One of the more frustrating issues that owners and veterinarians deal with is the headshaking horse. While not a common malady, it does occur in 2-4% of horses. Of that percentage, geldings are affected 75% of the time.
At the 2023 VMX Veterinary Meeting & Expo in Orlando, Florida, Sandy Taylor, DVM, PhD, DACVIM, of Purdue University, presented a comprehensive overview of idiopathic headshaking and potential treatments. She pointed out that the nerve itself is structurally normal and is not affected by inflammation or demyelinization. Instead, it is a functional problem that affects both sides of the trigeminal nerve.
One study she referenced (Aleman, et al., JVIM 2013) performed nerve conduction studies on affected horses under general anesthesia. A normal trigeminal nerve (TGN) doesn’t fire until stimulated with 20 mA of current. In contrast, the TGN of a headshaking horse fires at 2.5 mA. To provide context, a normal horse senses pain at 10 mA or more. Many veterinarians are familiar with shock from an electric fence, which delivers 120 mA of current.
Headshaking Triggers
Triggers of headshaking include exercise and/or sunlight—particularly in the spring and early summer—while other cases have no identifiable trigger and the nerve is hypersensitized continually. An affected horse typically shakes its head violently—80% of the time it is a vertical shake; 10% of the time it is horizontal; and 10% of horses display both vertical and horizontal headshaking. The horse experiences an “electric-shock sensation” that elicits varying degrees of anxiety, snorting, face rubbing on the legs, and striking at the face.
Taylor emphasized the importance of ruling out other possible differential diagnoses based on clinical signs that are associated with other health issues such as otitis externa/media/interna; temporohyoid osteoarthropathy (THO); TMJ (temporomandibular joint) arthritis; dental disease or tooth fracture; nuchal bursitis; nerve tumor; EPM (equine protozoal myelitis); or a behavioral issue.
In one study of 84 headshaking horses, only 22 had an identified primary cause that resulted in secondary headshaking. The other 62 horses were considered idiopathic headshakers because other causes were ruled out.
Potential Treatments for Headshaking
Referencing the “gate control” theory of pain, Taylor described how potential treatments might help.
Nose Net
A nose net applies “pressure” or touch that stimulates large A-beta fibers that close the “gate” in the central nervous system to block pain sensation to the brain from small C fiber transmission. In 50% of cases, nose nets achieve 50% relief; in 25% of cases, they achieve 70% relief.
Cyproheptidine
Cyproheptidine is a serotonin antagonist. While 50-75% of treated horses showed improvement in one study, the effects are often short-lived. In addition, this drug might cause drowsiness, is not approved for most competition venues, and is expensive ($35/day). Taylor cautioned against relying on compounded formulations.
Gabapentin
Gabapentin has been used to address headshaking, although it is poorly absorbed and only 16% bioabsorbable. High doses lead to sedation, and it is not allowed in competition. To date, gabapentin has not been studied for use in headshaking horses.
Nerve Stimulation
Nerve stimulation to desensitize the TGN has shown promise, but it requires multiple applications.
PENS (percutaneous electrical nerve stimulation) stimulates the nerve via a needle probed beneath the skin around the infraorbital nerve. Alternating current is applied for 25 minutes, up to three times, one week apart. In a 2019 study with 168 horses, 53% went into remission for at least 10 weeks. To date, PENS does not have FDA approval.
Electro-acupuncture uses electric current pulsed through an acupuncture needle over the infraorbital foramen. The sole study conducted thus far showed promising results.
Magnesium Supplementation
Magnesium supplementation has been used. The mechanism is not clear, but magnesium might block neuropathic pain signals. A study (Sheldon, et al., JVIM 2018) evaluated magnesium in multiple ways: a) Six horses received IV magnesium sulfate (MgSO4) to achieve 29% reduction in headshakes per minute while exercising; b) Six horses received four different diets: i) hay only, ii) Equine Senior and no supplement, iii) Equine Senior + magnesium supplement, and iv) Equine Senior + magnesium supplement + boron (boron increases intestinal absorption of magnesium). The combination of magnesium and boron gave the best results—a 64% decrease in headshakes per minute during exercise.
