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