Title: Portosystemic shunts in animals

{{Short description|Vascular anomaly in dogs and cats}}
'''Portosystemic shunts''' are a type of [[Blood vessel|vascular]] abnormality that causes blood to be emptied into the circulation, without passing through the liver. This prevents the liver from detoxifying the blood. The condition may be either [[congenital]] or [[acquired]].

==Classification==
Portosystemic shunts are either congenital or acquired and some will be categorised as being either extraheptic or intraheptic.&lt;ref name=&quot;Nelson and Nelson&quot;&gt;{{cite journal | last1=NELSON | first1=NATHAN C. | last2=NELSON | first2=LAURA L. | title=Anatomy of Extrahepatic Portosystemic Shunts in Dogs as Determined by Computed Tomography Angiography | journal=Veterinary Radiology &amp; Ultrasound | publisher=Wiley | volume=52 | issue=5 | date=2011-05-20 | issn=1058-8183 | doi=10.1111/j.1740-8261.2011.01827.x | pages=498–506| pmid=21599793 }}&lt;/ref&gt;
==Aetiology==
Intrahepatic shunts are caused by [[patent (medicine)|patent]] [[ductus venosus]]. The aetiology of extrahepatic shunts is complex and poorly understood.&lt;ref name=&quot;sasts&quot;&gt;{{cite book|first1=Eric|last1=Monnet|first2=Lisa|last2=Klopp|first3=Angela J.|last3=Marloff|last4=Webb|first4=Craig B.|chapter=Portosystemic shunt|pages=135–170|title=Small Animal Soft Tissue Surgery | publisher=Wiley | date=2023-05-12 | isbn=978-1-119-69368-0 | doi=10.1002/9781119693741}}&lt;/ref&gt;
== Symptoms ==
All forms of portosystemic shunts produce various neurological, gastrointestinal, and urinary symptoms.&lt;ref&gt;{{cite book |last1=Tobias |first1=Karen M. |title=Kirk's Current Veterinary Therapy |date=2009 |publisher=Saunders Elsevier |location=St Louis, Missouri |pages=581–586 |edition=14th |chapter=Portosystemic shunts}}&lt;/ref&gt;

Symptoms of congenital PSS usually appear by six months of age&lt;ref name=&quot;:0&quot;&gt;{{Cite book|title=Textbook of veterinary internal medicine : diseases of the dog and cat|date=1995|publisher=W.B. Saunders|others=Stephen J. Ettinger, Edward C. Feldman|isbn=0-7216-6795-3|edition=4th|location=Philadelphia|oclc=28723106}}&lt;/ref&gt; and include failure to gain weight, vomiting, and signs of [[hepatic encephalopathy]] (a condition where toxins normally removed by the liver accumulate in the blood and impair the function of brain cells) such as [[seizure]]s, depression, tremors, drooling, and [[head pressing]]. Urate [[Bladder stone (animal)|bladder stones]] may form because of increased amounts of [[uric acid]] in circulation and excreted by the kidneys.

Neurological symptoms include [[ataxia]] and seizure.&lt;ref name=&quot;sasts&quot;/&gt;
===Risk factors===
[[Irish Wolfhound]]s are predisposed to the condition with a [[digenic]] inheritance.&lt;ref&gt;{{cite web |last1=van Steenbeek |first1=Frank |url=https://dspace.library.uu.nl/bitstream/1874/275371/2/steenbeek.pdf|date=2013 |publisher=Utrecht University}}&lt;/ref&gt;

Extrahepatic shunts are more common in toy breeds whilst intrahepatic shunts are more common in large breeds.&lt;ref name=&quot;sasts&quot;/&gt;

== Pathology ==
Congenital PSS is caused by an anomaly in [[Fetus|fetal]] liver circulatory system development. Normally, blood from the [[placenta]] bypasses the liver and goes into circulation via the [[ductus venosus]], a blood vessel found in the fetus. A failure of the ductus venosus to close causes an intrahepatic shunt. On the other hand extrahepatic shunts usually result from a developmental abnormality of the [[vitelline veins]], which connect the [[portal vein]] to the [[Inferior vena cava|caudal vena cava]].&lt;ref&gt;{{Cite journal|last1=Miller|first1=Jonathan M.|last2=Fowler|first2=J. David|date=2006-03-01|title=Laparoscopic Portosystemic Shunt Attenuation in Two Dogs|url=https://meridian.allenpress.com/jaaha/article/42/2/160/176019/Laparoscopic-Portosystemic-Shunt-Attenuation-in|journal=[[Journal of the American Animal Hospital Association]]|language=en|volume=42|issue=2|pages=160–164|doi=10.5326/0420160|pmid=16527918|issn=0587-2871|url-access=subscription}}&lt;/ref&gt; Thus in the juvenile and adult animal with PSS, blood from the intestines only partly goes through the liver, and the rest mixes into general circulation. Toxins such as [[ammonia]] are not cleared by the liver. Most commonly, extrahepatic shunts are found connecting the portal vein or [[left gastric vein]] to the caudal vena cava.

