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Portosystemic Shunt (view source)

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'''Portosystemic shunts (PSS)''' are anomalous vascular connections between the portal and systemic venous systems. These vessels shunt blood from the '''hepatic portal vein''' (deriving from the stomach, intestines, [[Pancreas - Anatomy & Physiology|pancreas]] and [[Spleen - Anatomy & Physiology|spleen]]) directly into '''systemic venous system''', bypassing the [[Liver - Anatomy & Physiology|liver]].

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Portosystemic shunts may be '''congenital''' or '''acquired''' with diseases that cause portal hypertension. In the developing embryo, the cardinal veins form the systemic abdominal veins (the caudal vena cava, azygos, renal and gonadal veins) whereas the vitelline veins form the hepatic sinusoids, the portal vein and its tributaries, of which the largest are the left gastric, splenic, cranial and caudal mesenteric and gastroduodenal veins. The ductus venosus, a component of the foetal circulation that directs blood ~~directly~~ from the umbilical vein to the vena cava, is also part of the vitelline system. If a functional ~~connection~~ (ductus venosus or otherwise) ~~remains between~~ the vitelline and cardinal systems after birth, a PSS develops. However, in the normal animal, there are numerous non-functional ~~connection~~ between the two systems that may open if the pressure in the portal vein rises, leading to the formation of multiple extrahepatic acquired shunts.

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Portosystemic shunts may be '''congenital''' or they may be '''acquired''' with diseases that cause portal hypertension. In the developing embryo, the cardinal veins form the systemic abdominal veins (the caudal vena cava, azygos, renal and gonadal veins) whereas the vitelline veins form the hepatic sinusoids, the portal vein and its tributaries, of which the largest are the left gastric, splenic, cranial and caudal mesenteric and gastroduodenal veins. The [[Foetal Circulation - Anatomy & Physiology|ductus venosus]], a component of the foetal circulation that directs blood from the umbilical vein to the vena cava, is also part of the vitelline system. If a functional blood vessel (ductus venosus or otherwise) connects the vitelline and cardinal systems after birth, a PSS develops. However, in the normal animal, there are numerous non-functional connections between the two systems that may open if the pressure in the portal vein rises, leading to the formation of multiple extrahepatic acquired shunts in disease that cause portal hypertension. Although most congenital shunts run from the portal vein to the vena cava, their embryological origin explains why shunts may also run from the splenic or gastroduodenal veins to the vena cava or azygos vein.

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Congenital shunts represent approximately 70% of the total number diagnosed in dogs and constitute the majority of those diagnosed in cats. Congenital shunts usually involve a single (or occasionally double) anomalous vessel which may be located outside of the hepatic parenchyma (extrahepatic) or within it (intrahepatic). Extrahepatic shunts accounts for 63% of single shunts in the dog and they are most commonly found in miniature and toy breeds. Intrahepatic shunts are often located within the left lobes of the liver and occur due to persistence of the foetal [[Foetal Circulation - Anatomy & Physiology|ductus venosus]]. This form of shunt is most common in large breed dogs and patent ductus venosus is an inherited condition in Irish wolfhounds. Intrahepatic shunts running through the central or right liver lobes are recognised and these may have a different embryological origin.

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Congenital shunts represent approximately 70% of the total number diagnosed in dogs and constitute the majority of those diagnosed in cats. Congenital shunts usually involve a single (or occasionally double) anomalous vessel which may be located outside of the hepatic parenchyma (extrahepatic) or within it (intrahepatic). Extrahepatic shunts accounts for 63% of single shunts in the dog and they are most commonly found in miniature and toy breeds. Intrahepatic shunts are often located within the left lobes of the liver and occur due to persistence of the foetal [[Foetal Circulation - Anatomy & Physiology|ductus venosus]]<ref>White RN, Burton CA. '''Anatomy of the patent ductus venosus in the dog.''' ''Vet Rec. 2000 Apr 8;146(15):425-9.''</ref>. This form of shunt is most common in large breed dogs and patent ductus venosus is an inherited condition in Irish wolfhounds. Intrahepatic shunts running through the central or right liver lobes are recognised and these may have a different embryological origin.

Acquired shunts represent approximately 20% of those diagnosed in dogs and they often consist of multiple small vessels. They arise due to portal hypertension, following an increased resistance to portal blood flow. The increased pressure in the portal vein and its tributaries causes numerous non-functional microvascular communications with the systemic venous system to open. The causes of portal hypertension are numerous and they may be divided into pre-hepatic, hepatic and post-hepatic:

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**'''[[Thrombosis|Thrombosis of the portal vein]]''', '''hepatic arteriovenous fistulae''' and '''congenital hypoplasia of the portal vein.'''

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The pathophysiology of PSS relates to the shunting of portal blood directly into the systemic circulation, resulting in hyperammonaemia and [[Hepatic Encephalopathy|hepatic encephalopathy]]. Animals with PSS also have reduced function of the hepatic component of the monocyte phagocyte system (chiefly the Kupffer cells) and they often develop bacteraemia. Gram negative bacteria deriving from the GI tract are involved most commonly.

