OR­THO­PE­DICS The causes of string­halt

EQUUS - - Consultant­s - Bruce A. Con­nally, DVM, MS Wy­oming Equine Long­mont, Colorado

Q: This year I bought a horse to pre­vent him from go­ing to slaugh­ter. I im­me­di­ately rec­og­nized a sti­fle is­sue, per­haps a lock­ing patella, but be­cause I have the means and the space for a pas­ture pet, I pur­chased him from the auc­tion. I will never know his his­tory, but there is ev­i­dence that he has had an ac­ci­dent. He looks like he got into sheep wire and has a healed in­jury to one hoof that gives him no prob­lems. His is­sue is that oc­ca­sion­ally his hind leg locks in the up po­si­tion. He ap­pears to be in no pain. I am hop­ing you will out­line treat­ment op­tions for me. He is a very kind, re­spect­ful 8-year-old horse who de­serves a chance to live his life to the fullest. I don’t need him to be a rid­ing horse, but if he could be sound enough to ride it would be good. Lynn How­land Maple Val­ley, Wash­ing­ton

A: A lock­ing patella (up­ward fix­a­tion of the patella) will cause the rear leg to lock in ex­ten­sion. The toe will of­ten drag on the ground be­cause the horse is un­able to bend the sti­fle or the hock. What you have de­scribed sounds like the op­po­site con­di­tion: “The leg lock­ing in the up po­si­tion” is most com­pat­i­ble with a di­ag­no­sis of string­halt, which is char­ac­ter­ized by ex­ces­sive con­trac­tion of the dig­i­tal ex­ten­sor mus­cles---some­times to the point of the horse kick­ing him­self in the belly with each step. It is not thought of as an overtly painful con­di­tion, but it is an ex­treme mus­cle con­trac­tion or spasm.

There are two ma­jor cat­e­gories of string­halt:

• Aus­tralian string­halt (which also oc­curs in the United States) re­sults from in­ges­tion of a toxic plant. Flatweed, common dan­de­lion and lit­tle mal­low have all been im­pli­cated in this form of the dis­ease, although th­ese weeds are not con­sis­tently toxic. It is pos­si­ble cer­tain en­vi­ron­men­tal con­di­tions may in­crease toxicity be­cause this form of the dis­ease seems to oc­cur more of­ten in late sum­mer or fall. Horses with Aus­tralian string­halt are of­ten af­fected in both rear legs. Spasms can also some­times oc­cur in the front legs and the mus­cles of the lar­ynx. Th­ese horses usu­ally re­cover when they stop in­gest­ing the toxic plant, but it may take months.

• Clas­sic string­halt is not re­lated to plant toxicity. It may re­sult from a neck or back in­jury, but most com­monly the cause re­mains a mys­tery. Horses af­fected with clas­sic string­halt rarely re­cover and of­ten be­come pro­gres­sively worse with time. The dis­ease usu­ally af­fects one rear leg but some cases progress to in­volve both. Many treat­ments, in­clud­ing mus­cle re­lax­ants, an­ti­con­vul­sant drugs, Bo­tox (bo­tulinum toxin) and surgery, have been tried with vari­able suc­cess. Th­ese horses are not ap­pro­pri­ate for rid­ing and are of­ten very limited in their abil­ity to per­form ground­work ma­neu­vers.

Your horse is very lucky to have found a home that does not re­quire him to be an ath­lete. If you do de­cide to ride this horse I would strongly sug­gest a thor­ough neu­ro­mus­cu­lar exam first. Due to safety con­cerns, I would never ad­vise my clients to ride a horse af­fected with string­halt.

BIG STEP: In string­halt, the dig­i­tal ex­ten­sor mus­cles con­tract ex­ces­sively, caus­ing over­flex­ion of the hind limb.

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