Feature

Photic Head Shaking*

❯❯ Bonnie R. Rush, DVM, MS, ead shaking is a behavior in The mechanism of photic head shak- DACVIM which toss their heads, ing may be similar to photic sneezing in rub their noses, snort, and/or humans, in which exposure to bright light ❯❯ Jason A. Grady, DVM, MS, H sneeze in the absence of obvious exter- triggers sneezing episodes. Photic sneezing DACVIM nal stimuli. Some horses head shake with in humans is a heritable, nonallergic dis- Kansas State University such violence that they are dangerous to order. Photic head shaking may represent the handler or rider. a form of referred pain in which stimula- In a 1987 review of 100 head shak- tion of one of the cranial nerves enhances ing horses, the cause was undetermined irritability of the other—in this instance, in nearly 90% of cases. In 1995, Madigan optic-trigeminal summation. This may be and coworkers presented a series of seven associated with convergence between op- cases in which head shaking was trig- tic and trigeminal tracts in the brainstem. gered by natural sunlight, and darkness Therefore, neuropathic pain is the most provided relief from the condition; this plausible explanation for the signs associ- At a Glance disorder is called photic head shaking. ated with photic head shaking. Diagnostic Criteria Photic head shaking horses are suspected Bright sunlight is the most common Page 327 to experience a burning sensation or tin- trigger for neuropathic head shakers, but gling of the muzzle (neuropathic pain) in other stimuli, including specific feeds (as Treatment Recommendations response to bright sunlight. in gustatory head shaking), may also serve Page 329 as a trigger for infraorbital nerve irritability. Therefore, disorders other than neuropathic Differential Diagnosis TO LEARN MORE pain should be evaluated to eliminate other Page 330 causes of this behavior. In some horses, the Prognosis triggering stimulus cannot be identified; Page 331 Understanding Behavior: Head however, the head shaking seems to be char- Shaking (November/December acteristic of neuropathic pain, and affected 2008) patients respond to medical therapy. To see a video of a with head shaking, go to the Web exclusives Diagnostic Criteria at CompendiumEquine.com Historical Information The condition affects adult horses. The *Updated by the authors and re- mean age of onset is 7.5 to 9.2 years. printed with permission from Stan- Related content on dards of Care: Equine Diagnosis CompendiumEquine.com However, it has been documented in and Treatment 2001;1.1:1-4. horses younger than 5 years.

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FIGURE 1 Findings are unremarkable in photic head shakers. Horses with head shaking due to causes other than sunlight usually demonstrate intentional head tossing rather than rapid, vertical flips that resemble a reaction to a bee sting.

Laboratory Findings Complete blood count and serum chemistry analysis findings are unremarkable. Endoscopic examination of the upper respi- ratory tract (including guttural pouches) and radiographic examination of the skull should be performed in all horses with head shaking to rule out temporohyoid osteoarthropathy, a foreign body, or traumatic injury. The results are unremarkable in horses with photic head shaking.

A commercial mask with protective lenses. Other Significant Diagnostic Findings To determine whether the behavior is in- No breed predilection. duced by natural light, ideally, the clinician  are overrepresented. Affected mares should subject the horse to the following: should be evaluated for ovarian dysfunction. direct sunlight, blindfolding, the outdoors at The absence of testicular or ovarian hormones night, and the application of dark eye lenses may play a role in the pathophysiology. (Figure 1) in direct sunlight. The cessation of CriticalPo nt Clinical signs are often seasonal, abating dur- clinical signs during protection from direct ing the winter and returning in the spring. Horses may not The behavior is exhibited at rest and during FIGURE 2 exhibit the behavior exercise. Violent head shaking is common at during the exami- the beginning of exercise. nation; therefore, Affected horses attempt to avoid direct sun- the owner should light by seeking shade or hiding their heads be asked to make in unusual places. a videotape of the Photic head shaking may begin after an upper head shaking before respiratory tract infection. the appointment. Horses may not exhibit the behavior during the examination; therefore, the owner should be asked to make a videotape of the head shaking before the appointment.

Physical Examination Findings Sudden, violent, jerking movements of the head in the absence of obvious external stimuli. Characteristic quick vertical flips or jerking movements (as if stung by a bee on the end of the nose) may be interspersed with horizontal and rotary activity. Affected horses often snort, sneeze, and rub their noses. Ophthalmic, otic, and oral examinations should

be performed to rule out other causes of Paras Felecia by Illustration head shaking. Infraorbital nerve block.

