Liver shunt is a genetic disease...Unfortunately, I know many people whose yorkies have this condition. You just have to make sure the breeder that you choose has tested the sire and dam for this disease...
Basically, it means that there is a blood vessel that is causing the blood to bypass the liver. Therefore, the liver is not detoxifying the blood like it should before it goes into the rest of the dogs system.
When a dog ingests protein, the byproduct is ammonia which is filtered in the liver, but with a liver shunt, the ammonia is bypassing the liver and it's not being filtered. This can eventually cause the dog to head press, get seizures, become lethargic, get bladder stones, etc.
http://www.malteseonly.com/shunt2.html