15 Oct 2024
Veronica Roberts offers a background overview of this condition, followed by what is new based on literature.
Veronica Roberts
Job Title
Image (expanded) © Justin / Adobe Stock
Trigeminal-mediated (TGM) headshaking is an acquired condition. The median age of onset is nine years, but with a wide age range.
TGM headshakers usually show classic signs of vertical shaking; sharp vertical tics; and nasal irritation such as snorting, nose rubbing and striking at the nose (both sides). Signs are worse at (any) exercise, but may also be present at rest. They may be seasonal, with a complex and not understood association with environment.
No gross pathology exists (Roberts et al, 2017). The infraorbital branch (and potentially the other branches) are sensitised, firing at too low a threshold (Aleman et al, 2014). We think this then results in neuropathic pain (which in people is usually described as varying occurrence and intensity of burning, pins and needles or electric shock-like pain).
Many reasons exist as to why a horse might shake its head more than normal. This affects 4% of the UK equine population, with 1% (25% of the 4%) seeing a vet (Ross et al, 2018).
TGM headshaking is a diagnosis of exclusion, which is imperfect – mostly with a risk of over-diagnosis.
History and observation are hugely important for your index of suspicion for TGM headshaking, alongside ruling out potential causative gross pathology using CT, upper respiratory tract endoscopy, oroscopy/oral exam and ophthalmic exam. This may be different in cases with known seasonality.
The condition appears to have a 5% spontaneous remission rate and a 25% to 64% seasonal remission (depending on whether remission is classed as complete absence of signs or just improvement). The nerve appears to be sensitised, firing at too low a threshold and appears to be a functional, not structural abnormality (so could potentially be reversed).
Neuropathic pain is hard to treat even in people. We know little about TGM headshaking, with an incomplete understanding of trigeminal nerve sensitisation, and incomplete understanding of its role in the aetiopathogenesis of TGM headshaking. More than one cause with the same clinical manifestation may exist.
We need an objective measure of headshaking to be able to validate treatments as a significant placebo effect exists – about 30%.
We will consider the literature published over the past five years, in categories.
A survey of cases in Germany (Maxi Stange et al, 2020), France and Switzerland (Maxi Stange et al, 2022) found owners reported very similar signalment, histories and clinical signs, as has been previously reported from the US and UK. The most popular treatment was a nose net then a supplement.
After this, 11% of owners tried a scientifically tested treatment and 84% tried alternative therapies. A survey of Australian cases showed the same patterns as for the northern hemisphere (Bell et al, 2024).
The caecal microbiome of a small group of TGM headshakers differed to that of a small group of horses with orthopaedic disease, kept under the same management conditions (Aleman et al, 2022). Seasonally affected geldings did not have different levels of luteinising hormone, compared to healthy geldings in late summer (Sheldon et al, 2019a).
Cannabinoid receptors are expressed in the trigeminal ganglion of the horse (Zamith Cunha et al, 2023) and further work is required in this area.
A study assessing TGM headshakers, non-TGM headshakers, and forelimb lame horses found that using a tri-axial accelerometer taped to the headpiece was an accurate measure of severity and frequency of headshaking over a five-minute period on the lunge, in trot. Some measures showed good predicative values for diagnosis of TGM headshaking (Pickles et al, 2024). Further work is required to develop this for practical use.
History, rest and exercise score (Kloock et al, 2024a) is a valid and reliable score for assessing disease severity, independent of observers’ experience, confirmed in another study of 140 clinical cases (Kloock et al, 2024b) and now available on an app. It involves inputting data regarding the horse’s history and clinical signs, with the data being weighted by the app and a score being produced.
One of the challenges of reaching a diagnosis in TGM headshaking is that it is a diagnosis of exclusion. CT is currently our best practical imaging modality for the horse’s head and three studies have considered the use of CT in diagnosis of headshaking, finding likely causative gross pathology (so not TGM) in:
But not all gross pathology is relevant (Edwards et al, 2019) and the differences between the study results do not seem easily accounted for.
Magnesium sulphate intravenously decreased headshaking by 29% (Sheldon et al, 2019b).
Magnesium citrate and boron (increases absorption of boron) orally decreased headshaking by 64% (Sheldon et al, 2019c). However, it should be noted that horses were assessed over a five-minute period in walk and canter, and most severe signs are usually seen in trot.
It is not permitted to feed boron in the UK. We can measure total magnesium, but not ionised in the UK. You can calculate a dose (Marlin, 2023) or potentially can titrate a dose. Remember clinical signs of weakness and sweating at total mg greater than 2mg/dL.
The author thinks we should moderate owner expectations of magnesium supplementation to a slight reduction in signs, but to be used as part of a holistic approach. However, it maybe that if horses respond they do not see a vet (only 25% headshakers see a vet), so she may see those with a poorer response.
EquiPENS neuromodulation (Roberts et al, 2020) gave 50% remission (168 horses, 530 procedures) for an unpredictable period of time after three procedures. The author considers it an expensive gamble, but with some good successes. It can be considered as possible management, but unlikely a cure.
We are making some progress, but we still do not know the cause (and more than one may exist) of TGM headshaking. Without knowing the causes, we will always struggle to treat it.
We need to communicate science better to the public – it is disappointing that only 25% of headshakers see a vet and 84% are treated with alternative therapies, compared to 11% with scientifically tested therapies (Maxi Stange et al, 2020; 2022).
