Portosystemic shunting is a macroscopic diversion of portal blood from the liver into the systemic circulation, which most commonly occurs because of a congenital malformation of the portal circulation.
Blood flow to the liver may be decreased by up to 95% of normal in shunt patients, resulting in a bypass of the body’s normal homeostatic liver detoxification mechanisms. Additionally, this lack of normal blood flow results in a deficiency of oxygenated blood, insulin, and growth factors that the liver requires for normal growth and function. The anomalous connections may occur between the portal vein and the caudal vena cava, azygous vein, renal vein, phrenic vein, internal thoracic vein, and the umbilical vein remnant. As mentioned previously, in most cases, the shunt is congenital in nature; however, there are cases of acquired shunting secondary to an increase in portal hypertension.
The liver carries out a number of important functions, including but not limited to the synthesis of glucose, glycogen, cholesterol, lipoproteins, albumin, fibrinogen, bile acids, and detoxification of the blood to remove hazardous circulating chemicals, bacteria, and bilirubin. In addition, the liver functions as a reservoir to store important nutrients, minerals, vitamins, and hormones. All of these functions are significantly impaired in patients with portosystemic shunts. The degree of impairment is in direct correlation with the percentage of blood bypassing the liver.
Portosystemic shunts may be either intra or extra hepatic in nature. Congenital extrahepatic portosystemic shunts are most common, accounting for 61% to 94% of congenital shunts and are most prevalent in the smaller terrier breeds. They are frequently observed in the Maltese, Yorkshire Terrier, Shih Tzu, Dachshund, and Poodle. Congenital extrahepatic shunts are also frequently observed in cats. The Persian and Himalayan breeds are over represented; however, mixed breed cats may also be affected. Acquired Intrahepatic shunts represent between 6% and 40% of congenital shunts and are more common in large and giant breeds of dogs such as Irish Wolfhounds and Golden Retrievers. Hepatic microvascular dysplasia is an unusual form of intrahepatic portosystemic shunting in which no gross vascular abnormality can be identified. This rare condition is associated with somewhat milder clinical signs and appears to be the consequence of a developmental abnormality; it has a higher prevalence in Cairn Terriers, suggesting a hereditary basis. Since that time, this anomaly has been seen in many other smaller dog breeds. Clinical signs and diagnostic evaluations are similar to those discussed below, but no surgically identifiable or radiographically identifiable shunt is seen. The shunting is at the level of the hepatic portal vein capillaries, and thus, not correctable surgically.
Portosystemic shunts are most commonly observed and diagnosed while the animal is young, often less than six months of age. The affected pets are smaller in stature, have poor quality hair coats, appear weak, and often exhibit neurologic abnormalities. Particularly bizarre behavioral signs or loss of intellectual function, unpredictable bouts of maniacal or aggressive behavior, staggering, pacing, circling, head pressing, blindness, deafness, tremors, seizures, and coma may be seen. Other signs include pica and polyphagia. In many cases, the clinical signs have an episodic nature; they are present for a few hours to a day or two and the animal returns to normal. The clinical signs are often meal related neuro-encephalopathic signs or somnolence, nausea or vomiting, diarrhea or constipation, polyuria and polydipsia, intermittent fever, ptyalism, and signs attributable to ammonium biurate urolithiasis (chronic urinary tract infections, prolonged or difficult urination). Any dogs or cats in which a uric acid calculus is identified should be evaluated as a possible PSS case (except for Dalmations and Bulldogs). Dogs with portosystemic shunts also have increased susceptibility to infections because of the decreased function of their hepatic mononuclear phagocytes which serve to clear toxic cells from the body. Minor bite wounds, tick bites, subcutaneous infections, lacerations, and even vaccinations may lead to illness requiring hospitalization. The severity of clinical signs in symptomatic patients is highly variable and is largely affected by the quality of food being consumed and the percentage of shunting present.
The diagnosis of PSS is often suspect on presentation of the afore mentioned clinical signs, history, and physical examination. The diagnosis is confirmed by performing laboratory testing, which includes complete hematology, serum chemistry, urinanalysis, abdominal radiography, and pre and post prandial bile acid values. Hematologic evaluation may indicate microcytosis with or without a mild non regenerative anemia and a leukocytosis secondary to infection. Serum chemistry evaluation may indicate decreases in glucose, albumin, cholesterol, blood urea nitrogen, and total protein values because of decreased production and capacity for storage by the smaller than normal liver. An increase in certain liver enzyme values including the serum ALT, AST, and ALP, can be observed because of injury to the hepatic cells. Increased serum bile acid concentrations taken either after an overnight fast or 2 hours after a meal are usually strongly suggestive of a shunt abnormality.
A definitive diagnosis of intrahepatic, extrahepatic, or microvascular PSS is made with varying combinations of contrast radiography, ultrasonography, scintigraphy, CT scan, or MRI angiography. This generally requires specialized imaging techniques, and it is appropriate that these pets be referred to appropriate specialists. Survey radiographs normally taken in general practice veterinary facilities is an important first step because it may indicate the presence of a small liver and urinary bladder stone formation consistent with a presumed shunt abnormality.
