Portosystemic shunt in dogs and cats (Proceedings) - Veterinary Healthcare
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Portosystemic shunt in dogs and cats (Proceedings)


CVC IN KANSAS CITY PROCEEDINGS


Diagnosis

Clinical findings

No breed, or sex predilection have been shown in patients with PSS; however purebred dogs seems to have a higher incidence. Schnauzer, Yorkshire Terrier, Bernese Mountain dog, Husky, German shepherd dog, and Irish Wolfhound are most frequently represented. Most patients with PSS are diagnosed at approximately 1 year of age, however clinical signs can occur as early as 6 weeks or as late as 8 years of age.

The most consistent presenting signs in patients with PSS include small body stature, failure to grow, and weight loss. Frequently, patients show signs of hepatic encephalopathy, gastrointestinal disorders, urinary track abnormalities, and intolerance to various drugs. The most common signs of hepatic encephalopathy include listlessness, depression, ataxia, head pressing , cortical blindness and seizures. Behavior changes have been reported in dogs and cats. Cats frequently present with ptyalism. Because of the pathogenesis of HE, 25% of patients with PSS have an exacerbation of clinical signs with ingestion of a protein meal. Anorexia, vomiting and diarrhea unresponsive to medical therapy are common gastrointestinal signs. Infrequently, dogs may be polyphagic. Polakiuria, hematuria, dysuria are some common signs associated with urate crystals in patients with PSS. Enzyme uricase in the liver normally converts uric acid, a purine metabolism by-product, into water-soluble allantoin. Presumably, hyperuricemia and hyperammonemia in patients with PSS lead to increased urinary urate and ammonia excretion, which in turn leads to ammonium biurate crystal formation. Patients with a PSS have longer recovery times after surgery if benzodiazepin, barbiturate and phenothiazine have been used. These drugs have a longer half-life because their detoxification requires normal liver function. Clinical signs can be extremely variable at time of presentation making diagnosis confusing.

Laboratory findings

Clinical laboratory findings in patients with PSS are also variable. Fifty percent of dogs and 15% of cats have a microcytic normochromic anemia. Alkaline phosphatase (ALP) and alanine amino transferase (ALT) are usually mildly elevated. Increase in ALP is partially due to bone turnover in young animals. Bilirubin is generally normal as obstruction to bile flow is not present and only a small portion of functional liver is necessary to conjugate free bilirubin. Decrease BUN, secondary to decrease conversion of ammonia to urea, is found in 64% of patients with PSS. Hypoglycemia is a consistent finding due to decreased insulin metabolism by the liver and insufficient hepatic glycogen storage. Portosystemic Shunt may lead to hyperglucagonemia. Hypocholesterolemia is present in 65% of the cases. Protein and specifically albumin levels are reduced in patients with PSS. In one study 90% of dogs had a decreased albumin concentration. Hepatocytes are responsible for albumin synthesis. Because of decreased functional liver capacity in patient with PSS, albumin production is decreased. If hypoalbuminemia is severe enough it could lead to ascites.

Liver function tests are important for the diagnosis of PSS. Bromosulfophtaleine, ammonia tolerance, and bile acids are tests most commonly used. However, bile acids is the only test dependent upon portal systemic vascular flow. The plasma concentration of bile acid is dependent on enterohepatic circulation. Bile acid evaluation is as sensitive as ATT for the detection of circulatory abnormalities. Evaluation of postprandial bile acids improves the diagnostic capabilities of the fasting values for PSS. After 12 hours fasting a venous blood sample is collected for preprandial bile acid concentration. The animal is fed a high protein meal. Serum bile acid concentration is determined 2 hours after the meal. Normal value for fasting bile acids are < 5 microM/l in dogs and < 2 microM/l in cats. Normal postprandial values are < 15.5 microM/l in dogs and < 10.0 microM/l in cats.

Urinalysis may reveal ammonium biurate crystals. Ammonium biurate uroliths are an important diagnostic finding; they have been reported in 50 to 64% of patients with PSS.

Radiographic findings

Ultrasound can be used to diagnose a PSS. Ultrasound is very sensitive for intrahepatic shunt. Abdominal radiographs reveal a generalized decrease in contrast because of the decrease of abdominal fat. Size of the liver is commonly reduced (atrophy) and kidneys enlarged.

Per rectal portal scintigraphy using 99m Technetium Pertechnetate is the method of choice for the diagnosis of PSS. It is a non-invasive technique that tests the integrity of the portal circulation. After an enema, a dose of 1 mCi/kg of 99m Technetium Pertechnetate is placed in the descending colon with a soft plastic pediatric feeding tube. Images are recorded by a gamma-camera immediately after injection and for 3 minutes. In a normal dog, liver uptake of the radioisotope is higher than uptake by the heart. However, when a PSS is present, uptake by the heart is higher and earlier than uptake by the liver. A shunt fraction [the ratio of the heart activity/(liver + heart)activity] gives an idea of the significance of the shunt. Normal dogs have a shunt fraction < 10% while dogs with PSS have a shunt fraction > 50%.


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