okay, so ive been freaking out about this and reading threads about this for quite some time, but since my horses circumstances are sort of odd, i want to make my own post:D
so, my horse is laid up. he has been for 4 months now (broken coffin bone…different story) and is headshaking. It started in late march… and now that I think about it, it was after his vaccines. he got a 5-way, which i think includes flu-rhino.
at this time he was on complete stall rest- no hand walking, no nothing. for about 3 days he was going insane flipping his head, sneezing, rubbing nose on everything but ONLY when he would grab a bite of hay from the haynet. this went away after 3 days or so… then started up a month or so later (early-mid april), after he was allowed to handwalk and hand graze. he only did it for 1 day this time. he was outside and again only would headshake when he would go to eat grass. texted vet. she recommended antihistamines. started him on them same day, headshaking gone. then at the end of may, I was outside hand grazing him and he started heashaking when he would grab a bite of grass. put him back on allergy meds, and this time it didnt help. some days are better than others. when the fans are on in the barn, he headshakes like a maniac. fans off, hes much better. still very headshaky when he eats hay and grass. ticks and sneezes when we walk. hes much better walking in the indoor arena, and even the outdoor arena- little to no headshaking at all. seems worse when we are walking on grass, as odd as that sounds. he also rubs his eyes on stuff, as i think they itch from allergies because theyre often red and weepy. he also has some milky white snot 
we moved to a new barn last november, so there are probably new allergens there that hes not used to, hence the horrible allergies. a friend of mine also said her horse got allergies when she moved to this barn, and her horse had never had allergies before.
he is still on antihistamines, and i didnt think they helped much until i forgot to give him his meds a couple nights ago, then the next day he was horrible. gave him meds when i realized, and he was much better about an hour later.
he is worse at certain times of day. hes good until around noon, then gets really bad from about 1-4:00pm. its very hot and humid (nothern ohio) and hes worse the hotter and more humid it is.
im HOPING that once he can be turned out (hes still on stall rest…) his body will adjust to being exposed to the allergens like a normal horse and this problem will go away… he DOES improve the longer he is outside, which is encouraging. i also sort of think that him being cooped up and having so much energy is making everything much more exaggerated.
im thinking of trying hydroxyzine next, hes currently on generic zyrtec (cetirizine hydrochloride) but im thinking the hydroxyzine would be a little stronger and knock the allergies out completely.
also- hes on reserpine (fit tb on stall rest = lots of jumping around;)) just incase that could have anything to do with it.
so yeah, lots of factors to think about, just trying to figure this out. my vet did see him, checked eyes and found nothing. said she thinks its allergies. im still super concerned.
sorry it got kind of long, just trying to give you all the info 
Do you have a question or just venting? (either is ok!)
Sounds like it could be allergy related, especially since he has other symptoms (runny nose, red/weepy eyes) but it could be allergies plus other things causing the headshaking. My headshaker started as exercise-induced in the spring but over the four years since diagnosis he’s moved on to randomly year-round and has multiple triggers - it seems like a light breeze and/or rain/snow is the worst for him. He’s not photic though, so we’ve managed to avoid that for now. However, he now is randomly set off by eating, both grain and hay, and I don’t necessarily think it’s what he’s eating but more either the texture or action itself, it’s hard to tell.
It may not hurt to try hydroxyzine, or my cyproheptadine if the hydroxyzine doesn’t cut it. The hydro didn’t do anything for my guy but he didn’t have any other allergy symptoms other than headshaking so that could be why. If you try the cypro, I would suggest starting at a low dose and working your way up. I had seen several people warning about colic within a week or so of starting it and sure enough, my boy coliced around day 5 or so - not bad but enough that I had to call the vet out.
Good luck! Hope you’re able to figure out the exact cause and get it under control.
Have you tried wetting or steaming his hay?
Have you tried a nose net or a fly mask with a long nose attachment in his stall? Since the breeze from the fan is bothering him that might help. Good luck!
tried wetting and it gave him a little relief, but it was minimal
sort of wanted to get a feel for what people thought of his symptoms- allergies or headshaking? I realize that I got kinda rambly but I wanted to list all of his symptoms and see what people thought of them
I tried a nose net, didn’t help him did both the actual cashel quiet ride and the pantyhose- he hated the pantyhose and snorted and threw his head and tried to eat it so I can’t really say if it helped. the cashel did not help.
I’m actually standing outside right now hand grazing and he’s getting better the longer we are outside which is odd. really makes me think it’s allergies and it’s bad because he’s cooped up on stall rest
Try ulcer meds. My mare is a HSer and I inadvertently found out the HSing is greatly exacerbated by what I am pretty sure is gastric ulcers. She had to stay in when her filly had check ligament surgery and was on free choice hay, no HSing at all that year with filly on her. Last year she had to be trailered, did not eat hay well, came back HSing horribly, treated her with GG for 4 days and HSing gone.
I have heard that magnesium has a positive effect. Performance Equine MagRestore is inexpensive and very absorbable.
Khall - I’m about to put him on smartgut so we will see if there’s any improvement. I’m also trying smart breathe to see if it has to do with dust and dander in the barn. thanks!!
Magrestore helped my mare a little. She had photic headshaking. it was not a cure, but gave her some relief.
good news! changed my horses allergy meds dosing and poof! headshaking gone. he had been getting 10 Zyrtec tabs at am feeding at 6:30am and by the time it got hot and the dust was up the meds had stopped working. he now gets 5 tabs 4 times a day 3 1/2 hours apart and he is so much more comfortable. even today it’s 90 degrees and he’s still not sneezing! so thankful he’s getting some relief even if the dosage is a pain in the ass. let’s pray it keeps working!
If wetting the hay helped,
I would consider steaming the hay. Hay Mites are a very common allergy cause, a few are wash away by wetting, most are destroyed by steaming.
There have been several threads on steaming hay, plans for inexpensive home-made steamers can be found on the threads, or google.