Contrast radiography is a procedure whereby a marker dye is injected into the jejunal vein or splenic parenchyma, which provides excellent imaging of portal blood flow. The procedure requires general anesthesia and laparotomy to be performed and, as such, often performed in combination with surgical correction (so that the dog has to be anesthetized only once) and is often referred to as operative mesenteric portography. This procedure was the diagnostic technique of choice prior to the emergence of the non-invasive imaging techniques currently utilized in specialty practices such as the Animal Medical Center of Southern California.
Ultrasonography is a useful, noninvasive tool for acquiring information about the liver and circulatory system as well as the detection of portosystemic shunts. The exact location of intrahepatic shunts may be more amenable to diagnosis via ultrasound examination. Abnormalities observed upon ultrasound evaluation of pets with intrahepatic shunts include microhepatica, decreased numbers of hepatic and portal veins, detections of the anomalous vessel, and the presence of renal and bladder uroliths. Unfortunately, extrahepatic shunts are more difficult to diagnose using ultrasound because of the obscurity of the lesion from the surrounding visceral organs.
Nuclear scintigraphy is an advanced non-invasive technique which is currently the state of the art procedure used to evaluate portal venous shunting. In this technique, after the animal receives a warm water enema, the colon is infused with technetium 99m pertechnetate. The animal is scanned with a gamma camera and the images are registered. In normal animals, the radiochemical is transported through the portal system to the liver. In animals with PSS, a greater percentage of the radiochemical is carried to the heart. The deviation in the time curve provides the diagnosis of the presence of a shunt; however, it does not definitively prove its anatomical location. Normal dogs have a shunt fraction of less than or equal to 15 % whereas dogs with PSS have a shunt fraction of 60% or greater. An inaccurately low result can be caused by poor absorption of the radiochemical by the colon, excessive fecal material in the colon, or poor administration technique. A false positive result is occasionally observed with rectal administration of the radiochemical as uptake by the caudal rectal artery may be misinterpreted as an increased absorption indicative of an extrahepatic shunt.
MRI angiography and CT scan both provide excellent three dimensional images of the shunt. Both modalities require a general anesthesia and can be cost prohibitive. There are many other techniques available; transjugular retrograde portography, cranial mesenteric angiography, exploratory laparotomy, etc…The choice is often dependent on the patient’s stability, the urgency for surgical correction, availability, and physician preference.
Once a diagnosis is made, there are two standard management options: surgical ligation of the shunt and/or medical management of the effects of shunting. Surgery is the treatment of choice where feasible; however, the decision to pursue medical management should be made on a case-by-case basis depending on the type and location of the shunt, the age of the animal, and the severity of clinical signs. There may also be significant financial considerations on the part of the owner. Hepatic functional failure tends to progress in most animals that are diagnosed prior to one year of age. Even though medical therapy may keep them relatively asymptomatic, they eventually become refractory to medical management and succumb to their disease. Thus, surgical attenuation or ligation of the shunting vessel is the best therapeutic option in order to provide reversal of signs, and long duration quality survival. The objective of surgery is to redirect shunting blood back into the liver providing hepatic parenchyma with hepatotropic factors necessary for normal growth and metabolism.
Every patient diagnosed with a PSS should be appropriately medically managed prior to surgical intervention in order to provide more strength and stability to the patient in anticipation of undergoing an invasive surgical procedure. Initially, the correction of fluid and electrolyte abnormalities, glucose imbalance, and hepatic encephalopathy are of utmost importance. Other pre-operative goals include minimizing lower urinary tract disease and reducing the metabolic load on the liver. The chief components of medical management strategies are dietary modifications and the administration of oral antibiotics.
Dietary protein is restricted because it is a significant source of nitrogenous wastes and toxins that are in part responsible for symptoms of hepatic encephalopathy. Other additional toxins are derived from the bacterial flora found within the large intestine which is responsible for the production of ammonia and ammonia by products. The production of these toxins is reduced by limiting the total amount of protein ingested and ensuring that the dietary protein ingested is high quality and biologically available with little waste protein being generated during digestion. The utilization of these diets results in the reduction of the amount of waste protein that reaches the large intestine, thereby decreasing the substrate load the colonic bacteria have available to produce circulating toxins. Further reductions may be attained by feeding smaller meals more frequently to maximize the digestive capacity of the small intestine. These special high quality, lower total protein, high caloric diets are ideal because they provide a balanced protein-calorie intake, which is important for the control of hepatic encephalopathy. Including dietary fiber in the daily ration assists in acidifying the colonic environment and limiting toxin production and also acts as a mild laxative to increase the elimination of toxic factors in feces. Lactulose, a soluble fiber, is often prescribed as a supplement for this purpose. Antibiotics are used in most cases to reduce the bacterial population within the large intestine that are responsible for the production of circulating neurotoxins.
