Portal Vein Atresia

The liver carries out a vast array of essential functions that are vital for maintaining animal health, including detoxification of harmful substances, storage of glucose, and synthesis of proteins. The majority of blood reaching the liver for these processes is delivered through the portal vein, which drains blood from the intestines, stomach, pancreas, and spleen. Inside the liver, the portal vein divides into progressively smaller vessels, allowing blood to flow through the liver tissue and reach individual liver cells.

When these microscopic portal vessels appear abnormal on liver biopsy, the condition is referred to as hepatic microvascular dysplasia (HMD or MVD), also known as portal atresia. If these tiny vessels are poorly developed or absent, the liver becomes reduced in size (atrophic), and the animal loses the ability to effectively detoxify the blood or produce proteins required for normal growth and physiological function.

Hepatic microvascular dysplasia or portal atresia is a histopathologic diagnosis, meaning it describes the microscopic findings observed in liver biopsy samples rather than a single specific disease. A variety of disorders can lead to these histologic changes, including congenital portosystemic shunts. However, when abnormal liver microvasculature is identified without evidence of a congenital shunt, dogs are typically diagnosed with HMD as a distinct clinical condition.

Dogs affected by HMD may exhibit clinical signs and laboratory abnormalities similar to those seen in dogs with congenital portosystemic shunts; however, many dogs remain asymptomatic. Clinical signs often do not appear until the dog is 3 to 4 years of age. Some affected dogs are smaller than average and show poor muscle development. Neurologic signs such as reduced alertness, decreased intelligence, or a quieter demeanor may be observed due to the effects of circulating toxins on the brain.

Additional signs may include decreased appetite, intermittent vomiting, and diarrhea. Some dogs are more prone to infections or may develop urinary bladder stones. In more severe cases, dogs may show neurologic dysfunction such as unsteady gait, disorientation, apparent blindness, or seizure activity. Rarely, dogs may develop abdominal distension due to fluid accumulation associated with liver failure.

Yorkshire Terriers and Cairn Terriers are the breeds most frequently affected, although HMD has also been reported in many other small-breed dogs, including Maltese, Dachshunds, Miniature Poodles, Shih Tzus, Lhasa Apsos, Cocker Spaniels, and West Highland White Terriers.

In some dogs, routine biochemical blood tests may be within normal limits. In more severely affected individuals, bloodwork may reveal decreased concentrations of total protein, albumin, glucose, and blood urea nitrogen, reflecting impaired hepatic synthesis. Elevated liver enzyme levels may also be present in certain cases.

Urinalysis is performed to assess for infection and crystal formation. On rare occasions, dogs with HMD develop ammonium biurate crystals in the urine, which characteristically appear as spiky spheres or star-shaped structures.

Serum bile acid concentrations are measured after an overnight fast (preprandial) and again approximately two hours after feeding (postprandial). In dogs with HMD, one or both bile acid measurements are elevated. Because increased bile acids can occur in many liver disorders, this finding is not specific for either congenital portosystemic shunts or HMD.

A definitive diagnosis of HMD requires confirmation that no portosystemic shunts are present, along with identification of abnormal intrahepatic portal vessels on liver biopsy. Dogs with HMD demonstrate normal portal blood flow on nuclear scintigraphy, portography, and CT angiography, but microscopic examination of liver tissue reveals abnormal portal vasculature. Liver biopsy samples are typically obtained surgically through an abdominal incision or via laparoscopy to ensure adequate tissue is collected for vascular evaluation. Ultrasound-guided needle biopsies may not provide sufficient tissue for definitive diagnosis.

HMD must be distinguished from congenital portosystemic shunts; however, some dogs may suffer from both conditions simultaneously, a distinction that cannot always be made prior to surgery. If a dog undergoes surgical attenuation of a congenital portosystemic shunt and bile acid concentrations remain elevated 3 to 6 months postoperatively, concurrent congenital HMD is highly suspected. For this reason, liver biopsy is recommended at the time of shunt surgery.

Dogs with HMD are generally diagnosed at an older age than dogs with congenital shunts (typically between 2 and 5 years of age rather than under one year), and their laboratory abnormalities are often milder. In some cases, fasting bile acid levels may be normal, while postprandial bile acids are usually increased.

There are no surgical treatment options for hepatic microvascular dysplasia. Management is strictly medical and tailored to the severity of the disease. Some dogs require no treatment at all.

Dietary modification is the cornerstone of medical management, with emphasis on reducing protein intake. Commercial veterinary diets formulated for liver disease, such as Hill’s L/d, are commonly recommended. These diets contain highly digestible protein sources (often milk-based or soy-based) and are only mildly protein restricted. Appropriate diets for dogs with HMD typically contain approximately 15–20% protein (about 2 g/kg/day), 15–30% fat, and 30–50% highly digestible carbohydrates on a dry matter basis. They should also be enriched with zinc and vitamin E and contain reduced levels of manganese. Most dogs with HMD respond well to dietary management alone.

Reducing toxin production and absorption can also be achieved by altering the intestinal bacterial population. This may be accomplished through administration of lactulose syrup or yogurt. Short-term antibiotic therapy may also be prescribed by the veterinarian when indicated.

Nutraceuticals—substances not classified as pharmaceutical drugs—may further support liver function. Milk thistle (silymarin) has been shown to promote liver regeneration and improve hepatic function. Because over-the-counter supplements are not regulated, the use of veterinary-formulated products is recommended. Veterinary brands offering milk thistle supplements include Nutramax (Marin) and RxVitamins (Hepatosupport). In addition, veterinarians may prescribe Denosyl (SAM-e) to enhance liver function.

The prognosis for dogs with HMD is generally favorable. With appropriate medical management, most dogs remain clinically normal and many enjoy a normal life expectancy. However, dogs presenting with gastrointestinal signs or partial or focal seizures may show limited or no improvement, possibly due to concurrent diseases unrelated to HMD.

In some cases, HMD can progress to liver failure, and a small number of dogs may succumb to severe liver disease within 4 to 6 months following diagnosis.

Hepatic microvascular dysplasia or portal atresia is considered a hereditary condition. Dogs with abnormal bile acid levels should not be bred, and dogs born to parents with abnormal bile acids should also be excluded from breeding programs.

Additionally, although uncommon, urinary calculi (bladder stones) may develop and may require either medical or surgical treatment.