Head shaking can prevent otherwise healthy horses from performing normal driving or riding tasks. There are also safety concerns with a real risk of injury to the rider. The syndrome can affect any breed or discipline, and although most are affected at onset from 7 to 13 years of age, the reported age range is very wide, from 1 to 28 years of age. A survey of horse owners in the United Kingdom suggested a prevalence around 4% of the population.
Most horses begin showing signs under saddle or being driven with bit and bridle, and progress with signs becoming more violent and episodes lasting longer. Some horses will show signs at pasture and eventually around 60% of horse will head shake at rest. The condition can become distressing for both horse and rider.
What does head shaking look like?
It has been described as “involuntary sudden violent repetitive movements of the head”. Most commonly the head will move up and down in a vertical plane, although it can move side-to-side in a horizontal, or in a circular or rotatory plane. Affected horses may also rub their nose on their legs, ground or other inanimate objects. They may sneeze, snore or snort, and they may have a clear ocular or nasal discharge. Many do not like being touched around the face or ears, and may resent tack being placed on them.
The horse behaves as if there is a real or perceived noxious sensation about the nose or face. Imagine an insect flying up a nostril. Blocking of caudal nerves relieves signs temporarily in many horses with idiopathic head shaking. This suggests the likely basis of the distress in most horses is triggering or firing of sensory trigeminal nerve fibres in the caudal nasal passages. The trigeminal nerve is cranial nerve V.
There are many reported triggers for head shaking, including bright light, wind, rain, and riding on narrow roads. It has been estimated that about half of all cases are seasonal, beginning in spring and ending in late summer or early autumn. In Western Australia I believe this percentage to much higher. The horses that only shake in bright light are termed photic head shakers. Again this may be related to trigeminal nerve firing through optic-trigeminal summation pathways.
In some horses there is a clear trigger for head shaking. This list can be extensive, including (but not limited to) allergic rhinitis, vasomotor rhinitis, temporohyoid osteoarthropathy (THO), external or middle/inner ear infections, dental problems, sinus disease or surgery, guttural pouch infections, eye diseases (cystic corpora nigricans, particles in the front chamber of the eye), blocked nasolacrimal ducts, or osteoarthritis of the temporomandibular joints (TMJ). Psoroptic otitis, caused by the rabbit ear mite, Psoroptes cuniculi can cause head shaking and ear rubbing in horses, with a build up of thick greasy crusts in the ear canal. While ruling out physical causes is important, it is common in most investigations to not find an underlying physical basis of the head shaking.
Veterinary approach to diagnosis
The search for causes often involves a thorough physical examination, neurologic exam, dental exam, ophthalmologic examination, endoscopy of the nasal passages, guttural pouches and nasopharynx, radiographs of the skull, and bloodwork. Examination of the ear canal can be difficult. Another important part of the assessment is to assess with muzzle net and to determine whether the signs are linked to light.
Management
A good number of horses with seasonal head shaking, believed to be caused by allergic rhinitis, will respond to muzzle nets. In some horses the response will approach 100%. Some horses may also improve with dangling devices placed over the forehead or muzzle. There are also a number of medical approaches, but none will guarantee remission. Improvement has been seen with carbamazepine, gabapentin and pregabalin, although any positive response could be short-lived. Cyproheptadine has been useful in horses with photic headshaking, and it may potentiate the actions of carbamazepine in non-photic shakers. A few horses with seasonal head shaking have responded to corticosteroids or antihistamines. Melatonin has been advocated overseas. Other techniques including chiropractic therapy and dry needle acupuncture have been used but success seems rare.
Percutaneous electrical nerve stimulation (PENS) of the infraorbital nerves has been welcomed as a promising technique to manage horses. The technique can be performed using portable PENS probes or more recently success has been reported using an electroacupuncture/PENS technique. Most horses require three initial treatments to achieve a positive response for around 4 months, with some horses responding for around 6 months. It should be noted that the response is not universal and some horses only show short-lived or no response to the therapy.
Tags: Neurology