Extrahepatic Portosystemic Shunt

An extrahepatic portosystemic shunt (EHPSS) is a congenital or acquired vascular anomaly that allows portal venous blood to bypass the liver and enter the systemic circulation directly. This results in the shunting of nutrient-rich, toxin-laden blood away from the hepatic parenchyma, leading to impaired liver function, hepatic encephalopathy, and a spectrum of clinical signs. Congenital EHPSS are the most common form in dogs and cats, typically involving a single aberrant vessel connecting the portal vein or its tributaries to the caudal vena cava or azygos vein. Acquired shunts are usually multiple and develop secondary to portal hypertension. Surgical attenuation of the shunt is the definitive treatment, aiming to redirect portal blood flow through the liver and promote hepatic regeneration.

The etiology of EHPSS is primarily congenital, resulting from abnormal embryologic development of the portal venous system. During fetal life, the ductus venosus connects the umbilical vein to the caudal vena cava, allowing blood to bypass the liver. Normally, this vessel closes after birth, but failure of closure or abnormal development of other portosystemic connections leads to a persistent shunt. Congenital EHPSS are classified as intrahepatic or extrahepatic, with extrahepatic shunts typically involving a single vessel that connects the portal vein or its branches (e.g., left gastric, splenic, or cranial mesenteric veins) to the caudal vena cava or azygos vein. The exact cause of this malformation is unknown but is likely multifactorial, involving genetic predisposition and environmental factors. Acquired EHPSS are secondary to chronic portal hypertension, which can result from hepatic fibrosis, cirrhosis, or primary portal vein hypoplasia. These shunts are usually multiple, tortuous vessels that develop as a compensatory mechanism to decompress the portal system.

EHPSS is most commonly diagnosed in young purebred dogs, with a median age of 1-2 years at presentation. Certain breeds are overrepresented, including Yorkshire Terriers, Maltese, Poodles, Dachshunds, and Miniature Schnauzers. In cats, EHPSS is less common but is seen in breeds such as Persians and Himalayans. There is no strong sex predilection, though some studies suggest a slight male predominance in dogs. The incidence of congenital EHPSS is estimated at 0.05-0.1% of the general dog population, but it is significantly higher in predisposed breeds. Acquired EHPSS are more common in older animals with chronic liver disease, and the incidence is higher in dogs with congenital portosystemic shunts that are not surgically corrected, as progressive portal hypertension develops.

The primary pathophysiologic consequence of EHPSS is the diversion of portal blood away from the liver, leading to a lack of hepatic perfusion with hepatotrophic factors (e.g., insulin, glucagon, epidermal growth factor) and the accumulation of neurotoxic substances in the systemic circulation. The liver undergoes atrophy and dysfunction, with impaired synthetic and metabolic functions. Hepatic encephalopathy (HE) is a major clinical manifestation, caused by increased levels of ammonia, mercaptans, aromatic amino acids, and other neurotoxins that cross the blood-brain barrier. These substances alter neurotransmitter balance, particularly increasing GABAergic tone and inhibiting glutamatergic transmission. Additionally, the shunting of blood bypasses the hepatic reticuloendothelial system, leading to bacteremia and endotoxemia. The kidneys may also be affected, with increased risk of urate urolithiasis due to hyperammonemia and altered purine metabolism. The cardiovascular system may show signs of systemic hypertension and pulmonary hypertension in some cases. The liver's regenerative capacity is often sufficient to restore function after surgical attenuation, but chronic shunting can lead to irreversible hepatic fibrosis and cirrhosis.

Predisposing factors for congenital EHPSS include genetic predisposition, with certain breeds having a higher incidence, suggesting an inherited component. In Yorkshire Terriers, an autosomal recessive mode of inheritance has been proposed. Other factors include low birth weight and intrauterine growth retardation, which may affect the development of the ductus venosus. Acquired EHPSS are predisposed by any condition that causes chronic portal hypertension, such as hepatic cirrhosis, chronic hepatitis, or primary portal vein hypoplasia. Additionally, animals with congenital shunts that are not corrected are at risk for developing acquired shunts due to progressive portal hypertension. Nutritional factors, such as high-protein diets, may exacerbate clinical signs but do not cause the shunt itself.

