Imagine this clinical scenario: you are presented with a seven-year-old warmblood gelding used for eventing, who has been diagnosed with trigeminal-mediated headshaking.
Trigeminal-mediated headshaking is a neuropathic facial pain condition in horses. Clinical signs are of vertical headshaking, often accompanied by sharp vertical ticks and associated with signs of nasal irritation such as snorting, sneezing and rubbing the nose.
The gelding responds well to the use of a nose net but is not allowed to compete in one. The owner has heard that electroacupuncture is a treatment option and wants to know how it compares to EquiPENS (percutaneous electrical nerve stimulation) treatment, so you explore the evidence.
Three descriptive case studies were identified that investigated neuromodulation treatment for trigeminal-mediated headshaking in horses, which provides a weak strength of evidence.
The first study (Roberts et al., 2016) investigated seven horses diagnosed with trigeminal-mediated headshaking. The horses were not known to be seasonally affected and were all displaying clinical signs at the time of treatment. They were treated with EquiPENS neuromodulation – after sedation and local anaesthetic, the trigeminal nerve was stimulated at a frequency set to stimulate facial twitching while maintaining patient tolerance for 25 minutes bilaterally. The treatment course involved three sessions, with further treatments on re-emergence of clinical signs. The outcomes studied were efficacy in terms of remission – return to previous level of work or higher; safety in terms of adverse effects; and owner-reported outcomes – presence and severity of headshaking and competition record where applicable. Six horses entered remission after the first treatment and five continued to respond with further procedures. Median remission length was 15.5 weeks after three treatment sessions. Limitations include the small sample size and lack of control group.
In a second study by Roberts et al. (2020), 168 horses diagnosed with trigeminal-mediated headshaking were treated with EquiPENS neuromodulation. The treatment course involved three sessions, with further treatments on re-emergence of clinical signs. Remission was defined as a return to previous levels of work within three weeks of the last treatment. The assessment was made by the owners and obtained via telephone. Of the 136 horses available at follow-up, 53 percent went into remission following the initial course and the median remission duration was 9.5 weeks. Of all procedures, 8.8 percent had complications. Study limitations include the lack of a control group, subjective assessment of remission and some horses being lost to follow-up. The authors also acknowledge a level of support as a training centre for EquiPENS, so a potential risk of bias should be considered.
A final study (Devereux, 2019) included four horses and two ponies considered to be affected with trigeminal-mediated headshaking by the author and referring vet or referral practice. Three horses were sensitive to sunlight, and two were known to be seasonally affected. The horses received electroacupuncture treatment of the infraorbital nerve, under light sedation. Electrical stimulation occurred until a visible nostril twitch was maintained for 25 minutes. Treatment was repeated on re-emergence of clinical signs. Reduction in the grade of headshaking was seen in all horses and median remission length after three treatments was 18 days. No complications were reported, although post-treatment worsening of clinical signs was seen. Limitations include the small study size, lack of control group and that some subjects continued to wear a UV mask and nose net post-treatment, which could have confounded the results.
The available evidence for the use of neuromodulation in the treatment of trigeminal-mediated headshaking in horses is weak but indicates that it can provide safe and effective remission of varying timescales for a proportion of horses. The evidence for EquiPENS is more robust than for electroacupuncture and so could be recommended with greater confidence; however, all three studies are descriptive case series and provide no control group, blinding or randomisation. The strength of evidence is also confounded by the fact that headshaking may undergo spontaneous or seasonal remission, is dynamic in its severity and responds variably to treatments. Further research into both treatment options is required, ideally randomised controlled studies that will offer stronger evidence, although the ethical implications around withholding or delaying treatment should be considered.
The use of neuromodulation as a treatment for neuropathic pain stems from human medicine, with PENS being an approved treatment for disorders such as trigeminal neuralgia under the National Institute for Health and Care Excellence (NICE) guidelines. Despite differences in pathogenesis, similarities in symptoms between horses with trigeminal-mediated headshaking and people with trigeminal neuralgia may allow both to benefit from further studies into the use of neuromodulation for neuropathic pain.