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The pathophysiology of PSS relates to the shunting of portal blood directly into the systemic circulation, resulting in hyperammonaemia and [[Hepatic Encephalopathy|hepatic encephalopathy]]. Animals with PSS also have reduced function of the hepatic component of the monocyte phagocyte system (chiefly the Kupffer cells) and they often develop bacteraemia. Gram negative bacteria deriving from the GI tract are involved most commonly. This phenomenon may be of greater importance with acquired PSS <ref>Tobias KM, Besser TE. '''Evaluation of leukocytosis, bacteremia, and portal vein partial oxygen tension in clinically normal dogs and dogs with portosystemic shunts.''' ''J Am Vet Med Assoc. 1997 Sep 15;211(6):715-8.''</ref><ref>Howe LM, Boothe DM, Boothe HW. '''Detection of portal and systemic bacteremia in dogs with severe induced hepatic disease and multiple portosystemic shunts.''' ''Am J Vet Res. 1999 Feb;60(2):181-5.''</ref>.

==Signalment==

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Congenital portosystemic shunts are diagnosed regularly in dogs under one year of age, although they may be ten years old or more at presentation. Extrahepatic shunts occur most commonly in dogs of the small or toy breeds, including Shih tzus, West Higland white terriers, Maltese terriers and Yorkshire terriers. Intrahepatic shunts are much less common and occur in larger breeds such as Irish wolfhounds (in which patent ductus venosus is thought to be inherited), golden retrievers and Australian cattle dogs. The condition occurs occasionally in cats, horses, cows and pigs.

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Congenital portosystemic shunts are diagnosed regularly in dogs under one year of age, although they may be ten years old or more at presentation. Extrahepatic shunts occur most commonly in dogs of the small or toy breeds, including Shih tzus, West Higland white terriers, Maltese terriers and Yorkshire terriers. Intrahepatic shunts are much less common and occur in larger breeds such as Irish wolfhounds (in which patent ductus venosus is thought to be inherited<ref>van Steenbeek FG, Leegwater PA, van Sluijs FJ, Heuven HC, Rothuizen J. '''Evidence of inheritance of intrahepatic portosystemic shunts in Irish Wolfhounds.''' ''J Vet Intern Med. 2009 Jul-Aug;23(4):950-2. Epub 2009 May 30.''</ref>), golden retrievers and Australian cattle dogs<ref>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. 2004 Dec;82(12):746-9.''</ref>. The condition occurs occasionally in cats, horses, cows and pigs.

Acquired shunts occur in animals with portal hypertension, of which the most common causes are [[Hepatitis, Chronic|chronic hepatitis]] in dogs and cholangitis in cats.

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===Laboratory Tests===

====Haematology====

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Animals with congenital PSS often have '''microcytic normochromic red blood cells''' due to sequestration of iron. '''Leucocytosis''' may occur as a response to bacteraemia due to a reduction in hepatic phagocytic activity.

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Animals with congenital PSS often have '''microcytic normochromic red blood cells''' due to sequestration of iron <ref>Simpson KW, Meyer DJ, Boswood A, White RN, Maskell IE. '''Iron status and erythrocyte volume in dogs with congenital portosystemic vascular anomalies.''' ''J Vet Intern Med. 1997 Jan-Feb;11(1):14-9.''</ref>. '''Leucocytosis''' may occur as a response to bacteraemia due to a reduction in hepatic phagocytic activity.

Animals with inflammatory liver disease may have '''leucocytosis''' of various types and may have '''acanthocytic''' red blood cells.

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====Other Tests====

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*'''Increased postprandial ± preprandial [[Liver - Anatomy & Physiology #Bile Acids|bile acid]]''' concentrations. Bile acids are normally removed from the portal blood by the liver but, in an animal with PSS, they enter the systemic venous system directly. If the preprandial level is elevated beyond ~100 umol/l, a bile acid stimulation test will not achieve any higher sensitivity for the detection of PSS. A bile acid stimulation test is performed by measuring preprandial bile acids, feeding the animal and measureing post prandial bile acids two hours later. The preprandial sample may occasionally be higher than the postprandial if a gall bladder contraction occurred just before the first sample was taken.

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*'''Increased postprandial ± preprandial [[Liver - Anatomy & Physiology #Bile Acids|bile acid]]''' concentrations. Bile acids are normally removed from the portal blood by the liver but, in an animal with PSS, they enter the systemic venous system directly. If the preprandial level is elevated beyond ~100 umol/l, a bile acid stimulation test will not achieve any higher sensitivity for the detection of PSS. A bile acid stimulation test is performed by measuring preprandial bile acids, feeding the animal and measureing post prandial bile acids two hours later. The preprandial sample may occasionally be higher than the postprandial if a gall bladder contraction occurred just before the first sample was taken. Measurement of serum bile acid concentration has a sensitivity of >90% for the detection of congenital portosystemic shunts but its specificity is much lower than that of an ammonia tolerance test because it may be elevated in many other types of liver disease <ref name="one">Gerritzen-Bruning MJ, van den Ingh TS, Rothuizen J. '''Diagnostic value of fasting plasma ammonia and bile acid concentrations in the identification of portosystemic shunting in dogs.''' ''J Vet Intern Med. 2006 Jan-Feb;20(1):13-9.''</ref>.

