Porto Systemic Shunts (also known as Portacaval Shunts) are abnormal communications between the portal vein coming from the gastrointestinal tract and the posterior vena cava, which carries blood back to the heart.  The communications are normally present during fetal life and then close off shortly after birth.  When the communications fail to close properly, the vessels serve as shunts so that portal blood flow does not pass through the liver for detoxification before being delivered to the rest of the body. 

Most Porto Systemic shunts are congenital, that is, present at birth.  In some cases, an acquired Porto Systemic shunt can develop secondary to an underlying liver disease.  Congenital Porto Systemic shunts are most common in Miniature Schnauzers, Yorkshire Terriers, Irish Wolfhounds, Cairn Terriers, and Old English Sheepdog, but any breed can be affected. 

Symptoms include depression, seizures, ataxia (loss of muscle control), vomiting, excessive thirst and retarded growth.  Signs and symptoms are usually present by 6 months of age, but congenital Porto-Systemic shunts are occasionally not recognized until middle age.  Diagnosis is based on blood tests and x-ray imaging study.  Medication is often helpful, but definitive treatment is surgery to close off the shunt.