My parents had a dog that had frequent seizures. Interestingly, it always knew when it was about to have a seizure, so my mother knew when to hold onto it. I’m not sure but I think they said it was liver cancer. They had the dog put down eventually because it was either a matter of seeing it in pain all the time or just speeding up the inevitable.
Yes, I don’t want my dog to suffer. She actually seems fine 90 percent of the time, and does not seem to have pain; no panting, no whining, she does tremble a lot, though.