What is hepatic microvascular dysplasia?
Hepatic microvascular dysplasia (sometimes called portal vein hypoplasia) is an inherited abnormality of the liver. In affected dogs, the microscopic blood vessels within the liver are underdeveloped or absent. This decreases blood flow within the liver, causing atrophy (decrease in size) of the liver and its cells. Due to this atrophy and decreased blood flow, the liver is less capable of processing toxins or producing proteins needed for growth and development.
Hepatic microvascular dysplasia is often found in dogs that have other liver problems, such as a portosystemic shunt (a condition that prevents blood from being filtered by the liver). In some cases, however, hepatic microvascular dysplasia exists as an isolated abnormality.
What causes hepatic microvascular dysplasia?
Hepatic microvascular dysplasia is a congenital defect, meaning that animals are born with this condition. The condition is thought to have a genetic basis, although this is not fully understood. Yorkshire Terriers and Cairn Terriers are commonly affected by hepatic microvascular dysplasia. Other affected breeds include Miniature Dachshunds, Maltese, Miniature Poodles, Shih Tzus, Lhasa Apsos, Cocker Spaniels, and West Highland White Terriers.
What are the clinical signs of hepatic microvascular dysplasia?
The clinical signs of hepatic microvascular dysplasia vary significantly. Some dogs show no signs of liver disease, while others are severely affected. Most affected dogs show few clinical signs, and the disease may go undetected until adulthood. Clinical signs are more common and more severe in dogs who also have a portosystemic shunt or other liver abnormalities.
"Most affected dogs show few clinical signs, and the disease may go undetected until adulthood."
Puppies with hepatic microvascular dysplasia may show what is referred to as a “failure to thrive.” They may be abnormally small and slow to gain weight. As they get older, these dogs may demonstrate signs such as vomiting, diarrhea, or pica (eating non-food objects).
Hepatic microvascular dysplasia can also cause urinary problems. Affected dogs may exhibit increased thirst and urination, straining to urinate, or blood in the urine. Urinary stones may develop, and affected dogs may develop frequent urinary tract infections.
Depending on the degree of liver dysfunction, some dogs may develop a condition known as hepatic encephalopathy (neurological dysfunction as a result of liver disease). Dogs with hepatic encephalopathy often show behavioral changes, such as head-pressing, abnormal vocalization, and ataxia (acting wobbly, as if drunk). They may also have seizures.
How is hepatic microvascular dysplasia diagnosed?
Your veterinarian may suspect hepatic microvascular dysplasia, or another liver disease, based on your dog’s medical history, physical examination findings, and laboratory test results. Affected dogs may have elevated liver values. They may also be anemic (low red blood cell count) or have other abnormalities seen in their blood work results. A urinalysis may show abnormally dilute urine, caused by increased thirst and urination associated with the disease.
If your veterinarian suspects liver disease, a pre- and post-meal serum bile acids test may be recommended. This test measures the concentration of bile acids in your dog’s blood before and after a high-protein meal. If the bile acids test is abnormal, the next step may be an abdominal ultrasound.
"...your veterinarian will likely recommend a liver biopsy to assess for the presence of hepatic microvascular dysplasia."
Although hepatic microvascular dysplasia cannot be seen on ultrasound, it does allow your veterinarian to rule out a portosystemic shunt and other liver abnormalities. If no other abnormalities are seen on ultrasound, your veterinarian will likely recommend a liver biopsy to assess for the presence of hepatic microvascular dysplasia. This biopsy may be performed at the same time as the ultrasound, using the ultrasound to guide an instrument into the abdomen to obtain a sample. In some cases, your veterinarian may instead recommend a surgical biopsy of the liver.
Regardless of how the sample is obtained, a biopsy is the only way to definitively diagnose hepatic microvascular dysplasia.
How is hepatic microvascular dysplasia treated?
Hepatic microvascular dysplasia is a condition that is managed, not cured. There is no surgical or medical cure for the disease; therefore, therapy is directed at controlling the clinical signs of this condition. Dogs with asymptomatic microvascular dysplasia do not require any treatment.
"There is no surgical or medical cure for the disease; therefore, therapy is directed at controlling the clinical signs of this condition. "
In dogs with hepatic encephalopathy, treatment typically relies on two major components. First, the protein content of the diet is decreased. This results in the production of decreased quantities of protein breakdown products that contribute to the signs of hepatic encephalopathy. Secondly, antibiotics (e.g., metronidazole, amoxicillin, or neomycin) or other medications (e.g., lactulose) are used to alter the bacterial population within the intestines and further decrease the production of certain protein metabolites. With these measures, the signs of hepatic encephalopathy can often be minimized.
Hepatoprotective supplements intended to protect the liver are also frequently used in the management of hepatic microvascular dysplasia. Examples of commonly used hepatoprotectants include S-adenosylmethionine (SAMe), vitamin E, milk thistle, and ursodeoxycholic acid (Ursodiol). These supplements are thought to decrease ongoing liver damage and help to keep the remaining, functioning liver tissue healthy.
Dogs with hepatic microvascular dysplasia should not be bred, due to the suspected genetic component of this disorder.