Back to the Lab, Watson.

Lar had his date with the vet this morning.  I am thrilled that my favorite vet chose to return to KY after spending much of this year at a practice in Colorado, and happier still that he's had some success with headshaking cases previously.  He confirmed my observations and Lar's video clips as headshaking. syndrome.  General physical examination and ocular and facial examination didn't reveal anything significant, though while taking heart rate, Lar's heart rate shot up rapidly from 40 to 65 bpm before easing back to 40 bpm.  (Possible link to his training anxiety?)  The modern age of computers, video, etc. sure makes things a lot easier, as this entire week has been too dreary, cold, and rainy to ride or put Lar on the lunge line.  In Lar's case, the vet thought allergies seemed like the most likely trigger.  He's had luck with twice daily cyproheptadine, an anti-serotinergic and anti-cholinergic allergy medication.  I asked about testing for specific allergens, but he thought it would be very difficult to isolate a specific trigger allergen (not to mention then trying to eliminate it from the environment if it's something common like grass or tree pollen!).

Scientific Content Alert - avoid the next two paragraphs if you're not interested in the underlying science!  Serotonin acts as a neurotransmitter in the body.  It is linked with feelings of happiness and well-being; many antidepressants are targetted towards promoting it, and some illegal drugs act by making its levels spike in the body.  It also participates in the process of blood clotting.  It causes vasoconstriction, and quantities of it in the body can pain sensations (for example, wasps and some snakes inject it into their prey when they sting or bite).  Acetylcholine is a generally excitatory neurotransmitter in the body that acts presynaptically in the sympathetic nervous system and pre and post synaptically in the parasympathetic nervous system.  (It's pretty convenient that I've concurrently been studying some of these topics in my anatomy and physiology class.)  It seems logical that cypro could be effective in some headshakers by reducing symptoms associated with allergic response and/or excesses of these neurotransmitters.

The downsides are that it generally requires twice daily administration (in less severe cases, it may be reduced to once daily administration) at a cost of a few dollars a day.  It is also prohibited by most horse show sanctioning bodies, so horses must be taken off of it several days prior to competing (meaning that their symptoms may start to return just when you really need them to be on their best behavior).  Its efficacy can be hit or miss in headshakers, but my vet has had a personal success with it rate higher than the average, perhaps because KY is such an allergen riddled region.  Another treatment option is pulse dosing with dexamethasone, a steroidal antiinflammatory and immunosuppressant agent.  Allergies are basically the overreaction of the immune system to benign agents, so that coupled with mediation of inflammatory response explains the probable course of action in headshakers.  With long-term administration, immunosuppression, weight gain, and GI upset can be a problem.  The accepted course of treatment with the dex pulse therapy is to administer a fairly high dose for three consecutive days, then repeat every three to four weeks.  This relieves most of the risks associated with long term administration and (with proper planning) doesn't require mandatory cessation of treatment prior to sanctioned competitions.

So the current plan is to discontinue the melatonin (which didn't seem to do anything for him) and try the dexamethasone as both a diagnostic tool and treatment.  If it works, there's a decent chance that the cypro will also work if I need to alternate or change medications.

 I still wonder about headshaking being the root cause of all of his issues, as he does have some ability to suppress (but not eliminate) his behaviors when you really get after him (as I had to last week after he nearly pitched me while having a particularly bad episode).  I have no doubt the physical component is real.  The better questions are (1) can it be suppressed? (hence all of the neural experiments and pharmacology) and (2) can he be happy and comfortable doing what I want him to do?  Ultimately, the answer may be a combination of physical and behavioral, and it's quite possible that the life of showing I want to pursue with him may not be the best route for him.  

Better living through pharmacology?  I hope the answer lies at the bottom of the dose syringe. but only time (and more experimentation) will tell...