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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.”