Lower Palmar Digital Nerve Blocks Concerns (AAEP 2009)

When a horse becomes lame, one of the first things a veterinarian will do is perform nerve blocks on the affected limb(s) to identify the location of the pain. However, several recent studies have focused on whether these nerve blocks really block the areas we think they do. In some cases, that anesthetic might migrate and numb more structures than expected.

At the 2009 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas, Nev., one presentation examined diffusion of anesthetic following a common block veterinarians use for a lameness exam: the low palmar nerve block. The low palmar nerve block (or low four-point block, as it is often called) involves placing anesthetic at the medial and lateral palmar nerves and medial and lateral palmar metacarpal nerves, and it is thought to numb everything from the fetlock down.

"But are we really blocking what we think we are?" asked Kathryn Amend, DVM, a resident in clinical sciences at Colorado State University, who presented the study at AAEP. "Contrast studies can help us answer this question."

For this study, nine adult horses were injected with local anesthetic mixed with contrast medium so the injected solution would show up white on radiographs (X rays). Radiographs were taken at 5, 15, 30, 60, 90, and 120 minutes after injection, and the diffusion of the contrast was evaluated.

The investigators found the anesthetic diffused above and below the injection site. Inadvertently, contrast was placed within the deep digital flexor tendon sheath in 39% of limbs. The anesthetic ultimately traveled up to five centimeters from the injection site, and it tended to travel farther when the clinician performed the block while the limb was held up, than when they injected the horse as it was standing on the limb.

A follow-up examination of blocks done on horses soon to be euthanized for other reasons found that the anesthetic was also diffusing via the lymphatic system. However, the investigators didn't think the amount and distance of this diffusion was enough to anesthetize any lesions in the proximal suspensory region.

One notable finding was that 44% of the limbs had contrast in the fetlock joint. "This suggests that sterile preparation of the injection sites may be warranted (to avoid introducing joint infection)," noted Amend.

"It is also recommended that the palmar nerve block and palmar metacarpal nerve block be placed more proximally (higher up) on the limb," she concluded. "Raising the block up to 1.5 cm above the distal (lower) end of the splint bone will likely avoid tendon sheath and fetlock joint involvement."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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