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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 part of the abdomen next to the diaphragm. It is composed of six lobes. Blood is supplied to the liver by the hepatic artery and is drained by the hepatic veins. The liver also receives blood from the portal vein. 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 and systemic circulation. The blood from the abdominal organs which should be drained by the portal vein into the liver instead enters the systemic circulation thereby sending products absorbed by the intestines directly into this 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).
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.
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 always have extrahepatic shunts and the left gastric is the most common.
Signs and Symptoms
Animals with portosystemic shunts may present for small body stature, anesthetic intolerance or behavioral abnormalities. The signs are often episodic. Signs of abnormal neurologic function include ataxia (sway as if intoxicated), seizures, blindness and head pressing. Other signs may include anorexia (loss of appetite), vomiting, diarrhea, constipation, ptyalism (hypersalivation - in cats only), polyuria/polydipsia (excessive urination/drinking), stranguria (difficulty urinating) and hematuria (bloody urination). Animals may or may not possess a fever.
Exam, Screening Tests, and Imaging
If a portosystemic shunt is suspected, a full diagnostic work-up is advised. A full work-up includes bloodwork, radiographs, and ultrasound. Nuclear scintigraphy ( a non-invasive technique involving colonic administration 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. Bloodwork may demonstrate anemia (low red blood cell count), elevations in liver enzymes and low glucose, 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 may show a small liver and possibly large kidneys. Ultrasound is very helpful in locating a shunt but is dependent on the expertise of the ultrasonographer (Figure 1). Ultrasound is also helpful in determining if ammonium biurate bladder stones are present. Dogs and cats with PSS may develop stones due to high ammonia levels. These types of stones are not 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. Finally, the gold standard in identifying PSS is positive-contrast portography (Figures 2 & 3) which requires surgery. A mesenteric vein (vein draining the intestines) is catheterized and an H2O 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.
Figure 1. Ultrasound of abdomen. Arrow pointing to intrahepatic shunt.
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Figures 2. and 3. Normal portogram with arrows pointing to normal vasculature in liver. 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 managed 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 lactulose, neomycin and diet. The goals are to eliminate the bacterial population in the intestines and to minimize the production of toxins. Lactulose promotes the expulsion of fecal matter (it is a cathartic) and lowers the ammonia level while doing so thereby decreasing signs of hepatic encephalopathy. Neomycin helps to eliminate bacteria that promote the formation of ammonia. The ammonia level must be within a normal range and the animal must be neurologically normal in order to have the best chance for a surgical success. The diet should provide protein but may need to be moderately restricted depending on the clinical signs for each individual animal.
Surgical management
The treatment of choice is surgery by surgical attenuation or full ligation of the abnormal shunt vessel. The surgeon carefully explores the vasculature for any abnormal communication between the portal and systemic circulation. If a shunt cannot be identified a portogram is performed. (Figures 2 and 3) When the shunt is identified, full ligation (tying off the vessel) of the vessel is performed only if postligation 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). Elevation in this pressure, called portal hypertension can result in death.
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 (pressure is too high). Acute portal hypertension results in abdominal distension, pain, bloody diarrhea, ileus and endotoxic shock (shock due to bacterial toxins). At present, ligation of the shunt is done using an ameroid constrictor (Figure 6). The ameroid constrictor is made of a hydroscopic casein material (material that loves water) in a stainless steel ring. It is placed around the shunt and fluid is absorbed (Figure 7). The casein material of the ring becomes progressively larger and gradually occludes the vessel over 4-5 weeks. 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 cellophane. The tape will incite an inflammatory response and the vessel will slowly close down over a period of months.
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 still need to be hand-ligated because they are usually too large for the constrictor. At present most intrahepatic shunts are treated by placing coils in the vessel. This is performed by inserting a catheter into the vessel and releasing coils into the shunt. The shunt then closes off when the coil creates a clot in the vessel. The liver should always be biopsied at the time surgery takes place.
Postoperative Care
Routine postoperative management includes intravenous fluids and antibiotics. Lactulose, diet modification and neomycin/metronidazole are continued. 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 no further treatment whereas others may need some dietary restrictions or medications. 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
· The development of multiple acquired PSS secondary to surgically induced portal hypertension or fibrosis(scarring) of the liver.
Complications Following Surgery
Complications after surgery include portal hypertension, which leads to splanchnic ischemia (loss of blood to abdominal organs) and death. Animals will show signs of ascites (fluid distension in the abdomen), vomiting, diarrhea, depression and respiratory distress. Up to 21% of the patients used to die of portal hypertension or fatal hemorrhage. This percentage has decreased by more than 11% since the use of the ameroid constrictor. Animals should also be monitored for seizure activity postoperatively. The cause of these seizures is unknown. It is possible that seizures occur because the brain has adapted to an altered metabolism or because of withdrawal of the anticonvulsant effects of endogenous benzodiazepines (valium-like substances that the body produced when the shunt was open) after ligation of the shunt. Seizures are managed with intravenous medication (Phenobarbital), oral medication (KBr - potassium bromide), or intravenous propofol (anesthetic agent).
Prognosis
The prognosis is excellent if the animal survives the immediate postoperative period. With partial ligation the prognosis is not as good. After 3 years, signs recur in 40-50% animals with partial shunt ligation. Cats have a less favorable prognosis with shunt ligation. They may have some clinical improvement but relapses do occur and they may have neurological signs of blindness and seizures. Cats are more likely to develop acquired PSS.
Prevention
Portosystemic shunts can only be prevented by not breeding affected animals and refraining from breeding the sires and dams of affected animals.
—Susan Mitchell, DVM
Diplomate ACVS
Posted 8/13/2004
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