Portosystemic Shunts

Pathogenesis and Pathophysiology - Diagnosis - Treatment

 

Portosystemic sbunts (PSSs) are congenital or acquired vascular abnormalities that permit portal blood flow to bypass the liver and enter the systemic circulation directly. Portosystemic shunts are not uncommon in the dog and have been reported less frequently in the cat.  The disorder occurs in young cats and results in ptyalism (excessive salivation), hepatic encephalopathy, and G1 disorders.

Pathogenesis and Pathophysiology

Embryologic vascular development associated with portosystemic anomalies involves the vitelline, umbilical, and cardinal venous systems. The vitelline vein gives rise to the extrahepatic portal vein, hepatic sinusoids, and hepatic portion of the caudal vena cava; the caudal cardinal vein gives rise to the extrahepatic vena cava and azygous vein; and the umbilical vein gives rise to the intrahepatic portal vein and ductus venosus. The ductus venosus passes through the fetal liver carrying oxygenated blood directly from the umbilical vein into the caudal vena cava.  Two to three days after birth, the ductus venosus closes.

The portal vein is made up of tributaries of the cranial and caudal mesenteric, splenic, gastroduodenal, and cranial pancreaticoduodenal veins.  The left gastric vein, which is frequently the shunting vessel in the cat, enters the splenic vein. The portal vein divides within the liver into right, left, and caudal branches. Hepatotrophic factors, such as insulin and glucagon, pass from the pancreas into the liver via the cranial pancreaticoduodenal vein.

Portosystemic shunts occur when the ductus venosus remains patent (intrahepatic) or when abnormal communications exist between the fetal cardinal and vitelline venosus systems (extrahepatic). The anomaly may involve single or multiple vessels.  Single shunts are usually congenital whereas multiple shunts are acquired. in general, PSSs in cats are single and extrahepatic, although several intrahepatic shunts (patent ducts venosus) have been reported.

Portosystemic shunts frequently cause hepatic encephalopathy, a complex central nervous system (CNS) disorder resulting from endogenous neurotoxins and false neurotransmitters.  A variety of potentially neurotoxic substances are produced in the gut by resident microflora. The compounds normally pass into the liver for detoxification via the portal vein. Portosystemic shunts allow direct entrance of these substances into the systemic circulation, hence access to the CNS.

Ammonia has been considered the major compound responsible for hepatic encephalopathy.  However, the degree of clinical signs and magnitude of hyperammonemia do not always correlate, incriminating other substances in the pathogenesis, such as false neurotransmitters, mercaptans, short-chain fatty acids, indoles, and skatoles. Certain compounds (i.e., false neurotransmitters) may actually be synthesized within the CNS due to an abnormal accumulation of aberrant amino acids.   The cellular mechanism for CNS dysfunction is poorly understood although altered cell metabolism and neurotransmission probably play a key role.  Anatomic CNS lesions including cortical and hippocampal necrosis, leukopolymicrocavitation, and spongiform degeneration have been reported in cats with PSSs.

Clinical Signs

Portosystemic shunts have been reported in various breeds, including domestic shorthair, domestic long hair, Himalayan, Persian, and Siamese.  Considering the small number of cases, a breed predilection cannot be established. Males seem to be more often affected than females.  The disease affects young cats. Most cats are between 2 and 6 months of age when clinical signs develop, with a range of 2 to 30 months.

Clinical signs are variable and primarily involve the central nervous and digestive systems.  Ptyalism is the most common sign followed by ataxia, seizures, depression, and aggression.  Other less frequently reported signs include vomiting, diarrhea, stupor, head pressing, transient blindness, stunted growth, anorexia, polyphagia, paraparesis, polydypsia, polyuria, and pollakiuria due to urate urolithiasis.   Any combination of these signs may be present in an individual cat. Recognition of the disorder may be difficult since some cats exhibit vague subtle signs, although ptyalism is usually present.  Signs wax and wane with episodes lasting minutes to hours and normal periods lasting days to months. Signs may or may not be exacerbated by feeding. Once signs begin, they tend to worsen in severity and frequency with time.

Diagnosis

Differential diagnoses for ptyalism and encephalopathy in young cats primarily involve toxicoses and stomatitis.