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sunlight indicates that light is a stimulus for Favorable response to bilateral infraorbital head shaking. or posterior ethmoidal nerve block in horses Bilateral infraorbital nerve block (Figure 2; that do not respond to . 10 mL of mepivacaine over and within the infraorbital canal) should be performed in Treatment Recommendations horses with characteristic head shaking that Medical Therapy does not respond to medical management. If Cyproheptadine nerve block results in the abatement of clini- Cyproheptadine is an and sero- cal signs, the diagnosis is confirmed. tonin antagonist with anticholinergic effects. Bilateral posterior ethmoidal nerve block The mechanism of action of cyproheptadine (Figure 3; 5 mL of mepivacaine) using a in treating photic head shaking is unknown. 7-cm, 19-gauge spinal needle: the needle Administer a 7-day course of cyprohepta- is inserted below the zygomatic arch and dine (0.3–0.6 mg/kg PO q12h) to determine directed rostrally and ventrally toward the response to therapy. upper sixth cheek tooth (approximately 5 Horses that respond favorably should be cm; Figure 4). treated with cyproheptadine during the sea- CriticalPo nt son in which they exhibit head shaking. Approximately 70% Differential Diagnosis Adverse effects include transient lethargy, to 80% of photic See Box 1. depression, or anorexia. head shakers respond favorably Summary of Diagnostic Criteria for Photic to cyproheptadine. Head Shaking Carbamazepine (3–4 mg/kg PO q6–8h), which Negative findings from otic, ophthalmic, oral, is labeled for treating in Some horses may endoscopic (the upper airway, including the humans, may be administered alone or with respond initially but guttural pouches), and radiographic (skull) cyproheptadine in horses that fail to respond become more resis- examinations. to cyproheptadine alone. tant to therapy. Demonstration of sunlight-induced head shak- ing (sunlight may not be the only trigger for neuropathic head shaking). Melatonin (15–18 mg PO q24h administered Favorable response to a 7-day course of cypro­ between 5:00 and 6:00 p m ) has reduced clini- heptadine and/or carbamazepine (see below). cal signs in some horses.

FIGURE 3 Topical EMLA Cream Topical EMLA cream (AstraZeneca; lido- caine 2.5% and prilocaine 2.5%) may provide

FIGURE 4

Illustration by Mal Rooks Hoover Mal Rooks by Illustration Posterior ethmoidal nerve block. A 7-cm, 19-gauge spinal needle is Diagram of the landmarks for a posterior inserted below the zygomatic arch and directed rostrally and ventrally toward ethmoidal nerve block. the upper sixth cheek tooth.

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Mechanical Techniques Differential Diagnosis The use of a full-face net (covers ears to Box 1 muzzle) or fly fringe or netting over the Te mporohyoid osteoarthropathy: muzzle has reportedly been effective in con- ❯ Proliferation of the stylohyoid bone— trolling signs of head shaking. It is believed perform an endoscopic examination that the net or fringe over the muzzle pro- of the upper airway that includes the vides a counterstimulant that may reduce guttural pouches. responsiveness of the trigeminal nerve through ❯ Osteitis of the petrous temporal receptor inhibition or adaptation. bone—perform a radiographic A commercial face mask with eye protection examination. can be applied to control clinical signs when Foreign body, parasite, or infection in the horse is saddled or turned out during the external auditory canal—perform an daylight (Figure 1). otoscopic examination. Oral pain (wolf teeth or diastema), Surgical Intervention oral ulceration, periodontal disease, Bilateral infraorbital neurectomy is a periapical abscess—perform oral and ­salvage procedure for cases refractory to radiographic examinations. medical therapy. Surgical candidates must Iris cysts, uveitis, photophobia, demonstrate a consistent response to serial retinal lesions—perform an ophthalmic infraorbital nerve blocks. Infraorbital neurec- CriticalPo nt examination. tomy has been shown to be effective in Miscellaneous conditions: eliminating signs of head shaking. However, If sunlight has been ❯ Guttural pouch mycosis, epiglottic clinical improvement is inconsistent and identified as the entrapment—perform an endoscopic often only temporary. Postoperative com- examination. plications may include nasal pruritus (com- stimulus of infraor- ❯ Cervical pain—perform a radiographic mon, temporary), reinnervation, and neu- bital pain, reduced examination of the cervical vertebrae. roma formation. sunlight exposure ❯ Trombicula autumnalis larval Bilateral sclerosis of the posterior eth- is indicated for infestation (chiggers) of the muzzle— moidal branch of the trigeminal nerve perform a skin scrape. untreated horses is induced via perineural injection (5 mL) of or horses that do 10% phenol in almond oil. In an anesthetized not respond to transient relief when applied to the muzzle. patient, insert a 20-cm styletted needle into medical or surgical Cover area with plastic wrap (make holes for the infraorbital canal to the level of the max- management. nostrils). Leave on for 45 minutes. illary foramen; confirm needle location by fluoroscopy. Combination Treatment In a recent study on caudal compression of Cyproheptadine (as described above). the infraorbital nerve in 24 horses, 16 of 1 Magnesium supplementation (Quiessence, Fox­ 19 horses had a successful outcome. The pro- den Equine, Stuarts Draft, VA). cedure requires specialized equipment, and Increases threshold for nerve depolar- the authors report that the technique needs ization and may reduce irritability of refinement. trigeminal nerve. Patient Monitoring Administer 2 oz q24h PO. Horses should respond to cyproheptadine Increase to 4 oz q24h PO if no improve- within 7 days. ment. If head shaking is well controlled, the dose Evaluate serum magnesium after 2 wk, of cyproheptadine may be decreased to 0.12 then once monthly. mg/kg PO q12h. Spirulina Wafers (Springtime, Inc, Cockeys­ In many cases, medication may be discontin- ville, MD). ued during the winter. The mechanism is unknown, but they may increase the threshold for nerve Farm Management depolarization and decrease irritability If sunlight has been identified as the stim- of the trigeminal nerve. ulus of infraorbital pain, reduced sunlight

