Overview

• Portosystemic Shunts (PSS) are vascular abnormalities where blood bypasses the liver, flowing directly from the abdominal organs to the heart.

• Congenital PSS are present at birth, while acquired PSS can develop due to liver disease or portal hypertension.

• Treatment for congenital PSS is often surgical, with good outcomes, whereas acquired shunts require medical management.

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Etiology

• Congenital PSS: Typically occur as a single large vessel. Common forms include:

  • Intrahepatic portocaval shunts (e.g., patent ductus venosus).

  • Extrahepatic portocaval or portal-azygos shunts.

  • Absent prehepatic portal vein in some cases.

• Acquired PSS: Often result from chronic liver disease and portal hypertension, leading to multiple smaller shunts.

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Clinical Signs

• Signalment:

  • Congenital PSS: Often diagnosed in young animals, no clear sex preference.

  • Intrahepatic PSS: Seen more in larger dog breeds (e.g., Irish Wolfhounds, Golden Retrievers, Old English Sheepdogs).

  • Extrahepatic PSS: More common in small breed dogs (e.g., Yorkshire Terriers, Schnauzers, Maltese, Dachshunds).

  • Cats typically have extrahepatic shunts.

• Clinical Signs:

  • Nervous System: Lethargy, ataxia, seizures, behavior changes, blindness, head pressing, circling, head tilt.

  • Gastrointestinal: Anorexia, vomiting, diarrhea.

  • Urinary Tract: Some animals show signs of cystitis or urinary blockage.

  • Growth & Development: Poor growth, weight loss, and intolerance to anesthetics or tranquilizers.

  • Other: In cats, hypersalivation and copper-colored irises are observed in some cases.

• Hepatic Encephalopathy

  • Pathophysiology: Hepatic encephalopathy occurs due to a failure to detoxify ammonia and other toxins, leading to neurological dysfunction.

  • Clinical Signs: Depression, dementia, stupor, coma, muscle tremors, motor abnormalities, seizures.

  • Precipitating Factors: Infections, protein overload, hypovolemia, constipation, certain medications (e.g., diuretics), gastrointestinal hemorrhage.

  • Toxins: Ammonia, mercaptans, fatty acids, and other substances play a role in encephalopathy.

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Diagnosis

• Liver Function Tests:

  • Serum Bile Acids: Elevated in PSS due to impaired hepatic function. Fasting and postprandial samples are needed for accurate results.

  • Ammonia Levels: Increased blood ammonia may indicate PSS, but levels don’t always correlate with encephalopathy severity. Ammonia Tolerance Test (ATT) provides more reliable results.

• Imaging:

  • Radiographs: Microhepatica (small liver) and, in some cases, renal changes (e.g., urate calculi).

  • Ultrasonography: Detects shunts, microhepatica, and abnormalities in the portal system, though extrahepatic shunts may be harder to detect.

  • Nuclear Scintigraphy: A non-invasive technique using 99mTechnetium pertechnetate to identify PSS based on its circulation pattern.

  • Angiography & Intraoperative Mesenteric Portography: Offer excellent visualization of the shunt, particularly intrahepatic ones, but require surgical intervention.

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Medical Management of PSS

• Medical management of animals with Portosystemic Shunts (PSS) primarily focuses on controlling clinical symptoms, preventing complications (like hepatic encephalopathy), and supporting liver function.

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1. Fluid, Electrolyte, and Glucose Balance:

   • Correct fluid and electrolyte imbalances as they occur, especially in cases of hypoglycemia or dehydration.

   • Glucose may be administered to correct hypoglycemia, as impaired liver function can impair glucose production.

2. Prevention of Hepatic Encephalopathy:

   • Dietary Protein Restriction: A low-protein diet helps to reduce ammonia production by decreasing substrates available for ammonia formation in the gut.

   • Lactulose: This synthetic disaccharide is essential in managing hepatic encephalopathy. It:

     • Acidifies the colon, which traps ammonia in its ammonium form, preventing ammonia absorption.

     • Promotes osmotic diarrhea, decreasing the time ammonia has to be absorbed in the intestines.

     • May be given orally or as an enema—doses are adjusted to achieve soft but formed stool.

3. Management of Gastrointestinal (GI) Issues:

   • Gastrointestinal hemorrhage:

     • Ranitidine or sucralfate may be used if gastric irritation or ulcers are suspected.

     • Antihelmintics may be administered if parasites are contributing to GI issues.

   • Non-absorbable Intestinal Antibiotics:

     • Neomycin is effective against urease-producing bacteria, reducing ammonia production in the gut.

   • Enemas and Cathartics:

     • Used to clear the bowel of ammonia-producing bacteria and reduce substrates for ammonia formation.

