We did have a case of vestibular disease in a younger dog--she was 5 at the time--but she didn't have hind quarter weakness prior to the attack. Nor did the symptoms come and go--it was a prolonged attack with head tilt being the first symptom and the lack of coordination following. Took a few months to resolve completely.
Did the vets say why they thought the MRI was unnecessary?
The ticks are bad and we live in Tick Central, so tick-borne diseases always have to be ruled out as a cause of anything neurological here. Are ticks a problem where you live? Have you been to an area where your dog might have been exposed to them? If so, it might be something to explore as a cause.
One other thought--could your dog have gotten into something toxic (mushrooms, cleaning supplies, pesticides, etc)? Something that he might have access to at different times, showing symptoms only after exposure? Usually that sort of toxicity will affect the CBC, but sometimes the effects won't show in the blood work for a few weeks.
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