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exposure is indicated for untreated horses or horses that do not respond to medical or sur- Recommended Reading Did You gical management. Affected horses can be Know? turned out at night or during overcast days. C ook WR. Head shaking in horses: an after­ word. Compend Contin Educ Pract Vet If turned out during full sunlight, affected Infraorbital 1992;14:1369-1372. horses should have protection from the sun neurectomy via a three-sided shed, trees, or a commercial Lane JG, Mair TS. Observations on head­ was one of the shaking in the horse. Equine Vet J 1987;19: earliest treat- face mask that provides eye protection. 331-336. ments for head Milestones/Recovery Time Frames Madigan JE. In: Smith BP, ed. Large Animal shaking.a Cyproheptadine Internal Medicine, 4th ed. St. Louis: Mosby; Approximately 70% to 80% of photic head 2009:1044-1045. aWilliams WL. Invol- shakers respond favorably to cyprohepta- Madigan JE, Bell SA. Characterisation of untary shaking of the head and its treatment dine. Some horses may respond initially but headshaking syndrome—31 cases. Equine by trifacial neurectomy. become more resistant to therapy. Vet J Suppl 1998;27:28-29. Am Vet Rec 1899;23: More than 80% of horses respond favorably Madigan JE, Bell SA. Owner survey of head- 321-326. to carbamazepine alone or in combination shaking in horses. JAVMA 2001;219(3):334- with cyproheptadine. 337. Relief from head shaking may be observed Madigan JE, Kortz G, Murphy C, Rodger within 48 hours of initiation of therapy. L. Photic headshaking in the horse: 7 cases. Clinical signs typically recur 24 hours after Equine Vet J 1995;27:305-311. discontinuation of medical therapy. Mair TS. Assessment of bilateral infraor- bital nerve blockade and bilateral infraorbital Bilateral Infraorbital Neurectomy neurectomy in the investigation and treat- Approximately 30% to 40% of photic head ment of idiopathic headshaking. Equine Vet CriticalPo nt shakers improve after this procedure. J 1999;31:262-264. Careful case selection (consistent response to Mair TS. Headshaking associated with More than 80% of infraorbital nerve block) may increase the like- Trombicula autumnalis larval infestation in horses respond lihood of a positive postoperative outcome. two horses. Equine Vet J 1994;26:244-245. favorably to car- Mair TS, Howarth S, Lane JG. Evaluation bamazepine alone Nose Net of some prophylactic therapies for the idio- or in combination Clinical signs of head shaking completely pathic headshaker syndrome. Equine Vet J with cyproheptadine. resolve in 25% to 30% of horses with the use Suppl 1992;11:10-12. of a nose fringe or netting that applies pres- Mills DS, Cook S, Jones B. Reported re- sure to the skin over the muzzle. sponse to treatment among 245 cases of equine headshaking. Vet Rec 2002;150: Treatment Contraindications 311-313. Administration of , corticoster- Mills DS, Cook S, Taylor K, Jones B. Anal- oids, or NSAIDs is unrewarding. ysis of the variations in clinical signs shown Intradermal skin testing followed by aller- by 254 cases of equine headshaking. Vet gen-specific hyposensitization therapy is un- Rec 2002;150(8):236-240. successful. Mills DS, Taylor K. Field study of the effi- Prognosis cacy of three types of nose net for the treat- Favorable Criteria ment of headshaking in horses. Vet Rec 2003;152(2):41-44. Response to cyproheptadine within the first 7 days of treatment. Newton SA, Knottenbelt DC, Eldridge PR. Headshaking in horses: possible aetiopatho- Unfavorable Criteria genesis suggested by the results of diag- Failure to respond to recommended thera- nostic tests and several treatment regimens pies. used in 20 cases. Equine Vet J 2000;32: 208-216. Reference Wilkins PA. Cyproheptadine: medical treat- 1. Roberts VL, McKane SA, Williams A, Knottenbelt DC. Caudal compression of the infraorbital nerve: a novel surgical technique for ment for photic headshakers. Compend treatment of idiopathic headshaking and assessment of its efficacy Contin Educ Pract Vet 1997;19:98-111. in 24 horses. Equine Vet J 2009;41(2):165-170.

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