12. Ruland K, Fischer A, Hartmann K. Sensitivity and specificity of fasting ammonia and serum bile acids in the diagnosis of portosystemic shunts in dogs and cats. Vet Clin Pathol. March 2010;39(1):57-64 13. Tiemessen I, Rothuizen J, Voorhout G. Ultrasonography in the diagnosis of congenital portosystemic shunts in dogs. Vet Q. June 1995;17(2):50-3 14. Tisdall PL, Hunt GB, Tsoukalas G, Malik R. Post-prandial serum bile acid concentrations and ammonia tolerance in Maltese dogs with and without hepatic vascular anomalies. Aust Vet J. April 1995;72(4):121-6 15. Toulsa O, Center SA, Brooks MB, Erb HN, Warner KL, Deal W. Evaluation of plasma protein C activity for detection of hepatobiliary disease and portosystemic shunting in dogs. J Am Vet Med Assoc 2006; 229(11):1761. 16. Walker MC, Hill RC, Guilford WG, Scott KC, Jones GL, Buergelt CD. Postprandial venous ammonia concentrations in the diagnosis of hepatobiliary disease in dogs. J Vet Intern Med 2001; 15:463. 17. Winkler JT, Bohling MW, Tillson DM, Wright JC, Ballagas AJ. Portosystemic shunts: diagnosis, prognosis, and treatment of 64 cases (1993-2001). J Am Anim Hosp Assoc 2003; 39:169. 18. Zwingenberger A. CT diagnosis of portosystemic shunts. Vet Clin North Am Small Anim Pract. July 2009;39(4):783-92 Proceedings 1. Center SA: Portosystemic Vascular Anomalies & Hepatic MVD: Histological, Clinical, & Treatment Options. Proceedings ACVIM, 2008 2. Center SA: Portosystemic Vascular Anomalies & Hepatic MVD: Evidence of Common Genetics in Small Dogs. Proceedings ACVIM, 2008 Books 1. Berent AC, Weisse C: Hepatic Vascular Anomalies. In: Ettinger SJ, Feldman ED, ed. Textbook of veterinary internal medicine: diseases of the dogs and cats, 7th ed., St Louis: Elsevier Saunders; 2010:1649. 2. Center SA: Hepatic vascular diseases. In: Strombeck's small animal gastroenterology, 3rd ed., Philadelphia PA: Saunders, 1996:802. 3. Willard MD, Twedt DC: Gastrointestinal, pancreatic, hepatic disorders. In: Willard MD, Tvedten H, Turnwald GH, ed. Small animal clinical diagnosis by laboratory methods, 3rd ed., Philadelphia: Saunders; 1999. Rounds and Other resources 1. Bile Acids FAQ – VIN Medical FAQs This FAQ was reviewed by Sherri Wilson for the VIN Community Date Created:3/8/2011