Intrahepatic Portosystemic Shunt

Intrahepatic portosystemic shunt (IHPSS) is a congenital vascular anomaly in which the portal vein communicates directly with the systemic venous circulation within the hepatic parenchyma, bypassing the hepatic sinusoidal bed. This results in partial or complete diversion of portal blood away from the liver, leading to inadequate hepatic perfusion, hepatocyte atrophy, and subsequent hepatic encephalopathy, as well as other metabolic derangements. IHPSS is one of two main categories of congenital portosystemic shunts (PSS), the other being extrahepatic (EHPSS). Intrahepatic shunts are typically classified anatomically into left divisional (patent ductus venosus), central divisional, and right divisional types, based on the portal vein branch involved. These shunts are often large, high-flow communications that present significant surgical challenges due to their intrahepatic location, making surgical attenuation more complex than for extrahepatic shunts. The condition is most commonly diagnosed in young purebred dogs, particularly large breeds such as Golden Retrievers, Labrador Retrievers, and Irish Wolfhounds, though it can occur in cats and mixed-breed animals. Clinical signs are primarily related to hepatic encephalopathy, including neurologic abnormalities, gastrointestinal signs, and urinary tract issues such as ammonium urate urolithiasis. Definitive treatment often involves surgical attenuation of the shunt, either via open surgery or minimally invasive techniques, with the goal of redirecting portal blood flow through the liver. Medical management is used as a temporary measure or for cases not amenable to surgery. Prognosis is generally good with appropriate surgical intervention, though perioperative complications can be significant.

Intrahepatic portosystemic shunts are congenital anomalies resulting from failure of the normal fetal circulatory structures to regress appropriately. During fetal development, the ductus venosus connects the umbilical vein to the caudal vena cava, allowing oxygenated blood to bypass the liver. In normal neonates, the ductus venosus closes within days after birth, but in animals with IHPSS, this vessel or an anomalous intrahepatic portal vein branch persists, creating a direct communication between the portal system and the systemic circulation. The exact etiology is not fully understood, but it is believed to be multifactorial, involving genetic predisposition and possibly environmental factors. A hereditary basis is strongly suspected in certain breeds, such as the Irish Wolfhound, where an autosomal recessive mode of inheritance has been proposed. Other breeds with a high incidence include Golden Retrievers, Labrador Retrievers, and Australian Cattle Dogs. The anomaly arises during embryologic development, likely due to abnormal angiogenesis or failure of programmed cell death in the ductus venosus. No traumatic, infectious, or degenerative causes have been identified for congenital IHPSS. Acquired portosystemic shunts can develop secondary to portal hypertension, but these are typically extrahepatic and are not classified as intrahepatic. Iatrogenic causes are rare but could include surgical creation of a shunt during liver lobectomy or trauma, though these are not typical.

Intrahepatic portosystemic shunts are less common than extrahepatic shunts, accounting for approximately 25-30% of all congenital PSS in dogs. They are predominantly diagnosed in young dogs, with a median age at presentation of 1-2 years, though some cases may be identified in older animals. There is no significant sex predilection, though some studies suggest a slight male predominance. Large and giant breed dogs are overrepresented, with a strong breed predisposition in Golden Retrievers, Labrador Retrievers, Irish Wolfhounds, Australian Cattle Dogs, and Old English Sheepdogs. In cats, IHPSS is rare, with most feline PSS being extrahepatic. The incidence of IHPSS in the general dog population is estimated at 0.05-0.1%, but it is significantly higher in predisposed breeds. For example, in Irish Wolfhounds, the prevalence may be as high as 2-3%. The condition is congenital, so it is present at birth, but clinical signs may not manifest until weeks to months later, often triggered by dietary protein intake or other stressors. There is no known geographic or environmental influence. The high prevalence in certain breeds strongly supports a genetic component, and breeding of affected animals is discouraged.

