Portosystemic shunts (PSS), also known as liver shunts or portosystemic vascular anomalies, are anatomical defects where one or more veins let blood bypass a dog’s liver. These veins are remnants of embryonic blood vessels that are supposed to regress shortly after a puppy is born. What causes portosystemic shunts is unknown. They may be caused by some insult to fetuses inside the womb. There almost certainly is a strong genetic component. As the abnormal veins shunt blood around the liver, substances that normally would be filtered, metabolized or modified by the liver stay in circulation. Many of these, especially ammonia, are harmful - especially to nervous system tissue. Affected dogs usually develop symptoms by 1 year of age. The signs of PSSs are nonspecific and episodic. They include lethargy, weakness, disorientation, drooling, vocalization, vision disturbances, pacing, stunted growth, vomiting, diarrhea, constipation, appetite changes, changes in urination, itchy skin, tremors, seizures and collapse.