Page 747 - Clinical Small Animal Internal Medicine

66 Portosystemic Shunts and Microvascular Dysplasia 715

the hepatocytes, eventually resulting in cell damage and Although congestive heart failure or caudal vena caval
VetBooks.ir death. Cell damage is followed by accumulation of lipid obstruction causes hepatic venous distension, this does
not result in acquired portosystemic shunting because the
within the hepatocytes, cell death and phagocytosis by
macrophages, which then coalesce to form lipogranulo-
cava as well as the hepatic sinusoids and there is hence no
mas. Interstitial fibrosis eventually leading to cirrhosis hydrostatic pressure is increased within the caudal vena
has been observed in some patients with CPSS, but is far hydrostatic drive for the portosystemic shunts to occur.
more likely to be seen in patients with acquired portosys- Ascites resulting from increased hydrostatic pressure
temic shunts resulting from end‐stage hepatopathy of within the caudal vena cava (caval tumors, right‐sided
another etiology. congestive heart failure) tends to be a modified transu-
date due to leakage of protein‐rich hepatic lymph,
whereas ascites resulting from mild to moderate
Pathophysiology of Portal Hypertension
portal hypertension tends to be a transudate. Ascites
Portal hypertension (leading to acquired portosystemic occurring in fulminant portal hypertension may also be
shunts) is by definition an increase in portal venous sanguinous if the capillaries or splenic capsule rupture.
pressure above the expected normal of 8–12 mmHg Portal hypertension is likely to result in splenomegaly,
(note that patients with CPSS will usually have portal with a normal to small‐sized liver, unless the obstruction
pressures comparable or slightly higher than central occurs at the level of the hepatic venules or hepatic
venous pressure (typically 4–8 mmHg). Portal hyper- veins (Budd–Chiari syndrome).
tension occurs when the resistance to vascular flow
within the hepatic portal vein or sinusoids increases,
causing sequestration of blood within the viscera. Epidemiology
Reduction in the arterial to venous pressure gradient
across the capillary beds of the gut causes reduced Signalment
blood flow and hence reduced oxygen delivery to tis-
sues. Alteration of hydrostatic pressure, often accom- Acquired shunts may be seen in any species or breed of
panied by reduced plasma oncotic pressure resulting patient in which diseases causing hepatic fibrosis or cir-
from the patient’s underlying liver disease, allows fluid rhosis occur. A good example of such a disease in dogs is
to leak from the capillaries into both the intestinal chronic hepatitis. A subset of patients with acquired
lumen and the peritoneal cavity, resulting in ascites and shunts resulting from portal hypertension caused by
diarrhea. In acute, severe portal hypertension, frank congenital PVH‐MVD will present before the age of 3
blood may leak into the lumen of the bowel, resulting in years and may be indistinguishable from patients with
hematemesis and hematochezia. Edema and swelling of CPSS (see further discussion in Diagnosis section).
the intraabdominal organs causes stretching of the cap- Congenital shunts are seen most commonly in pure‐
sules and peritoneum, and hence this condition can be breed dogs, or cross‐bred dogs with parents of breeds that
quite painful. are overrepresented amongst the shunt population. Many
Reduction in blood flow through the hepatic sinusoids breeds have been reported as predisposed around the
results in lower venous return to the right atrium, with a world, the most notable being a number of terriers
resultant fall in cardiac output and the compensatory (Yorkshire, cairn, Maltese, Jack Russell), Chihuahuas, min-
changes of tachycardia and reduced stroke volume. iature schnauzers, pugs, toy poodles, Australian cattle dogs,
Hypovolemic shock will eventuate if the compensatory Labrador and golden retrievers, and Irish wolfhounds.
mechanisms are not sufficient to assure adequate tissue In general, extrahepatic shunts are more common in
oxygen delivery, or if ongoing fluid losses into the bowel terriers and other small‐breed dogs, whereas intrahe-
and peritoneal cavity cannot be replaced. patic shunts are more common in medium‐ to large‐
In patients that are able to compensate for mild to breed dogs. However, exceptions occur (e.g., toy and
moderate portal hypertension, small veno‐venous miniature poodles) and some form of shunt imaging is
communications open within a few days. These take recommended prior to surgical exploration. Patients
the path of least resistance from the portal circulation from breeds that are not known to be predisposed to
into a nearby branch of the systemic venous system, CPSS are more likely to “break the rules” and should be
and therefore tend to occur where the vascular sys- evaluated especially carefully.
tems are in closest proximity to one another (root of PVH‐MVD tends to occur in similar breeds to those
the mesentery, esophageal hiatus, perianal region). affected with CPSS, and has been best characterized in
These vessels are thin‐walled and tortuous and usually the terrier‐type breeds such as Maltese, Yorkshire and
cluster in plexi. cairn terriers.