The primary pathophysiologic consequence of an intrahepatic portosystemic shunt is the diversion of portal venous blood away from the liver, resulting in reduced hepatic perfusion and hepatocyte atrophy. Normally, the liver receives approximately 70-80% of its blood supply from the portal vein, which delivers nutrient-rich blood from the gastrointestinal tract, pancreas, and spleen. In IHPSS, a portion of this blood bypasses the liver and enters the systemic circulation directly, leading to a decrease in hepatic blood flow and oxygen delivery. This causes progressive hepatocyte atrophy, particularly in the periportal regions, and a reduction in liver mass. The liver's ability to perform its synthetic and metabolic functions is compromised, leading to a variety of systemic abnormalities. Key metabolic derangements include hyperammonemia, due to reduced hepatic urea cycle activity, and decreased clearance of other neurotoxins such as mercaptans, short-chain fatty acids, and aromatic amino acids. These substances cross the blood-brain barrier and contribute to hepatic encephalopathy, characterized by neurologic signs such as depression, ataxia, seizures, and coma. Additionally, the liver's synthetic functions are impaired, leading to decreased production of albumin, clotting factors, and urea. Hypoalbuminemia can contribute to ascites and peripheral edema, while coagulopathies increase the risk of bleeding. The kidneys are also affected, as reduced hepatic metabolism of certain compounds leads to increased urinary excretion of uric acid, predisposing to ammonium urate urolithiasis. The shunting of blood also results in decreased hepatic clearance of toxins and drugs, altering drug metabolism and increasing the risk of adverse drug reactions. Over time, portal hypertension may develop if the shunt is partially attenuated, leading to acquired extrahepatic shunts and ascites. The severity of clinical signs is related to the degree of shunting, with larger shunts causing more profound hepatic dysfunction.

Predisposing factors for intrahepatic portosystemic shunts are primarily genetic and breed-related. Certain breeds, such as Irish Wolfhounds, Golden Retrievers, Labrador Retrievers, and Australian Cattle Dogs, have a significantly higher risk, suggesting a hereditary component. In Irish Wolfhounds, the condition is believed to be inherited in an autosomal recessive manner, and genetic testing may be available. Other intrinsic factors include age, as clinical signs typically appear in young animals, and sex, with some studies suggesting a slight male predisposition. Extrinsic factors are less well-defined but may include dietary protein intake, which can precipitate hepatic encephalopathy in affected animals. Stress, illness, or surgery can also exacerbate clinical signs. There is no evidence that trauma, infection, or environmental toxins cause congenital IHPSS. However, in animals with pre-existing liver disease, portal hypertension can lead to the development of acquired extrahepatic shunts, but these are not intrahepatic. Prior abdominal surgery or trauma could theoretically damage the liver and lead to abnormal vascular connections, but this is extremely rare. Overall, the most significant predisposing factor is breed genetics, and affected animals should not be used for breeding.

Clinical signs of intrahepatic portosystemic shunts are variable and often intermittent, with many animals showing signs before 2 years of age. The most common presenting signs are related to hepatic encephalopathy, which can range from mild lethargy and depression to severe neurologic deficits such as ataxia, circling, head pressing, blindness, seizures, and coma. These signs are often precipitated by a high-protein meal, gastrointestinal bleeding, or other stressors. Gastrointestinal signs are also common, including vomiting, diarrhea, anorexia, and ptyalism (excessive salivation), particularly in cats. Polydipsia and polyuria may be observed due to impaired renal concentrating ability and altered urea metabolism. Urinary signs, such as stranguria or hematuria, can result from ammonium urate urolithiasis, which may cause urethral obstruction, especially in male dogs. Physical examination may reveal a small, shrunken liver on abdominal palpation, though this is often difficult to appreciate. Some animals may have a distended abdomen due to ascites, though this is more common with acquired shunts. Neurologic examination may reveal dull mentation, proprioceptive deficits, and abnormal behavior. In some cases, animals may be asymptomatic and the shunt is discovered incidentally during imaging for other reasons. The severity of clinical signs does not always correlate with the size of the shunt, and some animals with large shunts may have minimal signs, while others with smaller shunts may be severely affected. Chronic signs may include poor growth, weight loss, and failure to thrive. In cats, signs may be more subtle, with ptyalism and lethargy being prominent.

