MEDICINE POTPOURRI: NEUROLOGY, FOALS, MUSCLES, AND THE GUT


melatonin has been shown to attenuate an inducible model of trigeminovascular nociception in rats.23 Melatonin might, therefore, act to reduce headshak- ing by mechanisms other than photoperiod manipu- lation. Only 13% of owners had used melatonin treatment, and all except one of these were based in North America, where the drug is cheaply available as an “over the counter” product. Whereas melato- nin can be purchased with a veterinary prescription in other countries such as the UK, this does not appear to be occurring, most likely due to veterinar- ians being currently unaware of the protocol. Mel- atonin, as an endogenous hormone, also has the advantage of having no restrictions for use in com- petition horses. Given that almost half of head- shakers improved with melatonin therapy, with only winter coat retention as an adverse effect, this treat- ment warrants further attention. Magnesium supplementation had been used by


almost 50% of owners, with just under half of these (25/58) reporting an improvement in their horse’s headshaking. Magnesium increases the activation threshold of nerves such that a greater stimulus is required for depolarization.24 Magnesium, there- fore, appears a rational therapy given the reduced activation threshold of the trigeminal nerve in head- shaking horses.5 Formulations of magnesium vary in their oral bioavailability with that of magnesium oxide (the most frequently administered in this study) reported to be questionable in the horse.25 Generally availability of magnesium in horses is high and feeding high percentages of roughage en- hances its absorption from the gastrointestinal tract.26 From this study, toxicity from daily mag- nesium supplementation of up to 40 g appears to be rare, although it may be advisable to periodically measure ionized magnesium concentrations. Mag- nesium may be another useful treatment for com- peting


headshaking horses. Combining


magnesium and melatonin therapy appeared to be beneficial, resulting in an improved outcome than use of either treatment alone (55%; 6/11). Indeed, combined therapy had the highest overall positive outcome; however, this should be interpreted with caution, as only 11 owners had used this treatment. This study can be used to guide owners in the management of idiopathic headshaking. Owner- reported responses to treatment in 130 headshaking horses suggest that nose nets, face masks, and mag- nesium supplementation are the most efficacious nonmedical therapeutic options. Adverse effects such as facial irritation were reported in 1


⁄3 of horses


with nose nets. Cyproheptadine, corticosteroids, and melatonin were the most successful pharmaceu- tical treatments in this study; however, the number of horses using these therapies was low and, there- fore, these results should be interpreted with cau- tion. Of these pharmaceutical drugs, only melatonin is without significant adverse effects or allowed in competition horses. Veterinary diagno- sis of idiopathic headshaking is advised before com-


182 2014  Vol. 60  AAEP PROCEEDINGS


mencing any treatments. Further investigation of the pathophysiological mechanism underlying the aberrant trigeminal nerve activity in headshaking horses is warranted in order to develop more suc- cessful therapeutic options.


Acknowledgments


Conflict of Interest This study was funded by anonymous private dona- tion to the Equine and Comparative Neurology Re- search Group.


References and Footnotes


1. Williams WL. Involuntary twitching of the head relieved by trifacial neurectomy. J Comp Med Vet Arch 1897;18:426–428.


2. Williams WL. Involuntary shaking of the head and its treat- ment by trifacial neurectomy. Am Vet Rev 1899;23:321–326.


3. Madigan JE, Kortz G, Murphy C, et al. Photic headshaking in the horse: 7 cases. Equine Vet J 1995;27:306–311.


4. Newton SA, Knottenbelt DC, Eldridge PR. Headshaking in horses: Possible aetiopathogenesis suggested by the results of diagnostic tests and several treatment regimes used in 20 cases. Equine Vet J 2000;32:208–216.


5. Aleman M, Williams DC, Brosnan RJ, et al. Sensory nerve conduction and somatosensory evoked potentials of the tri- geminal nerve in horses with idiopathic headshaking. J Vet Int Med 2013;27:1571–1580.


6. Newton SA. The functional anatomy of the trigeminal nerve in the horse [dissertation]. Liverpool, UK: University of Liverpool; 2001.


7. Madigan JE, Bell SA. Owner survey of headshaking in horses. J Am Vet Med Assoc 2001;219:334–337.


8. Mills DS, Cook S, Taylor K, et al. Analysis of the variations in clinical signs shown by 254 cases of equine headshaking. Vet Rec 2002;150:236–240.


9. Mills DS, Cook S, Jones B. Reported response to treatment among 245 cases of equine headshaking. Vet Rec 2002;150: 311–313.


10. Taylor K. An evaluation of equine headshaking syndrome and its management [dissertation]. Leicester, UK: De Montfort University; 2004.


11. Lane JG, Mair TS. Observations on headshaking in the horse. Equine Vet J 1987;19:331–336.


12. Berger JM, Bell SA, Bradford BJ, et al. Successful treat- ment of head shaking by use of infrared diode laser deflation and coagulation of corpora nigra cysts and behavioral modi- fication in a horse. J Am Vet Med Assoc 2008;233:1610– 1612.


13. Mills DS, Taylor K. Field study of the efficacy of three types of nose net for the treatment of headshaking in horses. Vet Rec 2003;152:41–44.


14. Raj SN, Meyer RA, Ringkamp M, et al. Peripheral neural mechanisms of nociception. In: Wall PD, Melzack R, eds. Pain 4th ed. Edinburgh: Churchill Livingstone, 1999;11–57.


15. Tomlinson JE, Neff P, Boston RC, et al. Treatment of idiopathic headshaking in the horse with pulsed high dose dexamethasone. J Vet Intern Med 2013;27:1551–1554.


16. Watanabe S, Bruera E. Corticosteroids as adjuvant analge- sics. J Pain Symptom Manage 1991;9:442–445.


17. Lowe DA, Matthews EK, Richardson BP. The calcium an- tagonistic effects of cyproheptadine on contraction, mem- brane electrical events and calcium influx in the guinea-pig taenia coli. Br J Pharmacol 1981;74:651–663.


18. Mair TS. Assessment of bilateral infra-orbital nerve block- ade and bilateral infra-orbital neurectomy in the investiga- tion and treatment of idiopathic headshaking. Equine Vet J 1999;31:262–264.


19. Attal N, Cruccu G, Baron R, et al. EFNS guidelines on the pharmacological treatment of neuropathic pain: 2010 revi- sion. Eur J Neurol 2010;17:1113–1123.


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