Portosystemic Shunts

Chapter 145

Portosystemic Shunts

Portosystemic shunts (PSS) are vascular anomalies that divert blood from the abdominal to the systemic venous circulation while bypassing the hepatic sinusoids. Products absorbed from the intestines are delivered to the heart without undergoing the extraction and detoxification processes normally performed by hepatocytes. This reduction in hepatic blood flow and function leads to decreases in protein production and glycogen storage, reticuloendothelial dysfunction, and altered metabolism of ammonia and other toxins. PSS can occur as congenital anomalies or may develop secondary to liver disease and portal hypertension. Although clinical signs from multiple acquired shunts must be managed medically, congenital PSS have been treated successfully with surgery in many dogs and cats.

Congenital portosystemic shunts (CPSS) usually occur as single large vessels, although some animals have two or more shunts. Common types of CPSS include intrahepatic portocaval shunts, such as a patent ductus venosus and extrahepatic portocaval or portal-azygos shunts. In a small percentage of dogs with CPSS, the prehepatic portal vein is absent. CPSS are considered heritable in many breeds. Multiple acquired shunts develop secondary to liver disease and/or causes of portal hypertension. They are small, tortuous vessels that frequently join the caudal vena cava around the base of the mesentery or the renal veins.

Signalment, History, and Clinical Signs

CPSS usually are diagnosed in immature animals, although a few animals are diagnosed at 10 years of age or older. Breeds most commonly affected with extrahepatic CPSS are Yorkshire terriers, Havanese, Maltese, Dandie Dinmont terriers, pugs, and miniature schnauzers. Intrahepatic CPSS are found primarily in large-breed dogs such as Irish wolfhounds and in medium-sized breeds such as Australian shepherds and Australian cattle dogs.

General clinical signs of CPSS include small stature, weight loss or failure to gain weight, polydipsia, and anesthetic or tranquilizer intolerance. Neurologic dysfunction from hepatic encephalopathy (HE) is seen in most animals with CPSS and may include lethargy, restlessness or pacing, ataxia, head pressing, circling, seizures, behavioral changes, and amaurotic blindness. Precipitating factors of severe neurologic signs (HE) include protein overload, hypokalemia, alkalosis, hypovolemia, hypoxia, gastrointestinal hemorrhage, infection, azotemia, constipation, drugs, and transfusion of stored red cells. Gastrointestinal clinical abnormalities may include anorexia, vomiting, diarrhea, or pica and, in large-breed dogs, evidence of gastrointestinal bleeding such as melena or hematemesis. Some dogs have no apparent signs or signs of only lower urinary tract disease or urinary tract obstruction. Many cats have hypersalivation and seizures, and some have unusual copper-colored irises.


Routine Laboratory Tests

In dogs, common blood work abnormalities include microcytosis and decreases in blood urea nitrogen, protein, albumin, glucose, and cholesterol. Serum alanine aminotransferase and alkaline phosphatase may be increased. Increase in alkaline phosphatase is most likely from bone growth because cholestasis is not usually a feature in animals with CPSS. Cats with CPSS may have normal albumin, glucose, total protein, and cholesterol concentrations but usually have increased liver enzymes. Up to half of dogs with CPSS have prolonged partial thromboplastin times; however, this usually does not result in a clinically significant problem.

Urine abnormalities include low urine specific gravity, ammonium biurate crystalluria, and occasionally abnormal urine sediment suggestive of cystitis secondary to crystalluria or (urate) urolithiasis. Some dogs may have silent urinary tract infections; therefore urine culture is performed routinely in many animals.

Common hepatic histologic changes in animals with CPSS include lobular atrophy, increased numbers of hepatic arterioles and bile ductules because of proliferation or tortuosity, decreased number or size of intrahepatic portal tributaries, and deposition of lipid and pigment within cytoplasmic vacuoles (lipogranulomas). These pathologic changes are variable and also can be seen in dogs with congenital portal vein hypoplasia with secondary microvascular dysplasia (PVH-MVD) that do not have CPSS and in dogs with other hepatic diseases such as noncirrhotic portal hypertension (see Chapter 146).