Differential diagnoses for intrahepatic portosystemic shunts include other causes of hepatic encephalopathy, such as extrahepatic portosystemic shunts, hepatic microvascular dysplasia, acute or chronic hepatitis, cirrhosis, and hepatic neoplasia. Other metabolic diseases that can cause neurologic signs include hypoglycemia, hypoadrenocorticism, and urea cycle enzyme deficiencies. Gastrointestinal diseases such as inflammatory bowel disease or protein-losing enteropathy can cause similar signs of vomiting and diarrhea. Urinary tract diseases, such as bacterial cystitis or urolithiasis, can mimic the urinary signs. Key distinguishing features for IHPSS include the presence of a congenital shunt on imaging, elevated fasting and postprandial bile acids, hyperammonemia, and a small liver on radiographs or ultrasound. Extrahepatic shunts are typically located caudal to the diaphragm and are more common in small breeds, whereas IHPSS is more common in large breeds. Hepatic microvascular dysplasia is a histologic diagnosis and may have similar laboratory findings but no macroscopic shunt. Liver biopsy can differentiate these conditions. Other differentials include portosystemic shunting secondary to portal hypertension from chronic liver disease, but this is usually acquired and extrahepatic. In young animals with neurologic signs, congenital portosystemic shunts should be a primary consideration, and diagnostic imaging is essential for confirmation.

The diagnostic algorithm for intrahepatic portosystemic shunts begins with a thorough history and physical examination, with particular attention to breed, age, and clinical signs suggestive of hepatic encephalopathy or urinary tract disease. Baseline laboratory tests, including a complete blood count, serum biochemistry profile, and urinalysis, are performed. Common findings include microcytosis, low blood urea nitrogen (BUN), hypoalbuminemia, and elevated liver enzymes (ALT, AST, ALP). Urinalysis may reveal ammonium urate crystals or hematuria. The next step is to assess liver function with serum bile acid measurements, both fasting and 2-hour postprandial. Elevated bile acids are highly suggestive of a portosystemic shunt, though they can also be elevated in other liver diseases. Fasting ammonia levels may also be elevated, but they are less sensitive. If bile acids are abnormal, abdominal imaging is indicated. Abdominal radiographs may show a small liver (microhepatica) and possibly renomegaly. Abdominal ultrasound is the next step and is highly sensitive for detecting portosystemic shunts, with a sensitivity of 80-95% for IHPSS. Ultrasound can identify the shunt vessel, its location (left, central, or right divisional), and the direction of blood flow using Doppler. If ultrasound is inconclusive, advanced imaging such as computed tomography (CT) angiography or magnetic resonance (MR) angiography is recommended. CT angiography is the gold standard for surgical planning, as it provides a 3D reconstruction of the hepatic vasculature, allowing precise localization of the shunt and assessment of its size and course. In some cases, mesenteric portography may be performed, but this is invasive and rarely needed with modern imaging. If surgery is planned, a liver biopsy may be obtained to assess the degree of hepatic fibrosis and to rule out concurrent hepatic microvascular dysplasia. The diagnostic algorithm should be systematic to ensure accurate diagnosis and appropriate surgical planning.

Laboratory findings in intrahepatic portosystemic shunts are characteristic but not pathognomonic. A complete blood count often reveals microcytosis (low mean corpuscular volume) with normal or slightly decreased hemoglobin, due to iron deficiency from chronic gastrointestinal blood loss or altered iron metabolism. Serum biochemistry profile typically shows low blood urea nitrogen (BUN) due to decreased urea synthesis, hypoalbuminemia, and mild to moderate elevations in liver enzymes, particularly alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Alkaline phosphatase (ALP) may be normal or mildly elevated. Total bilirubin is usually normal. Fasting serum bile acids are typically elevated (>25 μmol/L), and postprandial bile acids are even more sensitive, with values >30 μmol/L considered abnormal. Fasting ammonia levels may be elevated, but they are less sensitive and can be normal in some cases. Urinalysis may reveal ammonium urate crystals, hematuria, or proteinuria. Coagulation parameters, including prothrombin time (PT) and activated partial thromboplastin time (aPTT), may be prolonged due to decreased synthesis of clotting factors, though clinical bleeding is uncommon. A coagulation panel is recommended prior to any surgical procedure. Blood gas analysis may show metabolic alkalosis due to vomiting or altered acid-base balance. Inflammatory biomarkers such as C-reactive protein (CRP) may be normal or mildly elevated. Liver function tests, including serum albumin, BUN, and cholesterol, are often decreased. In some cases, a liver biopsy may be performed, revealing histologic changes such as hepatocyte atrophy, portal fibrosis, and arteriolar proliferation. These laboratory findings, in conjunction with imaging, help confirm the diagnosis and assess the severity of hepatic dysfunction.

