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PORTOSYSTEMIC SHUNTS
Anatomy
The liver has a number of important functions in the body, from making proteins and clotting factors to detoxification of blood. It rests in the front top) part of the abdomen underneath the diaphragm, and is composed of six lobes. Blood is supplied to the liver by the hepatic artery and is drained from the liver by the hepatic veins. The liver also receives blood from the portal vein, a large vein that drains most of the lower abdominal organs, especially the small and large intestines. The liver receives ¼ of total cardiac output: 75% of this is transported by the portal vein and 25% by the hepatic artery. The portal vein carries blood from the abdominal organs including the spleen, stomach, pancreas, intestines and greater omentum, and carries it to the liver. The liver then metabolizes nutrients in the blood and removes certain toxins.
Overview
A portosystemic shunt (PSS) is an abnormal connection between the portal vein and systemic circulation. Blood from the abdominal organs which should be drained by the portal vein into the liver, is instead shunted to the systemic circulation by the PSS. This means that a portion of the toxins and nutrients absorbed by the intestines bypasses the liver and is shunted directly into the systemic circulation. There are two categories of shunts, extrahepatic (outside the liver) and intrahepatic (inside the liver). While most portosystemic shunts are congenital (the dog or cat is born with the shunt), under certain circumstances portostystemic shunts may also be acquired, i.e., developed secondary to another problem with the liver.
There are numerous types of extrahepatic shunts, named by the vessels that created the abnormal shunt. A portocaval shunt is an abnormal connection between the portal vein and the vena cava; a portoazygous shunt is an abnormal connection between the portal vein and azygous vein. Other abnormal shunts include left gastric, left gastroduodenal, splenic and cranial and caudal mesenteric. Shunts allow portal blood to reach the systemic circulation without first passing through the liver. Normally blood exiting the intestines, spleen, and pancreas enters the portal vein, which then takes blood to the liver. The liver metabolizes and detoxifies this blood. If a shunt is present the liver is deprived of factors that enhance liver development (hepatotrophic factors) which results in failure of the liver to reach normal size (hepatic atrophy). A common result of hepatic atrophy is hepatic insufficiency, which then frequently results in hepatic encephalopathy (a clinical syndrome of altered central nervous system function due to failure of normal liver function).
Intrahepatic shunts are usually located in the left side of the liver. The most common is the ductus venosus, which is the left umbilical vein. In the fetus, this is a working shunt that deliberately allow blood to bypass the liver, as the maternal circulation will detoxify the fetal blood. After birth, this shunt should close on its own; failure to do so results in intra-hepatic PSS. Rarely, an anomalous connection of blood vessels in a liver lobe, called an arterio-venous fistula, can cause shunting of blood like a single PSS. These can be congenital or acquired.
The genetic basis of PSS in dogs is unknown but it is considered congenital and breeds affected include Miniature schnauzers, Yorkshire terriers, Irish wolfhounds, Cairn terriers, Maltese, Australian cattle dogs, Golden retrievers, Old English sheepdogs and Labrador retrievers. Single extrahepatic shunts are typically congenital and affect small and toy breeds whereas single intrahepatic shunts affect large breeds. Cats nearly always have extrahepatic shunts and the left gastric is the most common.
Acquired PSS are almost always multiple vessels, which develop in response to hepatic hypertension. They can occur in any breed or age of animal. They are a compensatory mechanism to prevent or delay liver failure. As such, they cannot be ligated without causing severe symptoms, and medical management is the only option for treatment.
Signs and Symptoms
Animals with congenital portosystemic shunts may present for small body stature, anesthetic intolerance or behavioral abnormalities. The signs are often episodic and may be more noticeable after eating. Signs of abnormal neurologic function include ataxia (swaying as if intoxicated), seizures, blindness and head pressing. These signs are refeerd to as “hepatic encephalopathy”. Other signs may include anorexia (loss of appetite), vomiting, diarrhea, constipation, ptyalism (hypersalivation) – most frequently seen in cats), polyuria/polydipsia (excessive urination/drinking), stranguria (difficulty urinating) and hematuria (blood in the urine).
