Survey findings from a recently published article on Equine Headshaking Syndrome help shine a light on the issue of light - and season - for this poorly understood syndrome that affects 1% of horses. One of the most significant findings was that bright sunlight is the primary trigger reported for affected horses, suggesting that future research investigate this connection further.
Equine headshaking syndrome is considered a neuropathic pain condition, with an extensive range of symptoms:
Spontaneous and involuntary violent flicking of the head.
Snorting.
Rubbing the face against leg or objects.
Striking at the nose with the forelimb.
Equine headshaking is also referred to as trigeminal-mediated headshaking syndrome (TMHS), since it has been associated with hyperexcitability of the trigeminal nerve, but the pathophysiology of this condition is still not clear.
There are consistent triggers that are connected to the display of symptoms, and this recent study reported that the most common triggers are bright sunlight, wind, high pollen count, and dust.
Based on survey responses, symptom onset varies across season:
All year round: 41.6%
Springtime: 32.8%
Summer: 12.9%
Fall: 3.2%
Unsure: 4.6%
Other less-commonly reported triggers include light rain, cool breeze, heavy rain, exercise, artificial light, dark places, high grass/sugar diet, high legume diet, hot weather, insects, bathing or washing, tack or headcollars, change in light or humidity, and previous injury or surgery. In addition, geldings are more commonly affected than mares (76% vs 24%).
Therapeutic options for this condition are limited because the underlying cause is not fully understood. Existing treatment options are often reported to be only partially successful, and remain an area of equine science that needs more research focus.
This is reflected in survey responses, with the majority of affected horse owners using 2+ treatments to help manage their horse's condition. The most common treatments included nutritional supplements, gear such as nosenets and light-blocking masks, and therapies such as bodywork.
Despite bright sunlight being the most predominant trigger, use of light-blocking masks was reported to be ineffective by 50% of respondents.
The authors of this study believe that the survey results suggest that there is a seasonal intensification of symptoms, which may result from day length and photoperiod-related hormone changes.
Reproductive hormone involvement is shown in pain mediation in humans, as well as human patients with trigeminal neuralgia, which speaks to the potential for such a connection in the equine condition.
Based on the higher occurrence in geldings, there is interest in whether the absence of testicular testosterone may impact hormone release contributing to instability of the trigeminal nerve, causing the neuropathic pain involved.
This study speaks to just how much more there is to learn about our beloved equine companions so that we can continue our understanding of their physiology, and further improve lives!
Reference
Bell,T., Kyriazopoulou, P., Mowbray, C., Murphy, BA. 2024. Equine Headshaking Syndrome: Triggers, Seasonality, and Treatment Efficacy in Australia. Animals 14(6): 875.
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