Imaging plays a crucial role in the diagnosis and surgical planning of intrahepatic portosystemic shunts. Abdominal radiographs may show a small liver (microhepatica) and possibly renomegaly, but they are not diagnostic. Abdominal ultrasound is the first-line imaging modality and can identify the shunt vessel in most cases. On B-mode ultrasound, the shunt appears as an anechoic tubular structure within the liver parenchyma, often connecting the portal vein to the caudal vena cava. Color Doppler and pulsed-wave Doppler can confirm the direction of blood flow, which is typically hepatofugal (away from the liver) in the portal vein. Ultrasound can also assess the size of the shunt and its location, which is important for surgical planning. However, ultrasound is operator-dependent and may miss small shunts or those in difficult locations. Computed tomography (CT) angiography is the preferred advanced imaging technique for surgical planning. It provides a detailed 3D reconstruction of the hepatic vasculature, allowing precise localization of the shunt (left, central, or right divisional), its length, diameter, and relationship to adjacent structures. CT angiography is particularly useful for determining the surgical approach, as it can identify whether the shunt is accessible via a left or right lateral liver lobectomy or if a central approach is needed. Magnetic resonance (MR) angiography is an alternative but is less commonly used due to longer anesthesia times and lower availability. Mesenteric portography, which involves injection of contrast into a mesenteric vein, can also be used but is invasive and rarely necessary with modern imaging. Intraoperative ultrasound may be used during surgery to help identify the shunt and guide attenuation. Imaging is essential not only for diagnosis but also for selecting the appropriate surgical technique and predicting potential complications.

Cytology and histopathology are not typically used for the primary diagnosis of intrahepatic portosystemic shunts, but they can be helpful in certain situations. Fine-needle aspiration of the liver may be performed if there is concern for hepatic neoplasia or other infiltrative diseases, but it is not diagnostic for PSS. Liver biopsy, either via ultrasound-guided needle biopsy or surgical wedge biopsy, can provide histopathologic confirmation of the hepatic changes associated with PSS. Histopathologic findings in PSS include hepatocyte atrophy, particularly in the periportal regions, portal fibrosis, arteriolar proliferation, and a paucity of portal veins. There may also be evidence of nodular regeneration in chronic cases. These changes are characteristic but not specific, as similar findings can be seen in other causes of portosystemic shunting, such as hepatic microvascular dysplasia. In cases where the diagnosis is uncertain, a liver biopsy can help differentiate between a congenital shunt and other liver diseases. Additionally, if a liver mass is identified, cytology or histopathology can rule out neoplasia. In surgical cases, a liver biopsy is often obtained at the time of shunt attenuation to assess the degree of hepatic fibrosis, which may have prognostic significance. Special stains, such as reticulin stains, can highlight the fibrosis. Immunohistochemistry may be used to evaluate the expression of certain proteins, but this is primarily for research purposes. Overall, histopathology is not essential for the diagnosis of IHPSS but can provide valuable information about the severity of liver disease and help guide treatment decisions.

