Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
This doesn't sound like heat stroke to me. It sounds like your dog has Vestibular Disease. The vestibular system is the connection from the brain
stem to the inner ear
and it coordinates balance. Vestibular disease is a disruption in that pathway.
The cause in young dogs is usually due to an inner ear infection. In older dogs it's usually due to an unknown cause and we call it old dog vestibular. Rarely it is caused by a brain stem tumor. Common presenting signs are: head tilt, nystagmus (rapid eye
movement), falling, circling, rolling and nausea.
The good news is that old dog vestibular disease will usually resolve slowly over 7-10 days with no specific therapy. I usually put them on Meclizine (Antivert) to help with the nausea and I think it helps to resolve the signs faster than with no therapy. The dose I use is 25 mg once daily. Meclizine is available over the counter at most human drug stores.
If the signs progress over the next couple days, then an MRI would be indicated to rule out a brain stem tumor. Try not to get too discouraged as it can take some dogs up to 5-7 days to regain balance and to be able to walk normally in the severely affected.
I hope this helps, let me know if you have any other questions.