What is it?
Trigeminal-mediated headshaking is a neuropathic facial pain syndrome. Neuropathic pain is pain from nerves and is different to ‘normal’ pain. People who suffer neuropathic pain often describe burning sensations, ‘pins-and-needles’ and even electric-shock like pain. We can only assume that horses experience the same or similar. Clinical signs in horses include vertical headshaking, often accompanied by sharp vertical ticks and signs of nasal irritation such as snorting or sneezing, rubbing the nose and even striking at the nose. Signs are usually worse at exercise but may also be present at rest. Some horses are only, or worse, affected seasonally, usually in the Spring and Summer. Severely affected horses can become impossible or dangerous to ride and even less severely affected horses may be unable to perform at their intended level. Horses affected at rest cannot even have relief through being retired.
Which horses are affected?
Trigeminal-mediated headshaking is an acquired condition rather than being one the horse is born with. It usually starts when the horse is a young adult and, in most cases, comes on suddenly. In some studies, geldings are more often affected. There is no strong breed pre-disposition and it is does not appear to be directly hereditary. There seems likely to be a complex interaction with the horse’s environment but we don’t know what or how that is; it is not ‘normal’ allergy.
How common is it?
Research that we did found that 1% of the UK equine population has had to see a vet for headshaking.
What causes it?
We don’t know. There have been many studies, including by ourselves, to try to find the cause but while we have ruled out many possible causes, we have not found the answer. It is possible that there is more than one cause, with the same clinical manifestation. This would make treatment and prevention even harder.
What goes wrong?
So far, the only fact about the pathogenesis of Trigeminal Mediated Headshaking of which we can be sure is that the trigeminal nerve of affected horses is sensitised, firing at too low a threshold. Seasonally affected horses, when out of season and free of clinical signs, appear to have a normal threshold. It should be noted though, that this information is based on a very small number of horses. The test is complex and carried out under general anaesthesia.
We don’t know why or how this happens. The nerves appear completely normal when examined under a microscope.
This leads us to hope that the condition is reversible. However, as we don’t know why or how it happens, preventing or reversing it is currently close to guesswork.
Can horses spontaneously recover?
There is very little what is called ‘longitudinal data’ on headshakers, looking at what happens to them over time. However, one study has suggested we can expect about 5% of affected horses to recover spontaneously.
As stated above, we don’t know why or how trigeminal-mediated headshaking occurs. This makes treatment – and indeed prevention - very challenging. It is possible that there is more than one cause, making treatment and prevention even harder. It is recognised in human medicine that response to treatment for neuropathic pain varies amongst individuals, even with the same diagnosis. Therefore, even if all trigeminal-mediated headshakers have the same underlying condition, response to the same treatment may vary.
When considering what has been published about treatments, consideration must be made of the reliability of diagnosis, the mechanism of assessment of response used and placebo effect. The assessment of response to treatment can be challenging. Diagnosis is currently made through a combination of exclusion and clinical suspicion, rather than a definitive test, which allows for the possibility of mis-diagnosis. Horses can vary from day to day and season to season in the severity of their signs. Furthermore, interpretation of these signs is heavily influenced by placebo effect – that is we hope so much that the horse is improved that we think it is. Therefore, the most objective measure currently available is classification of success as being return to ridden work at the previous level. Where possible, trials of treatments for headshaking should include placebo or control groups. However, this is not always possible due to welfare considerations in a group of horses suffering a painful condition.
Here we will consider published treatments. There are many unpublished treatments used by owners of headshakers. Assessment of efficacy of unpublished treatments is particularly challenging; it is not known if a veterinary diagnosis was made, or whether the criteria for success are sufficiently robust as to compensate for placebo effect. Here, we will only consider scientifically proven, rather than disproven, treatments.
The rationale behind more established therapies with some evidence of success appears to be to reduce sensory input from the trigeminal nerve, intended or not, even before sensitisation was shown to occur.
Scientifically proven treatments
This is the first treatment to try as it is cheap, non-invasive, risk-free and is allowed in most competition at most levels. It is reported to give up to 70% relief in 25% cases. The mechanism by which a nose-net may work is thought likely to be similar to (but not the same as it does not travel through the spinal cord) gate control theory – the same reason why if you bang your elbow, rubbing it makes it feel better. A nose-net is non-invasive, affordable and accepted in most by many, but not all, competition regulatory bodies. It can be useful to try a few different types (we recommend to try three different types although do not recommend specific products) and a full facemask.
There are drugs available for the treatment of neuropathic pain in people and some of these have been used in trigeminal-mediated headshakers to see if their neuropathic pain could be managed this way. Even in people with neuropathic pain, these drugs have inconsistent results and may confer side-effects including drowsiness. These are likely to be more of a problem still in horses as we do not know the dose to give or how frequently. Of first-line drugs used for neuropathic pain in people, use of gabapentin has been published in the horse, but not in cases of trigeminal-mediated headshaking, although it is trialled in some cases. The use of carbemazepine and/or cyproheptadine is published in trigeminal-mediated headshakers but results are mixed, and as with people, occasionally a positive response is merely short-term. Some horses may be affected by drowsiness so there may need to be consideration as to whether they are safe to ride/handle. It is our interpretation of these results that, as some individuals can respond well, these drugs can be worth trying as long as expectations are managed. It should be noted that use of these medications would not be allowed in competition, and this fact may be sufficient to limit further research in this area.
