Hepatic Microvascular Dysplasia or Portal Atresia: Client Handout 2006
Karen Tobias, DVM, MS, Diplomate ACVS
Overview of the condition
The liver performs an incredible number of functions to maintain health of animals, including filtering out toxins, storing
sugar, and making proteins. Most of the blood that is carried to the liver for these processes arrives via the portal
vein, which drains the intestines, stomach, pancreas, and spleen. Within the liver, the portal vein branches into smaller
and smaller vessels so that the blood can percolate throughout the tissues to each liver cell. When these microscopic
vessels are abnormal on liver biopsy, the condition is called “hepatic microvascular dysplasia (HMD or MVD)” or
“portal atresia”. When the microscopic vessels within the liver are underdeveloped or absent, the liver becomes small
(“atrophied”) and the animal can no longer process toxins or make proteins necessary for growth and normal function.
Hepatic microvascular dysplasia (HMD) or portal atresia is a histologic diagnosis, meaning it only describes the biopsy
findings. In fact, there are many conditions that can cause these findings, including congenital portosystemic shunts;
however, when the diagnosis is made without evidence of a congenital shunt, then the dogs are often given the
diagnosis of HMD as a specific disease.
Signs/Clinical presentation
Dogs with HMD can present with signs similar to dogs with congenital portosystemic shunts; however, many dogs have
no clinical signs at all. Often affected dogs are 3 to 4 years old before they have clinical signs. Some affected dogs
are smaller than normal, with poor muscle development. They may seem less intelligent or quieter because of the
toxins that depress their brains. They may have a loss of appetite or occasional bouts of vomiting and diarrhea. Some
dogs may have a greater risk of infections or develop bladders stones. Severely affected dogs may be wobbly or act
drunk or blind and can even seizure. Rarely, dogs will develop fluid filled bellies from liver failure.
Risk Factors
Yorkshire terriers and Cairn terriers are most commonly affected, but the condition is also seen in many other small
breeds, including Maltese, dachshund, miniature poodles, Shih tzu, Lhasa apso, cocker spaniel, and West Highland
white terriers.
Diagnostic tests
In some dogs, basic biochemical tests are normal. Severely affected dogs may have low blood protein, albumin,
glucose, and urea nitrogen levels because their livers are not making enough of these chemicals. Some dogs have
increased liver enzymes. Urine is evaluated for evidence of infection and crystals; rarely, dogs with HMD will develop
ammonium biurate crystals in the urine that look like spiky balls or starfish.
Bile acids are measured after an overnight fast (“preprandial” or fasting) and then 2 hours after eating
(“postprandial”). In dogs with HMD, one or both sets of bile acids are increased. Bile acids can increase with any liver
disease, so high bile acids are not specific to congenital portosystemic shunts or HMD.
A definitive diagnosis of HMD is made by proving that the dogs do not have any shunts but that they do have
abnormal vessels on their liver biopsies. Dogs with HMD have normal portal blood flow on scintigraphies (nuclear
scans of liver blood flow), portograms (x-ray studies of liver blood flow), and CT angiograms (Cat scans of liver blood
flow), but they have abnormal microscopic portal blood vessels on liver biopsy. The liver biopsy is usually taken
surgically through a belly incision or with a laparoscope so that enough liver tissue can be obtained to evaluate the
blood vessels. Needle biopsy using ultrasound may not provide enough tissue to make the diagnosis.
Differential diagnoses
HMD must be differentiated from congenital portosystemic shunts; unfortunately some dogs can have both diseases,
and this cannot be determined before surgery (Figure 1). If a dog undergoes surgical closure of a congenital
portosystemic shunt and its bile acids remain high 3-6 months after surgery, it is quite possible it also had congenital
HMD. Dogs with HMD are usually older than dogs with shunts when they are diagnosed (2 to 5 years instead of less
than one year), and often their blood work changes are less
severe than dogs with shunts. They may even have normal fasting bile acids, but usually their postprandial bile acids
are increased.
