Headshaking in the horse: what do we know?

Introduction

Trigeminal-mediated headshaking (HSK) in horses is a potentially life-threatening disorder that can affect performance, daily activities, and quality of life that might lead to euthanasia. Common clinical manifestations include a violent vertical headshake that resembles an electrical shock to the horse’s nose. Horses have an anxious facial expression, rub their noses in the ground or against objects, strike at their nose wiht their thoracic limbs, and display excessive snorting. Episodes of headshaking can be spontaneous or associated with exposure to UV light, wind, exercise (triggers or exacerbates), and sharp sounds. Most horses are young mature geldings (72-76%) with a mean age at presentation of 8 to 10 years old. Clinical manifestations can occur all year round but most commonly present in a seasonal fashion (spring, summer, and early fall). These observations have led our research group to suspect a multifactorial etiology including hormonal, dietary, and environmental factors all playing a role in the pathogenesis of disease. The exact cause(s) remains unknown.

Pathophysiology

Trigeminal-mediated headshaking is associated to inappropriate firing of the trigeminal nerve due to a low threshold for firing which underlying cause is unknown. The trigeminal complex is composed by central (brainstem, spinal cord tracts) and peripheral components (branches of the trigeminal nerve and trigeminal ganglia). Another central component is the sensory cerebral cortex which is responsible for the perception (awareness) of pain. Histochemical studies have not shown abnormalities of the trigeminal complex. Therefore, this disorder involves a functional abnormality. Sensory nerve conduction of the trigeminal complex from affected horses appears normal in terms of amplitude, duration, and velocity. However, the difference between affected and healthy horses is the low threshold for firing in horses with trigeminal-mediated headshaking (stimulus intensity < 5 mA) compared to healthy horses (> 10 mA). This low stimulus for threshold explains why some apparently innocuous stimuli (e.g., wind on face, exposure to light, sound, exercise) can trigger violent headshaking.

Diagnosis

It is important to consider and rule out other causes of headshaking such as middle and inner ear infection, temporohyoid osteoarthropathy, 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, sinus surgery (complications), nuchal crest avulsion, cervical vertebral osteoarthritis, apparent neck pain, inappropriate tack, bridle or bit, and poor rider skills. Herpes virus infection causing neuritis and/or ganglionitis of the trigeminal ganglia must also be considered. Anesthesia of the infraorbital and maxillary nerves is not specific for trigeminal-mediated HSK because this perineural block will anesthetize any area innervated by these nerves. Diagnostic modalities to rule out some of the disorders listed here include radiographs (skull, neck), endoscopy (upper airway including guttural pouches), ophthalmic and otoscopic examination, and computed tomography and magnetic resonance of the head when available. Nasal swabs for PCR testing for equine herpesvirus 1 should be considered. Allergy testing or allergy medicine can be tried as a diagnostic aid (evaluate response).

Treatment

Several physical devices such as nose and head nets, masks, and goggles with UV light protection have been used successfully in some horses. Riding practices at night for the photo activated headshakers has helped but rendered impractical. Various medications including antidepressants, anticonvulsants, channels blockers, and anesthetics have been used with variable results. Pharmacokinetics of many of these drugs and their mode of action in the horse are unknown, plus possible adverse effects must be considered. Several other therapies such as pulse therapy with steroids, compression or excision of the infraorbital nerve, and percutaneous electrical nerve stimulator (PENS) of the infraorbital nerve have had variable results.

Novel findings

Transient effects of acidification and alkalinization of blood in headshaking

It is known that alterations in blood pH can influence binding of calcium and magnesium to proteins, further influencing the amount of free physiologically active calcium and magnesium (ionized calcium and magnesium). These ions are important modulators of nerve function and have been implicated in neuropathic pain. Therefore, we investigated the effects of various intravenous solutions with varying pH and electrolytes to study the effects on pH, concentrations of Ca++, Mg++, and headshaking in diseased and healthy horses. These IV solutions included 8.5% sodium bicarbonate (2 mmol/kg to alkalinize), 7.5% hypertonic saline (4 mL/kg to acidify), and 5% dextrose (control solution). Changes in blood composition were transient and the greatest decrease in headshaking (> 50%) occurred with IV sodium bicarbonate.

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