Young cats with ptyalism, CNS signs, or vague GI signs should be suspected of having PSS. Diagnosis is based on clinical signs, routine serum chemistries, urinalysis, abdominal radiography, liver function tests, liver biopsy, and visualization of the shunt during contrast portography or angiography, abdominal ultrasonography, or celiotomy.

Routine laboratory testing is helpful in recognizing cats with PSS. Serum chemistry abnormalities include elevated alkaline phosphatase and alanine transaminase activity (approximately 25 percent of cats with PSSs), low blood urea nitrogen concentration (approximately 33 percent of cats with PSSs) and the presence of urinary ammonium biurate crystals (approximately 33 -percent of cats with PSSs). Hemograms are usually normal.

Liver function tests are indicated in cats suspected of having PSS. Sulfobromophthalein (BSP) retention, fasting serum ammonia, ammonia tolerance test (ATT), and serum bile acid concentration are typically abnormal; however, fasted and postprandial serum bile acid determinations are regarded as the screening test of choice for PSS. Sulfobromophthalein retention at 30 minutes is prolonged in 50 percent of cats with PSSs. Greater than 3 percent retention is indicative of hepatic dysfunction.

Hyperammonemia is a more consistent finding than prolonged BSP retention in cats with PSS and is present in the majority of cases.  Ammonia tolerance tests are not indicated if elevated fasting blood ammonia values are present.   Administration of ammonium chloride may exacerbate signs and does not provide significant additional information. If fasting ammonia values are not diagnostic, an ATT may be performed. Cats with postsystemic shunts have two- to sevenfold increases in blood ammmonia levels compared with fasting values.  Side effects of the ATT include exacerbation of CNS signs and vomiting.

Serum bile acid concentrations are the preferred hepatic function test for diagnostic screening for PSS.  The test is performed by obtaining a fasting serum sample, and a 2-hour postprandial sample. Normal bile acid concentrations after eating are 8.3 0.8 umol/L. Cats with PSS have a greater than 10-fold postprandial increase in bile acid concentration as compared to normal animals. Serum bile acids concentration are more sensitive than BSP retention, and equally sensitive compared with ATT in detecting PSS. Many commercial laboratories are capable of performing the test and serum bile acids remain stable for two days at room temperarture.

Microhepatica, defected by radiology or direct visualization, is present in some cats with PSS. Liver biopsy may reveal the typical microscopic changes, including hepatic atrophy, bile duct hyperplasia, obliteration of hepatic sinusoids, and portal veins and lymphatic dilation.  Definitive diagnosis may be obtained by direct visualization of the shunt at surgery or by portography (angiographic visualization of the portal system).  If the shunt is not recognized during celiotomy or if a definitive diagnosis is preferred prior to surgical correction, portography is indicated.

Three methods have been used to angiographically outline PSSs in cats: operative portography, transabdominal splenoportography, and cranial mesenteric arterial portography.

Operative portography requires celiotomy and cannulation of a mesenteric Jejunal) or splenic vein using a catheter of the largest possible diameter (19gauge, 8-inch is ideal for rapid injection). The catheter may be inserted through the accompanying needle or via a catheter introducer. Once in place, it is secured by placing a ligature around the cannulated vein. The catheter is brought through the abdominal incision and the abdomen is closed in a routine fashion. Aqueous iodide contrast medium (5 to 15 ml) is manually injected as rapidly as possible and a radiograph taken as the last milliliter is delivered. Following diagnosis the abdomen may be reopened and the shunt surgically corrected, or the catheter may be removed and surgery planned for a later date. The decision whether to perform portography and surgery during one anesthetic period depends on stability of patient, length of anesthetic time, presence and severity of hypothermia, and surgeon's personal preference.

Splenoportography is performed under aseptic conditions. An 18- or 20-gauge plastic-sheathed catheter is passed percutaneously into the splenic pulp. Venous backflow indicates proper placement. Manual injection of 5 to 15 ml aqueous contrast material is given, and a radiograph is made as the last milliliter is delivered. The main advantage is percutaneous methodology. Possible complications include inability to locate and inject the spleen and hemorrhage following splenic puncture or laceration.

