Page 633 - Small Animal Internal Medicine, 6th Edition

CHAPTER 36 Hepatobiliary Diseases in the Dog 605

grossly at surgery. Scintigraphy can demonstrate shunting portovenography wherever possible (see Chapter 34 for
but is not helpful for differentiating congenital from acquired more details).
VetBooks.ir PSS, so some other imaging method is necessary for treat- Overview of Treatment Options
ment decisions. See Chapter 34 for more information on
and Prognosis
imaging PSSs.
If possible, it is important to try to estimate how well Surgical occlusion of the anomalous vessel to restore normal
developed the remaining hepatic portal vasculature is by portal circulation has long been recommended as the treat-
repeating the portovenography after ligation and/or by eval- ment of choice. In many cases this will restore normal or
uating the histologic findings on liver biopsies taken at the near-normal liver function. However, owners need to be
time of ligation. This is a work in progress, but there is a aware of the small but definite risk of postoperative mortality
strong suspicion that the postligation prognosis may depend as a result of portal hypertension and/or refractory seizures,
on the potential for the intrahepatic vasculature to open up and of the potential that the PSS may be only partially and
after surgery, and dogs that do poorly postoperatively may not totally ligated. It is more common to be able to ligate the
have concurrent portal vein hypoplasia and/or MVD (see PSS partially at the first surgery, because the portal vascula-
later). ture cannot initially accommodate all the shunting blood. In
Nonspecific clinicopathologic findings in more than 50% some cases it is possible to repeat the surgery at a later date
of affected dogs, regardless of the type of vascular anomaly, to ligate the PSS further, but this is often unnecessary to
are microcytosis, hypoalbuminemia, mild increases in serum control clinical signs. A few dogs with partially ligated shunts
AP and ALT activities, hypocholesterolemia, and low blood develop portal hypertension and multiple acquired PSSs
urea nitrogen concentration. Fasting bile acid concentrations with a recurrence of their clinical signs. There are several
may be normal or high, but postprandial bile acid concentra- different surgical procedures described for ligation of PSS,
tions are high in all cases. However, this does not distinguish but they are outside the scope of this text. In addition to
congenital PSS from acquired PSS or early cholestasis, which surgical ligation, a PSS may be attenuated with ameroid con-
also causes increases in bile acid concentration. The post- strictors (Fig. 36.12) or embolized with coils. Laparoscopic
prandial ammonia concentration can also be measured and ligation of PSS has been reported in two dogs (Miller et al.,
will be high, whereas fasting ammonia concentration may be 2006). As a general rule, ligation of a PSS requires an expe-
high or normal (see Box 34.3 for details of how to perform rienced surgeon.
an ammonia challenge test). Ammonia tolerance or chal- Medical treatment is required to stabilize the patient be-
lenge tests are potentially dangerous because they can pre- fore surgery and for about 8 weeks after surgery while the
cipitate an encephalopathic crisis. Other tests have been hepatic vasculature and mass recover. This involves care-
evaluated for their sensitivity and specificity in the diagnosis ful dietary management combined, in many cases, with an-
of PSS. The level of protein C, a liver-derived anticoagulant, tibiotics and soluble dietary fiber. In some cases medical
is also decreased in dogs with PSS and increases after liga- management may continue successfully over the course of
tion; this can help differentiate PSS from MVD.
Puppies of high-risk breeds could be screened for con-
genital PSS by measuring bile acid or ammonia concentra-
tions before they are placed into homes, but there are
potential false-positives with both of these tests; no puppy
should be euthanized or labeled as having a definite con-
genital PSS on the basis of a high bile acid and/or ammonia
concentration without further evidence. Normal Irish Wolf-
hounds can have a transiently high blood ammonia concen-
tration between the ages of 6 to 8 weeks; this normalizes at
3 to 4 months of age. Zandvliet et al. (2007) have demon-
strated that this is caused by a clinically insignificant urea
cycle defect. Postprandial bile acid concentrations can be
falsely elevated in Maltese puppies without PSS for unknown
reasons, again confusing any efforts at screening tests in this
breed (Tisdall et al., 1995).
On diagnostic imaging, the liver is frequently but not
always small. Ultrasonography now has a high sensitivity
and specificity for the diagnosis of both intrahepatic and FIG 36.12
extrahepatic PSS; furthermore, their anatomy can usually Lateral abdominal radiograph of a 3-year-old Miniature
also be described ultrasonographically. Bubble studies may Schnauzer that had an extrahepatic portosystemic shunt
ligated with an ameroid constrictor 2 years previously. Note
help visualization of a PSS with ultrasonography (Gómez- that the ameroid is visible as a radiodense ring in the
Ochoa et al., 2011). If the shunting vessel cannot be fully craniodorsal abdomen. (Courtesy Diagnostic Imaging
visualized or characterized by ultrasonography, CT angiog- Department, Queen’s Veterinary School Hospital, University
raphy is now the imaging technique of choice, replacing of Cambridge, Cambridge, England.)