LI­DO­CAINE NERVE BLOCKS CAN BE TRICKY

EQUUS - - Eq Medical Front -

An Aus­tralian study sug­gests that vet­eri­nar­i­ans need to be cau­tious when in­ter­pret­ing the re­sults of nerve blocks done with the drug li­do­caine.

Nerve blocks are used to pin­point the source of pain in a lame horse’s limb. The vet­eri­nar­ian in­jects an anes­thetic agent over nerve branches that sup­ply a spe­cific area, then trots the horse to see if the lame­ness dis­ap­pears. If it does, this is ev­i­dence that the pain is orig­i­nat­ing from the numbed area.

Re­cently, re­searchers at Mur­doch Univer­sity in Perth set out to de­ter­mine whether the numb­ing agent used for nerve blocks in­flu­ences the di­ag­nos­tic ef­fi­cacy of the lame­ness in­ves­ti­ga­tions. “We wanted to col­lect in­for­ma­tion in re­gard to the on­set, du­ra­tion and ef­fi­cacy of the com­monly used lo­cal anes­thetic agents in equine prac­tice to al­low for in­formed de­ci­sion­mak­ing and tailored use of these agents to spe­cific cases,” says Mona Ho­erde­mann, Dr. Med. Vet.

The re­searchers in­duced lame­ness in a sin­gle fore­limb of eight horses us­ing a spe­cial­ized shoe that puts pres­sure on the sole in the heel area of the hoof. They then per­formed pal­mar dig­i­tal nerve blocks on the horses with ei­ther li­do­caine or mepi­va­caine and tracked the loss of skin sen­sa­tion in the heel re­gion and mea­sured the re­duc­tion of lame­ness. The study was then re­peated so each horse was given both drugs for com­par­a­tive pur­poses.

The data showed that although li­do­caine re­sulted in skin numb­ness, which is con­sid­ered a sign that the block has taken ef­fect, it elim­i­nated lame­ness in only three of the eight cases. In con­trast, mepi­va­caine re­solved lame­ness in all eight cases.

Ho­erde­mann says the fact that li­do­caine numbs the skin with­out re­duc­ing lame­ness could con­found di­ag­nos­tic ef­forts. “In a clin­i­cal sit­u­a­tion, in­com­plete lame­ness res­o­lu­tion in the face of com­plete skin de­sen­si­ti­za­tion could be mis­judged as the pres­ence of more than one source of pain in the limb or as a sin­gle ori­gin of pain, which ex­ceeds the ex­tent of the block,” she says. “Ad­di­tion­ally, there is a risk of falsely in­ter­pret­ing the re­main­ing lame­ness as a neg­a­tive re­sponse to the block, which would lead us to look else­where. There­fore, it cer­tainly could lead to a sig­nif­i­cant de­lay in com­ing to the cor­rect di­ag­no­sis.”

These find­ings do not nec­es­sar­ily mean that li­do­caine can’t be used at all, adds Ho­erde­mann, only that vet­eri­nar­i­ans must re­al­ize its po­ten­tial lim­i­ta­tions and that its use in lame­ness in­ves­ti­ga­tions might need to be re­con­sid­ered.

“Our study showed that the re­sponse to skin sen­si­tiv­ity test­ing has to be scru­ti­nized care­fully and that it can­not be re­lied on as an in­di­ca­tor of lame­ness res­o­lu­tion.” She adds that, as a shorter-act­ing numb­ing agent, li­do­caine could be use­ful in sit­u­a­tions be­yond de­tect­ing limb lame­ness. “[It] can still be use­ful for other re­gional blocks, for ex­am­ple in oph­thalmic ex­am­i­na­tions.”

Ref­er­ence: “Du­ra­tion of ac­tion of mepi­va­caine and li­do­caine in equine pal­mar dig­i­tal per­ineu­ral blocks in an ex­per­i­men­tal lame­ness model,” Vet­eri­nary Surgery, July 2017

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