The horse may also suffer from epilepsy. I have a friend with a horse who has epilepsy. The local vet school told her her would be unridable etc etc however that was years ago and he trail rides and has a very happy life. He was older when this started too. He is not on a bromide medication daily but it works and I don't think he's seized in years. Before the medication it was happening fairly frequently. There were a series of blood tests they did to diagnose this condition. I can try to find out more from her about what tests they did if it would be helpful.