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==Introduction==

 

==Description==

'''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]].

'''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]].

*'''Urinary tract signs''', including dysuria, stranguria, haematuria and pollakiuria.  These signs occur because an increased blood ammonia concentration decreases the ability of enzymes to convert [[Urate Metabolism - Pathology|uric acid]] to allantoin, resulting in urate urolithiasis and urethral obstruction.  Ammonium biurate are the most common uroliths to be formed.

*'''Urinary tract signs''', including dysuria, stranguria, haematuria and pollakiuria.  These signs occur because an increased blood ammonia concentration decreases the ability of enzymes to convert [[Urate Metabolism - Pathology|uric acid]] to allantoin, resulting in urate urolithiasis and urethral obstruction.  Ammonium biurate are the most common uroliths to be formed.

*'''Polyuria and polydipsia''' occur for reasons that are not fully understood.  Urea synthesis is reduced in animals with PSS (because ammonia is not taken up by the liver in the normal way) and a reduction in the urea concentration of the renal medulla reduces the ability of the kidney to concentrate urine.  The hypofunctional liver probably also degrades cortisol at a reduced rate, increasing the circulating concentration of this diuretic hormone.   

*'''Polyuria and polydipsia''' occur for reasons that are not fully understood.  Urea synthesis is reduced in animals with PSS (because ammonia is not taken up by the liver in the normal way) and a reduction in the urea concentration of the renal medulla reduces the ability of the kidney to concentrate urine.  The hypofunctional liver probably also degrades cortisol at a reduced rate, increasing the circulating concentration of this diuretic hormone.   

*Intermittent '''gastro-intestinal signs''' such as [[Stomach and Abomasum Consequences of Gastric Disease - Pathology|vomiting]] and [[Diarrhoea|diarrhoea]].

*Intermittent '''gastro-intestinal signs''' such as [[Vomiting|vomiting]] and [[Diarrhoea|diarrhoea]].

*'''Ascites''' may occur due to [[Hypoalbuminaemia|hypoalbuminaemia]].

*'''Ascites''' may occur due to [[Hypoalbuminaemia|hypoalbuminaemia]].

*'''Bleeding tendencies''' due to coagulopathy, but this condition is often subclinical.

*'''Bleeding tendencies''' due to coagulopathy, but this condition is often subclinical.

==Treatment==

==Treatment==

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.

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

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.   

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.   

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

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

*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>.     

*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.

*'''Cellophane banding''' involves the application of cellophane around the shunt. Real cellophane is made from regenerated cellulose. <ref>Smith et al [http://www.ncbi.nlm.nih.gov/pubmed/23550728] </ref>evaluated commonly used sources of cellophane and determined that only one was cellulose based and not a plastic. This may be why historical outcomes of cellophane banding for attenuation of portosystemic shunts has been variable, especially in cats. Cellophane has reportedly variable outcomes in cats, and some surgeons will prefer it not be used in this species, however recent literature suggests that good outcomes can still be achieved<ref>[http://www.ncbi.nlm.nih.gov/pubmed/21194327]</ref>.Cellophane made from cellulose is a foreign material and is passed around the shunting vessel and secured in place to stimulate fibrosis, resulting in gradual occlusion. There is now a commercial supplier of cellophane for veterinary use<ref>[https://www.cellovet.com]</ref>.

*'''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.

*'''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.

==Prognosis==

==Prognosis==

The prognosis for outcome depends on the age of presentation and the urea concentration at presentation, with older dogs with higher levels of urea having better outcomes.  Dogs usually respond very well to total surgical ligation if they do not experience severe adverse effects in the week following the procedure but the response in cats is less favourable.

The prognosis for outcome depends on the age of presentation and the urea concentration at presentation, with older dogs with higher levels of urea having better outcomes.  Dogs usually respond very well to total surgical ligation if they do not experience severe adverse effects in the week following the procedure but the response in cats is less favourable.

==References==

==References==

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

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

*Nelson, R.W. and Couto, C.G. (2009) '''Small Animal Internal Medicine (Fourth Edition)''' ''Mosby Elsevier''.

*Nelson, R.W. and Couto, C.G. (2009) '''Small Animal Internal Medicine (Fourth Edition)''' ''Mosby Elsevier''.

*Watson, P. (1997) '''Decision making in the management of portosystemic shunts''' ''In Practice'' 19;106 - 120 [http://inpractice.bvapublications.com/cgi/reprint/19/3/106?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=portosystemic+shunt&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT]

* [http://inpractice.bvapublications.com/cgi/reprint/19/3/106?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=portosystemic+shunt&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT Watson, P. (1997) '''Decision making in the management of portosystemic shunts''' ''In Practice'' 19;106 - 120]

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