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 decision- making 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 re- peated so each horse was given both drugs for com-parative 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, incom-plete 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