Portosystemic Shunts

Portosystemic vascular shunts (PSS) are anomalous (abnormal) blood vessels that shunt blood away from the liver).

This diversion of blood leads to small liver size and the collection of ammonia and toxins in the blood. The result of these toxins often leads to abnormal neurological signs such as: behavioral changes, seizures, or coma.

The classification of PSS is often confusing. Portosystemic shunts can be divided into congenital or acquired shunts. Congenital shunts originate from the portal vein draining the intestines and empty into the caudal vena cava or azygous veins. This effectively allows all of the blood draining from the intestines, pancreas, and spleen to bypass the liver. Without the liver to process the blood from the intestines, some toxins (i.e. ammonia) are sent directly to the heart for circulation within the body. Acquired shunts are usually secondary to severe liver disease (e.g. Cirrhosis) Acquired shunts are typically found in animals that are older and affected by cirrhotic liver disease, microvascular dysplasia, or noncirrhotic portal hypertension. They often appear as a “spider web of vessels” seen around the kidneys.

This type of shunting is often involves multiple vessels and surgical correction is often not possible. Most congenital shunts are extrahepatic (outside the liver) and consist of a single vessel.

These are easily found on surgical exploration and are amenable to attenuation via ameroid constrictor. Congenital, single, extrahepatic shunts are generally found in small breeds of dogs and most breeds of cats. If multiple extrahepatic shunts are present, surgical correction may not be possible.

Clinical Signs

There are a variety of clinical signs associated with portosystemic shunts. However, some dogs may be completely normal at physical exam. It is important to always look for other congenital defects if suspicious of a PSS. Dogs and cats can have evidence of neurological disease, which is often most prominent after ingestion of a meal. These signs are related to the increased levels of toxins and ammonia circulating throughout the body, which are produced by proteins in the meal. Many animals affected by PSS are less than one year of age, but animals may exhibit signs of the disease at any time. Most patients are considered to be small in size when compared to litter mates. Neurological signs noted most frequently can involve seizures, personality changes, visual deficits, head pressing, disorientation, stupor, or coma. Symptoms of PSS are exacerbated by high protein diets and may worsen after eating even an average meal. Some dogs will have evidence of gastrointestinal signs, including: vomiting and anorexia. Because of the inability to process ammonia byproducts appropriately in the liver, PSS patients are at a higher risk of urate bladder stones as well.

Diagnosis

A tentative diagnosis of a PSS is made based on the history, laboratory results, physical, and neurological exam. Any or all of the following laboratory abnormalities may be seen. The most common abnormalities include microcytic anemia, increased white blood cell count, mild elevation in liver enzymes, low blood urea nitrogen, low cholesterol, elevated ammonia, and the most commonly performed test, elevated bile acids. The bile acids test is an excellent indicator of abnormal liver function. The test is performed after fasting and repeated 2 hours after feeding a medium sized meal.

Abdominal radiographs (x-rays) and ultrasound are often the most useful diagnostics for diagnosis of PSS. Both diagnostic modalities may reveal a small liver. Ultrasound can be used to evaluate liver size, evaluate for urate bladder stones, and identify the vascular shunt. Portography may be performed prior to or after surgery for PSS. The portogram involves injecting contrast into the portal vasculature, in order to preoperatively identify the anomalous vessel or to confirm postoperative attenuation of the vessel.

Treatment

The management of portosystemic shunts consists of both medical and surgical modalities. Medical treatment of shunts will often decrease the severity of the clinical signs and is often performed prior to surgery to control clinical signs and to decrease the patient’s surgical and anesthetic risk. It is important to remember that medical therapy is like a temporary band-aid for the clinical signs; however it does not address the underlying problem.

Medical therapy administered at South Paws often consists of: a low protein diet (Science Diet L-D or K-D), antibiotics to decrease ammonia producing bacteria, and lactulose, an ammonia binding agent. Betadine or lactulose enemas to lower nitrogen (ammonia) absorption may also be indicated in times of severe hepatoencephalopathy.

Surgical Intervention

The goal of surgical intervention for extrahepatic PSS is to identify and ultimately close the shunting vessel completely. However, complete ligation of the vessel is often not possible, due to the inability of the liver to accept normal blood flow so quickly. Complete, rapid ligation would result in portal hypertension, shock, and death. To decrease the risk of portal hypertension and allow the liver time to adapt to its increased blood flow, an ameroid constrictor can be used. The ameroid constrictor is designed with an outer metal casing and an inner casein core. When the ameroid constrictor is placed around the shunting vessel, water in the body causes the casein core to swell, causing slow constriction of the vessel. Complete constriction takes place over a period of approximately 15-30 days. Because of the slow attenuation, the liver has time to adapt to its increasing blood flow, greatly eliminating the risk of portal hypertension. At the time of surgery, a liver biopsy is taken and the bladder is evaluated for large urate bladder stones.

Postoperative Care

Animals are monitored carefully in the ICU during the postoperative period. The first 48 hours is the most critical time, allowing for monitoring of seizures or portal hypertension. These seizure episodes generally last for 48-72 hours and are controlled with intravenous anticonvulsant medication. Generally, patients are placed on prophylactic oral anticonvulsant therapy for at least 48 hours prior to surgery. This protocol has dramatically decreased the risk of post-operative seizures. Once these animals are sufficiently recovered to go home from the hospital, they are continued on previously recommended medical therapy of: low protein diet, antibiotics, and lactulose. At 8 weeks, the bile acids test may be repeated, to ensure there is no further evidence of PSS. In most cases, the prognosis for a normal, active life is excellent with complete attenuation. Approximately, 10-15% may develop multiple acquired shunts.

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