Then I picked up Equus one day and read the article. That was EXACTLY what he was doing. At least now it had a name, instead of just being something wrong with my horse.
He only shakes while riding or lunging, with or without any tack. HSing during turnout is more recent. Mostly at trot and canter. Does not and has not done it when in his paddock. Does it in light or almost night (late dusk). Some days worse than others, have not been able to say definitely worse when windy or cloudy or anything. Random.
It does not seem to matter. After a few months he seemed to be less jerky about it, as if he anticipates the sensations and tries to deal with them by bobbing his head or trying to carry it more parallel to the ground. More flipping of his lips than initially, and less sneezing than initially. Only occasional sudden jerks now. He does not generally shake when being led around at a walk. Only very slightly some times. He will shake when turned out with his little brother to play. Even at a walk. He definitely is doing something funny with his nose, pinching it in almost where the cartilage and the nasal bone meet. He does not have a nasal discharge. His eyes run very infrequently. At times he is very reluctant to tuck his head towards or beyond the vertical as needed for some tricks on the ground at a standstill. Other times no problem. 
When it started I thought, sheesh he's only a month overdue for his teeth. I know he's sensitive but... He also had a bout of hives in March/April which went away and stayed away after two acupuncture treatments and the first herbs. He had hives three years ago as well, also went away with two acupuncture treatments. No herbs. And no shaking until this year.

To make a long story a little less long, over the past 6 months I hauled him to work three times (I work at a specialty hospital for
horses) to scope his upper airway and guttural pouches, do a full cervical spine and skull series of x-rays, not mention, feet,
fetlocks and hocks (subtle lameness perhaps a cause???) and had his ears examined twice. I had blood panels run and did a full allergy screen. I have also had a series of acupuncture and chiropractic treatments done and have tried two special formulas of Chinese herbs. I had two people do his teeth and a third one look to confirm the first two did a good job! The first person had only done half a job. That's why the second, and so on... How I miss my Canadian equine dentist!!! But I have not had him bone scanned (aka nuke scan, gamma, scan, nuclear scintigraphy). I remember reading someone here had that done. Upper airway, skull, cervical spine, hocks, feet, fetlocks, teeth, blood panels were all unremarkable. He does have an abnormally formed opening to one guttural pouch, but the pouches themselves were fine. Two doctors were doubtful that was the cause. One ear exam turned
up sores of unknown origin waaaaaay deep down inside. I had that done because twice I have come to find him carrying one ear flat back which lasted for a for a few days then went away. Each time was different ear. The first ear exam by a different doctor showed nothing. But it doesn't necessarily mean nothing was there. Allergy tests showed sensitivity to several allergens in his immediate environment. The barn owners did not go for my idea of cutting down all 150+ cottonwood tress on the property. Don't understand why...:-) And trying to rid southern California of sage would be no small task.

I found that he is much much worse in the arena and roundpen than when working on the hard dirt roads around the ranch. I tried the nose net pantyhose thing and he hated it, shaking worse than ever. He is rideable, I suppose, but I haven't been riding him for a while. It must be miserable for him, though he doesn't really feel unsafe.
And he makes me smile just by his very existence.

All the vets at work I have talked to have told me to put him on Cypro (except one). I haven't totally ruled it out yet, but I didn't really know why I have been so hesitant to try it until I read here about it potentially leading to impaction and colic. Must be
intuition. He had been prone to colic from sand/mild ulcers (stress during travel). The sand is long gone and I know how to manage him for travel, though he is way less stressed by everything in general now. I do think his gut is a weak point though, and I would not be at all surprised if he were one to colic on Cypro. None of the doctors would say there were any side effects to Cypro, all they'd say was "it's a relatively safe drug." That wasn't good enough, thus the attempts at acupuncture, etc. first. And I'm not very "western" in my medical mentality, though I work with those of a very strong western medicine mentality. The one doctor who did not suggest Cypro suggested I try Lidocaine gel up his nostrils before trying Cypro. He thought it could possibly be of diagnostic value, reasoning that if those nerve endings were numbed, and he stopped HSing, the nerve was definitely the culprit. If the nerve endings were numbed and he still did it, it was something else. He was at Davis with Madigan, but was not involved in the studies. He just heard a lot about them while there. I have the Lidocaine gel, but haven't tried it yet.

Theresa, USA, August 2004

Footnote: the owner will keep us updated on the progress of this gorgeous gelding