THE MERCK VETERINARY MANUAL
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Portosystemic Vascular Malformations in Small Animals

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The most common circulatory anomalies of the liver in dogs are microvascular dysplasia (MVD) and portosystemic vascular anomalies (PSVA). Cats also are affected with PSVA. MVD and PSVA are gene-linked congenital inherited disorders affecting small-breed dogs. Large-breed dogs also may develop intrahepatic PSVA.

Microvascular Dysplasia

MVD is far more common than PSVA in affected kindreds of small “terrier” type dogs. A diagnosis of MVD denotes abnormal development of the fine (tertiary) branches of the intrahepatic portal veins. Dogs with MVD have high TSBA but do not demonstrate clinical illness or other laboratory abnormalities found in dogs with PSVA. They do not demonstrate HE, do not develop ammonium biurate crystalluria, and have a normal protein C activity. A normal lifespan is expected in dogs with MVD; this diagnosis does not warrant feeding of a special diet or liver-specific medications. However, because dogs with MVD may have trouble metabolizing drugs that require rapid delivery and extraction in the liver, care is necessary when prescribing certain medications. Because MVD is genetically linked with PSVA, TSBA should be measured in all young pups of predisposed breeds for future health-care related issues and to select breeding stock. Once high bile acids are detected in young (<6 mo) small terrier type breeds lacking clinical signs of PSVA, repeated bile acid measurements are not warranted as they will remain variably increased for the patient's life. Knowing that a dog has high bile acids likely caused by MVD will define utility of the TSBA test for future health assessments.

Definitive diagnosis of MVD is possible only by liver biopsy combined with vascular studies. Liver biopsy demonstrates lesions similar to those associated with PSVA; in fact, biopsy usually cannot differentiate the two disorders. Liver biopsy by tru-cut method is strongly discouraged because diagnosis is based on examination of multiple acinar units to detect lobular atrophy, portal triad arteriolarization, and segmental contraction of the smooth muscle of the hepatic venule that typify these disorders. Because there is variation among liver lobes in the extent of vascular malformation, samples from 3 different lobes are recommended. However, steps to pursue a definitive diagnosis are not recommended in most dogs. Instead, it is prudent to consider that an at risk dog for hepatic vascular malformations has MVD as the underlying cause of high TSBA unless it exhibits clinical (HE) or clinicopathologic features (RBC microcytosis, low BUN, creatinine, and cholesterol concentrations, low Protein C activity) associated with PSVA.

Portosystemic Vascular Anomalies (PSVA)

A PSVA is a grossly apparent aberrant connection between the extrahepatic portal vasculature and the systemic circulation (usually connecting a branch of the portal vein to the vena cava) that diverts blood to the systemic circulation, bypassing the liver. Reduced portal flow to the liver causes hepatic atrophy. Because the portal circulation transports microorganisms, toxins, nutrients, and other materials from the intestines to the liver, detoured blood is not cleansed or processed before circulation to the brain and systemic circulation. Consequently, neurotoxic substances that can provoke encephalopathic effects can be circulated directly to the brain.

Congenital PSVA are seen primarily in purebred dogs. There are two types of PSVA. Extrahepatic PSVA occurs predominantly in small purebred terrier-type dogs, eg, Yorkshire Terriers, Maltese, Shih Tzu, Havanese, Miniature Schnauzers, Pugs, Cairn Terriers, Norfolk Terriers, Tibetan Spaniels, and others. Extrahepatic PSVA usually arise from the portal vein, left gastric vein, or splenic vein and connect to the caudal vena cava (most common), the azygous vein, or rarely another systemic vessel. An intrahepatic PSVA represents the retention of an embryonic vessel that carries fetal blood from the placenta to the heart, through the middle of the liver but bypassing the hepatic circulation. This malformation primarily affects large-breed dogs, including for example (but not exclusively) Irish Wolfhounds, Old English Sheepdogs, Labrador Retrievers, and Golden Retrievers.

Fig. 1

Congenital PSVA in cats are seen more frequently in mixed breeds, but purebred Himalayans and Persians may have an increased prevalence. However, the higher prevalence of polycystic liver disease and associated portal hypertension in these breeds complicates PSVA diagnosis. In cats, extrahepatic PSVA involving the left gastric vein are most common.

Animals with PSVA are often smaller than littermates, fail to thrive, and can have other congenital abnormalities (eg, cryptorchidism in dogs and cats, heart murmurs in cats). Clinical signs are highly variable, and 10–20% of affected animals may be asymptomatic. The presence of clinical signs depends on the severity of portosystemic shunting. In symptomatic animals, clinical signs are usually evident by 6 mo of age in cats and before 1 yr in dogs. HE is the most common clinical feature; other clinical signs include nausea, vomiting, diarrhea, pica, intermittent anorexia, PU/PD (dogs), and hematuria, pollakiuria, stranguria, or urethral obstruction associated with formation of ammonia biurate uroliths. Signs referable to urinary tract calculi may be the only presenting complaint. Hypersalivation is a common clinical sign associated with HE in cats. Cats with PSVA also have a unique homogenous copper-colored iris that appears to be genetically linked; the exception is blue-eyed cats. However, a copper-colored iris is common in Persians and Russian blue cats that do not have a PSVA. Episodic blindness and excessive vocalization also may be seen.

