Trigeminal-mediated headshaking is a frustrating condition to manage. It has significant ramifications for horse owners wishing to compete their horses or even just ride them outside due to a variety of behaviors associated with episodic facial pain. Unfortunately, the peak of head-shaking behavior coincides with the most active riding seasons, i.e., spring and summer. Owners try nose nets, UV-protective facemasks, and a number of medications and nutritional supplements. Some also resort to anesthetic nerve blocks, neurectomy, nerve compression, or electrical neuromodulation of the infra-orbital nerve.
A common therapy attempt uses oral administration of magnesium sulfate (MgSO). Anecdotal reports suggest a 40% decrease in headshaking behavior when horses are on this substance.
With that in mind, a study was done to investigate the intravenous use of MgSO for its role in nerve impulse transmission [Sheldon, S.A.; Aleman, M.; Costa, L.R.R.; Santoyo, A.C.; Howey, Q.; Madigan, J.E. Intravenous infusion of magnesium sulfate and its effect on horses with trigeminal-mediated headshaking. J Vet Intern Med Jan 2019].
Intravenous infusion of MgSO elicits a temporary elevation of blood concentrations of magnesium, thus might help mitigate discomfort caused by abnormal firing of the trigeminal nerve.
Six geldings with seasonal headshaking syndrome were included in the study at the University of California’s School of Veterinary Medicine. The study design was that of a randomized, controlled crossover. Each horse was exercised with a one-minute walk, then 1-3 minute trot, 1 minute canter, and finally a one-minute walk. For these horses, exercise exacerbated the headshaking behavior. Right before starting the exercise, the horses were given either 5% dextrose IV or 50% MgSOsolution IV over 5-10 minutes through a catheter. Blood samples were obtained just before IV infusion, then at 15, 30, 50 and 120 minutes post-infusion.
All headshaking behaviors—vertical headshakes, head tossing, snorting, nose rubbing, dropped head—were recorded during each exercise level and at the five time points for blood collection. None of the evaluators knew which treatment the horses received.
Blood magnesium concentrations peaked at five minutes after intravenous infusion of MgSO and remained increased by 120 minutes post infusion.
For MgSO-treated horses, headshakes per minute decreased by 29% at first walk, trot, canter, and second walk and decreased by 51% at canter when compared to treatment with dextrose. The best effect in lessening headshaking was seen between 30-120 minutes post infusion with MgSO, although the study only covered two hours.
Overall, when factoring in time, at 120 minutes, there was 55% decrease in headshaking at trot and an 80% decrease at canter for horses treated with MgSO. In contrast, horses treated with dextrose showed a 10-fold increase of headshakes/minute at all gaits other than canter, which increased 22-fold.
Such a large bolus of MgSO has the potential to induce some adverse effects, such as sweating, agitation, muscle fasciculations, arrhythmias and collapse. However, no horses in the study experienced any of these concerns.
The authors noted, “Our study highlights the possible blunting effects of magnesium during faster gaits that are likely to exacerbate head-shaking signs.” Long-term use might ultimately rely on oral supplementation with magnesium products that have high bioavailability.