Melanie Perrier, Senior Lecturer in Equine Soft Tissue Surgery

Occasional shaking of the head may be normal, however when becoming frequent, violent and when affecting riding or handling the horse, a pathological process must be considered. Conditions which can cause headshaking include otitis interna, cranial nerve dysfunction, cervical injury, ocular disease, guttural pouch mycosis, dental disease, sinusitis and behavioural or rider issues.

In 98% of cases, when no physical cause could be determined, headshaking is classified as idiopathic headshaking. Idiopathic headshaking is a complex pathology, which can be frustrating for both owner and veterinarian.

Trigeminal mediated headshaking or idiopathic headshaking is defined as an acquired disorder of the horse. The headshaking is typically worst at exercise with only some horses being affected also at rest. Seasonality of clinical signs has been reported in about 60% of cases, the majority being in spring and summer.

Diagnosis

The diagnosis is usually one of exclusion and should include a complete and thorough physical examination and a ridden examination. Further diagnostics may include dental examination, dental endoscopy, otoscopy, maxillary nerve block (if the clinical signs are reproducible and consistent), upper airways endoscopy, skull radiographs and more recently Computed Tomography (CT).

CT is a useful tool to rule out primary disease for which treatment could result in improvement of headshaking. An RVC study found that, in more than 20% of the cases, a treatable primary condition was identified – leading to complete resolution of clinical signs.

Clinically relevant primary diseases in that study included dental fracture, primary sinusitis, temporo-mandibular joint arthritis, nuchal bursitis, musculoskeletal pathologies, basisphenoid fracture, otitis externa and a mass affecting the infra-orbital nerve.

Treatment

While several treatments are available, success rates are extremely variable. There is also considerable placebo effect when interpreting results of treatments and many unpublished treatments used by owners.

Among treatment used, nose nets are found to be cheap, non-invasive, risk-free and allowed in most competition at most levels. They are reported to give up to 70% relief in 25% cases and are believed to act through the gate control theory.

The drug gabapentin is used in equine headshaking but it has poor bioavailability and only anecdotal reports of success. Carbamazepine, which reduces central nerve conduction, is also used. Cyproheptadine, a centrally acting antihistamine and serotonin antagonist, is used, but research findings on efficacy are mixed.

Magnesium may have neuroprotective effects on nerve firing that potentially dampen signs of neuropathic pain. In one study, intravenous infusion of magnesium sulphate was used in six horses with headshaking signs. Horses were evaluated up to two hours after IV Mg infusion and an increased in magnesium blood levels was noted. This resulted in a 29% reduction in headshaking.

Sodium cromoglycate eyedrops have also been used for their mast cell stabilising properties. Three seasonally affected horses were treated and all returned to ridden exercise.

Percutaneous Electrical Nerve Stimulation (PENS) is a treatment translated from electrical nerve neuromodulation. As such it may normalise neural function. Complication rates were reported in up to 8.8% of cases and are usually mild and transient. Remission of headshaking following the initial course occurred in 53%. Median length of time recorded in remission was 9.5 weeks.

PENS is believed to be a good first-line treatment for horses which do not respond to a nose net. More recently, electroacupuncture has been used to modulate perception of afferent nociceptive stimuli by inhibiting afferent pain signals and by activation of descending inhibitory pathways. Electroacupuncture is believed to be less invasive than PENs and may yield similar results.

Finally, surgical treatment includes bilateral infraorbital neurectomy, which has been associated with serious side effects, and caudal ablation of the infraorbital nerve via coil compression. This procedure has been shown to yield 50% success rate in 57 horses, with 26% relapsing within a median time of nine months. Most horses developed side effects of nose rubbing, which was deemed short-term in most cases, but in four of 58 cases, horses were euthanised due to severity or nonresolution of these side effects.