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Portosystemic Shunts


Pathophysiology

Clinical signs associated with portosystemic shunts commonly involve the nervous system, gastrointestinal tract, and urinary tract. General clinical signs include poor growth rate, weight loss, fever, and anesthetic or tranquilizer intolerance. Neurologic dysfunction is seen in most animals with PSS and includes lethargy and depression, ataxia, seizures, behavioral changes, and blindness.   Head pressing, circling, and development of a head tilt have also been reported.  Gastrointestinal clinical abnormalities include anorexia, vomiting, and diarrhea. Some dogs have no apparent signs or present with signs of cystitis or urinary tract obstruction. Many cats have hypersalivation and some have unusual copper colored irises.

Abnormalities found on hemograms of animals with PSS include leukocytosis, anemia, and microcytosis. Most animals with congenital PSS have normal coagulation profiles.  Biochemical abnormalities associated with PSS include decreases in blood urea nitrogen, protein, albumin, glucose, and cholesterol; and increases in serum alanine aminotransferase and alkaline phosphatase. Increase in alkaline phosphatase is most likely from bone growth, since cholestasis is not usually a problem in animals with shunts. Cats with PSS usually have normal albumin concentrations. Urinalysis abnormalities include low urine specific gravity and ammonium biurate crystalluria. At magnifications of 400x or more, ammonium biurate crystals often have a spikey, thornapple or starfish shape and golden color. Because of increased urinary excretion of ammonia and uric acid, dogs and cats may also develop uroliths. Urate uroliths are often radiolucent and therefore may not be detectable on survey radiographs unless they are combined with struvite. Abnormal urine sediment suggestive of cystitis (hematuria, pyuria, and proteinuria) has been described in animals with PSS and may be associated with crystalluria or urolithiasis.

Hepatic histologic changes in animals with PSS include generalized congestion of central veins and sinusoids, lobular collapse, bile duct proliferation, hypoplasia of intrahepatic portal tributaries, proliferation of small vessels and lymphatics, diffuse fatty infiltration, hepatocellular atrophy, and cytoplasmic vacuolization. These pathology changes can also be seen in dogs with hepatic microvascular dysplasia that do not have single congneital shunts. Pathologic changes may be present in the central nervous system, especially in encephalopathic animals.

Hepatic Encephalopathy

Hepatic encephalopathy has been recognized in animals with PSS, end-stage liver disease, and congenital urea cycle enzyme deficiencies. Clinical signs include depression, dementia, stupor, and coma. Muscle tremors, motor abnormalities, and focal and generalized seizures have also been reported. The etiology of hepatic encephalopathy is probably dependent on several factors, including circulating toxins, alterations in amino acid concentrations, and increased cerebral sensitivity to drugs and toxins. Toxins that have been implicated in hepatic encephalopathy include ammonia, mercaptans, short chain fatty acids, indoles, aromatic amino acids, and biogenic amines.

Precipitating factors of hepatic encephalopathy include diuretics, protein overload, hypokalemia, alkalosis, and transfusion of stored red cells, hypoxia, hypovolemia, gastrointestinal hemorrhage, infection, and constipation. Increased cerebral sensitivity to sedative, analgesic, and anesthetic agents may induce coma in animals with PSS, even when normal dosages are used. Protein overload and gastrointestinal hemorrhage provide substrates for bacterial production of ammonia, and constipation can increase retention and absorption of ammonia and other encephalopathic substances. Blood which has been stored for 24 hours contains 170 ug of ammonia/dL, and ammonia concentrations will continue to increase with prolonged storage.

 

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