EQUUS - - Medical Front -

Re­searchers are a step closer to un­der­stand­ing the role played by the trigem­i­nal nerve in equine head­shak­ing.

Char­ac­ter­ized by spon­ta­neous and re­peated flip­ping of the nose, head­shak­ing can have many causes in­clud­ing al­ler­gies or poorly fit­ting tack. But when other fac­tors are ruled out, veterinarians fo­cus on the trigem­i­nal nerve, which runs along each side of the face.

In peo­ple, a con­di­tion known as trigem­i­nal neu­ral­gia causes burn­ing or sting­ing sen­sa­tions on parts of the face. Of­ten trig­gered by ac­tiv­i­ties such as eat­ing or drink­ing, trigem­i­nal neu­ral­gia has been linked to de­myeli­na­tion, which is dam­age to the pro­tec­tive cov­er­ing sur­round­ing nerve fibers.

“De­myeli­na­tion causes some­thing called ephap­tic con­duc­tion, so when the nerve im­pulse---which is an elec­tri­cal sig­nal---trav­els along the nerve and gets to the de­myeli­nated area, it is the same as get­ting to an unin­su­lated part of an elec­tri­cal wire. The im­pulse goes a bit crazy and trig­gers all the nerve fibers in the area, in­clud­ing the pain ones, to fire,” ex­plains Veron­ica Roberts, MRCVS, of the Uni­ver­sity of Bris­tol.

Roberts and her re­search team set out to de­ter­mine whether a sim­i­lar process is at work in horses with un­ex­plained head­shak­ing. For their study, the re­searchers re­moved and dis­sected the trigem­i­nal nerves of six horses eu­tha­na­tized be­cause of in­tractable head­shak­ing. For com­par­i­son, they also re­moved and dis­sected the same nerve from four horses eu­tha­na­tized for other rea­sons.

They found no ev­i­dence of de­myeli­na­tion of the trigem­i­nal nerve or sur­round­ing struc­tures in any of the horses, even those eu­tha­na­tized be­cause

of se­vere head­shak­ing.

These findings don’t mean the trigem­i­nal nerve isn’t in­volved in head­shak­ing, says Roberts; in­stead they sug­gest that the pathol­ogy is dif­fer­ent from the one in hu­mans--and that may be a good thing.

“It’s def­i­nitely trigem­i­nal nerve pain, just not due to de­myeli­na­tion,” she says. “The nerve is sen­si­tized and fir­ing at too low a thresh­old. We don’t know why that hap­pens, but with no struc­tural ab­nor­mal­ity de­tected in the nerve, it sug­gests it’s a func­tional ab­nor­mal­ity. It gives hope for treat­ment like neu­ro­mod­u­la­tion to work.”

Ref­er­ence: “Trigem­i­nal nerve root de­myeli­na­tion not seen in six horses di­ag­nosed with trigem­i­nal me­di­ated head­shak­ing,” Fron­tiers in Vet­eri­nary Sci­ence, May 2017

trigem­i­nal nerve

oph­thalmic nerve max­il­lary nerve mandibu­lar nerve TROU­BLE SPOT: Trigem­i­nal nerve pain is be­lieved to be re­spon­si­ble for in­tractable head­shak­ing in some horses.

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