Portosystemic shunt (PSS) is a blood vessel birth defect that's common in small breeds like the Yorkie. The portal vein carries toxins from the intestines to the liver, which cleans the blood. With PSS, the vein bypasses the liver, and toxins aren't removed. PSS may cause poor growth, vomiting, confusion, and seizures. Most of the time, PSS can be corrected with surgery, and the dog will go on to live a normal, healthy life.