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| Also known as:
| Also known as:
| '''Portocaval Shunt<br>
| '''Portocaval Shunt<br>
'''Patent Ductus Venosus'''
'''Patent Ductus Venosus<br>
'''Portosystemic Vascular Anomalies'''
|-}
|-}
==Diagnosis==
==Diagnosis==
===Clinical Signs===
===Clinical Signs===
All portosystemic shunts are likely to cause [[Hepatic Encephalopathy|hepatic encephalopathy]](HE) and PSS is the major cause of this disease. HE mainly causes waxing and waning neurological signs, including central blindness, seizures, depression and bizarre behaviour. Cats often show hpersalivation/ptyalism.
All portosystemic shunts are likely to cause [[Hepatic Encephalopathy|hepatic encephalopathy]](HE) and PSS is the major cause of this disease. HE mainly causes waxing and waning neurological signs, including central blindness, seizures, depression and bizarre behaviour. Cats often show hypersalivation/ptyalism.
In additon, animals with congenital PSS may show the following signs:
In additon, animals with congenital PSS may show the following signs:
*'''Reduced serum iron concentration''' due to sequestration.
*'''Reduced serum iron concentration''' due to sequestration.
*'''Clotting times''' can be measured in animals where a coagulopathy is suspected.
*'''Clotting times''' can be measured in animals where a coagulopathy is suspected.
*'''Analysis of urine''' will show that it has a low specific gravity and cytological examination may reveal the presence of urate crystals which resemble 'thorn apples' or 'mites'.
*'''Analysis of urine''' will show that it has a low specific gravity and cytological examination may reveal the presence of urate crystals which resemble 'thorn apples' or 'mites'. If the animal is suffering from cystitis or urethral obstruction due to urolithiasis, haematuria, pyuria or proteinuria may also be detected.
===Diagnostic Imaging===
===Diagnostic Imaging===
====Radiography====
====Radiography====
A definitive diagnosis relies on visualisation of the shunting blood vessel but radiography may reveal changes that are supportive of a diagnosis of PSS:
A definitive diagnosis relies on visualisation of the shunting blood vessel but radiography may reveal changes that are supportive of a diagnosis of PSS:
*'''Renomegaly''' is thought to occur as the kidneys attempt to remove and metabolise ammonia and
*'''Renomegaly''' is thought to occur as the kidneys attempt to remove and metabolise ammonia
*'''Microhepatica''' probably occurs because the liver is deprived of growth factors from the pancreas and other abdominal organs which are usually carried to it in the portal blood, for example the somatomedins (insulin-like growth factors, IGFs).
*'''Microhepatica''' probably occurs because the liver is deprived of growth factors from the pancreas and other abdominal organs which are usually carried to it in the portal blood, for example the somatomedins (insulin-like growth factors, IGFs).
*'''Urate uroliths''' are '''radiolucent''' and will not be detected by radiographs.
*'''Urate uroliths''' are '''radiolucent''' and will not be detected by radiographs.
====Ultrasonography====
====Ultrasonography====
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 right kidney. It may require some skill to find a shunting vessel.
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.
Scans of the urinary bladder and urethra may reveal the presence of '''urate uroliths''' and '''thickening of the bladder wall''' due to cystitis.
Scans of the urinary bladder and urethra may reveal the presence of '''urate uroliths''' and '''thickening of the bladder wall''' due to cystitis.
====Positive Contrast Portography====
====Positive Contrast Portography====
The gold standard technique for diagnosis of PSS is mesenteric portovenography, which involves catheterisation of a mesenteric vein and injection of a water soluble iodine-containing contrast medium into the portal circulation. Radiographs are then made to follow the course of the medium through the portal system and liver. The medium can be injected percutaneously into the spleen (under ultrasound guidance) in a less invasive variant of this technique but the spleen may overly the location of the shunt in the subsequent radiographs.
The gold standard technique for diagnosis of PSS is mesenteric portovenography, which involves catheterisation of a mesenteric vein and injection of a water soluble iodine-containing contrast medium into the portal circulation. Radiographs are then made to follow the course of the medium through the portal system and liver. The medium can be injected percutaneously into the spleen (under ultrasound guidance) in a less invasive variant of this technique but the spleen may overly the location of the shunt in the subsequent radiographs.
