That was the same thing I thought of - our dog. He too had been on gabapentin for quite awhile for pain (3 to 4 years??? My wife knows all that stuff!). The last 6 months of his life he would do lots of pacing and walking in circles. The vet wasn’t sure if it was due to pain/discomfort or dementia. I don’t know about dementia, he still obeyed all of his commands, stayed out of rooms he wasn’t allowed in, remembered the various ways we would walk around the neighborhood, etc.
At his peak he was 90lbs (Great Dane/Black Lab). We finally put him to sleep this spring at the age of 15 1/2. It was only about those last 6 months that he acted “old”.
I never understood how folks got so heartbroken when a dog died. After our first dog died years ago - I got it.
My dog was not quite 12 years old, but the dementia was becoming quite severe. She was not obeying any commands at the time we had to have her put down.
There is a worksheet for assessing dementia in dogs, called DISHAA (Disorientation; Social Interactions; Sleep/Wake Cycles; House soiling, Learning and Memory; Activity; and Anxiety).
According to what I have read on the internet, one of the first symptoms that dog owners notice is pacing during the night. That is definitely what we observed—sleeping almost all day, then pacing around for most of the night.
The dog we had put down was completely different than the dog we rescued four years ago.