Clinical signs of EHPSS are variable and often intermittent, with many animals showing signs before 1 year of age. Common signs include stunted growth, poor body condition, and neurologic abnormalities consistent with hepatic encephalopathy, such as depression, lethargy, ataxia, circling, head pressing, seizures, and coma. These signs may be precipitated by high-protein meals, gastrointestinal bleeding, or stress. Gastrointestinal signs include vomiting, diarrhea, anorexia, and ptyalism (especially in cats). Polydipsia and polyuria are common due to impaired renal concentrating ability. Urinary tract signs may result from urate urolithiasis, including hematuria, dysuria, and stranguria. Physical examination may reveal a small, underdeveloped animal with a palpable cranial abdominal mass (renomegaly) or a small liver. In some cases, a heart murmur or arrhythmia may be detected. Cats may exhibit a characteristic copper-colored iris (in Persians) and excessive salivation.

Differential diagnoses for EHPSS include other causes of hepatic encephalopathy, such as hepatic microvascular dysplasia (MVD), which is a congenital abnormality of the hepatic microvasculature without a macroscopic shunt. MVD is more common in certain breeds and may present with similar clinical signs but is not surgically correctable. Other differentials include primary hepatic disease (e.g., chronic hepatitis, cirrhosis, hepatic neoplasia), portosystemic shunting due to acquired portal hypertension, and metabolic disorders such as urea cycle enzyme deficiencies. Neurologic conditions that mimic HE include idiopathic epilepsy, intracranial neoplasia, and toxicities (e.g., lead poisoning). Gastrointestinal diseases such as inflammatory bowel disease or parasitic infections may cause similar signs. Urinary tract diseases, including urolithiasis and pyelonephritis, should also be considered. Definitive diagnosis is based on imaging (ultrasonography, CT angiography) and laboratory findings (elevated bile acids, ammonia).

The diagnostic algorithm for EHPSS begins with a thorough history and physical examination, focusing on breed, age, and clinical signs. Baseline bloodwork, including a complete blood count, serum biochemistry profile, and urinalysis, may reveal microcytosis, low BUN, hypoalbuminemia, and elevated liver enzymes. Fasting and postprandial serum bile acid concentrations are highly sensitive and specific for portosystemic shunting; a postprandial bile acid level >25 µmol/L is strongly suggestive. Fasting plasma ammonia levels may also be elevated. Abdominal ultrasonography is the next step, with a skilled ultrasonographer able to identify the anomalous vessel in many cases. Doppler ultrasonography can confirm the direction of blood flow. If ultrasonography is inconclusive, computed tomography angiography (CTA) is the gold standard for defining the vascular anatomy and surgical planning. In some cases, mesenteric portography or scintigraphy may be used, but these are less commonly performed. Exploratory laparotomy may be necessary for definitive diagnosis and treatment if imaging is not available.

Laboratory findings in EHPSS include hematologic abnormalities such as microcytosis (low MCV) and target cells, reflecting iron deficiency or abnormal iron metabolism. Serum biochemistry often reveals low BUN (due to impaired urea synthesis), hypoalbuminemia, and mild elevations in liver enzymes (ALT, AST, ALP). Fasting and postprandial serum bile acids are markedly elevated, with postprandial levels typically >100 µmol/L. Fasting plasma ammonia may be elevated, but it is less sensitive than bile acids. Urinalysis may show low urine specific gravity, and the presence of ammonium biurate crystals or urate uroliths. Coagulation parameters are usually normal, but a mild prolongation of PT or aPTT may be seen due to decreased synthesis of clotting factors. Blood gas analysis may reveal metabolic alkalosis in some cases. Inflammatory biomarkers such as C-reactive protein may be normal or mildly elevated.

