Idiopathic trigeminal-mediated headshaking (HSK) in horses is a debilitating disorder characterized by facial pain that compromises performance, daily activities and overall quality of life, often leading to euthanasia.

This disorder is associated with inappropriate firing of the tri geminal nerve due to a low threshold for activation manifesting as violent vertical jerking shakes in 90% of cases, resembling an electrical shock to the horse’s nose. Anxious facial expression, rubbing the nose in the ground or against objects, striking of the face with the thoracic limbs, excessive snorting, sneezing and blinking are also common. Manifestations of pain in between episodes also occurs as shown by increased facial grimace scores.

These horses become unrideable and even dangerous for those around them. The episodes can occur spontaneously or be trig gered in association to exposure to UV light, wind, light touch to the nose (hay), exercise (triggers or exacerbates) and sharp sounds (e.g., metal to metal). The mean age at first episode is usually at 8 to 10 years old. Clinical manifestations can occur year-round but most commonly present in a seasonal fashion (spring, summer and early fall).

The underlying cause(s) remains unknown but multifactorial factors such as environment (e.g., UV light, temperature, season of the year), hormones (more common in geldings – 76% – and diet (acid diets, low ionized magnesium) play an important role.

It is essential to rule out other disorders that can present with head shaking, including:

• External, middle and inner ear infection
• Temporohyoid osteoarthropathy (THO)
• Ear ticks (Otobius megnini)
• Ear mites (Trombicula autumnalis)
• Guttural pouch disorders
• Periapical dental osteitis
• Other dental problems (e.g. wolf teeth abnormalities)
• Allergic rhinitis
• Vasomotor rhinitis
• Ocular disorders (cysts, masses)
• Intranasal masses
• Sinusitis
• Nuchal crest avulsion
• Cervical vertebral osteoarthritis, especially of the cranial cervical vertebrae
• Apparent neck pain
• Inappropriate tack, bridle or bit
• Poor rider skills
• Herpes virus infection causing neuritis and/or ganglionitis of the trigeminal ganglia

Caution must be practiced when performing facial surgeries involving the paranasal sinuses because the infraorbital nerve is contained in the infraorbital canal, a thin osseus canal that can be easily damaged and trigger signs undistinguishable to the idiopathic headshaker. Masses or any other alteration or damage to the infraorbital canal will trigger signs of headshaking due to severe pain. Idiopathic trigeminal-mediated headshaking has no known macroscopic nor histochemical abnormalities; therefore, presumed to be a functional disorder. What makes the nerve trig ger in a seasonal manner or all year remains uncertain.

Because the cause(s) is unknown, it has been challenging to man age this disorder. However, several modalities from environmen tal (moving to a different geographical area, avoiding exposure to UV light, modifying riding practices) and dietary (alkalinization of diet) modification, supplements (magnesium with boron), physical devices (e.g., nose nets, facemasks or goggles with UV light protection), various types of electrostimulation (e.g., percu taneous infraorbital nerve stimulation, electroacupuncture, and transmagnetic stimulation) and drugs with various mechanisms of action (channel blockers, antihistamines, antiseizure medication) have been tried with variable results. The goal of electrostim ulation is to alter the sensory input to the trigeminal complex. Environmental and dietary modification, physical devices and electrostimulation should be attempted first whenever possible before considering pharmacological intervention because of unknown pathophysiology of disease; therefore, lack of targeted therapy, extra-labeled use of some drugs, limited pharmacokinet ic studies, unknown established dosages and potential adverse effects with variable results.

Recent studies have shed some light with regards to possible biomarkers of disease (e.g., calcitonin gene-related peptide in CSF, although not specific) and the identification of cannabinoid receptors in the trigeminal ganglion which might be a potential target for therapeutic management while the underlying cause is identified. The search for answers must continue to address this important welfare issue that affects our equine companions.

Editor’s note: This is an excerpt from Equine Disease Quarterly, Vol. 35, Issue 3, funded by Equus/Standardbred Station Inc. and M&J Insurance. It was written by Monica Aleman, MVZ Cert., PhD, Dipl. ACVIM, professor at the University of California, Davis.