3. Sodium cromoglycate eye drops
Administration of sodium cromoglycate (a mast cell stabilizer) eye drops was effective in three seasonally affected horses, returning them to ridden exercise. This could suggest allergic conjunctivitis as a cause in these horses. Unfortunately, it appears that conventional allergy is not the cause in the majority of headshaking cases so usually horses do not respond, but trialling this treatment is low risk.
Cutting the nerve (bilateral infraorbital neurectomy) helped 3/19 horses with serious side effects being common. As such it is not a suitable surgery to do now but at the time it helped to identify the nerve as being involved.
The nerve can be compressed with coils (caudal ablation of the infraorbital nerve via coil compression) which had better results, with about a 50% success rate in 57 horses but 26% relapsed with a median time of nine months (range two months to five years). Most horses developed side effects of nose rubbing which were short-term in most cases but 4/58 were euthanased due to severity or non-resolution of these side effects. It is our opinion that due to the risk of side effects and the development of better procedures, this surgery is not usually to be recommended.
5. EquiPENS™ Neuromodulation
This is a technique we have pioneered, working with a human neurosurgeon and neuropathologist. We have trained hospitals across the UK and Europe.
The technique was translated from percutaneous electrical nerve stimulation neuromodulation, a minimally invasive procedure which can have efficacy for the management of neuropathic pain in people. Potentially, the procedure is effective by normalizing the way the nerve functions, but no basic science has been performed to investigate the mechanism of action.
An initial trial, using an electrical protocol based on humans and an initial three procedure course, was published in 2016. This demonstrated that the procedure was possible to perform under standing sedation, had minimal risk of side effects and returned 5 of 7 horses to ridden work.
The procedure is carried under sedation and takes about an hour and a half and is an outpatient procedure, either at Breadstone or your own vets. Competition withdrawal from the procedure is only a few days for the sedative drugs and local anaesthetic. There is an initial three procedure course, carried out with approximately 1 week between the first and second procedures and approximately 14 days between the second and third. If the horse has not gone into remission after 3 weeks after the end of the course then the procedure has not worked.
We have recently published data (Equine Veterinary Journal, 2019) on 168 trigeminal-mediated headshakers receiving EquiPENS™ neuromodulation (530 procedures) has followed.
The complication rate was very low, with only 8.8% of procedures affected and in all but one case, complications were mild and transient, with no self-trauma. The most common complication was that of likely neuritis which is nerve inflammation. Signs were a worsening of headshaking and nasal irritation. They resolved in a few days without treatment, or in hours if steroid was given to reduce inflammation. Affected horses could still then go into remission. If a horse had experienced neuritis after a procedure, they were more likely to have it again at the next procedure and so pre-treatment with steroid would be recommended where suitable.
Remission of headshaking following the initial course occurred in 53% (72/136) of horses. Average (median) length of time recorded in remission was 9.5 weeks (range 2 days to 5 years ongoing). Where signs recurred, most horses went back into remission following future procedures usually for longer than from the previous procedure. No predictors for outcome were determined. EquiPENS™ neuromodulation, whilst clearly carrying limitations, is a good first-line treatment for horses which do not respond to a nose-net.
Electroacupuncture was attempted for the treatment of trigeminal-mediated headshaking, following the initial indication of possible success of EquiPENS™ neuromodulation. There was possible success in a very small group of horses but unless work is done on a large number of horses it cannot be compared to EquiPENS ™.
Electrolytes are important for nerve function and therefore it is likely that they are somehow involved in trigeminal-mediated headshaking. However, there is probably a complex relationship; it is not ‘normal’ allergy. There is some evidence that supplementing feed with magnesium and boron can reduce headshaking signs; however headshaking was still present and there was little improvement beyond what could be achieved by just feeding concentrates as compared to a horse just eating hay or grass. Horses can be supplemented with magnesium easily – there is a supplement made by Platinum Performance containing magnesium citrate and boron in the same mix as was used in the study but it is currently only available in the US. There is no UK licence to feed boron. However, the boron is just there to increase uptake of magnesium. There is evidence that magnesium citrate is more easily absorbed than other types of magnesium. The team at B and W can supply you with a supplement to use alongside other treatments. There is some work suggesting grass from soil with high potassium may contribute to headshaking but no published study yet. There is also work suggesting we should avoid rye grass - very hard in UK equine pasture but it may be easier with hay or haylage to find a meadow or timothy grass source. This area is very promising but we know very little as yet.
We currently recommend EquiPENS™ as a first line treatment for trigeminal mediated headshaking where a nosenet fails or is prohibited in competition. It can have brilliant results, but not in every case and we cannot predict who it will help. We recommend giving a magnesium supplement alongside. We need research into trigeminal-headshaking to progress so that we can improve treatment for the condition.
Dr Veronica Roberts MA(Oxon) MA VetMB (Cantab) PhD PGCert(HE) DipECEIM FRCVS