Portal Vein Hypoplasia

The liver carries out many essential functions in animals, such as detoxifying harmful substances, storing sugars, and producing proteins. Most blood delivered to the liver for these tasks comes 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 pass through the liver tissue and reach the hepatocytes.

When these tiny vessels appear abnormal on a liver biopsy, the finding is termed hepatic microvascular dysplasia (HMD / MVD) or portal atresia. If the microscopic vessels are poorly formed or missing, the liver becomes small (atrophic), and the animal may lose the ability to properly detoxify and to synthesize proteins needed for growth and normal function.

HMD/portal atresia is a histologic (biopsy-based) description rather than a single cause. Multiple conditions can produce the same microscopic changes, including congenital portosystemic shunts. When biopsy changes are present without evidence of a congenital shunt, dogs are often categorized as having HMD as a distinct disease.

Dogs with HMD may show clinical signs and lab abnormalities similar to those seen with congenital portosystemic shunts, but many dogs have no obvious signs. When signs do occur, they often appear when dogs are around 3–4 years old. Some affected dogs are smaller than expected and have reduced muscle development. Toxins affecting the brain may make them seem quieter or less responsive. Possible signs include reduced appetite, intermittent vomiting and diarrhea, increased susceptibility to infections, and development of bladder stones. Severely affected dogs may appear wobbly, act intoxicated, seem blind, or have seizures. In uncommon cases, liver failure can lead to abdominal fluid accumulation.

Yorkshire Terriers and Cairn Terriers are most frequently affected, but HMD is also reported in other small breeds including Maltese, Dachshunds, Miniature Poodles, Shih Tzus, Lhasa Apsos, Cocker Spaniels, and West Highland White Terriers.

In some dogs, routine biochemical testing may remain normal. In more severe cases, bloodwork can show low total protein, albumin, glucose, and blood urea nitrogen because the liver is not producing adequate amounts. Liver enzymes may be elevated. Urinalysis is used to check for infection and crystals; rarely, dogs with HMD form ammonium biurate crystals that can resemble spiky balls or starfish.

Bile acids are typically measured after fasting (preprandial) and again about two hours after eating (postprandial). With HMD, one or both values may be increased. However, bile acids can rise with many liver disorders, so elevated bile acids are not specific to shunts or HMD.

A definitive HMD diagnosis requires demonstrating no shunt is present while confirming microscopic portal vessel abnormalities on biopsy. Dogs with HMD can have normal portal blood flow on tests such as nuclear scintigraphy, portography, or CT angiography, yet still show abnormal portal microvasculature on biopsy. Biopsy is usually obtained surgically via an abdominal incision or laparoscopic approach to ensure adequate tissue; ultrasound-guided needle biopsies may not provide enough sample for diagnosis.

HMD must be distinguished from congenital portosystemic shunts, but some dogs may have both conditions, and this may not be clear prior to surgery. If a dog has shunt surgery and bile acids remain elevated 3–6 months later, concurrent congenital HMD is a possibility—one reason liver biopsy is recommended during shunt-attenuation procedures. Compared with shunt patients, dogs diagnosed with HMD are often older (about 2–5 years rather than under one year) and may have milder lab changes. Fasting bile acids can be normal, while postprandial bile acids are more often elevated.

There is no surgical correction for HMD. Management is medical and depends on severity; some dogs require no treatment. The main approach is dietary protein reduction. Prescription liver diets (e.g., Hill’s L/d) use highly digestible proteins (often milk- or soy-based) and are only mildly protein-restricted. Diets for HMD are described as containing roughly 15–20% protein (about 2 g/kg/day), 15–30% fat, and 30–50% highly digestible carbohydrates on a dry matter basis, with higher zinc and vitamin E and lower manganese. Many dogs improve with diet change alone.

Reducing intestinal toxin production and absorption can also help by altering gut bacteria, for example using lactulose syrup or yogurt. Short courses of antibiotics may be prescribed.

Some nutraceuticals may support liver function. Milk thistle (silymarin) is used to support liver function and regeneration; because over-the-counter products are not government-regulated, veterinary-formulated supplements are recommended. Examples mentioned include Nutramax (“Marin”) and RxVitamins (“Hepatosupport”). Veterinarians may also prescribe Denosyl (SAM-e) to support liver function.

Overall prognosis is good for most dogs with HMD. With medical management, many dogs remain clinically normal and can have normal life spans. Dogs that have gastrointestinal signs or partial/focal seizures may not improve, possibly because other conditions are present alongside HMD. In some cases HMD can progress to liver failure, and a small number of dogs may die within 4–6 months after diagnosis due to severe liver disease.

HMD/portal atresia is considered hereditary. Dogs with abnormal bile acids should not be bred, and offspring from parents with abnormal bile acids also should not be bred.

Rarely, urinary stones can occur and may require medical or surgical management.