====Nuclear Scintigraphy====
A further technique available in some veterinary hospitals is nuclear scintigraphy. A radioactive substance (pertechnetate Tc 99m) bound to a marker substance is administered by high colonic infusion and a gamma camera is used to image the liver and heart. In animals with a shunt, the material will reach the heart at a faster rate (within 2 seconds) than in normal animals.
==Treatment==
==Treatment==
In animals with acquired PSS, the underlying cause should be treated and HE should be managed as described [[Hepatic Encephaolopathy|here]]. Acquired shunts should never be ligated as they occur as a compensatory response to portal hypertension. Ligation would increase portal pressure further with negative consequences for the animal.
In animals with acquired PSS, the underlying cause should be treated and HE should be managed as described [[Hepatic Encephaolopathy|here]]. 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. 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. 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===
===Medical Management===
A suitable regime would incorporate the following features:
A suitable regime would incorporate the following features:
*A '''diet''' with '''protein of high quality''' and with both soluble and insoluble fibre. Severe protein restriction is only necessary in animals showing signs of HE and it is otherwise detrimental because animals with PSS are often hypoalbuminaemic. A protein of high quality is selected so that excess amino acids are not available to colonic bacteria and a suitable choice would be cottage cheese. Soluble fibre acts in a similar manner to lactulose (below) whereas insoluble fibre decreases intestinal transit time and helps to prevent constipation.
*A '''diet''' with '''protein of high quality''' and with both soluble and insoluble fibre. Severe protein restriction is only necessary in animals showing signs of HE and it is otherwise detrimental because animals with PSS are often hypoalbuminaemic. A protein of high quality is selected so that excess amino acids are not available to colonic bacteria and suitable sources would be cottage cheese in dogs or white fish in cats. Soluble fibre acts in a similar manner to lactulose (below) whereas insoluble fibre decreases intestinal transit time and helps to prevent constipation.
*'''Lactulose''', a synthetic disaccharide that causes acidification of the colonic environment when it is fermented by bacteria. This environment promotes the conversion of ammonia to ammonium and the latter ions are not easily absorbed due to their electrical charge. The trapped ammonia is therefore excreted in faeces. Lactulose is also an osmotic laxative that reduces the amount of time available for colonic bacteria to act on surplus amino acids.
*'''Lactulose''', a synthetic disaccharide that causes acidification of the colonic environment when it is fermented by bacteria. This environment promotes the conversion of ammonia to ammonium and the latter ions are not easily absorbed due to their electrical charge. The trapped ammonia is therefore excreted in faeces. Lactulose is also an osmotic laxative that reduces the amount of time available for colonic bacteria to act on surplus amino acids.
*Administration of '''oral antibiotics''' to reduce the numbers of colonic bacteria. Neomycin, ampicillin or metronidazole are commonly used for this purpose.
*Administration of '''oral antibiotics''' to reduce the numbers of colonic bacteria. Neomycin, ampicillin or metronidazole are commonly used for this purpose.
===Surgical Ligation===
===Surgical Ligation===
This is the treatment of choice for congenital PSS but it may cause adverse effects in the post-operative period. Consideration must also be given to the choice of anaesthetic drugs as animals with PSS have reduced ability to metabolise drugs in the liver and often undergo prolonged recoveries from sedation or anaesthesia.
This is the treatment of choice for congenital PSS but it may cause adverse effects in the post-operative period. Consideration must also be given to the choice of anaesthetic drugs as animals with PSS have a reduced ability to metabolise drugs in the liver and often undergo prolonged recoveries from sedation or anaesthesia. Highly protein bound drugs should also be avoided as their effective concentration will be increased in hypoalbuminaemia.
====Anaesthesia====
====Anaesthesia====
Benzodiazepines should be avoided as they are highly protein-bound and they may also worsen any pre-existing HE. Phenothiazine tranquilisers (acepromazine) and barbiturates should be avoided because they are metabolised in the liver.