Laboratory abnormalities may include microcytic RBC, mild nonregenerative anemia, poikilocytosis (cats), target cells (dogs), mild hypoproteinemia and hypoalbuminemia, hypoglycemia (especially in toy-breed dogs, may be symptomatic), low BUN and creatinine, hypocholesterolemia, normal to mildly increased liver enzyme activity (ALT, AST, and AP), normal bilirubin, dilute urine (hyposthenuria or isothenuria), and ammonia biurate crystalluria. Fasting and postprandial TSBA are usually markedly increased; however, measurement of TSBA or ammonia after a prolonged fast may yield normal values. Postprandial TSBA and ammonia after NH4Cl administration are markedly abnormal. Routine coagulation assessments are usually within normal limits, but Protein C activity is usually <70%.

Abdominal radiographs reveal microhepatica and “plump” kidneys. Ammonium biurate uroliths are radiolucent and thus not detected with radiographic imaging. Ultrasonography is a useful noninvasive tool for identifying PSVA if done by an experienced operator using color-flow Doppler. While discovery of intrahepatic PSVA is relatively easy, identification of extrahepatic PSVA can be challenging and requires a systematic approach. Bowel gas and animal cooperation may limit imaging in critical regions. Ultrasonographic examination can identify radiolucent uroliths in the renal pelvis or urinary bladder. Colorectal portal scintigraphy, available in some specialty clinics or teaching hospitals, can clearly determine the presence of portosystemic shunting. However, colorectal scintigraphy is unable to identify the anatomic location of involved shunting vasculature. Splenoportal scintigraphy requires percutaneous injection of isotope into the spleen, is considered an invasive test, and does not provide better resolution, specificity or sensitivity over routine colorectal scintigraphy. In contrast radiographic portography, a branch of the portal vein is cannulated and a radiodense iodinated contrast “dye” injected to illuminate portal vascular anatomy. Multisector CT allows the best anatomic mapping of portal vasculature and is best done under short-term anesthesia with contrast injected into a peripheral vessel. This imaging modality permits 3-dimensional anatomic reconstruction of the vascular anomaly and adjacent viscera. A liver biopsy is always indicated in PSVA patients during surgical shunt ligation, or if multiple shunts are noted, to determine whether a primary underlying disease or acquired liver disease coexists.

The treatment of choice for symptomatic PSVA is surgical attenuation or ligation. The most common postsurgical complication is short-term benign abdominal effusion that typically resolves within a few days. The most serious postsurgical complication is acute portal hypertension, characterized by development of abdominal effusion, bloody diarrhea, abdominal pain, ileus, endotoxic shock, and cardiovascular collapse. This complication requires immediate removal of the shunt ligature. Other complications include seizures (rare) and formation of blood clots. APSS can silently develop at varying intervals after PSVA surgery, necessitating longterm follow-up. The ligation site also may be circumvented by formation of a medusa of vessels or a recanalized PSVA that reestablishes portosystemic shunting several years after initial surgery. The greatest risk for insidious post-operative complications is associated with application of ameroid constrictors. Best outcomes have been observed with careful intra-operative graded ligation of extrahepatic PSVA (observation of portal pressure and visceral response).

Overall, prognosis after surgical ligation of a single PSVA is usually good. Prognosis is less favorable in dogs with multiple acquired shunts secondary to severe intrahepatic portal vein atresia, and those with intrahepatic shunts. Surgery is less successful in cats than in dogs. Cats are more likely to develop multiple APSS after PSVA ligation. Staging surgeries to gradually attenuate PSVA in cats has not improved outcome. Surgical management of intrahepatic PSVA is more difficult than for extrahepatic PSVA. Recently, intervention with placement of intravascular coils has been used to attenuate blood flow within intrahepatic PSVA not amenable to surgical attenuation. Outcome (acute and chronic) of treated dogs has not yet been documented.

Dogs with relatively asymptomatic PSVA can often be managed with special diets indicated for hepatic insufficiency. Lifelong dietary support is required, and lifespan can be normal. The best protein sources are soy and dairy quality protein with a protein intake starting at 2.5 g/kg/day. Red meat, fish, and organ meats should be avoided. Addition of dairy quality protein is usually easily tolerated and increases the protein and phosphate intake. Treats of raw vegetables (eg, broccoli, carrots), cheese, probiotic yogurt, popcorn, modest numbers of dog biscuits, and rawhide bones (with supervision) can be offered without adverse consequences. Medically managed dogs remain at risk for developing HE. These dogs also appear to have increased susceptibility for systemic infections because of their reduced hepatic macrophage function.

Last full review/revision March 2012 by Sharon A. Center, DVM, DACVIM

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