Maeve,
After my surgery, I had swelling in my neck that compressed all those nerves and gave me a right arm brachial plexus injury as well as Horner's syndrome. I was told there is no cure for Horner's. What you can have done is aesthtic surgery to make things appear "more normal". The docs told me that as the nerves that were compressed start to heal, that over time, the symptoms of the Horner's syndrome (droopy eyelid and mis-matched pupil size) may possibly go away. If not, plastic surgery by a doc specializing in eyelids could be done to lift the droopy one. Apparently, there are also eye drops used for glaucoma patients that can make your pupil size vary in order to have it match your other eye.
For me, I don't have any problems with my sight, so I am sitting tight for right now. Yes, the appearance isn't what I'd like, but I can live with it, at least for now. Anything to avoid more medicines and surgeries....I've had enough
Hopefully you can find a doc that can help you with what you need.
Good luck and let me know if you have any questions for me...
Amy