Feature Photic Head Shaking* ❯❯ Bonnie R. Rush, DVM, MS, ead shaking is a behavior in The mechanism of photic head shak- DACVIM which horses 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 horse 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. CompendiumEquine.com | September 2009 | Compendium Equine: Continuing Education for Veterinarians® 327 Photic Head Shaking 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 Geldings 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. 328 Compendium Equine: Continuing Education for Veterinarians® | September 2009 | CompendiumEquine.com Photic Head Shaking 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 cyproheptadine. 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 antihistamine 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 Carbamazepine to cyproheptadine. Head Shaking Carbamazepine (3–4 mg/kg PO q6–8h), which Negative findings from otic, ophthalmic, oral, is labeled for treating trigeminal neuralgia 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 Melatonin 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 PM ) 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. CompendiumEquine.com | September 2009 | Compendium Equine: Continuing Education for Veterinarians® 329 Photic Head Shaking Mechanical Techniques Differential Diagnosis The use of a full-face net (covers ears to BOX 1 muzzle) or fly fringe or netting over the Temporohyoid 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,