Myxomatous Valve Degeneration

The liver carries out many essential tasks that keep animals healthy, including detoxifying harmful substances, storing glucose, and producing proteins. Most blood reaching the liver arrives through the portal vein, which drains the intestines, stomach, pancreas, and spleen. Inside the liver, the portal vein divides into progressively smaller vessels so blood can flow through the tissue and reach individual liver cells.

When these microscopic portal vessels are abnormal on a liver biopsy, the finding is termed hepatic microvascular dysplasia (HMD/MVD) or portal atresia. If these tiny vessels are underdeveloped or absent, the liver may become small (atrophic) and less able to clear toxins or make proteins needed for growth and normal function.

Importantly, HMD/portal atresia is a histologic (biopsy-based) description, not a single cause. Similar biopsy changes can be seen with other conditions, including congenital portosystemic shunts (PSS). When the biopsy pattern is present without evidence of a congenital shunt, clinicians often use “HMD” as the working disease diagnosis.

Dogs with HMD can have signs and lab abnormalities resembling those seen with congenital PSS, but many dogs have no obvious clinical signs. When signs occur, they may not appear until 3–4 years of age.

Possible findings include:

• smaller-than-expected size and poor muscle development
• subdued behavior or dullness (thought to be related to circulating toxins affecting the brain)
• reduced appetite
• intermittent vomiting and diarrhea
• increased susceptibility to infections
• development of urinary bladder stones
• in more severe cases: wobbliness (“drunken” gait), blindness-like behavior, or seizures
• rarely: abdominal fluid accumulation associated with liver failure

Breeds most often reported include Yorkshire Terriers and Cairn Terriers, but HMD is also described in other small breeds such as Maltese, Dachshund, Miniature Poodles, Shih Tzu, Lhasa Apso, Cocker Spaniel, and West Highland White Terrier.

Some affected dogs have normal routine bloodwork. In more severe cases, blood tests may show low:

• total protein and/or albumin
• glucose
• blood urea nitrogen

Liver enzymes may be increased in some dogs. Urinalysis is used to look for infection and crystals; rarely, dogs may form ammonium biurate crystals, described as spiky “balls” or starfish-like shapes.

Bile acids testing is performed after an overnight fast (preprandial) and again about 2 hours after eating (postprandial). In HMD, one or both bile acid measurements may be elevated. However, elevated bile acids are not specific because they can increase with many liver disorders.

A definitive HMD diagnosis requires demonstrating two things:

• No portosystemic shunt is present, and
• Microscopic portal vascular abnormalities are present on liver biopsy.

Dogs with HMD typically have normal portal blood flow on imaging studies such as nuclear scintigraphy, portography, or CT angiography, yet show the characteristic microvascular changes on biopsy. Biopsies are usually obtained via an abdominal incision or laparoscopy to ensure adequate tissue; ultrasound-guided needle biopsies may not provide enough sample for reliable assessment of the portal vessels.

HMD must be distinguished from congenital PSS. Some dogs can have both disorders, and this may not be identifiable before surgery. If a dog has had surgical attenuation of a congenital shunt but bile acids remain high 3–6 months later, concurrent HMD is a reasonable possibility. For this reason, a liver biopsy is commonly recommended during shunt surgery.

Compared with shunt patients, dogs with HMD are often diagnosed later (about 2–5 years, rather than <1 year) and may show milder lab changes. Some may have normal fasting bile acids with elevated postprandial values.

Figure 1. Example of a Yorkshire Terrier that underwent surgery for a congenital portosystemic shunt but continued to have abnormal bloodwork, poor muscle development, coat changes, and episodic weakness/confusion; later diagnosed with HMD.

There is no surgical correction for HMD. Management is medical, tailored to severity. Some dogs require no treatment.

Core management is usually dietary protein modification—typically a mildly protein-restricted diet using highly digestible protein sources (commercial hepatic diets are commonly used). The text describes target diet composition on a dry matter basis, including approximate protein targets and additional micronutrient considerations (e.g., zinc/Vitamin E emphasis and low manganese). Many dogs do well with diet change alone.

Reducing toxin production/absorption in the intestines may help. Approaches described include:

• lactulose (to alter colonic environment and reduce toxin absorption)
• sometimes short-term antibiotics
• dietary measures such as yogurt (as described in the source text)

Supportive supplements may be recommended. Examples mentioned include:

• milk thistle (silymarin) (with a preference for veterinary-formulated products)
• SAM-e (e.g., Denosyl) as prescribed by veterinarians

Overall prognosis is good for most dogs. Many are clinically normal with medical management and can have normal life spans. Dogs with persistent gastrointestinal signs or partial/focal seizures may show limited improvement, possibly due to additional concurrent disease.

In some cases, HMD can progress to liver failure. A small subset may deteriorate rapidly and die within 4–6 months of diagnosis when liver disease is severe.

HMD/portal atresia is described as hereditary. Dogs with abnormal bile acids, and dogs born to parents with abnormal bile acids, should not be bred.

Urinary bladder stones can occur in rare cases and may require medical and/or surgical management.