Nerve blocking is tricky. While this practice of injecting a short-acting local anesthetic at various sites along the nerves of the lower limb is very useful for diagnosing lameness, researchers also know it’s possible to complicate blocks–particularly blocks that are commonly used for diagnosing suspensory ligament disease–by inadvertently numbing the wrong structure. A University of Georgia College of Veterinary Medicine research team recently took a closer look at the diffusion patterns resulting from two injection techniques and the accompanying risk of unintentionally invading synovial structures.

Something as simple as the needle’s insertion angle can mean accidental joint or tendon sheath blocking and a false positive reading of the nerve block, said Kevin Claunch, DVM, who described the study at the 2013 American Association of Equine Practitioners’ Convention, held Dec. 7-11 in Nashville, Tenn. In other words, the horse’s lameness might resolve not because a structure in the area supplied by the nerve in question was the problem but, rather, because other structures were inadvertently numbed.

From reviewing clinical case outcomes, the team had “a high suspicion based on clinical impression that this particular block may not be as specific as we would like,” Claunch said.

A veterinarian might make an inaccurate diagnosis or recommend ineffective treatments based on such results, so he and colleagues compared the risks of inadvertent injection of synovial (joint or tendon sheath) structures and the diffusion of anesthetic when taking two approaches to block the deep branch of the lateral plantar nerve (DBLPN).

They compared two needle placement techniques and diffusion of low-volume (2 mL) and high-volume (8mL) injections of mepivacaine (a local anesthetic) combined with a radiographic contrast medium by taking serial radiographs. They found that both proximal (upward) and distal (downward) diffusion of anesthetic was surprisingly high with this nerve block at the higher volume The team also noted inadvertent penetration of the tarsometatarsal (lower hock) joint and tarsal sheath which surrounds the deep digital flexor tendon at the level of the hock.

Based on these findings, the team concluded that the low-volume injection with a needle placed perpendicular to the skin is likely a superior option for this nerve block because it produces less proximal diffusion than the high-volume injection and, thus, has less potential to block unintended areas.

“The take-home messages for practitioners would be that the DBLPN block is not specific for suspensory disease, a smaller volume of anesthetic may help it to be more specific, and that it is certainly possible if not likely to inadvertently enter the tarsal sheath and/or tarsometatarsal joint when performing this block,” Claunch concluded.