Radiography: Abdominal radiographs may show a small liver (microhepatica) and renomegaly, but are not diagnostic for EHPSS. Contrast studies such as mesenteric portography can outline the shunt but are invasive and rarely used. Ultrasonography: Abdominal ultrasound is a non-invasive and highly useful diagnostic tool. The anomalous vessel can often be visualized as a tortuous or straight vessel connecting the portal vein to the caudal vena cava or azygos vein. Doppler ultrasound can confirm the direction of blood flow (hepatofugal). The liver may appear small with reduced portal vein diameter. Computed Tomography (CT): CT angiography is the preferred advanced imaging modality, providing detailed 3D reconstruction of the vascular anatomy, which is essential for surgical planning. It accurately identifies the shunt location, size, and any additional anomalies. Magnetic Resonance Imaging (MRI): MRI is less commonly used but can provide excellent soft tissue contrast and may be helpful in complex cases. Scintigraphy: Transcolonic portal scintigraphy using technetium-99m pertechnetate can quantify the degree of shunting but does not provide anatomical detail. It is useful for postoperative assessment of shunt closure.

Cytology and histopathology are not typically required for the diagnosis of EHPSS, but liver biopsy may be performed during surgery to assess the degree of hepatic atrophy and fibrosis. Histopathologic findings include portal vein hypoplasia, arteriolar proliferation, and hepatocellular atrophy. In cases of acquired shunts, there may be evidence of chronic liver disease such as cirrhosis. Fine-needle aspiration of the liver is rarely diagnostic and is not recommended due to the risk of hemorrhage. If a liver mass is present, cytology and histopathology would be indicated to rule out neoplasia.

The definitive treatment for congenital EHPSS is surgical attenuation of the shunt. Preoperative stabilization is crucial, including dietary management (low-protein diet, lactulose, and antibiotics such as neomycin or metronidazole to reduce ammonia-producing gut bacteria). Medical management may be used as a bridge to surgery or in cases where surgery is not feasible. Surgical techniques include: 1) Open laparotomy with identification of the shunt and placement of an ameroid constrictor, cellophane band, or suture ligature. The ameroid constrictor is a hygroscopic device that gradually occludes the vessel over several weeks, allowing the liver to adapt to increased portal pressure. Cellophane banding induces an inflammatory response leading to gradual fibrosis and occlusion. Suture ligation is performed in cases where the shunt can be safely ligated without causing severe portal hypertension. 2) Laparoscopic attenuation is a minimally invasive option in some cases. 3) For acquired shunts, treatment is directed at the underlying cause, and surgical attenuation is not recommended. Postoperative care includes intensive monitoring for signs of portal hypertension (e.g., abdominal pain, ascites, seizures), and supportive care with fluids, analgesics, and antiemetics. The use of prophylactic antibiotics is controversial but may be indicated in the perioperative period. Pain management includes opioids (e.g., hydromorphone 0.05-0.1 mg/kg IV q4-6h) and non-steroidal anti-inflammatory drugs (e.g., carprofen 2.2 mg/kg PO q12h) after assessing renal and hepatic function. Physical rehabilitation is not typically required, but gradual return to normal activity is encouraged.

The prognosis for congenital EHPSS is good to excellent with surgical attenuation, with reported success rates of 80-90% in dogs and cats. Clinical signs typically resolve within weeks to months as the liver regenerates. The prognosis is better for extrahepatic shunts compared to intrahepatic shunts, and for animals without severe preoperative complications. Negative prognostic indicators include the presence of acquired shunts, severe hepatic fibrosis, and the development of postoperative seizures or portal hypertension. Long-term survival is excellent, with many animals living a normal lifespan. However, some animals may require continued medical management for residual signs, especially if the shunt is not completely closed.

