One of the hardest types of lameness to diagnose is a slight lameness in a horse’s rear leg. The signs are not as easy to see in a back leg as in a front leg. Many times you cannot see a lameness at all, but an experienced rider can feel that the horse is off and not moving correctly. The rear legs are also more difficult to diagnose because it is more dangerous to examine the rear legs of some horses than the forelegs. Many horses resent it when someone picks up and flexes their back legs and checks for pain, and this places the veterinarian in a potentially dangerous situation. Also, one of the main techniques we use to diagnose lameness in a horse is nerve blocks, which involves sticking a needle under the skin and numbing a nerve to see if it improves the lameness. This is routinely performed on the forelimbs but it can be much more dangerous on the back limbs, and some horses will simply not allow you to do nerve blocks without sedation. However, if you sedate the horse, then it is difficult to determine the effect the sedative has on the lameness.
For this reason, a group of veterinarians in Brazil used a device to cause a temporary lameness on the rear leg of a group of horses and then administered various sedatives and pain medications to see the effect on the lameness. They determined that it was possible to use a low dose of xylazine and tobuterol intravenously prior to a nerve block, and this did not have an effect on visualizing the lameness in a rear leg for up to 40 minutes after the sedation. However, the same could not be said of the forelegs as the sedation significantly affected a lameness there and prevented an accurate diagnosis. This information will allow veterinarians to examine rear leg lameness in difficult horses and do so with less danger.