There are many possible reasons why a horse may shake its head. These include poor riding, challenging behaviour, the presence of a foreign body in the nasal passages, a tooth root abscess and even back pain. Facial pain – or dysaesthesia (abnormal sensation) – can certainly be a reason for a horse to shake its head.
Facial pain may be present as a result of detectable gross pathology, such as a tooth root abscess, or, unfortunately, in the majority of headshakers due to a trigeminal neuropathy of unknown cause, now best described as “idiopathic trigeminal-mediated headshaking”. This has been the reported cause in 98 percent of cases (Lane and Mair, 1987) though the number has decreased to 90 percent since the advent of CT scans (Fairburn et al., 2022).
What is trigeminal-mediated headshaking?
Trigeminal-mediated headshaking appears to be an acquired condition, with age of onset usually young adult (in the author’s experience, most commonly 5 to 10 years, but with a fairly wide range), and it may be more common in geldings.
Clinical signs
Signs are often acute in onset (to the point that owners can often give you an exact date and time), but some may be insidious. A classic history owners give is that while out riding, “a bee must have flown up his nose, he went crazy and has never recovered”. The signs may occur at rest but are usually worse during exercise. They may be seasonal (about one-third) and if so, usually occur in spring/summer. They may also be worse outdoors than indoors.
There is little longitudinal data regarding long-term disease progression, however horses are often reported to progress for the first few weeks and then plateau
Clinical signs are usually classic and involve predominantly vertical head and neck movements. These often occur with some sharp vertical twitches/flicks and are accompanied by signs of nasal irritation such as snorting, twitching lips and rubbing and/or striking at the nose. Both sides of the face appear affected.
There is little longitudinal data regarding long-term disease progression, however horses are often reported to progress for the first few weeks and then plateau. It seems that spontaneous remission is rare but not impossible.
Causes
From the above information, we can conclude that the condition is acquired. However, we do not know why this occurs, except that it likely involves exposure to some unknown factor. There may be some kind of environmental influence, given that it is seasonal in some horses, but we do not know what this may be, though we do know it is not an allergy. There is no evidence for direct heritability.
The fact that headshaking symptoms can be seasonal suggests the condition could be reversible, if only temporarily. We assume seasonally affected trigeminal-mediated headshakers suffer the same condition as those affected all year round, but of course we may be dealing with two different conditions.
New discoveries
In the last few years, practitioners have made several advances in the knowledge of trigeminal-mediated headshaking, even if we are far from a complete understanding.
It has been discovered that the trigeminal nerve in affected horses is sensitised; that is, it fires at too low a threshold. Threshold potentials can be tested under general anaesthesia (Aleman et al., 2013; Pickles et al., 2014). Certainly, this increased sensitivity fits with the experiences owners often report, such as the horse being very reactive to the presence of insects or dust.
The nerve appears normal microscopically (Aleman et al., 2013; Roberts et al., 2014), but is functionally abnormal, which does fit with a potentially reversible condition that could be consistent with our observations of seasonality in some cases. Importantly, this discovery gives some hope for treatment.
How to approach a case in first opinion practice
1) Triage
First establish whether the horse needs to be seen urgently. The vast majority of headshaking cases do not need to be seen urgently, but occasionally signs have a sudden onset, with the horse affected at rest. These horses may warrant a prompt visit for their own welfare. (The author has had two cases where owners reported headshaking, but in fact one horse was seizing and the other had hepatic encephalopathy!)
The vast majority of headshaking cases do not need to be seen urgently, but occasionally signs have a sudden onset, with the horse affected at rest
For most horses that do not require an urgent visit, the author suggests asking the owner to collect video footage before a visit to avoid the not uncommon situation where the horse appears perfectly normal on the day you examine them. The author asks the owner, if it is safe to do so, to collect videos of the horse:
- headshaking in the stable and/or field, if affected at rest
- on the lunge with a headcollar
- with a bridle
- with a bridle and saddle
- with a bridle, saddle and side reins/pessoa
Videos of the horse ridden in the arena and when hacking are also helpful, although it is useful to explain that these are better taken from the side rather than from on top! These videos help with history and observation – probably the most important parts of investigation of a headshaking case.
2) History and observation
The author finds patient history and observation to be of great importance when determining the index of suspicion for trigeminal-mediated headshaking.
