Katharine Arnell, DVM, DACVIM
Veterinary Specialty Hospital, San Diego, CA
Posted on 2016-08-30
The two most common types of congenital hepatic disease include portosystemic vascular anomalies (PSVA) and microvascular dysplasia (MVD, now technically termed portal vein hypoplasia without a macroscopic anomaly). The clinical course and treatment options differ depending on the underlying disease, and an accurate diagnosis is essential for future management and prognosis. Most affected dogs will have a mildly but variably elevated ALT, and may be symptomatic (variable for PSVA) or asymptomatic (almost always true for MVD, and variable for PSVA). The following tests are commonly utilized to differentiate between these two congenital diseases.
Pre- and post-prandial bile acids test
Bile acids are produced by the liver, secreted into the biliary system, released into the duodenum to help digest fats, then resorbed by the ileum into the portovenous circulation, and re-excreted into bile. This test is easy to perform and only requires a pre-testing 12-hour fasting period and two blood samples drawn two hours apart. We are most concerned about the post-prandial bile acids result, with dogs having hepatobiliary disease typically showing elevated (>25 umol/L) levels. Normal bile acids concentrations levels do not rule out hepatobiliary disease, although most dogs with PSVA will have markedly elevated post-prandial concentrations (usually >100 umol/L) and dogs with MVD will have mild to moderately elevated concentrations. Occasionally, the pre-prandial bile acids levels are higher than the post-prandial levels (this has no diagnostic significance; just go by the highest value); this is likely due to inadequate pre-testing fasting, spontaneous gallbladder contraction, differing gastric emptying or intestinal transit times, or varying cholecystokinin release. Results can also be affected by exogenous bile acids, mainly ursodeoxycholic acid (ursodiol), which need to be discontinued for at least 48 hours before measure serum bile acids. Finally, Maltese and Maltese mixes may have elevated post-prandial bile acids in the absence of hepatic disease; this is likely due to substances within their blood that interfere with the assay.
Abdominal ultrasound exam
The sensitivity of ultrasound exams for differentiating between a PSVA or MVD is largely dependent on the ultrasonographer/machine, however, this can be a relatively sensitive test (up to 97% sensitivity in experienced hands). Ultrasound is used to visualize a definitive extra- or intra-hepatic PSVA, as well as assess for microhepatica, decreased hepatic vascularity, renomegaly, nephroliths or cystoliths (most commonly ammonium urate), acquired hepatic shunts, and abdominal effusion. Two ultrasonographic measurements have been shown to help differentiate between a PSVA and MVD; a PV:Ao (portal vein to aorta) ratio ≥ 0.65 is most consistent with an extra-hepatic portosystemic shunt, and a PV:CVC (portal vein to caudal vena cava) ratio ≥ 0.75 helps to rule out an extra-hepatic PSS.
Protein C activity assay
This assay is now available through Cornell University and helps to further differentiate between dogs with PSVA and MVD. Protein C is a plasma anticoagulant factor synthesized by the liver, and protein C deficiency can be a marker for hepatic dysfunction. Decreased protein C activity develops in dogs with PSVA and hepatic synthetic failure, and a level < 70% supports a diagnosis of PSVA, whereas activity greater than 70% are more typically seen in dogs with MVD. The protein C level requires 1 mL separated citrated plasma, and the laboratory cost is $37.50. Samples are run daily Mon-Fri, so the turn-around time is fairly quick.
Transplenic or rectal scintigraphy
These tests involve a nuclear scan that measures radioactive uptake by the heart and liver after the technetium pharmaceutical is given by rectal enema or a splenic injection.
A shunt fraction is calculated after determining the relative uptake. Normal dogs have a shunt fraction < 15% ( > 85% of the blood goes to the liver first rather than the heart), whereas dogs with a PSVA typically have a shunt fraction > 65% ( < 35% of the blood goes to the liver first rather than the heart). Usually, only mild sedation at most is required. Nuclear scintigraphy can determine whether or not a PSVA is present with very high sensitivity, though does not provide anatomic detail.
Abdominal CT scan
This is the most sensitive test for the evaluation of PSVA. It does require anesthesia and is the most costly test to perform, however, it can accurately determine if a PSVA is present and elucidate the specific location by providing anatomic detail; this is essential to assess if surgery is an option and for surgical planning.
Ultimately, a liver biopsy is the only means of determining the specific etiology for elevated hepatic enzymes and hepatic function tests when a macroscopic shunt is ruled out. A liver biopsy cannot differentiate between a PSVA and MVD, so the tests above are always recommended if there is a concern for a PSVA.
About the author
Dr. Arnell has clinical interests in all aspects of small animal veterinary internal medicine with special interests in gastroenterology, nephrology, and immune-mediated disease. She is skilled in ultrasonography and endoscopic procedures.