What all can cause this in dogs? Is it curable? ?

My parents just called to tell me that our dachshund had a seizure earlier this afternoon. He couldn't move his back legs or his front right leg after it happened. He's at the vet's office now. He's fifteen years old and he was holding up really well for his age until now. What all can cause this? I'm expecting that we'll probably have to put him down because the vet said that at his age it's probably not going to be anything easy to treat.