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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Photic headshaking is a condition where, in the absence of any external stimuli other than light, a horse vigorously and violently shakes its head in horizontal, vertical, or rotary directions. Clinical signs are alleviated immediately upon reduction of light.

Pathophysiology!!navigator!!

  • Photic headshaking is probably a form of optic–trigeminal nerve summation in which retina and optic nerve stimulation produces referred sensation to facial sensory branches of the trigeminal nerve in the nasal cavity, including the infraorbital and sphenopalatine nerve, with possible involvement of the trigeminal ganglion. The irritability in the nasal cavity causes the horse to shake its head
  • Recent studies confirmed the involvement of the trigeminal nerve in headshaking. It has been shown that the threshold for activation of the infraorbital nerve (a branch of the maxillary division of the trigeminal nerve) is significantly lower in affected horses than in healthy controls
  • This condition shares similarities with the photic sneeze syndrome in human patients, when sudden exposure to bright light immediately leads to a single or a series of sneezes. Convergence in the brainstem between trigeminal and optic nerves has been suggested as a possible mechanism in the photic sneeze and may also play a role in photic headshaking
  • Extracranial vasodilation via release of vasoactive peptide as a result of stimulation of the trigeminal nerve or ganglion has also been suggested as a possible cause for this condition

Systems Affected!!navigator!!

  • Ophthalmic
  • Nervous

Genetics!!navigator!!

Heritability and genetic background in the horse are unknown; however, photic sneeze in human patients appears to be determined by innate and hereditary factors.

Incidence/Prevalence!!navigator!!

N/A

Geographic Distribution!!navigator!!

Worldwide

Signalment!!navigator!!

This condition affects adult horses with a mean age of 9 years. In one study, Quarter Horses were overrepresented while Thoroughbreds were overrepresented in another. Geldings are more frequently affected than mares.

Signs!!navigator!!

  • Excessive and occasionally violent rubbing, sneezing, snorting, and flipping of the nose usually accompany the headshaking
  • Although clinical signs may be seen at rest or during exercise, most horses show clinical signs shortly after the beginning of exercise
  • This condition is seasonal, and the most common onset of clinical signs is during the spring and early summer; however, in some cases, headshaking can develop in the fall. Usually clinical signs regress spontaneously in the winter and recur in the spring/summer
  • Low head position and active seeking of shade is seen occasionally

Causes!!navigator!!

Sunlight may stimulate referred sensation via parasympathetic activity in branches of the trigeminal nerve, resulting in irritating nasal sensations with ensuing headshaking.

Risk Factors!!navigator!!

  • Bright daylight
  • Spring, summer, and to a lesser extent, the fall season
  • The triggering factor leading to long-term photoperiod-dependent headshaking is unknown

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Middle or inner ear disorders
  • Ear mites
  • Cranial nerve dysfunction
  • Maxillary osteomas
  • Premaxillary bone cysts
  • Guttural pouch disease
  • Upper respiratory tract disease
  • Vasomotor rhinitis
  • Allergic rhinitis or sinusitis
  • Equine protozoal myelitis
  • Cervical injury and pain
  • Dental diseases
  • Ocular disease
  • Iris cysts
  • Cystic corpora nigra
  • Infraorbital neuritis
  • Behavioral abnormalities

CBC/Biochemistry/Urinalysis!!navigator!!

Usually normal

Other Laboratory Tests!!navigator!!

N/A

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

Photic headshaking is diagnosed based on characteristic clinical signs seen on bright sunny days and with a decrease or cessation of clinical signs when light is reduced or eliminated (e.g. blindfolding, evaluation during the evening). Clinical signs should be evaluated at rest and during exercise. Other etiologies for headshaking (see Differential Diagnosis) should be ruled out utilizing the appropriate diagnostic testing.

Pathologic Findings!!navigator!!

N/A

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

The complete workup may require overnight hospitalization. The medical management can be done by the trainer/owner.

Nursing Care!!navigator!!

Riding affected horses only during times of dim light or darkness is impractical. However, alleviation of clinical signs can be achieved by lowering the amount of light entering the eye. This can be accomplished by the use of a face mask and gray lenses. Placement of two overlying face masks sprayed with black paint was also reported to improve the photic headshaking. In some cases, more than one layer of gray lenses was needed to improve clinical signs. one study utilized tinted contact lenses and reported only a partial and temporary positive effect; however, the lenses used were blue or green (and not gray). It is possible that dark gray contact lenses will provide improvement of clinical signs.

Activity!!navigator!!

Headshaking can occur at rest or during exercise. Because most headshakers show signs shortly after the onset of activity, there may be unforeseen risks in working an uncontrolled photic head shaker.

Diet!!navigator!!