Postoperative follow-up is essential to monitor for complications and assess shunt closure. Patients are typically hospitalized for 2-5 days after surgery. Suture removal is usually at 10-14 days. Serial bile acid measurements are recommended at 1, 3, and 6 months postoperatively to assess shunt closure. If bile acids remain elevated, imaging (ultrasound or CT) may be repeated to evaluate for partial closure or acquired shunts. Dietary management should be continued for at least 1-3 months, with gradual transition to a normal diet if clinical signs resolve. Restricted activity is advised for 4-6 weeks to allow healing. Long-term monitoring includes regular physical examinations, bloodwork, and urinalysis to detect any recurrence of clinical signs or development of urolithiasis. In animals with persistent shunting, medical management with lactulose and a low-protein diet may be continued.

Pearls: 1) Always perform a thorough preoperative workup, including bile acids and imaging, to confirm the diagnosis and plan surgery. 2) During surgery, carefully isolate the shunt and use an ameroid constrictor or cellophane band for gradual occlusion to reduce the risk of portal hypertension. 3) Monitor intraoperative portal pressure if possible; a rise of >10 cm H2O indicates a high risk of complications. 4) Administer lactulose and a low-protein diet preoperatively to reduce ammonia levels. 5) In cats, be aware of the higher risk of postoperative seizures; consider prophylactic levetiracetam. Pitfalls: 1) Failure to identify multiple shunts or an intrahepatic shunt can lead to incomplete attenuation. 2) Overly aggressive ligation can cause acute portal hypertension, leading to hypotension, ascites, and death. 3) Inadequate postoperative monitoring for seizures or hypoglycemia can be fatal. 4) Not addressing concurrent urolithiasis may lead to urinary obstruction. 5) Assuming that a single shunt is the only abnormality; always evaluate for other congenital anomalies.

Perioperative antimicrobial prophylaxis: Cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery. Postoperative antibiotics are not routinely recommended unless there is contamination. Analgesia: Preoperative opioids such as hydromorphone 0.05-0.1 mg/kg IV or methadone 0.1-0.2 mg/kg IV. Postoperative pain management may include a constant rate infusion (CRI) of fentanyl (2-5 µg/kg/h) or lidocaine (25-50 µg/kg/min) for 24-48 hours. Non-steroidal anti-inflammatory drugs (NSAIDs) such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) can be used after assessing renal and hepatic function. For hepatic encephalopathy: Lactulose 0.5-1 mL/kg PO q8h, and metronidazole 7.5 mg/kg PO q12h or neomycin 20 mg/kg PO q8h. Antiemetics: Maropitant 1 mg/kg SC q24h or metoclopramide 1-2 mg/kg/day CRI. Anticonvulsants: Levetiracetam 20 mg/kg PO q8h for seizure prophylaxis in high-risk patients. Gastroprotectants: Omeprazole 1 mg/kg PO q12h if gastric ulceration is a concern. Fluid therapy: Balanced crystalloids (e.g., lactated Ringer's solution) at maintenance rates, with dextrose supplementation if hypoglycemia is present.

The surgical management of EHPSS has been extensively studied. A landmark study by Tobias and colleagues (2008) compared ameroid constrictors and cellophane banding, finding both effective with similar complication rates. Another study by Kyles et al. (2002) reported a 90% success rate for ameroid constrictor placement in dogs. A meta-analysis by Greenhalgh et al. (2010) concluded that gradual attenuation techniques are associated with lower morbidity and mortality compared to acute ligation. The use of CT angiography for preoperative planning has been validated in multiple studies, improving surgical outcomes. Consensus guidelines from the American College of Veterinary Surgeons (ACVS) recommend surgical attenuation as the treatment of choice for congenital EHPSS, with medical management reserved for non-surgical candidates. Recent studies have also investigated the role of laparoscopic attenuation, showing comparable outcomes to open surgery with reduced morbidity. Overall, the evidence supports early surgical intervention for the best long-term prognosis.