Headshaking in Horses
Robert J. MacKay
Headshaking renders an otherwise normal horse unsuitable for riding or driving. There is no obvious predilection for age, breed, or gender. In the United Kingdom, owner-reported prevalence of headshaking of horses in 2017 was 4.2%.1 Surveys have usually demonstrated that more than 50% of headshakers are geldings, and mean ages reported in published series are 7.3 to 12.8 years (range, 1 to 28 years).2,3 Headshaking develops mostly in horses used for pleasure riding, dressage, or show jumping, but horses of any breed or discipline can be affected.4,5 Signs of headshaking usually first occur while the horse is being ridden or driven (i.e., with the horse in a bit and bridle) and become more pronounced over time, both in severity and duration.2 The condition usually renders a horse unsafe or at least unsatisfactory for riding and in severe cases may cause inhumane suffering to the affected horse.
Once established, the condition occurs during any form of exercise, and approximately 60% of affected horses eventually also exhibit headshaking at rest.2,4 Additional trigger factors that exacerbate or provoke headshaking have been identified: bright light, sun, heat, cold, stress, wind or rain, eating hay, extreme neck flexion, riding on narrow roads, hearing people's voices, and riding near trees or water.6,7 On average, approximately half of cases from multiple series are either partially or completely seasonal; signs begin suddenly or get worse in spring and end or become milder in the late summer or fall.2,6,8 A few cases begin in the fall and end in late winter. Signs may stop for weeks, only to recur for no apparent reason.Headshaking is classified as either symptomatic or idiopathic. Symptomatic headshaking has an identifiable physical cause that, if removed, permanently resolves the problem.
Idiopathic cases either have no discernable basis or continue after removal of a putative cause. Reported or postulated symptomatic causes have included skull lesions, allergic rhinitis, vasomotor rhinitis, otitis externa or media/interna, Trombicula autumnalis (harvest mite) infection, fungal sinusitis, THO, temporomandibular degenerative joint disease, mites or ticks in the external ear canals, guttural pouch mycosis, Onchocerca dermatitis, dental problems, sinus surgery, cervical arthritis, and ocular lesions, such as cystic corpora nigricans, “floaters” in the anterior chamber, allergic conjunctivitis, and blocked lacrimal ducts.9-12 Because horses are normally programmed for headshaking as, for example, for fly avoidance,13 almost any unpleasant condition around the head (or possibly elsewhere on the body) seems to have the potential to amplify the behavior to abnormal levels. Despite the seemingly impressive (and incomplete) number of potential causes just listed, no physical basis for headshaking was found in 89 of a series of 100 reported cases.2 Headshaking does not appear to be associated with latent equine herpesvirus 1 infection.14Headshaking may be defined as involuntary sudden violent repetitive movements of the head, either dorsally and ventrally in the median plane (most common), horizontally, and/or in a rotatory manner. Other signs, particularly in horses with idiopathic headshaking, have included snorting, sneezing, snoring breath sounds, rubbing the face on stationary objects, keeping the head low while trotting or even rubbing the nose along the ground, acting as if a bee has flown up the nose, excessive nasal and lacrimal discharge, hyperesthesia/ hyperresponsiveness around the face, anxiety and agitation, hopping off the ground in one or both thoracic limbs, head pressing, shade seeking, muscle fasciculations, sweating, and striking at the nose. Many of these signs seem directed by the horse toward noxious sensations coming from the nose or face.
In one study, anesthesia of the infraorbital nerves at the infraorbital foramina prevented headshaking in 3 of 19 horses,15 whereas in another, 24 of 27 horses stopped headshaking when the caudal nasal nerves were blocked.16 The latter technique probably blocks the maxillary nerve in addition to branches such as the caudal nasal nerve and infraorbital nerve.17 These results indicate that idiopathic headshaking is triggered in the caudal parts of the nasal passages in most horses.Sensory nerve conduction studies have been used to show that the threshold for activation of the infraorbital nerve of headshakers is lower than that for normal horses.7 Idiopathic headshaking that can be relieved by sensory nerve blocks is termed trigeminal-mediated headshaking. Some cases of headshaking are almost entirely induced by bright light and appear to represent summation of stimuli arriving via both the optic and trigeminal nerves.3 It is not yet known how these clear-cut cases of photic headshaking are related to the much more numerous cases in which light is not directly involved or is only a weak trigger. It may be that photoperiod (i.e., the proportion of the day that is light) is more important than light intensity in such horses and may explain the role of season in many cases.3 Whatever the inciting cause, idiopathic headshaking appears to be a nasal/facial pain syndrome associated with hyperexcitability of trigeminal pain pathways.7 In humans, trigeminal neuralgia is often caused by vascular impingement on the trigeminal ganglion or nerve root adjacent to the brainstem. Trigeminal nerve root demyelination, a characteristic finding in humans with trigeminal neuralgia, has not been found in headshakers.18
Horses with headshaking should undergo a complete physical examination; ophthalmic, otoscopic, neurologic, and dental examinations; endoscopic examination of nasal passages, pharynx, and guttural pouches; radiography of the skull; and a complete blood cell count and chemistry panel. When available, CT is the preferred imaging technique.