Exam, Screening Tests, and Imaging
If a portosystemic shunt is suspected, a full diagnostic work-up is advised. A full work-up includes blood work, radiographs, and ultrasound. Nuclear scintigraphy (a non-invasive technique involving colonic administration – enema– of the radioisotope 99mTC) or portography (an x-ray dye study that specifically highlights the portal system) may also be necessary to confirm the diagnosis. Blood work may demonstrate anemia (low red blood cell count), elevations in liver enzymes, hypoglycemia (low blood sugar), low cholesterol, low protein and low BUN (blood urea nitrogen -a waste product of protein metabolism). Liver function tests (bile acids and ammonia) are also evaluated to determine if liver function is normal. These values are elevated, indicating abnormal function, in animals with PSS.
Radiographs (x-ray study) may show a small liver and possibly large kidneys. Ultrasound may very helpful in locating a shunt but is dependent on the expertise of the ultrasonographer (Figure 1). Intrahepatic shunts are seen more often; extra-hepatic shunts are identified less than 50% of the time. Ultrasound is also helpful in determining if bladder stones are present. Dogs and cats with PSS may develop stones (usually composed of ammonium biurate) due to high ammonia levels excreted by the kidneys. These types of stones are not usually visible on radiographs. If ultrasound cannot identify a shunt, a rectal technetium scan can be performed. This type of imaging demonstrates blood as it travels from the colon. In a normal animal the blood should travel first to the liver and then to the heart. If a shunt is present, the scan will show blood traveling from the colon directly to the heart. Positive-contrast portography (Figures 2 & 3) requires surgery, but is usually definitive. A mesenteric vein (vein draining the intestines) or splenic vein is catheterized and a water-soluble contrast agent (a dye) is injected into the vein. Radiographs are taken. The course of the dye will demonstrate a shunt. If one is located surgery may be performed. A non-selective type of positive contrast study that doesn’t require surgery is done in conjunction with abdominal CT (CAT scan). This is more expensive, but can be used to identify hard to locate shunting vessels.
Figure 1. Ultrasound of abdomen. Arrow pointing to intrahepatic shunt.
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Figure 2. Normal portogram with arrows pointing to
normal vasculature in liver.
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Figure 3. Abnormal portogram with large arrow pointing to shunt and small arrows pointing to lack of vasculature in liver.
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Treatment
Medical management
Before surgery can be performed the patient may need to be stabilized medically. The goal of medical management is to improve the patient's health to a point where the risk of anesthesia and surgery is minimal. Medical management consists of a low protein diet and oral administration of antibiotics and lactulose. The goals are to decrease the bacterial population in the intestines and to minimize the production of toxins. Lactulose is a cathartic, which promotes the expulsion of fecal matter, as well as decreasing the bacterial load in the colon. It lowers the ammonia level while doing so, thereby decreasing signs of hepatic encephalopathy. Antibiotics help to eliminate bacteria that promote the formation of ammonia. The blood ammonia level should be within a normal range and the animal should be neurologically normal in order to have the best chance for a surgical success. The diet should provide high-quality protein but may need to be moderately restricted in amount of protein, depending on the clinical signs for each individual animal. If seizures are a part of clinical signs, anti-siezure medication may also be used.
Surgical management
The treatment of choice for a single PSS is surgical attenuation (narrowing) or full ligation (tying off) of the abnormal shunt vessel. This full ligation may be done instantaneously using suture material or intra-venous injection of an embolus of special glue material, or delayed full ligation with an ameroid constrictor, methyl cellulose band or an intra-venious embolic coil (see below). Find an ACVS Veterinary Surgeon.
The surgeon carefully explores the vasculature for any abnormal communication between the portal and systemic circulation. The liver should always be biopsied at the time surgery takes place, to determine the extent of liver damage and aid in determining prognosis and need for long term medical management.
If a shunt cannot be identified at surgery, an intra-operative portogram is performed (Figures 2 and 3). When the shunt is identified, pressure in the portal vein is measured before ligation is attempted. (Figure 4). The shunt is then temporarily occluded while measuring pressure again. Full ligation of the vessel is performed only if post-ligation portal pressure does not exceed 10 cm H20 (8 mmHg) above baseline pressures or 20-23 cm H2O (15-18 mmHg) (Figures 4 and 5). Excessively high portal system pressure, called portal hypertension, can result in death. Acute portal hypertension results in abdominal distension, pain, bloody diarrhea, ileus (stasis of bowel with gas build-up) and endotoxic shock (shock due to bacterial toxins).
Figure 4. Mesenteric vein catheterized to measure portal pressure.
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Figure 5. Manometer for measuring portal pressure.