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*'''Increased blood ammonia concentration''' is diagnostic of PSS for the same reasons as are elevated bile acids. An ammonia tolerance test can be performed by administering ammonium chloride (or a small meal) to a fasted animal and measuring the change in blood ammonia level. Samples must be frozen immediately after collection as continued red blood cell metabolism will artificially raise ammonia levels in stored samples.

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*'''Increased blood ammonia concentration''' is diagnostic of PSS for the same reasons as are elevated bile acids. An ammonia tolerance test can be performed by administering ammonium chloride (or a small meal) to a fasted animal and measuring the change in blood ammonia level. Samples must be frozen immediately after collection as continued red blood cell metabolism will artificially raise ammonia levels in stored samples. Elevated fasting ammonia concentration is reported to have a sensitivity of 100% for the detection of congenital portosystemic shunts <ref name="one">nothing</ref>.

*'''Reduced serum iron concentration''' due to sequestration.

*'''Clotting times''' can be measured in animals where a coagulopathy is suspected.

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====Ultrasonography====

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The shunting vessel may be identified by ultrasound scan, especially if a '''colour flow Doppler''' facility is available. Congenital shunts are usually thin-walled, tortuous vessels with '''turbulent flow''' and it should be noted that they may run between the tributaries of the portal vein and the azygos vein, rather than the portal vein itself. Acquired shunting vessels can often be located around the cranial pole of the left kidney. It may require some skill to find a shunting vessel.

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The shunting vessel may be identified by ultrasound scan<ref>d'Anjou MA, 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.''</ref>, especially if a '''colour flow Doppler''' facility is available. Congenital shunts are usually thin-walled, tortuous vessels with '''turbulent flow''' and it should be noted that they may run between the tributaries of the portal vein and the azygos vein, rather than the portal vein itself. Acquired shunting vessels can often be located around the cranial pole of the left kidney. It may require some skill to find a shunting vessel.

Scans of the urinary bladder and urethra may reveal the presence of '''urate uroliths''' and '''thickening of the bladder wall''' due to cystitis.

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In animals with acquired PSS, the underlying cause should be treated and HE should be managed as described [[Hepatic Encephalopathy|here]]. '''Acquired shunts should never be ligated''' as they occur as a compensatory response to portal hypertension and ligation would increase portal pressure.

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Animals affected by congenital PSS may be managed either medically or surgically but a recent study has shown that those undergoing surgical ligation of the shunting vessel have a longer median survival time. Medical management is often employed in those animals that show few clinical signs on presentations, are older or which have a shunt that is not amenable to ligation.

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Animals affected by congenital PSS may be managed either medically or surgically but a recent study has shown that those undergoing surgical ligation of the shunting vessel have a longer median survival time<ref>Greenhalgh SN, Dunning MD, McKinley TJ, Goodfellow MR, Kelman KR, Freitag T, O'Neill EJ, Hall EJ, Watson PJ, Jeffery ND '''Comparison of survival after surgical or medical treatment in dogs with a congenital portosystemic shunt.''' ''J Am Vet Med Assoc. 2010 Jun 1;236(11):1215-20.''</ref>. Medical management is often employed in those animals that show few clinical signs on presentations, are older or which have a shunt that is not amenable to ligation.

===Medical Management===

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If the shunt cannot be ligated, attenuation may be achieved by three major techniques:

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*An '''ameroid constrictor''' consists of a ring of stainless steel with an inner collar of casein. When the ring is applied around a vessel, the casein sheath swells gradually to occlude blood flow and it may also stimulate some fibrosis.

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*An '''ameroid constrictor''' consists of a ring of stainless steel with an inner collar of casein. When the ring is applied around a vessel, the casein sheath swells gradually to occlude blood flow and it may also stimulate some fibrosis. Placement of an ameroid constrictor also results in a much shorter surgical time than complete ligation <ref>Hurn SD, Edwards GA. '''Perioperative outcomes after three different single extrahepatic portosystemic shunt attenuation techniques in dogs: partial ligation, complete ligation and ameroid constrictor placement.''' ''Aust Vet J. 2003 Nov;81(11):666-70.''</ref>.

*'''Cellophane banding''' involves the application of cellophane around the shunt. This material is sutured in place to stimulate fibrosis around the vessel, resulting in gradual occlusion. Cellophane does not stimulate a strong inflammatory response in cats and should not be used in this species.

*'''Partial ligation''' is achieved by applying a loose ligature to the shunt so that some blood flow still occurs. In a proportion of animals, a complete ligation will then have to be performed at a later date.

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==References==

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*Ettinger, S.J. and Feldman, E. C. (2000) '''Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2''' (Fifth Edition) ''W.B. Saunders Company''.

*Fossum, T. W. et. al. (2007) '''Small Animal Surgery (Third Edition)''' ''Mosby Elsevier''.

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