Magnesium increases the threshold for nerve firing, hence increasing the stimulus needed for depolarization. As headshaking was found to be associated with a reduced activation threshold of the trigeminal nerve, magnesium seems to be a rational therapy for this condition. Daily oral supplementation with 10–20 g of magnesium led to improvement in headshaking in about 40% of the horses in one report. Nevertheless, optimal serum magnesium levels have not been established in the horse. Evaluating ionized magnesium serum concentration before and 14 days after the beginning of supplementation is recommended to avoid possible toxicity.

Client Education!!navigator!!

  • Severely affected horses that do not respond to therapy may inflict considerable self-trauma and may be dangerous to handle or ride
  • Avoiding bright light, such as riding at night or in an indoor facility, may be helpful. However, this is impractical in many cases, especially for performance horses

Surgical Considerations!!navigator!!

  • Infraorbital neurectomy—the response to this procedure is poor with high complication rates and, therefore, is not recommended
  • Sclerosis of the posterior ethmoidal nerve is associated with high rate of recurrence
  • Compression of the caudal infraorbital nerve has been described for the treatment of headshaking, with a reported overall success rate of about 50%. However, recurrence of clinical signs was observed in some of the horses requiring additional surgery. A transient increase in severity of clinical signs was seen in 63% of the horses postoperatively, and 4/58 horses had to be euthanized due to the severity of clinical signs and/or nonresolution. As such, this procedure should be considered only for horses that did not respond to medical therapy and euthanasia is the only other option

Medications ⬆ ⬇

Outline

MEDICATIONS

Medical therapy controls, but does not cure, the condition. If effective, medical treatment may only be needed during the season in which the horses exhibit headshaking behavior.

Drug(s) of Choice!!navigator!!

  • Cyproheptadine (H1 blocker), a serotonin antagonist that alters proopiomelanocortin metabolism, has the potential to alleviate the clinical signs at a dose of 0.3 mg/kg PO BID
  • Carbamazepine is an anticonvulsant that stabilizes voltage-gated sodium channels. A dose of 4 mg/kg PO TID–QID may be effective; however, such a frequent administration schedule could be problematic. In one case, a higher dose of 8 mg/kg was needed to alleviate clinical signs
  • A combined treatment of cyproheptadine and carbamazepine may be synergistic

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

  • Treatment of performance horses should comply with the rules of the governing organization
  • Mild lethargy, anorexia, and depression have been reported in horses treated with cyproheptadine
  • Drug eruptions have also been reported with cyproheptadine

Possible Interactions!!navigator!!

Additive central nervous system depression may be seen if cyproheptadine is combined with barbiturates or tranquilizers. Cyproheptadine has anticholinergic effects, which may be intensified by monoamine oxidase inhibitors (e.g. furazolidone).

Alternative Drugs!!navigator!!

To mimic winter conditions physiologically, melatonin therapy at a dose of 12 mg PO on a sugar cube once between 17:00 and 18:00 daily may also be beneficial. For horses with seasonal headshaking, best results are achieved when melatonin therapy is started before the onset of spring.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Although improvement or cessation of clinical signs may be seen within 3–4 days after initiation of medical therapy, a 7 day trial may be needed to determine whether the patient will respond favorably to medical management. Therapy can be stopped periodically and reinstated if the behavior recurs and there are no side effects.

Prevention/Avoidance!!navigator!!

Horses kept in a dark environment may not show severe clinical headshaking. Light-blocking protectors or a “nose net” may also help control the clinical signs for some horses. This type of management, however, may not be practical for the horse or its owner and trainer.

Possible Complications!!navigator!!

A horse with uncontrolled headshaking may develop unwanted head trauma, and the owner or trainer may find it difficult for the horse to optimally work or perform.

Expected Course and Prognosis!!navigator!!

Long-term, seasonal medical therapy can control, but not cure, this disease. Infraorbital nerve blocks may relieve clinical signs in some horses. In some cases, cessation of clinical signs was seen for months to several years. The reasons for the resolution and recrudescence of clinical signs are unclear.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Photic sneezing in humans.

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Synonyms!!navigator!!

Head tossing

See Also!!navigator!!

Headshaking

Suggested Reading ⬆ ⬇

Brooks DE.Ophthalmology for the Equine Practitioner, 2e. Jackson, WY: Teton NewMedia, 2008.

Gilger BC. Equine ophthalmology. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary Ophthalmology, 5e. Ames, IA: Wiley Blackwell, 2013:1560–1609.

Gilger BC, ed. Equine Ophthalmology, 3e. Ames: Wiley Blackwell, 2017.

Author(s) ⬆

Author: Gil Ben-Shlomo

Consulting Editor: Caryn E. Plummer

Acknowledgment: The author and editor acknowledge the prior contribution of Dennis E. Brooks.