Surgical ligation of the shunt vessel is a sophisticated technique which requires an experienced surgeon and support staff in order to best ensure a successful surgical outcome. At the Animal Medical Center, our Board Certified Surgeon has successfully diagnosed and treated PSS patients for over 27 years. A thorough exploratory laparotomy is performed. All the viscera in the abdomen are gently explored. The venous drainage system is explored and the shunt located. Single extra hepatic shunts are usually identified as tortuous, abnormal vessels. The most common locations for an extrahepatic shunt include an abnormal vessel visualized rostral to the renal veins at the level of the caudal vena cava, along the greater or lesser curvature of the stomach, along the venous drainage of the spleen, and an abnormal vessel coursing dorsally to the azygous vein and disappearing beneath the diaphragm. The shunt is ligated as close to it’s insertion site as possible so that the blood flow from potential tributaries of the shunt is redirected. More specifically, portacaval shunts are occluded at their termination at the caudal vena cava and portoazygous shunts can be occluded at the abdominal side of the diaphragm.
A potentially fatal complication from shunt ablation is portal hypertension, which occurs when intrahepatic vessels are unable to cope with the additional volume of blood that is diverted to the liver after closure of the shunt vessel. Failure to recognize and alleviate hypertension and resulting pain can lead to the development of bloody diarrhea and endotoxic shock leading to death in 2 to 24 hours after surgery. Approximately 40-68% of dogs and cats that undergo shunt attenuation can only tolerate partial ligation. Before permanent ligation, the shunt is temporarily occluded using gentle digital pressure for 3-5 minutes and the portal and systemic blood pressures are carefully monitored for hypertension and the intestines and pancreas are inspected for signs of cyanosis indicative of the inability of the body to cope with the redistribution of blood flow through the liver. These complications were much more prevalent when shunt attenuation was accomplished by ligature placement around the shunting vessel. In order to decrease the incidence of complications observed with the older technique of immediate, complete occlusion of the shunting vessel by ligature placement, an ameroid ring constrictor was developed. An ameroid ring is made of casein on its inner side and stainless steel on the outside. The casein interior is composed of a hygroscopic substance that slowly swells as it slowly absorbs fluid from the abdominal cavity. The ameroid ring is placed around the shunt vessel and locked in a closed position. The resultant inward swelling causes the vessel to gradually occlude over a 4-5 week period.
Dogs generally have dramatic clinical and biochemical improvement following shunt correction. They are usually clinically improved within 24 to 48 hours. Follow-up function studies significantly improve, but rarely return to normal postoperatively. Occasional animals will have persistently abnormal, but improved, ammonia tolerance tests or slightly elevated bile acid values but remain clinically asymptomatic and require no special therapy. A number of post-operative portal angiograms have been reported and all show improved portal circulation of the liver, but the perfusion pattern is aberrant. The overall prognosis for dogs with single extrahepatic shunts appears to be excellent if they survive the surgery and immediate post-operative period. For intrahepatic shunts, the outlook is more guarded, but numerous successful surgeries have been performed by surgeons knowledgeable in the intricacies of this procedure. If urinary calculi cannot be removed during the surgery to correct the shunt, they may be either removed at a later date or managed medically. Spontaneous dissolution of renal calculi (presumed to be ammonium urate) following shunt correction alone may be observed. Calculi present in the bladder may also be successfully dissolved using a combination of a reduced purine diet (Prescription diet-u/d) combined with allopurinol, 10 mg/kg every 8 hours, and sufficient sodium bicarbonate to alkalinize the urine. Calculi may disappear in 4 to 12 weeks on this therapy. Recurrences of calculi should not be a problem once the shunt is corrected, as metabolic abnormalities causing increased urinary ammonia and uric acid excretion should cease.
Immediate postoperative therapy should include fluid therapy, temperature and blood pressure monitoring, packed cell volume, total protein and glucose testing. All dogs undergoing surgical ligation should continue to receive medical therapy as discussed above for 2 to 4 weeks postoperatively. Liver regeneration occurs rapidly and returns to normal to near normal hepatic function can be observed by 8 weeks post-operatively.
In conclusion, the portosystemic shunt is a common disease process encountered in dogs and cats. While the origin of the disease is obscure the disease is considered to be congenital; however, breed predisposition does exist, indicating a hereditary origin. Symptoms vary but all result in central nervous system disorders called hepatic encephalopathy. Puppies and kittens that fail to maintain the same growth rate as their siblings are suspect. Specific urinary crystals develop due to increased concentrations of ammonium. The degree or percentage of vessel shunting may explain why some animals are without symptoms until later in life. Small liver size coupled with bizarre neurological signs in young dogs and cats is highly suggestive of the disease. Medical or surgical treatment is available; however, without corrective surgery, an animal will eventually succumb to the progressive disease process. Prompt and effective surgical intervention utilizing an ameroid constrictor and medical management results in effective and long lasting alleviation of the clinical signs induced by this congenital anomaly.