Liver Function Tests

Results of bile acids, ammonia, and protein C activity provide additional information about liver function. Serum bile acids are measured after a 12-hour fast and again 2 hours after a meal. Bile acid concentrations are usually greater than 75 µmol/L in dogs with CPSS but also can be increased with any significant liver disease. Occasionally postprandial bile acid concentrations are less than the prefasting sample (in approximately 20% of dogs) because of spontaneous interdigestive gallbladder contraction.

Most animals with CPSS also have increased ammonia concentrations, particularly if measured 6 hours after feeding or after oral or rectal administration of ammonia (ammonia tolerance test). Concentrations of blood ammonia are not well correlated with severity of hepatic encephalopathy, and ammonia concentrations may be normal with effective medical treatment. Ammonia can be increased falsely with improper sample handling.

Protein C is a component of the coagulation cascade that decreases clot formation. In normal dogs, protein C activity is at least 70%. Protein C activity is decreased in many dogs with severe liver disease and in most dogs with CPSS but is usually normal (>70%) in most dogs with PVH-MVD.

Diagnostic Imaging

Common findings on survey radiographs include a small liver and enlarged kidneys. Urate calculi are usually not visible unless combined with other compounds such as struvite or calcium.

Often CPSS can be identified on ultrasonography by experienced operators, particularly if color-flow Doppler is available. The combination of small liver, large kidneys, and uroliths is highly suggestive of shunting in dogs, and dogs and cats with extrahepatic shunts have reduced portal vein-to-aorta ratios. Extrahepatic CPSS can be more difficult to diagnose with ultrasonography because the patient is usually small and structures such as ribs and intestines can obscure the vessel.

Nuclear scintigraphy with technetium 99m provides a diagnosis of shunt or no shunt. If the radionuclide is injected directly into the spleen, the operator often can tell where the shunt terminates, how many shunts are present, and whether they are likely to be congenital or acquired. Normal shunt fraction is less than 5% for transsplenic scintigraphy and less than 15% for per rectal. Isolation for 12 to 24 hours usually is required in animals undergoing rectal scintigraphy.

Direct injection of contrast into the splenic or jejunal veins (portovenography) usually provides excellent information regarding the presence, number, and location of shunts; however, at most practices a celiotomy is required to obtain the images. In addition, some CPSS are not visible on portograms performed in dorsal or right lateral recumbency.

Computed tomographic (CT) angiography is considered the standard for definitive diagnosis of CPSS and is particularly useful for preoperative planning in animals with intrahepatic CPSS and hepatic arteriovenous malformations. Magnetic resonance angiography also can be used to detect shunts, but it is more expensive and image quality is not as good.

Differential Diagnoses

Single congenital portosystemic shunts must be differentiated from neurologic conditions such as hydrocephalus and epilepsy and from other primary hepatic diseases, including congenital PVH-MVD and multiple acquired shunts secondary to portal hypertension. Differentiation usually is based on results of advanced imaging; however, history, clinical signs, and results of blood work also may be helpful. Conditions other than CPSS should be suspected when neurologic or other clinical signs do not resolve with medical management of the HE.

Unlike animals with CPSS, ascites is a common finding on physical examination or ultrasonography in dogs with portal hypertension secondary to severe hepatocellular disease or hepatic arteriovenous malformations. With development of multiple acquired PSS, these animals have increased shunt fractions on scintigraphy.

Congenital portal vein hypoplasia is found in many small breeds predisposed to CPSS. Although results of liver biopsy are the same for both conditions, laboratory changes are usually less severe in dogs with PVH-MVD, unless noncirrhotic portal hypertension is present (see Chapter 146). Dogs with PVH-MVD are more likely to have normal red cell size (MCV); glucose, albumin, total protein, and cholesterol concentrations; and urine specific gravity and fewer if any at all clinical signs than dogs with CPSS. In addition, 90% to 95% of dogs with PVH-MVD have normal protein C activity and 81% have postprandial bile acids less than 75 µmol/L at the author’s institute. Results of scintigraphy, portography, and CT angiography are usually normal in dogs with PVH-MVD.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Portosystemic Shunts

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