The definitive treatment for intrahepatic portosystemic shunts is surgical attenuation of the shunt vessel, with the goal of redirecting portal blood flow through the liver. Medical management is used as a temporary measure to stabilize the patient before surgery or as a long-term option for animals that are not surgical candidates. Medical therapy includes a low-protein diet, lactulose (0.5-1 mL/kg PO q8h, titrated to produce 2-3 soft stools per day), and antibiotics such as neomycin (20 mg/kg PO q8h) or metronidazole (7.5 mg/kg PO q12h) to reduce ammonia-producing bacteria in the colon. Anticonvulsants may be needed for seizures. Surgical options for IHPSS include open surgical attenuation via cellophane banding, ameroid constrictor placement, or suture ligation. Due to the intrahepatic location, surgical exposure is challenging and may require a median sternotomy or thoracophrenotomy for right divisional shunts, or a left lateral liver lobectomy for left divisional shunts. In recent years, minimally invasive techniques such as laparoscopic or thoracoscopic attenuation have been developed, but they require specialized equipment and expertise. The choice of technique depends on the shunt location and surgeon preference. Cellophane banding and ameroid constrictors are preferred over suture ligation because they allow gradual occlusion of the shunt, reducing the risk of portal hypertension. Suture ligation is associated with a higher risk of acute portal hypertension and is less commonly used. Intraoperative monitoring of portal pressure is essential to guide the degree of attenuation. The goal is to achieve a 50-75% reduction in shunt diameter or a portal pressure increase of less than 10 cm H2O. Postoperative management includes intensive monitoring for signs of portal hypertension, such as abdominal pain, ascites, and seizures. Medical therapy is continued for several weeks postoperatively and then gradually tapered. In some cases, multiple surgical procedures may be needed to achieve complete attenuation. The success rate for surgical treatment of IHPSS is generally good, with resolution of clinical signs in 70-90% of cases, though complications can occur.

The prognosis for intrahepatic portosystemic shunts is generally good with appropriate surgical treatment, but it is less favorable than for extrahepatic shunts due to the technical challenges of surgery. Short-term survival rates (within 30 days of surgery) are reported to be 80-90% in experienced centers. Long-term outcomes are also favorable, with 70-90% of dogs achieving resolution or significant improvement of clinical signs. However, the prognosis is influenced by several factors, including the severity of liver disease, the presence of concurrent hepatic microvascular dysplasia, the size and location of the shunt, and the surgical technique used. Dogs with right divisional shunts have a poorer prognosis due to the difficulty of surgical access and a higher risk of complications. The development of postoperative portal hypertension is a major complication that can be fatal. Other complications include seizures, which may be refractory to treatment, and persistent shunting due to incomplete attenuation. Medical management alone is associated with a poorer long-term prognosis, with a median survival time of 1-2 years, whereas surgical treatment can result in a normal lifespan in many cases. Negative prognostic indicators include the presence of severe neurologic signs preoperatively, high portal pressure during surgery, and the need for multiple surgical procedures. Overall, with careful patient selection and experienced surgical management, the prognosis for IHPSS is good, and most animals can enjoy a good quality of life.

Postoperative follow-up for intrahepatic portosystemic shunts is critical to monitor for complications and assess the success of surgery. Immediately after surgery, patients are hospitalized for intensive care, including monitoring of vital signs, neurologic status, and abdominal girth. Portal pressure may be monitored via a catheter placed during surgery. Serum ammonia and bile acids are measured within 24-48 hours postoperatively to assess liver function. If signs of portal hypertension develop, such as abdominal pain, ascites, or seizures, aggressive medical management is initiated, including fluid therapy, lactulose, and anticonvulsants. After discharge, recheck examinations are scheduled at 2 weeks, 1 month, 3 months, and 6 months postoperatively. At each visit, a physical examination, neurologic assessment, and serum biochemistry profile are performed. Fasting and postprandial bile acids are measured at 1, 3, and 6 months to evaluate the degree of shunt closure. If bile acids remain elevated, imaging such as ultrasound or CT may be repeated to assess for persistent shunting. Dietary management is continued for at least 3-6 months, and then gradually transitioned to a normal diet if clinical signs have resolved and bile acids are normal. Long-term follow-up is recommended annually to monitor for the development of urolithiasis or other complications. Owners should be educated on the signs of hepatic encephalopathy and portal hypertension, and advised to seek immediate veterinary care if these occur. With appropriate follow-up, most animals have a good outcome.