Cranial mesenteric arterial portography requires special catheters and fluoroscopy.  A cutdown is made into a femoral artery and a polyethylene catheter is inserted, advanced in the aorta and directed fluoroscopically into the cranial mesentery artery. Contrast medium is automatically pressure injected, and multiple radiographs are made using an automatic film changer. The contrast material flows through the cranial mesenteric artery into the bowel and drains via the cranial mesenteric vein. Normally, the medium passes into the portal vein and enters the liver. In cats with PSSs, the contrast medium shunts through the anomalous vessel.

Treatment

Most cats with PSSs treated by surgical ligation completely recover. Medical management is palliative at best.  Surgery, on the other hand, offers a potential cure. The medical therapy for hepatic encephalopath, includes a low-quantity protein diet, antibiotics (e.g., neomycin, metronidazole), and lactulose. These measures and other supportive treatments for hepatic dysfunction may be used to strengthen debilitated cats prior to surgery.

 Anesthesia requires special consideration in cats with PSSs. These cats may have insufficient hepatic function to metabolize drugs and also have compromised hepatic blood flow.   Tranquilizers such as phenothiazines and butyrphenones require a high intrinsic hepatic clearance and may cause prolonged anesthetic recovery periods. Narcotics rely on hepatic blood flow for elimination but are reversible and at low doses offer an option for preanesthetic sedation. Narcotics, however, may cause hyperexcitability.

Several alternatives exist for anesthetic induction. Mask or tank induction using inhalant agents is ideal. Isoflurane undergoes minimal hepatic biotransformation and is the agent of choice, followed by halothane and finally methoxyflurane. Ketamine is excreted unchanged by the kidney and may be used. Ultrashort-acting barbituates, such as thiamylal and thipental, require biotransformation by the liver for ultimate elimination, but recovery from anesthetic effect results from redistribution of the drug from the brain to peripheral tissues. Because of this feature, ultrashort-acting barbituates may be administered for induction.  Anesthesia should be maintained with an inhalant agent and again, isoflurane is ideal followed by halothane and methoxyflurane. Cats with PSSs may have low-normal blood glucose concentration, and intraoperative fluid therapy consisting of dextrose solution may be beneficial.

The technique for surgical correction of PSS in dogs has been reported.   A similar method is used in cats.  Following standard preparation, ventral midline celiotomy is performed and the shunt identified. The portal vein is exposed at the base of the mesoduodenum by elevating the duodenum and retracting it to the left. Extrahepatic shunts are usually found by examining the portal vein and caudal vena cava or the lesser curvature and dorsal surface of the stomach.  A catheter is placed in the portal vein or a tributary (mesenteric or splenic vein). The same catheter may be used for operative portography and pressure measurements. The catheter is connected to a water manometer or transducer for pressure determination. Normal portal pressure is 8 to 10 cmH20. Cats with PSSs usually have decreased pressure. After determining preligation pressure, a strand of nonabsorbable suture is passed around the shunt. Right angle forceps are helpful to complete this maneuver. Tension is placed on the suture to occlude the shunt. If portal pressure does not exceed 20 cmH20 or increase by more than 10 cm above preligation pressure, and the bowel does not become congested, the shunt is totally ligated. In previous reports, portal pressures have not exceeded 18 cmH20 following complete attenuation in cats. On the basis of studies on dogs, pressures greater than 20 cmH20 lead to fatal portal hypertension, and 'in -this situation partial ligation would be indicated.

Postoperative care includes carefully monitored fluid therapy and evaluation of vital signs. The cat should be encouraged to eat following surgery. Blood glucose concentration is of particular concern since liver glycogen stores may be inadequate. Blood glucose concentrations should be monitored postoperatively and intravenous dextrose administered as needed.

Possible postoperative complications include ascites and bowel ischernia due to portal hypertension and recurrence of clinical signs. Most cases treated by surgical ligation have been successful, although reported results are not quite as favorable as in dogs with PSSs.

portosystemic_shunt_extrahepatic.jpg (120214 bytes)

Operative portography demonstrates a single extrahepatic portosystemic shunt. The splenic vein is cannulated and contrast medium is seen to pass from the portal venous system in the caudal vena cava.

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