4. Urinary Tract Management:

   • Cystitis should be managed with appropriate antibiotics, based on culture and sensitivity. If urate uroliths are present, a low-protein diet may help resolve them.

5. Other Supportive Care:

   • Probiotics may be considered to help restore normal intestinal bacterial flora.

   • Hepatic support: Supplements like SAMe (S-adenosylmethionine) and milk thistle are sometimes used to protect liver function.

• Prognosis with Medical Management

  • Outcome: With proper medical management, weight and quality of life often stabilize or improve, and some animals may live for several years without surgical intervention.

  • Survival Rates:

    • One-third of dogs with medical management alone live well into 7 years or older.

    • However, over half of dogs may be euthanized within 10 months due to uncontrolled neurological signs or progressive liver disease.

  • Factors Influencing Survival:

    • Age at onset and BUN (blood urea nitrogen) concentration at diagnosis influence survival—older animals and those with lower BUN concentrations tend to survive longer.

  • Liver Disease Progression: In some cases, progressive hepatic fibrosis and portal hypertension can develop, leading to poor outcomes.

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Surgical Treatment

• While medical management can stabilize many animals, surgery is generally the treatment of choice for animals with a single congenital PSS, especially in younger animals or those with significant clinical signs of hepatic dysfunction.

• Anesthetic Management of Animals with PSS

  • Anesthesia in animals with PSS requires careful management due to impaired liver function and the associated risk of complications like hypoalbuminemia and altered drug metabolism.

• Considerations for Anesthesia:

  • Anesthetic Drugs to Avoid:

    • Barbiturates, phenothiazine tranquilizers, and hepatotoxic agents like halothane should be avoided as these can exacerbate liver dysfunction.

    • Drugs that are highly protein-bound (e.g., diazepam, barbiturates) should be used with caution.

  • Preferred Anesthetic Regimen:

    • Sedation: Mild sedation with agents like acepromazine and butorphanol.

    • Induction: Mask induction is often preferred, followed by maintenance with isoflurane.

    • Opioids: These can be safely used as premedicants and for preemptive analgesia, as their effects are reversible.

• Exploratory Laparotomy for PSS Diagnosis

  • Exploratory laparotomy is often necessary to definitively diagnose and localize extrahepatic PSS.

  • Anatomical Landmarks:

    • Caudal vena cava (CVC): Most extrahepatic PSSs terminate here, and its anatomy must be well understood for successful identification.

    • Epiploic Foramen: Common site for extrahepatic PSS termination. It is located to the right of midline at the base of the mesoduodenum.

    • Renal and Phrenicoabdominal Veins: These help differentiate normal and abnormal venous structures.

  • Intraoperative Techniques:

    • Portosystemic Shunt: Detection involves a careful examination of the caudal vena cava, particularly at the epiploic foramen.

    • Portoazygos Shunts: These shunts may traverse the diaphragm and can be more difficult to access. Sometimes, the omental bursa must be opened for better access.

    • Intrahepatic Shunts: These are more challenging to identify during surgery but can be found via careful palpation or additional diagnostic techniques (e.g., portal pressure measurement, ultrasound).

• Portosystemic Shunt Occlusion Techniques

  • The goal of surgery is to occlude or ligate the shunt to redirect blood flow through the liver.

  • Ligation: The shunt is ligated as close to its terminus on the caudal vena cava as possible.

    • Portocaval shunts: Should be occluded at the caudal vena cava.

    • Portoazygos shunts: Can be occluded at the abdominal side of the diaphragm.

  • Techniques for Ligation:

    • Suture Ligature: Using silk sutures (for dogs) or monofilament sutures (for cats) to tie off the shunt.

    • Ameroid Constrictors: Gradual occlusion over 2-5 weeks via the use of casein rings, which slowly swell and close off the shunt. Preferred for extrahepatic PSS as they reduce the risk of portal hypertension.

  • Postoperative Care:

    • Animals are monitored closely for complications like seizures, hypoglycemia, and portal hypertension.

    • Analgesia: Opioids and low-dose acepromazine may be used for pain management.

    • A protein-restricted diet and lactulose are continued post-surgery until liver function improves.

• Prognosis After Surgical Treatment

  • Extrahepatic PSS: Surgery often yields a good prognosis. Ameroid constrictors offer a higher success rate and lower complication rate (85% of dogs with ameroid constrictors are normal within 3 months).

  • Intrahepatic PSS: These are more challenging to treat surgically and tend to have a guarded prognosis.

  • Postoperative Seizures: Around 5-10% of dogs may experience seizures, which may require ongoing treatment with lactulose or neomycin.

  • Mortality and Complications: Mortality is higher in animals with severe liver disease, low albumin, or those with significant postoperative complications like portal hypertension or multiple shunts.