Hepatic Microvascular Dysplasia

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Also known as: Intrahepatic Portovascular Dysplasia — MD

See also: Portosystemic Shunt


Microvascular dysplasia (MD) is a congenital defect of the hepatic vasculature that results in abnormal communication between the portal and systemic venous circulation. Unlike portosystemic shunting (PSS), microvascular dysplasia is not grossly evident and occurs due to vascular connections between microscopic blood vessels. The defect is thought to occur at the level of the terminal portal veins. MD may occur in conjunction with a gross shunting vessel and it does cause clinical signs which are very similar to those observed in animals with portosystemic shunts.


MD occurs as a congenital disease in several small breeds of dog, particularly the Yorksire terrier and Cairn terrier (in which the disease inherited autosomally). The average age at presentation is 3 years, which is older than most animals presenting with congenital portosystemic shunts and the condition is more common in female dogs than males.


Clinical Signs

Signs are very similar to those observed in animals with congenital portosystemic shunts. Animals with MD are often less severely affected than those with PSS and they may even by asymptomatic. Briefly, common clinical signs include:

  • Stunted growth compared to littermates.
  • Neurological signs due to Hepatic Encephalopathy.
  • Intermittent gastro-intestinal signs, including vomiting and diarrhoea.
  • Urinary tract signs resulting from urate urolithiasis.
  • Prolonged recovery from sedation or anaesthesia due to reduced hepatic metabolism of drugs.

Laboratory Tests

The results of blood tests will be similar to those obtained from animals with PSS. In some cases however, raised serum bile acid concentrations may be the only abnormality detected in young animals with MD and concentrations of albumin and cholesterol are less reliable in making a diagnosis. Microcytic red blood cells are also an uncommon finding in animals with MD.

Other Tests

Diagnosis relies on ruling out the presence of a gross shunting vessel by diagnostic imaging. A liver biopsy is then performed and histological analysis of the sample will show a pattern identical to that of a congenital portosystemic shunt. Most of these changes occur as compensatory responses to the loss of oxygen and growth factors usually provided in the portal blood:

  • Arteriolarisation of central veins
  • Smooth muscle proliferation (segmental) within the walls of central veins
  • Random distribution of small calibre vessels
  • Endothelial hyperplasia within portal triads
  • Dilation of periacinar vascular spaces
  • Decreased diameter of intrahepatic veins


Medical management should be implemented, as for portosystemic shunts.


With symptomatic treatment for hepatic encephalopathy, animals with MD can be expected to live with good quality of life for up to 5 years [1].

Hepatic Microvascular Dysplasia Learning Resources


  1. Christiansen JS, Hottinger HA, Allen L, Phillips L, Aronson LR. Hepatic microvascular dysplasia in dogs: a retrospective study of 24 cases (1987-1995). J Am Anim Hosp Assoc. 2000 Sep-Oct;36(5):385-9.

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