Consider the signalment and when the signs occur, for example if they are worse at exercise than at rest. Furthermore, determining whether headshaking occurs not only during exercise when ridden, but also when loose, on the lunge or in the stable and/or field helps rule out many possible causes, such as poor riding, bad behaviour and ill-fitting tack – even in cases where those factors are concurrent! If headshaking only occurs when at rest or eating then even where there are signs consistent with neuropathic pain, you are most likely to find gross pathology (for example, dental disease).
If headshaking only occurs when at rest or eating then even where there are signs consistent with neuropathic pain, you are most likely to find gross pathology
Most trigeminal-mediated headshakers will be young to middle-aged and display the typical signs of vertical headshaking, accompanied by sharp vertical tics and signs of nasal irritation, and crucially be worse at any exercise than at rest.
3) Grade the severity
From history and observation, the author then grades the severity of the headshaking. This enables a more subjective assessment of response to treatment.
The author developed the following grading system as a practical approach (Roberts, 2014):
- 0/3 – no headshaking
- 1/3 – headshaking at exercise but not sufficient to interfere with ridden exercise
- 2/3 – headshaking at exercise, making the horse impossible or dangerous to ride
- 3/3 – headshaking even at rest
4) Response to treatment
Even before owners have called a veterinarian to see their horse, they are likely to have tried a nose net. Nose nets broadly fall into three different categories: an often competition-legal one that looks a little like a hairnet, a stiff plate style or a nosebag style. Most owners have tried the first variety, but it may be useful for them to try the others as well. If the horse responds to a nose net, the headshaking is likely caused by neuropathic pain. However, about three-quarters of trigeminal-mediated headshakers do not respond to a nose net, so failure to respond does not help with diagnosis.
It can be useful to try a facemask (the Guardian mask appears to be the best at reducing exposure to UV light), which can be done concurrently with a nose net. Response to a Guardian mask could infer trigeminal-mediated headshaking, photic headshaking or ocular pathology.
If the horse’s signs are not typical for trigeminal-mediated headshaking, there may be merit in a non-steroidal anti-inflammatory drug (NSAID) trial, more so if the owner understands that a negative response does not rule out musculoskeletal pain. Inhaled corticosteroids as a trial may help alleviate any concerns that signs are due to an allergy, which is often considered when cases appear seasonal. Trigeminal-mediated headshaking does appear to have a complex association with the environment but is not a classic allergy, so a trigeminal-mediated headshaker will not respond to steroid treatment.
5) Is the headshaking due to facial pain?
Since facial pain, neuropathic or otherwise, is a common cause of headshaking, diagnostic local anaesthesia may be considered. This is because it may have a place in proving, though not disproving or showing why, there is facial pain. Further investigations may be rapidly directed towards the head if the presence of facial pain is proven.
Diagnostic local anaesthesia may be considered […] because it may have a place in proving, though not disproving or showing why, there is facial pain
Not every case is invasive, minor complications are common and major complications are possible for diagnostic anaesthesia as the horse must tolerate a procedure under only light sedation and be shaking consistently and reproducibly on the day of the exam. Beyond this, bilateral maxillary anaesthesia is invasive, and common minor and major complications are possible. Bilateral rostral infraorbital anaesthesia is not very reliable and while simple to perform, can be quite aversive to some horses.
A positive result will confirm facial pain, but a negative result will not rule it out. Wilmink et al. (2015) also showed an influence of lack of experience on the likelihood of a false negative result. They found that needle placement was accurate in 80 percent of cases if performed by an “experienced” vet but in only 20 percent for those who did not perform the technique regularly. The author recommends, therefore, to only perform this procedure where horses present as atypical, and it may be best placed within a referral setting.
6) Is there gross pathology causing facial pain? If not, is the pain likely to be from a trigeminal neuropathy?
The next stage of the investigation is to perform a standard and thorough clinical exam, including an oral exam (ideally with oroscopy), an ophthalmic exam, upper respiratory tract and guttural pouch endoscopy, and computed tomography (CT).
All but CT can be performed in the field. Equally, many vets choose to refer cases for further investigation once they have ruled out causes that are easy to identify in the field. This is a sensible approach, especially as the majority of information in these cases is gained from history, signalment and observation.
If the author detects possibly significant abnormalities, they are likely to advise treatment where possible. Significance can then be determined if headshaking is seen to resolve (Lane and Mair, 1987).