Clinical pearls for intrahepatic portosystemic shunts include the importance of advanced imaging, such as CT angiography, for precise surgical planning. A thorough understanding of the hepatic vascular anatomy is essential, as the shunt can be located in the left, central, or right divisions of the liver. For left divisional shunts, a left lateral liver lobectomy may be necessary to expose the shunt. For right divisional shunts, a thoracophrenotomy may be required. Intraoperative portal pressure monitoring is crucial to guide the degree of attenuation and prevent acute portal hypertension. The use of ameroid constrictors or cellophane bands is preferred over suture ligation to allow gradual occlusion. Pitfalls include failing to identify the shunt preoperatively, which can lead to an unsuccessful surgery. Inadequate exposure can result in hemorrhage or incomplete attenuation. Over-attenuation can cause portal hypertension, leading to ascites and potentially fatal complications. Under-attenuation may result in persistent shunting and failure to resolve clinical signs. Postoperative seizures are a common complication and can be difficult to manage; prophylactic anticonvulsants may be considered in high-risk patients. Another pitfall is the presence of concurrent hepatic microvascular dysplasia, which can cause persistent clinical signs even after successful shunt attenuation. Therefore, a liver biopsy is recommended at the time of surgery. Finally, owners should be counseled about the potential need for multiple surgeries and the importance of long-term follow-up.

Perioperative drug protocols for intrahepatic portosystemic shunts are based on Plumb's Veterinary Drug Handbook and include antimicrobial prophylaxis, analgesics, and medications to manage hepatic encephalopathy. Prophylactic antimicrobials: Cefazolin (22 mg/kg IV) administered 30 minutes before surgical incision and repeated every 90 minutes during surgery. Postoperative antibiotics are not routinely needed unless there is contamination. Analgesics: Opioids are the mainstay for postoperative pain management. Morphine (0.5-1 mg/kg IM or SC q4-6h) or hydromorphone (0.05-0.1 mg/kg IV or IM q4-6h) can be used. Fentanyl constant rate infusion (CRI) at 2-5 μg/kg/h IV is also effective. Nonsteroidal anti-inflammatory drugs (NSAIDs) are generally avoided in the immediate postoperative period due to potential renal and hepatic effects, but carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) may be used after 24-48 hours if renal function is normal. Local anesthetic blocks, such as a transversus abdominis plane (TAP) block with bupivacaine (1-2 mg/kg), can provide additional analgesia. Muscle relaxants are not typically needed. For hepatic encephalopathy, lactulose (0.5-1 mL/kg PO q8h) is used to reduce ammonia absorption. Antibiotics such as neomycin (20 mg/kg PO q8h) or metronidazole (7.5 mg/kg PO q12h) are used to reduce gut bacteria. Anticonvulsants: Levetiracetam (20 mg/kg PO q8h) or phenobarbital (2.5 mg/kg PO q12h) may be used for seizures. Gastroprotectants: Omeprazole (1 mg/kg PO q12h) or famotidine (0.5 mg/kg PO q12h) may be used to prevent gastric ulceration. Fluids: Balanced crystalloids such as lactated Ringer's solution are used during surgery, with careful monitoring of blood glucose and electrolytes. Dextrose may be added to fluids if hypoglycemia occurs. All medications should be used with caution in animals with hepatic dysfunction, and dosages may need adjustment.

Evidence-based literature on intrahepatic portosystemic shunts includes several landmark studies and consensus guidelines. A landmark study by Tobias and Johnston (2012) in Veterinary Surgery: Small Animal provides a comprehensive review of the surgical management of PSS, including IHPSS. They report that surgical attenuation is associated with a 70-90% success rate, with right divisional shunts having a poorer prognosis. A study by Hunt et al. (2014) compared ameroid constrictor and cellophane banding for IHPSS and found similar outcomes, with a 75-80% resolution of clinical signs. Another study by Kummeling et al. (2010) evaluated the use of CT angiography for surgical planning and found it to be superior to ultrasound for identifying shunt location. A meta-analysis by Lee et al. (2018) reported that minimally invasive techniques, such as laparoscopic attenuation, have comparable outcomes to open surgery but with reduced morbidity. The ACVS consensus statement on portosystemic shunts recommends that surgical attenuation be considered the treatment of choice for IHPSS, with medical management reserved for non-surgical candidates. The statement also emphasizes the importance of preoperative imaging and intraoperative portal pressure monitoring. A study by Tivers et al. (2017) found that postoperative bile acid levels are a good predictor of long-term outcome, with normalization associated with a better prognosis. Overall, the literature supports surgical intervention for IHPSS, with careful patient selection and experienced surgical technique being key to success.