Headshaking horses improved significantly when given magnesium sulfate intravenously, researchers report.
The University of California, Davis, study involved six geldings whose headshaking related to their trigeminal nerve had not responded well to conventional treatments.
Trigeminal-related headshaking is a little-understood nerve-related facial pain condition in horses.
It affects around 1% of horses to a degree that requires veterinary attention.
Affected horses display violent, mostly vertical, head shakes, and other common signs including apparent itching of the skin, tingling, and presumed burning or electric-like sensation in the face, similar to that described for neuropathic pain in people.
Horses may snort, rub their nose on their limbs, or attempt to strike at their face with their thoracic limbs in attempts to get relief.
The response to a range of treatments is variable, and badly affected horses are often euthanized.
Shara Sheldon and her colleagues, writing in the Journal of Veterinary Internal Medicine, noted that magnesium may have neuroprotective effects on nerve firing that potentially dampen signs of neuropathic pain.
Anecdotally, magnesium supplementation leads to a decrease in head-shaking signs in 40% of affected horses.
The study team set out to investigate head-shaking behavior in affected horses after receiving an intravenous infusion of magnesium sulfate (more commonly known as Epsom salts).
The six horses used in the study comprised four Quarter Horses and two Thoroughbreds, aged 5-13.
The horses were known to be seasonal head shakers and their condition had been resistant to various treatments, including antihistamines for possible allergies.
The horses were all displaying head-shaking behavior at the time of the study.
The researchers used a randomized crossover design for the experiment, in which the horses served as their own controls.
All were kept on the same diet for the five-week duration of the study, in which they received infusions once a week.
The horses received either an intravenous infusion of 5% dextrose solution, at 2 millilitres per kilogram of body weight; or the magnesium sulfate 50% solution at 40 milligrams per kilogram of bodyweight diluted in the 5% dextrose solution.
Head-shaking behavior was recorded before infusion, and then 15, 30, 60, and 120 minutes afterwards at the walk, trot and canter. Blood samples were also taken and tested.
The study team reported several blood changes in the horses that received the magnesium sulfate, including higher ionized magnesium concentrations (Mg2+), compared to when receiving only the dextrose.
They reported a 29% reduction in the head-shaking rate occurred following the magnesium sulfate treatment, but no change after the dextrose-only treatment.
The observed decrease in head-shaking was significant, they said.
Discussing their findings, the study team noted that, as exercise intensity increased from walk to trot to canter, median head shakes per minute also increased, suggesting that exercise might have contributed to the triggering or worsening of the observed signs.
They said the effect from the magnesium sulfate was expected to be observed over a short time-frame because of the blood and tissue distribution of the solutions after a single infusion.
The largest decrease in head shakes per minute was seen at the canter, with a 51% reduction.
“The faster the gait, the larger the decrease in median head shakes/minute after magnesium sulfate treatment.
“Therefore, our study highlights the possible blunting effects of magnesium during faster gaits that are likely to exacerbate head-shaking signs.”
The effect of the treatment was seen from the 30-minute mark to the 120-minute mark.
“This blunting effect might have persisted longer than our study period of two hours,” they said. “Therefore, further evaluation to determine how long this blunting effect persists is warranted.”
They noted that the infusion of magnesium sulfate corrected the lower concentrations of magnesium ions observed at baseline, which also was seen in a previous study of horses affected by trigeminal-mediated headshaking.
“The decrease in head-shaking behavior could be a result of Mg2+ inhibiting nerve transmission.
“The exact mechanism of how magnesium acts as a pain reducer is not known but may involve inhibition of neurotransmission by blockade of receptors involved in cell excitation.
“Intravenous magnesium supplementation in our study was expected to have short-acting effects on magnesium concentrations and head shaking as fluid distribution occurred in the body. However, magnesium concentrations increased and remained increased 120 minutes after infusion.
“The time required for normalization of plasma magnesium concentrations after infusion was not determined in our study.”
They continued: “Our study supports anecdotal reports of the perceived benefits of magnesium supplementation in affected horses.”
Future studies should investigate practical supplementation of magnesium with high bioavailability for long-term use.
“Doing so might help address a major welfare challenge in equine medicine and possibly avoid euthanasia.
“Further investigation is warranted to determine if a cutoff concentration of Mg2+ would be associated with the prevention of or decrease in head shaking in affected horses.”
The full study team comprised Shara Sheldon, Monica Aleman, Lais Rosa Costa, Ana Santoyo, Quinn Howey and John Madigan.
Intravenous infusion of magnesium sulfate and its effect on horses with trigeminal-mediated headshaking
Shara A. Sheldon, Monica Aleman, Lais Rosa R. Costa, Ana C. Santoyo, Quinn Howey, John E. Madigan.
First published 22 January, 2019. https://doi.org/10.1111/jvim.15410