Hello, this is Dr. Linda. I hope I can help with your question today.
I am so sorry for your concerns about Sunni's head shaking.
Head shaking can be classified as grade 1 - mild & intermittent with or without facial twitching
grade 2 - more developed signs
grade 3 - difficult to control
grade 4 - unrideable
grade 5 - dangerous & bizarre behavior
The rule outs are many & include infectious rhinitis ( upper respiratory infection ). sinus infection, dental disease, tumors, polyps, ethmoid hematomas, seizure disorders, OCD, ear mites, ear infection/foreign body or material in ear, ear disease or even allergies.
In a 25 year old horse, my first guess would be a tooth problem, possibly an infected molar, but you really need to have your veterinarian out to see what is bothering her. She may need some light sedation to really examine the inside of her mouth, & her ears, but that would be a good place to start, especially if this is a new behavior or it seems to be getting worse.
I hope this makes sense & you find the information helpful. Please let me know if you have any further questions or concerns.