HMD must also be differentiated from other conditions that cause seizures (epilepsy, hydrocephalus, toxins, etc.) or
liver disease (viral or bacterial infections, chronic active hepatitis).
Treatment Options
There is no surgical treatment for HMD. Dogs with the condition are managed medically, and treatment is based on
the severity of the condition. In some dogs no treatment is needed. The mainstay of medical management is to reduce
the amount of protein in the diet. Specific veterinary diets such as Hill’s L/d have been formulated for dogs with liver
disease. The protein is highly digestible (often milk based or soy) and is only mildly protein restricted. Diets for dogs
with HMD should contain about 15-20% protein (roughly 2 g/kg per day of protein), 15-30% fat, and 30-50% highly
digestible carbohydrates on a dry matter basis. They should also be high in zinc and Vitamin E and low in manganese.
Most dogs with HMD do well on diet change alone.
Changing the type of bacteria that live in the intestines can also decrease toxin production and absorption. This can
be accomplished by giving lactulose syrup or yogurt. Some veterinarians may prescribe antibiotics for a short time as
well.
Nutriceuticals- compounds that are not considered “drugs”- can also improve liver function. Milk thistle (“silymarin”)
can help improve liver function and regeneration. Because the government does not regulate over-the-counter
compounds, purchase of specially formulated veterinary supplements is recommended. Two veterinary companies
that sell milk thistle include Nutrimax (“Marin”) and RxVitamins (“Hepatosupport”). Veterinarians may also prescribe
Denosyl (SAM-e) to improve liver function.
When to seek referral
Specialized tests that are used to diagnose HMD and rule out portosystemic shunts are often not available in general
veterinary practices. Your veterinarian may consider referral for scintigraphy, portography, ultrasonography, or
surgical or laparoscopic liver biopsies to rule out other liver diseases. Additionally, dogs that do not improve with
dietary management (reduced protein in the diet), particularly dogs with seizures or vomiting and diarrhea, may have
other conditions that are actually causing the clinical signs. These veterinarians should be seen by and ACVIM
Veterinary Internist or Neurologist for further evaluation.
Prevention and prognosis
Hepatic microvascular dysplasia or portal atresia is a hereditary condition. Dogs with abnormal bile acids should not
be bred, and dogs that come from parents with abnormal bile acids should also not be bred.
Prognosis is good for most dogs with HMD. Most dogs are clinically normal with medical management and many have
normal life spans. Dogs with gastrointestinal signs or partial or focal seizures, however, may show no improvement,
possible because they have other diseases besides HMD. Occasionally dogs with HMD can progress to liver failure,
and a few dogs will die within 4-6 months of diagnosis because of the severity of their liver disease. At this time, there
is no way to predict which animals will have normal lifespans; however, we expect that dogs without neurologic signs
and that have normal protein production are more likely to live long lives, particularly if they do not get other liver
diseases (or medications that cause liver disease) as they get older.
Milk Thistle dosage: Dried herb: 15-20mg/pound once a day; concentrated or alcohol extract: 2-5 mg/pound every 8-
12 hours. Veterinary formulations are recommended since they have been tested for safety and quality; to find the
correct dose for your dog, see the manufacturer’s recommendations.
Hepatic encephalopathy
Hepatic encephalopathy is brain and nervous system damage that occurs as a complication of liver disorders. It causes
different nervous system symptoms including changes in reflexes, and behavior changes that can range from mild to
severe.
Watch for signs of Hepatic Encepthalopathy (H.E.):
drooling
vomiting
Pacing
circling
inability to sit still for any length of time and drinking lots of water
a stuperous look
head pressing
staggering
listlessness
runny nose
blank look
stare
lethargy
Not all dogs display all the symptoms, but you will get to know the ones your dog displays. It is best to head it off
early although you cannot always do this.
General Information on signs
and symptoms of Liver Shunt
and MVD
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