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Partial ligation is done if there is a risk of portal hypertension (occlusion pressure is too high). Partial ligation of the shunt may be done by partially enclosing the vessel with a suture ligature until pressure rise is at its acceptable limit. About half of patients using this method will go on to scar closed their shunts; but about half will maintain some shunting of blood and need a second surgery months later, when the liver has adapted to its new circulation and can withstand full ligation. This method is rarely used anymore, due to the availability of ameroid constrictors, intravenous coils, and methyl cellulose bands. The ameroid constrictor (Figure 6) is made of a hydroscopic casein material (material that loves water) in a stainless steel, “C”-shaped ring. It is placed around the shunt, partially ligating it, and the ring is closed with a small obdturator. Over the next few weeks, the casein swells and gradually occludes the shunt (Figure 7). The most rapid phase of occlusion occurs within the first 3-14 days and then the closure of the constrictor slows down for the remaining 3-4 weeks. This is considered a method of gradual occlusion. The vessel may also be occluded using a special cellophane band. The tape will incite an inflammatory response and the vessel will slowly close down over a period of months. Trans-venous coiling is usually used for larger, intra-hepatic shunting vessels.
Figure 6. Ameroid constrictor band.
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Figure 7. Ameroid constrictor band placed on a shunt vessel during surgery.
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Intrahepatic shunts usually are usually too large for the ameroid constrictor. At present most intrahepatic shunts are treated by either had ligation or by placing intravenous coils in the vessel. This is done by inserting a catheter into the vessel and releasing one or more coils into the shunt. The shunt then closes off when the coil creates a clot in the vessel.
Postoperative Care
Routine postoperative management includes intravenous fluids and antibiotics. Lactulose and diet modification are continued, as it takes time for the liver cells to regenerate and adjust to the new circulation. These medications may be tapered depending on follow-up bile acid test results. Because serum bile acids values may or may not improve, some dogs may need long term treatment whereas others may only need some dietary restrictions or no medical restrictions. After ligation, the liver should regenerate. Failure of the procedure can occur for any of the following reasons:
- failure of shunt to close
- recanalization of the shunt (the shunt reopens)
- the presence of a second, unrecognized shunt (extremely unlikely)
- the development of multiple acquired PSS secondary to portal hypertension or fibrosis (scarring) of the liver.
Complications Following Surgery
Complications after surgery include portal hypertension, which leads to splanchnic ischemia (loss of proper blood circulation to abdominal organs) and death. Animals can show signs of ascites (fluid distension in the abdomen), vomiting, diarrhea, depression and respiratory distress. Use of the ameroid constrictor or delayed ligation of the shunt has reduced the chance of death by portal hypertension by more than 50%. Ascities may develop without posrtal hypertension in many animals as the liver adjusts to its new circulation. This fluid will eventually be reabsorbed by the body and disappear.
One of the most problematic post-op complications is development of seizures that are refractory to treatment. This occurs most frequently in toy breed dogs, in the first 4-5 days post-op. The cause of these seizures is unknown, but is thought to be due to the fact that in the shunting state, toxins in the circulation bathing the brain prompt the body to produce endogenous(made within the body) benzodiazepines (valium-like substances) that act as anti-seizure medication. Post-op, it is possible that seizures occur because the brain has adapted to that altered metabolism or because of sudden withdrawal of the anticonvulsant effects of the endogenous benzodiazepines that are no longer being produced since the shunt was ligated. Seizures can be are managed with anti-seizure medication (Phenobarbital, potassium bromide). In severe cases, intravenous administration of pentobarbital, propofol (anesthetic agent) and/or calium may be required to control seizures. Post-op development of seizures that are poorly controlled by medication is a very poor prognosis.
Prognosis
The prognosis is excellent if the animal survives the immediate postoperative period and full ligation of the shunt is achieved. With partial ligation, the prognosis is not as good. If continuing shunting of blood occurs, after 3 years, signs recur in 40-50% animals with partial shunt ligation. In many cases, full ligation is possible in animals that were partially ligated 4-6 months previously, so follow-up bile acids tests and portal scintography should be done to monitor for shunt function.
Prevention
Portosystemic shunts can possibly be prevented by not breeding affected animals and refraining from breeding the sires and dams of affected animals. Inheritance is not proven, but is strongly suspected. In cats, very orange/amber eyes have been noted in a number of affected cats. The significance of this is unknown.
—Susan Mitchell, DVM
Diplomate ACVS
Posted 8/13/2004
Reviewed and updated 2/19/2009 by Holly S. Mullen, DVM, Diplomate ACVS
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