Headshaking in horses can be a frustrating condition to manage. The clinical signs are not specific; there are many possible reasons why a horse may shake its head. In a significant proportion of horses that headshake no underlying structural abnormalities are detected. Many of these horses are believed to have facial pain caused by a hypersensitivity of the trigeminal nerve. These horses are referred to as Idiopathic headshakers, although the term trigeminal-mediated headshakers has also been proposed.

Clinical presentation

Headshaking is characterised by a sharp, usually vertical movement of the head, although rotatory or horizontal headshaking is occasionally seen. This has been described as a rapid downward jerk of the nose followed by an upwards fling of the nose. Affected horses may also experience nasal irritation leading to rubbing of the face and self-trauma. The signs are typically worse during exercise although in some cases they may also be present at rest. Approximately two thirds of cases are seasonal headshakers; with characteristic signs most frequently observed during the spring and summer months of the year. Environmental triggers vary between individual cases; these can include bright sunlight, wind, rain, noise, pollen or dust. Affected horses are typically middle-aged, with geldings more frequently affected than mares.

 Diagnostic investigations

Observing the horse’s behaviour, both in the stable and when ridden, is important. This will allow problems with the tack or behaviour to be evaluated and excluded. It is often useful to observe the horse worked on different days and in different environmental conditions to enable potential trigger factors to be established.  Some horses will stop headshaking when moved to a different environment. If the horse does not display the described behaviour when examined it can be useful to review video footage of the described episodes. A thorough and systematic investigation is then required in order to rule out other pathological conditions that can cause a horse to shake its head. Such conditions would include diseases affecting the bones of the head, the ears, ophthalmic abnormalities, guttural pouch disorders, dental abnormalities and sinus disease.

In addition to a thorough clinical examination, appropriate diagnostic tests should include:

• Imaging of the horses head. Radiographs (x-ray), computed tomography (CT) or MRI of the head, can all be useful.
• Endoscopy of the upper respiratory tract including the guttural pouches.
• Ophthalmic examination
• Aural examination.
• Dental examination

If the classic clinical signs are present and these diagnostic tests are within normal limits, a diagnosis of idiopathic headshaking is often made.

Evaluating the clinical response to anaesthesia of the caudal part of the infraorbital nerve and the caudal nasal nerve has been proposed. A positive response to this nerve block, in the absence of identifiable pathology, should increase the index of suspicion of trigeminal-mediated headshaking. However this technique has proved unreliable due to inconsistent results. The technique will block pain caused by other pathology in the areas innervated by the nerve therefore false positives may also be observed. This renders the results of diagnostic analgesia difficult to interpret.

Treatment of Idiopathic headshaking

A number of treatments have been proposed for the management of horses with idiopathic headshaking (table 1). These are aimed at minimising particular trigger factors, for example by the use of tinted contact lenses for photic headshakers, reducing inflammation of or desensitising the trigeminal nerve. Often these treatments are not curative, with the main aim of treatment to manage the horse to minimise the clinical signs to an acceptable level.

Table 1: Treatments used for the management of horses with idiopathic headshaking.

Nose nets are readily available and have been shown to improve the clinical signs in 25% cases. Similarly full-face masks or fly masks that protect from UV light may also help. Antihistamines such as cyproheptadine have resulted in mixed responses. Side effects include lethargy, drowsiness and anorexia. Carbamezepine is an anticonvulsant drug that can be used alone or in combination with cyproheptadine. Gabapentin is frequently advocated for neuropathic pain and is thought to act on voltage gated calcium channels. There are anecdotal reports of successful treatment in horses with headshaking however the poor oral bioavailability is a concern. Sodium cromoglycate drops have been used to treat horses with headshaking in conjunction with photophobia and excessive tear production. Inconsistent responses have been observed to ant-inflammatory administration such as corticosteroids. Magnesium supplements are readily available and may help by reducing the threshold for nerve firing.

As some horses do seem to respond to some of the above medications, treatment can be rewarding. This may involve sequential trials of each drug, or in severe cases combined therapy followed by step wise withdrawal to determine the significant contributer to the clinical imporvement. In many cases no response is observed, therefore a guarded prognosis should be given.

Neurectomy of the infraorbital nerve has been attempted using various techniques such as laser cautery or the application of platinum coils. Unfortunately these techniques have resulted in unacceptable side effects, often with worsening of the clinical signs.  Such techniques should therefore only be undertaken if euthanasia of the animal is being considered.

Recently percutaneous electrical nerve stimulations (PENS) has been developed by Dr Veronica Roberts at the University of Bristol. This involves repeated treatments of nerve stimulation that result in periods of remission, of increasing duration, between successive treatments. The latest data suggests moderate-term improvement or resolution in clinical signs allowing return to previous work in 39% of cases. PENS is now available at a number of referral centres throughout the UK.

Conclusions

The aetiology of idiopathic headshaking is not fully understood. The condition is associated with a hypersensitivity of the trigeminal nerve. To date there is no reliable diagnostic test, therefore the condition is diagnosed on the presence of characteristic clinical signs in the absence of identifiable structural disease. Treatment response is variable but generally success rates are low to moderate with therapeutic strategies aimed to manage the clinical signs rather than achieving long-term cure.

Dr Rosie Naylor BVetMed MVetMed DipACVIM PhD MRCVS