During anaesthesia, blood glucose concentration should be measured regularly in animals with congenital PSS as they quickly become hypoglycaemic. Consideration should also be given to warming strategies (such as heat pads, warmed intra-venous fluids and insulation) as they also rapidly become hypothermic.
====Surgical Procedure====
====Surgical Procedure====
It is highly advisable to refer animals with PSS to a specialist centre for surgical management and post-operative intensive care. The abdomen should be explored thoroughly in case of multiple shunts and the bladder should also be examined as it may contain urate uroliths that could be removed during the same procedure.
=====Extrahepatic Shunts=====
=====Extrahepatic Shunts=====
A ventral midline coeliotomy is performed and the shunting vessel is identified. If pre-operative imaging revealed the location of the shunt, it should be found easily but intra-operative mesenteric portovenography can be performed by catheterising a jejunal vein and instilling contrast medium. As a guide, the vena cava accepts blood from the renal veins and the small phrenico-abdominal veins before it reaches the hepatic hilus. Any vessel that joins the vena cava bewteen the phrenico-abdominal and hepatic veins is abnormal.
Direct ligation of the shunt will cause a large rise in portal pressure, reducing the venous drainage of the organs contributing to the portal vein. For this reason, a tributary to the portal vein (such as a jejunal vein) should be catheterised to allow the portal pressure to be measured directly during the procedure. When the shunt is ligated, the portal pressure should not rise by more than 10 cm of water from its baseline value and it should not rise above an absolute value of 20-23 cm of water. If the portal pressure rises above this level, the surgeon is likely to notice that the intestines are blanched (due to reduced perfusion), that the pulses in the mesenteric arteries are hyperkinetic, that the intestines show vigorous contractility and that the pancreas appears oedematous. If any of these signs are observed, the shunt should be attenuated rather than completely ligated.
For complete ligation, silk is often used as a suture material in dogs because its presence promotes a fibrous reactions which reinforces the ligature. Although silk is often categorised as a non-absorbable suture material, its strength is lost after 6 months and it is often fully absorbed after approximately 2 years. Better knot security can be achieved if the silk is wet when the ligatures are tied. Since silk does not stimulate a strong inflammatory response in cats, it is preferable to use the synthetic non-absorbable monofilament polypropylene in this species.
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 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.
*'''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. This material 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.
=====Intrahepatic Shunts=====
=====Intrahepatic Shunts=====
Intrahepatic shunts are much more difficult to locate and ligate than are extrahepatic shunts and owners should be warned of this fact before a procedure is undertaken. The shunt may be located by palpation of a soft area over a hepatic lobe or, if the vessel runs within a left lobe, it may be located where it joins the hepatic vein. Intra-operative ultrasound can also be employed to find a shunt. Extensive dissection may be required to expose a shunt running through the hepatic parenchyma before it can be ligated and an ultrasonic aspirator can be used for this purpose.
Another option involves temporarily occluding the hepatic blood flow and incising into the caudal vena cava (an '''intravascular approach'''). The shunt is identified where it enters the vena cava (cranial to the liver) and pledget-buttressed sutures are used to close the mouth of the vessel.
====Post-operative Care====
The following syndromes may be encountered after ligation of a shunt:
*'''Intra-abdominal haemorrhage''', which may require a further surgical procedure if severe or may be managed with a belly bandage.
*'''Portal hypertension''', causing ascites, haemorrhagic diarrhoea, septic shock and a painful abdomen. The abdominal girth is often measured regularly after the surgical procedure to assess the degree of abdominal distension. Some ascites is to be expected but severe clinical signs warrant a further surgical procedure to loosen the ligature around the shunt. Ascites should resolve over time but it can be managed with the diuretic spironolactone (because it occurs in part due to inappropriate activation of the renin-angiotensin-aldosterone system).
*'''Portal vein thrombosis''' because portal blood flow may be drastically reduced.
*'''Seizures''' or '''status epilepticus''', particularly in small breeds of dog. Seizures occur most commonly 1-3 days after ligation and probably result from rebalancing of neurotransmitter levels in the CNS. Long-term anticonvulsant therapy may be required and some animals may acquire permanent defects, such as blindness.
==Prognosis==
==Prognosis==