Hematologic abnormalities often reveal evidence of marked hepatic dysfunction. Twelve-hour fasting preprandial and two-hour
postprandial serum bile acid concentration tests are preferred for detecting liver dysfunction in patients suspected of having
portosystemic vascular anomalies.2 Samples are easy to obtain because serum harvested from whole blood samples is all that is required. The sensitivity for
preprandial and postprandial serum bile acid concentration tests is 100% for detecting hepatobiliary disease.8
Ammonia tolerance tests and fasting or postprandial blood ammonia concentration tests are less commonly used because samples
must be processed within 20 minutes of the blood draw.2 In fact, Antech Diagnostics does not process ammonia concentrations because of the instability of the samples.9 Ammonia tolerance tests also require that 100 mg/kg of ammonium chloride be given to patients either orally or rectally.
The use of fasting ammonia concentrations and fasting serum bile acid concentrations in identifying portosystemic shunts was
recently compared, and the sensitivity and specificity were 98% and 89.1% for fasting ammonia and 88.1% and 17.9% for fasting
serum bile acids.10 A second study evaluated only postprandial ammonia concentrations as a diagnostic test for detecting hepatic disease. The
sensitivity and specificity for detecting portosystemic vascular anomalies in dogs increased to 100% and 91% six hours postprandially.11 Elevated results in any of the liver function tests indicate hepatic insufficiency but are not specific for hepatic vascular
THE ROLE OF DIAGNOSTIC IMAGING
Diagnostic imaging is the only method to specifically diagnose hepatic vascular disease. The goals of imaging are to confirm
a vascular anomaly, quantify the degree of shunting, and characterize the morphology and location of the anomaly. The information
that diagnostic imaging can provide is critical to the development of a therapeutic plan, including surgical planning when
Differentiating between intrahepatic and extrahepatic portosystemic shunts depends on where the shunt is found to diverge
from the portal vein. If the shunting vessel diverges cranial to T13, then it is usually an intrahepatic portosystemic shunt.
If it is caudal to T13, it is usually an extrahepatic portosystemic shunt.12 Prognosis is also variable depending on the type of vascular disease and severity of shunting.
Available techniques include portovenography, ultrasonography, scintigraphy, computed tomography (CT), and magnetic resonance
imaging (MRI) (Table 1). Each imaging modality varies in its sensitivity and specificity, cost, and availability.
Table 1 Advantages and Disadvantages of Available Techniques for Imaging Hepatic Vasculature
Portovenography is the gold standard diagnostic imaging modality to anatomically diagnose congenital or acquired portosystemic
Intraoperative techniques that have been described include mesenteric portography and transsplenic portovenography.
Figure 1. A normal operative mesenteric portogram. These fluoroscopic images, from left to right, depict the flow of contrast
injected through a jejunal vein into the portal vein. The bottom three images are digitally subtracted to increase the contrast
between the vessels and surrounding tissue. Note the normal arborizing appearance of the portal branches within the liver.
(Figures 1 & 2 courtesy of Philip Steyn, BVSc, MS, DACVR, director of professional services and chief radiologist for Antech
Mesenteric portography is performed through surgical catheterization of a jejunal vein, injection of 2 ml/kg of a nonionic
water-soluble contrast medium such as iohexol, and visualization of the portal venous system by fluoroscopy.14,15 An example of a normal operative mesenteric portogram is shown in Figure 1; an abnormal one is shown in Figure 2.
Figure 2. An operative mesenteric portogram of an extrahepatic portosystemic shunt. By using fluoroscopy, the flow of contrast
is noted entering the azygous vein through a shunt vessel that unites the portal vein and azygous vein. The image on the right
is digitally subtracted to enhance contrast between the vessels and surrounding tissue. Note the absence of flow through the
liver because of the shunt.
Transsplenic portovenography involves surgical catheterization of a splenic venule followed by injection of contrast.16,17
With either technique, the caudal vena cava or azygous vein opacifies before or at the same time as the liver when shunting
is present. The normal arborizing appearance of the intrahepatic portal vasculature is attenuated or absent and may remain
so after corrective ligation in cases of multiple macroscopic portosystemic shunts or primary hypoplasia of the portal vein.7
Operative portovenography of a jejunal vein or splenic vein is invasive but provides excellent imaging of shunting vessels'
morphology and location. The sensitivity of operative portovenography is between 85% and 100%.2 Disadvantages include the requirement of mobile fluoroscopy equipment and laparotomy; however, laparotomy is indicated regardless
for surgical treatment of portosystemic shunts.
Methods of visualizing the portal venous system that do not require laparotomy include percutaneous splenoportography and
cranial mesenteric arterial portography.
Percutaneous splenoportography is accomplished through ultrasound-guided injection of 2 to 4 ml/kg of a nonionic water-soluble
contrast medium into the splenic parenchyma.2 Fluoroscopy is used to visualize venous drainage from the spleen, followed by systemic venous flow in the presence of portosystemic
shunts. One drawback of splenoportography is that mesenteric-systemic shunts located caudal to the splenic vein are not opacified.12,17 Splenoportography has been extensively investigated and reviewed, but its sensitivity and specificity have not been reported.12,17
Cranial mesenteric arterial portography is accomplished through catheterization of a femoral artery and injection of 1.5 to
3 ml/kg of nonionic water-soluble contrast medium.12 The contrast medium opacifies the portal vein, following circulation through the intestinal vasculature.