EQUUS - - Contents - By Chris­tine Barakat and Mick McCluskey, BVSc, MACVSc Ref­er­ence: “Corneal abra­sion and mi­cro­bial con­tam­i­na­tion in horses fol­low­ing gen­eral anaes­the­sia for nonoc­u­lar surgery,” Ve­teri­nary Anaes­the­sia and Anal­ge­sia, Jan­uary 2018

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A new study un­der­scores the need to pro­tect horses from an of­ten over­looked risk of gen­eral anes­the­sia: eye in­jury.

The re­search, con­ducted at the Univer­sity of Liver­pool in Eng­land, was based on 40 horses slated to un­dergo elec­tive, non-oph­thalmic surgery. Prior to the surgery, each horse’s eyes were ex­am­ined us­ing flu­o­res­cein dye, which re­veals abra­sions on the cornea. While the horses were un­der anes­the­sia, oph­thalmic oint­ment was ap­plied to their eyes, a stan­dard prac­tice in­tended to pro­tect the cornea. Then, 24 hours af­ter the surgery, the flu­o­res­cein tests were re­peated.

The re­searchers found that none of horses had eye dam­age prior to un­der­go­ing anes­the­sia, but 17.6 per­cent sus­tained mild corneal abra­sions dur­ing surgery. Af­ter an­a­lyz­ing var­i­ous po­ten­tial risk fac­tors, in­clud­ing the age and weight of the horses, the du­ra­tion of anes­the­sia and the length of re­cov­ery pe­ri­ods, the re­searchers found that only one fac­tor ---re­cum­bency on the op­er­at­ing ta­ble---seemed to in­crease the risk of eye in­jury. But they aren’t sure why.

“It is im­pos­si­ble to know how ex­actly the eyes were in­jured dur­ing the pe­ri­op­er­a­tive pe­riod,” says Ste­fa­nia Scara­belli, DVM. “It could be due to po­si­tion on the ta­ble, me­chan­i­cal dam­age dur­ing mon­i­tor­ing of palpe­bral re­flexes by the anes­thetist or by sur­geons dur­ing ma­nip­u­la­tion of the head if that is needed dur­ing surgery.” She adds that a horse’s eyes stay open dur­ing surgery with no blink re­flex due to the ef­fects of anes­the­sia drugs. The same ef­fect is seen in peo­ple, but our eye­lids are typ­i­cally taped shut while an­i­mals’ eye­lids are not.

Stud­ies of oc­u­lar dam­age in peo­ple af­ter gen­eral anes­the­sia have found vastly dif­fer­ent rates---rang­ing from less than 1 per­cent to 44 per­cent---depend­ing on the set­ting and pre­ven­tive mea­sures taken. Sim­i­lar stud­ies of dogs and cats un­der­go­ing gen­eral anes­the­sia found rates of dam­age be­tween 2 per­cent and 19 per­cent.

“Un­for­tu­nately, there is not a com­pletely re­li­able method to pro­tect eyes dur­ing gen­eral anes­the­sia in hu­man medicine; there­fore, I think that do­ing this in horses could be even more dif­fi­cult,” says Scara­belli. Dur­ing the pro­ce­dure, maybe the oint­ment could be ap­plied more of­ten---even though there is ev­i­dence that this doesn’t help in dogs---and tak­ing care to not touch the eye when per­form­ing pro­ce­dures around it.”

None of the study horses showed out­ward signs of their corneal in­juries and all healed within 24 hours of treat­ment with an­tibi­otic oint­ment, but it’s still im­por­tant to con­sider the risk, says Scara­belli. “Horses are not reg­u­larly checked for eye trauma post­op­er­a­tively. The aim of the study was to un­der­stand if this pro­ce­dure should be done. The num­ber of horses in the study is prob­a­bly not enough to sug­gest per­form­ing a check in each horse re­ceiv­ing gen­eral anes­the­sia, but I think it is some­thing we should be aware of. We would like to per­form an­other study to see if there are pre­dis­pos­ing fac­tors to the de­vel­op­ment of corneal abra­sion in or­der to try to pre­vent them.”

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