Roberts, V. (2019). Trigeminal-mediated headshaking in horses: prevalence, impact, and management strategies. Veterinary Medicine: Research and Reports, 10. https://doi.org/10.2147/VMRR.S163805 Publisher's PDF, also known as Version of record License (if available): CC BY Link to published version (if available): 10.2147/VMRR.S163805 Link to publication record in Explore Bristol Research PDF-document This is the final published version of the article (version of record). It first appeared online via Dovepress at https://www.dovepress.com/veterinary-medicine-research-and-reports-journal. Please refer to any applicable terms of use of the publisher. University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Journal name: Veterinary Medicine: Research and Reports Article Designation: Review Year: 2019 Volume: 10 Veterinary Medicine: Research and Reports Dovepress Running head verso: Roberts Running head recto: Trigeminal-mediated headshaking in horses open access to scientific and medical research DOI: http://dx.doi.org/10.2147/VMRR.S163805 Open Access Full Text Article REVIEW Trigeminal-mediated headshaking in horses: prevalence, impact, and management strategies Veronica Roberts Abstract: Trigeminal-mediated headshaking is a little-understood neuropathic facial pain con- dition of the horse. The condition may affect around 1% of the equine population to a degree of Bristol Vet School, Faculty of Health Sciences, University of Bristol, severity sufficient to require veterinary attention. As a pain condition, this represents a significant Langford, North Somerset BS40 5HB, welfare issue. Horses are usually more severely affected at exercise which can leave them unable UK to perform, or even dangerous to ride and handle. With little known about the condition and variable response to treatments, severely affected horses are often euthanized. This review article considers the literature on trigeminal-mediated headshaking, focusing on what is known, and indeed not known, about the prevalence of the condition and its impact. The current published management options are considered, along with their challenges and limitations. Keywords: neurology, neurological disorder, neuropathic pain, facial pain, welfare Introduction Occasional shaking of the head is a normal equine behavior. However, where the headshaking is any or all of frequent, violent, accompanied by flicks or jerks of the head, accompanied by signs of nasal irritation (such as snorting, sneezing, rubbing the nose, striking the nose), accompanied by signs of distress, or affects riding or handling the horse, then a pathological process must be considered.1 Conditions which can cause headshaking may include ear mite infestation, otitis interna, cranial nerve dysfunction, cervical injury, ocular disease, guttural pouch mycosis, dental periapi- cal osteitis,2 protozoal myeloencephalitis,3 and sinusitis4 as well as a behavioral or rider issue.1 However, where shaking was such that referral veterinary advice was required, there was a 98% chance that no physical cause could be determined, lead- ing to a diagnosis of idiopathic headshaking.2 As knowledge of this condition has developed, it is likely that most of these horses were suffering what is now termed trigeminal-mediated headshaking.1 Trigeminal-mediated headshaking appears to be an acquired disorder of the horse. Clinical signs are usually of predominantly vertical headshaking, which may be violent.1 These movements are often accompanied by sharp vertical flicks and signs of nasal irritation.2 Typically, signs are worst at exercise,2,5,6 with only some horses being affected Correspondence: Veronica Roberts at rest also. Seasonality of clinical signs is usually reported in ~60% of headshaking Bristol Vet School, Faculty of Health horses, with the majority of these being spring/summer affected,5,7 although a more Sciences, University of Bristol, Stock Lane, Langford, North Somerset recent study found that owner’s reported horses were only completely free of signs BS405HB, UK seasonally in 25% cases (unpublished data). Even in nonseasonally affected horses, Tel +44 117 394 0786 Email [email protected] signs may vary with an apparent correlation to weather conditions.2,5,7 submit your manuscript | www.dovepress.com Veterinary Medicine: Research and Reports 2019:10 1–8 1 Dovepress © 2019 Roberts. 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Roberts Dovepress The clinical signs of trigeminal-mediated headshaking sis.32 Therefore, even if all trigeminal-mediated headshakers are consistent with neuropathic pain of the trigeminal nerve. have the same underlying condition, response to the same Although this was suspected even in the 19th century,8,9 there treatment may vary. have been many other theories, with trigeminal involvement It is against this challenging background that this article being only recently confirmed.10,11 These studies demon- considers the prevalence, impact, and current management strated that the infraorbital branch of the trigeminal nerve strategies for trigeminal-mediated headshaking. There of affected horses was sensitized, with a lower threshold for is a need for further research into this condition to allow activation than unaffected horses. Somatosensory-evoked progression in treatment and even prevention. Establishing potentials were recorded under general anesthesia. These prevalence and welfare and economic impacts contributes to determined the threshold for activation for control horses determining research priority for this condition, as does an to be >10 mA, but <5 mA for affected horses. There were understanding of the limited success of current treatments. no differences in the neurophysiological characteristics of This review considers and consolidates the current literature the action potentials. Additionally, there were no differ- in these areas. ences between left and right sides, consistent with bilateral involvement of the trigeminal nerve. One seasonally affected Prevalence horse was tested out of season and returned normal threshold Knowledge of the prevalence of headshaking in the world- activations, but was not tested when showing clinical signs, to wide population is fundamental to understanding the scale see if then, threshold was abnormal. Should clinical signs be of the welfare issue that is trigeminal-mediated headshaking. shown to change as threshold changes, then not only would While there are reports of horses with signs consistent with that confirm causation but also suggest a potentially reversible trigeminal-mediated headshaking from many developed condition. Further work is required in this area. countries, only the UK horse population has been used for The reason for the sensitization of the trigeminal nerve prevalence studies. There may be a complex interaction with remains unknown. While herpes virus is involved in the environment in this acquired condition of unknown etiology. pathogenesis of some human neuropathic pain syndromes, For this reason, further information as to whether there is a the virus was not involved in the pathogenesis of trigeminal- difference in prevalence between countries, and in particular mediated headshaking.12 While trigeminal nerve root demy- between developed and developing countries, would be of elination is the most frequent cause of trigeminal neuralgia, interest. a neuropathic facial pain syndrome in people,13 no histo- The prevalence of headshaking in the UK equine popula- pathological abnormality of the nerve has been discovered.14 tion was first reported by Slater33 in 2013 as part of a general Despite the apparent clinical similarities between trigeminal equine health survey. The prevalence of horses reported to neuralgia and trigeminal-mediated headshaking, this result have shaken their heads on a particular day of the year was was not inconsistent with signs as human trigeminal neuralgia reported to be between 1% and 1.5%. A dedicated study into sufferers are usually unilaterally affected and would have the prevalence of headshaking in the UK equine population abnormal conduction patterns.13 It can be considered to be was published in 2018.15 Owners reported that 4.6% of their a potentially positive result, as this condition would be less horses had shaken their heads in the last year, with 6.2% at likely to be reversible. The results of these studies suggest a any time point since ownership was considered. However, functional, rather than structural abnormality of the nerve, veterinary advice was required in only 30% of these horses. leading to the potential for developing a treatment which This suggests that medically significant headshaking can reverses the