Portosystemic Liver Shunt. The blood goes through a blood vessel that avoids filtering through the liver resulting in toxins building in the bloodstream. (ammonia is the main one). Makes the dog have neurological signs, staggering, seizures, behavioral.
Ammonia is the breakdown product of protein, hence the protien restricted diet.
After firm diagnosis, surgery to close the shunt usually results in normal lifespan for the dog in question. Medical management alone can sometimes allow 2 to 3 years.
I had a case of this today in a Yorkie. Yorkies are super common, as are Maltese. Dachshunds are lower on the list, but on the list.