Congenital shunts are usually solitary. Acquired shunts are usually multiple, and are caused by portal hypertension in dogs with liver disease. This is most commonly seen in older dogs with cirrhosis, but may also be seen in younger dogs with liver [[fibrosis]] caused by lobular dissecting [[hepatitis]].&lt;ref&gt;{{Cite journal|last=Agg|first=Emma J.|date=July 2006|title=Acquired extrahepatic portosystemic shunts in a young dog|journal=The Canadian Veterinary Journal |volume=47|issue=7|pages=697–699|issn=0008-5286|pmc=1482449|pmid=16898115}}&lt;/ref&gt;

== Diagnosis ==
Initial diagnosis of PSS is through laboratory bloodwork showing either elevated serum [[bile acid]]s after eating or elevation of fasting blood ammonia levels, which has been shown to have a higher [[Sensitivity (tests)|sensitivity]] and [[Specificity (tests)|specificity]] than the bile acids test.&lt;ref&gt;{{Cite journal|last1=Gerritzen-Bruning|first1=M. J.|last2=Ingh|first2=T. S. G. A. M. van den|last3=Rothuizen|first3=J.|date=2006|title=Diagnostic Value of Fasting Plasma Ammonia and Bile Acid Concentrations in the Identification of Portosystemic Shunting in Dogs|journal=Journal of Veterinary Internal Medicine|language=en|volume=20|issue=1|pages=13–19|doi=10.1111/j.1939-1676.2006.tb02818.x|pmid=16496918|issn=1939-1676|doi-access=free}}&lt;/ref&gt; Various diagnostic imaging techniques are used to demonstrate PSS. Ultrasonography is a rapid, convenient, non-invasive, and accurate method for diagnosis of PSS.&lt;ref&gt;{{Cite journal|last=Lamb|first=Christopher R.|date=1996|title=Ultrasonographic Diagnosis of Congenital Portosystemic Shunts in Dogs: Results of a Prospective Study|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1740-8261.1996.tb01231.x|journal=Veterinary Radiology &amp; Ultrasound|language=en|volume=37|issue=4|pages=281–288|doi=10.1111/j.1740-8261.1996.tb01231.x|issn=1740-8261|url-access=subscription}}&lt;/ref&gt; Ultrasonographic diagnosis of congenital PSS depends on finding an anomalous vessel either in the liver or just caudal to the liver in the dorsal abdomen, usually draining into the caudal vena cava. Ultrasonography can also be used to estimate hepatic volume and vascularity, and to identify related lesions affecting other abdominal structures, such as urinary calculi. Computed tomography (CT) may be considered when ultrasound expertise is lacking or ultrasonography is considered sub-optimal (e.g. because of the conformation of the patient). Control of respiration and careful timing of CT acquisition after contrast injection is necessary for optimal depiction of PSS. Rectal portal [[Nuclear medicine|scintigraphy]] using [[Technetium|&lt;sup&gt;99m&lt;/sup&gt;technetium pertechnetate]], a technique of imaging involving detection of [[gamma ray]]s emitted by [[radionuclide]]s absorbed through the rectum and into the bloodstream, demonstrates the blood vessel bypassing the liver. In certain institutions, scintigraphy is the preferred diagnostic technique, but this leaves the patient radioactive for 24h, which may be inconvenient depending on nursing needs. Portal venography is the definitive method for demonstrating PSS, but is invasive, hence it is best reserved for animals with a known shunt or those considered highly likely to have a shunt that was not detectable by ultrasonography.&lt;ref&gt;{{Cite journal|last1=Lee|first1=Karla C. L.|last2=Winstanley|first2=Alison|last3=House|first3=Jacqueline V.|last4=Lipscomb|first4=Victoria|last5=Lamb|first5=Christopher|last6=Gregory|first6=Susan|last7=Jalan|first7=Rajiv|last8=Mookerjee|first8=Rajeshwar P.|last9=Brockman|first9=Daniel J.|date=September 2011|title=Association between hepatic histopathologic lesions and clinical findings in dogs undergoing surgical attenuation of a congenital portosystemic shunt: 38 cases (2000–2004)|journal=Journal of the American Veterinary Medical Association|language=en|volume=239|issue=5|pages=638–645|doi=10.2460/javma.239.5.638|pmid=21879964|issn=0003-1488|doi-access=free}}&lt;/ref&gt;

== Treatment ==
Surgical treatment is best, when it can be performed. Pressure within the portal vein is measured as the shunt is closed, and it must be kept below 20&amp;nbsp;cm H&lt;sub&gt;2&lt;/sub&gt;O or else portal hypertension will ensue.&lt;ref name=&quot;:0&quot; /&gt; Methods of shunt attenuation should aim to slowly occlude the vessel over several weeks to months in order to avoid complications associated with portal hypertension. These methods include ameroid ring constrictors, cellophane banding, intravascular or percutaneous silicone hydraulic occluders. The most common methods of attenuation used by veterinarians are ameroid ring constrictors&lt;ref&gt;{{Cite journal|last1=Falls|first1=Emily L.|last2=Milovancev|first2=Milan|last3=Hunt|first3=Geraldine B.|last4=Daniel|first4=Leticia|last5=Mehl|first5=Margo L.|last6=Schmiedt|first6=Chad W.|date=November 2013|title=Long-term outcome after surgical ameroid ring constrictor placement for treatment of single extrahepatic portosystemic shunts in dogs: Long-Term Outcome After Ameroid Ring Constrictor Placement in Dogs|url=https://onlinelibrary.wiley.com/doi/10.1111/j.1532-950X.2013.12072.x|journal=Veterinary Surgery|language=en|volume=42|issue=8|pages=951–957|doi=10.1111/j.1532-950X.2013.12072.x|pmid=24118005|url-access=subscription}}&lt;/ref&gt; and cellophane banding.&lt;ref&gt;{{Cite journal|last1=Cabassu|first1=Julien|last2=Seim|first2=Howard B.|last3=MacPhail|first3=Catriona M.|last4=Monnet|first4=Eric|date=January 2011|title=Outcomes of cats undergoing surgical attenuation of congenital extrahepatic portosystemic shunts through cellophane banding: 9 cases (2000–2007)|journal=Journal of the American Veterinary Medical Association|language=en|volume=238|issue=1|pages=89–93|doi=10.2460/javma.238.1.89|pmid=21194327|issn=0003-1488|doi-access=free}}&lt;/ref&gt; Both methods have reportedly good outcomes in both cats and dogs, although the true composition of readily sourced cellophane has been found to be made from plastics (inert) and not cellulose (stimulates a fibrous reaction). Recently, a commercial supplier of regenerated cellulose based [[cellophane]] for veterinarians has been established for use of cellophane banding for portosystemic shunts in dogs and cats. Complete closure of extrahepatic shunts results in a very low recurrence rate, while incomplete closure results in a recurrence rate of about 50 percent. However, not all dogs with extrahepatic shunts tolerate complete closure (16 to 68 percent).&lt;ref&gt;{{Cite journal|last1=Frankel|first1=Daniel|last2=Seim|first2=Howard|last3=MacPhail|first3=Catriona|last4=Monnet|first4=Eric|date=May 2006|title=Evaluation of cellophane banding with and without intraoperative attenuation for treatment of congenital extrahepatic portosystemic shunts in dogs|journal=Journal of the American Veterinary Medical Association|language=en|volume=228|issue=9|pages=1355–1360|doi=10.2460/javma.228.9.1355|pmid=16649938|issn=0003-1488|doi-access=free}}&lt;/ref&gt; Intrahepatic shunts are much more difficult to surgically correct than extrahepatic shunts due to their hidden nature, large vessel size, and greater tendency toward portal hypertension when completely closed.&lt;ref&gt;{{Cite journal|last1=Adin|first1=Christopher A.|last2=Sereda|first2=Colin W.|last3=Thompson|first3=Margaret S.|last4=Wheeler|first4=Jason L.|last5=Archer|first5=Linda L.|date=December 2006|title=Outcome associated with use of a percutaneously controlled hydraulic occluder for treatment of dogs with intrahepatic portosystemic shunts|journal=Journal of the American Veterinary Medical Association|language=en|volume=229|issue=11|pages=1749–1755|doi=10.2460/javma.229.11.1749|pmid=17144820|issn=0003-1488|doi-access=free}}&lt;/ref&gt; When surgery is not an option, PSS is treated as are other forms of [[liver failure]]. [[Antibiotic]]s such as [[neomycin]] or [[metronidazole]] and other medicines such as [[lactulose]] can reduce ammonia production and absorption in the intestines. The [[prognosis]] is guarded for any form of PSS.

== Genetics ==
The intrahepatic shunts found in large dog breeds are passed on in a simple autosomal recessive way.&lt;ref&gt;{{Cite journal|last=Rothuizen|first=Jan|date=2002|title=Molecular Genetics-Diseases of the Liver. Proceedings of the 27th WSAVA Conference.|website=Vin.com|url=https://www.vin.com/apputil/content/defaultadv1.aspx?meta=&amp;pId=11147&amp;id=3846235|access-date=2021-08-29}}&lt;/ref&gt; Extrahepatic shunts are believed to be a [[polygenic]] trait. It is not [[sex linked]].&lt;ref name=&quot;sasts&quot;/&gt;

== References ==
{{reflist}}
[[Category:Genetic dog diseases]]
[[Category:Syndromes affecting the hepatobiliary system]]
[[Category:Vascular diseases]]
[[Category:Cat diseases]]