No abnormalities are usually noted in these examinations.■ Treatment
PHYSICAL. Mild cases of headshaking that occur during exercise can often be partially suppressed by the use of muzzle nets or devices that dangle over the face and nostrils. Approximately 30% of horses managed with muzzle nets showed at least 70% improvement, although the durability of any salutary effect has not been established.19 There was no difference between the effectiveness of full face nets and that of nasal nets. A number of organizations and jurisdictions now allow horses to be shown wearing a muzzle net. A few horses respond to a heavy hair net or a dangling device that makes contact with the nose area or upper forehead. This type of contact may prevent the nerve from firing, in a manner similar to blocking a sneeze by placing a finger under the nose and applying pressure.20 Facemasks and contact lenses that substantially reduce ambient light are available and are very effective in preventing photic headshaking.3 Some researchers have argued that bitless bridles may be helpful if the action of the bit triggers headshaking.21 These devices have been used widely.6
SURGICAL. Surgical approaches to headshaking are controversial because considerable morbidity and even severe suffering are frequent side effects of the procedures.6 Notwithstanding these problems, surgery does appear to achieve a permanent cure 151722
in some horses.1, Ihe objective of surgery is interruption of pain pathways in the trigeminal nerve or its branches. Two approaches have been used: infraorbital neurectomy and caudal infraorbital nerve compression. In the first procedure, a 2-cm section of nerve is removed just rostral to the infraorbital foramen.15,22 To effect nerve compression, up to eight platinum embolization coils are introduced via the infraorbital foramen and packed around the infraorbital nerve within the caudal aspect of the infraorbital canal.16,17 Five of 26 horses (19%)15,22 and 26 of 57 (46%)16 horses had long-term elimination of headshaking after infraorbital neurectomy and caudal compression, respectively.
Temporary remission of headshaking was achieved in other horses but was followed by relapse weeks to months after surgery. Further long-term remissions were achieved when the procedure was repeated in some of the horses that suffered relapse.16,22 Exacerbation of headshaking and self-inflicted trauma around the muzzle and lips were problems in most horses after these procedures. These complications typically resolved within weeks but were persistent in some; four of these horses were euthanized because of the severity of signs. The results of nerve blocks was not completely predictive of response to surgeries; however, two of three horses that responded to infraorbital blocks achieved complete remission with infraorbital neurectomy.15 Tracheostomy has been shown to be effective as a salvage procedure, presumably by bypassing the passage of air through the nasal passages.6MEDICAL. It is unlikely that complete and permanent remission can be achieved medically. Cyproheptadine, 0.3 mg/kg PO twice daily, has been used with some success for photic headshaking3 and with limited success in other cases of idiopathic headshaking.4,15,23 Mild lethargy and colic are occasional side effects. Headshaking can be suppressed for variable periods by the potent analgesic antiseizure drugs carbamazepine and gabapentin, but effects are often not complete, and signs return within several weeks after treatment begins.6,24 Cyproheptadine may potentiate the analgesic actions of carbamazepine. In one horse, persistent 95% remission was obtained by the CT-guided injection of glycerol into the trigeminal ganglia.25 Individual horses are reported to have responded to corticosteroids or antihistamines, which perhaps indicates a role for allergic rhinitis in those few cases, but intermittent treatment with high-dose dexamethasone or NSAIDs is ineffective.2,15,26 Melatonin is advocated as a way to suppress the neurologic responses to increasing photoperiod (i.e., seasonal effect), so it should be started before the anticipated seasonal onset of headshaking.3 This has reportedly been effective in a few cases.
Various other supplements and traditional or alternative products have anecdotal or conceptual support with (as yet) no evidence. Most of them are inexpensive and nontoxic at recommended doses. There are protocols for chiropractic therapy and acupuncture for headshaking. These have not been successful in the author's experience.ELECTRICAL NERVE STIMULATION. Percutaneous electrical nerve stimulation (PENS) of the infraorbital nerves is a promising technique for the management of trigeminal-mediated headshaking. Nerves are stimulated by subcutaneously positioned portable PENS probes.27 After three treatments with a neurostimulation device, five of seven horses had durable positive responses, returning to work at the same level as before the onset of headshaking. Median remission time was 15.5 weeks (range, 0 to 24 weeks). PENS treatments need to be repeated when clinical signs return. The procedure has now been used in more than 130 horses, of which approximately 50% have returned to their previous level of activity.28 The logical next step would be the development of programmable implantable nerve stimulators that could conveniently be used for long-term control of trigeminal-mediated headshaking.