How Can You Tell with Ultrasound that a Patient with High Blood Ammonia Level has a Congenital or Acquired Portosystemic Shunt or No Shunt at All?
*Utrecht University, Faculty of Veterinary Medicine, Division of Diagnostic Imaging
Elevated blood ammonia level in dogs and cats can be caused by congenital or acquired portosystemic shunts, and urea-cycle enzyme deficiencies. Only congenital portosystemic shunt requires surgery, thus its non-invasive differentiation from the other conditions is essential. The purpose of this study was to assess the value of abdominal ultrasonography in the exclusion, or detection and characterization of portosystemic shunts in dogs and cats with elevated blood ammonia levels.
Of the patients that underwent abdominal ultrasonography because of elevated blood ammonia level at the Division of Diagnostic Imaging in Utrecht between May 2000 and April 2002 those were selected in which sufficient additional information was available (e.g., from surgery, liver biopsy, necropsy) to diagnose or exclude a portosystemic shunt. During this period, 87 canine and 8 feline patients met these criteria. Portosystemic shunt was diagnosed if the abnormal vessel was ultrasonographically visualized, and ruled out when no shunt was found and the rest of the abdomen was ultrasonographically normal.
Congenital shunt was diagnosed by ultrasound in 63 dogs (in one only the secondary changes were found), and was confirmed during surgery in each case. Acquired shunts were diagnosed by ultrasound in 11 cases: in 9 dogs liver biopsies revealed cirrhosis, fibrosis, or portal vein hypoplasia, and in two arterio-portal fistula was diagnosed. In 13 dogs no abnormalities were found by ultrasound: in 10 urea-cycle enzyme deficiency was suspected. Ultrasonography of the 8 cats revealed congenital shunt in 3, and this was confirmed during surgery. Acquired shunts were diagnosed in 4 cats, and the diagnosis was confirmed either by ultrasound (a tumour that compressed the portal vein in one and arterio-portal fistula in another), or by ultrasound and liver biopsies (polycystic kidney disease with liver fibrosis). In 3 dogs and 1 cat with normal abdomen the ammonia level was falsely high.
Ultrasonography was sensitive and specific not only in excluding portosystemic shunts, but also in differentiating congenital from acquired shunts. The shunt was considered as congenital if it arose from the right or left portal branch or from the gastroduodenal vein. If it arose from the portal trunk and could be followed to the point where it entered the thorax or the caudal vena cava, it was also considered as congenital. An acquired shunt may also arise from the portal trunk, but the point where it enters the thorax or the caudal vena cava cannot usually be seen. The most frequently found acquired shunts were the splenorenal collaterals, which were seen by ultrasound as a wide vein (i.e., the dilated left gonadal vein) that runs parallel to the caudal vena cava and enters the left renal vein from caudal. A double caudal vena cava may look alike, but these are symmetric, moreover not tortuous.