Willow 2 months following ameroid ring placement on a porto-azygous shunt.
Isa 3 months following ameroid ring placement on a porto-azygous shunt.
Portosystemic shunts (PSS) are abnormal vessels that permit blood draining the gastrointestinal tract to bypass the liver and travel directly to the systemic circulation. These shunts may be intrahepatic or extrahepatic. Intrahepatic shunts occur primarily in large breed dogs while extrahepatic shunts are found most commonly in small breed dogs and cats. Certain breeds are highly predisposed in particular Yorkshire terriers which have been shown to have a genetic predisposition to PSS. The most common similar disease in humans are acquired multiple shunts which occur secondary to sclerosis of the liver.
Portosystemic shunts are most commonly identified in young animals as clinical signs occur at a relatively early age. The most common clinical signs are neurologic abnormalities due to lack of filtering by the liver of substances such as ammonia and mercaptans. Neurologic findings include seizures and mental dullness as well as ataxia or more severe mental deficiencies. Cats frequently show episodic blindness.s Other signs of PSS include failure to grow or stranguria secondary to bladder stones.
Diagnosis of PSS is based on history, physical examination findings, blood work, and diagnostic imaging. Abdominal palpation of patients with PSS may reveal microhepatica and in some cases enlargement of the kidneys. Cats may have copper colored irises and exhibit ptyalism.
Routine blood work of dogs and cats may include many abnormalities such as microcytosis, anemia, low albumin, cholesterol and BUN and occasionally elevations in ALT, AST, and alkaline phosphatase. Urinalysis may show ammonium biurate crystals.
Measurement of bile acids and ammonia remain the most common blood analysis for diagnosis of PSS, but studies have shown that there are significant normal variations of both of these chemicals in the blood and the tests must be interpreted carefully. A recent study by Ruland et al (Vet Clin Path 2009) showed that increased fasting bile acids and ammonia are accurate indicators of PSS particularly when interpreted with specific cut off points. A study in 2006 (Bruning et. al. J Vet Int Med) demonstrated that fasting plasma ammonia elevation is significantly more specific for PSS than bile acids.
Diagnostic imaging is critical to the diagnosis of PSS in dogs and cats. Numerous techniques have been reported for confirming PSS including positive contrast portography and nuclear scintigraphy; however portography is moderately invasive and technically demanding and scintigraphy requires specialized equipment and isolation facilities.
Ultrasound has become the most accepted method for localizing PSS. At our facility Drs Saunders and Walker of the Lynks Group have extensive experience in localization of PSS with color flow Doppler and in many cases can specify the origin and destination of the shunt.
Treatment of PSS can be separated into medical and surgical options. Medical treatment is directed at decreasing the toxins responsible for hepatic encephalopathy and protect the liver from additional damage. This is primarily achieved through feeding a diet low in protein and high in carbohydrates. In addition, antibiotics (neomycin, metronidazole, ampicillin) and lactulose are used to further reduce the encaphalopathic hepatotoxins. But surgery should be recommended as the primary treatment for PSS in almost all cases. The life expectancy for dogs and cats medically treated has been reported as 2 months to 2 years. There is still some controversy regarding the best treatment of PSS in older dogs (more than 5 years of age) although a recent study (Worley et. al. JAVMA 2008) reported excellent results following surgery in 17 dogs over 5 years of age.
Surgical treatment of PSS is directed at occluding or attenuating the shunting vessel. Previously surgical treatment involved partially or completely ligating the vessel with silk suture under the guidance of portal pressure profiles to minimize the risk of postoperative portal hypertension. Portal hypertension would frequently result in seizures (particularly in cats), ascites, severe pain, hemorrhagic diarrhea and in many cases death. Appropriate attenuation was difficult because of the unpredictability of the portal pressures in the awake patient and the acute nature of the change to the shunt diameter.
Two methods of progressive shunt attenuation are now available: ameroid constrictor rings and cellophane banding.
At Burlington Veterinary Specialists we primarily perform ameroid constrictor ring placement except in the smallest of patients when there is concern that the weight of the ring may acutely occlude the shunt. In these cases we perform the cellophane band technique. Careful anesthesia of PSS patients is critical because of their small size and altered hemodynamics. Dr Caroline Horn directs anesthesia on all of our PSS cases and the surgery is performed as quickly as possible to minimize anesthetic complications and hypothermia.
When performing PSS surgery the abdomen is routinely explored and a liver biopsy is obtained. The portal vasculature is then examined for the shunting vessel. Once identified, it is dissected just enough to permit placement of the ring or band. Because acute attenuation or occlusion is not going to be performed it is not necessary to measure portal pressures and there is almost no risk of portal hypertension. The abdomen is then closed routinely and the patient recovered as quickly as possible.
The use of ameroid rings and cellophane banding has dramatically changed our treatment of PSS. While some patients and cats in particular may experience postoperative seizures for unknown reasons, the majority of patients recover no differently from a routine spay. The shunt closes down gradually over a period of several weeks, after which the patient can be slowly returned to a normal diet and the medications stopped. These patients usually demonstrate no further neurologic signs, begin to gain weight, and in most cases have a normal life expectancy.