We had a dog in the clinic this weekend that was pretty sick. And she had very little chanced of getting better. Not because the disease in untreatable, but because the veterinarian in charge of the case is bowing to the client's wishes. See, the client doesn't want to spend any money. So instead of telling the client the truth, the veterinarian is just placating the client until the dog passes away. The dog truly has a portosystemic shunt.
A portosystemic shunt is a fairly uncommon occurrence. In the womb, a fetus doesn't need to use their liver to detoxify the blood. Mom's liver does that for both the mom and the fetus. Mom's liver also makes enough glucose for both as well. Sometime (hopefully soon!) after birth a little vessel, or shunt, between the intestines and the vena cava closes. In some dog's this may take up to a year, if they live that long. The other way this disease can occur is if these shunts develop within the liver itself. Dog's that are born with this condition are fairly easy to diagnose. They typically have trouble regulating blood glucose and are much smaller than the rest of the litter. (Don't confuse this with the "runt", which statically speaking grows up to be the largest adult.) Many times these dogs present because they are disoriented, or actually have seizures. Simple blood tests usually confirm the diagnosis, but an ultrasound of the abdomen and a biopsy of the liver are sometimes need. Older dogs can acquire a shunt at any time, but these are more difficult to diagnose.
Treatment for mild cases is conservative. Feeding the dog a special diet, along with some medication to help control toxin buildup in the blood stream may be all some dogs need while the body stabilizes. Surgery to repair shunts outside the liver are controversial. The last paper I read about 50% of the dogs did not survive very long after the correction. However, some dogs did do very well, so it may be a good choice if the symptoms are too sever to control with diet and medication.
This site gets more in-depth on the topic: