If only a horse could come to us and tell us exactly where they are hurting! When there are no external signs of trauma or swelling this can be quite difficult. It is usually clear which limb is affected, but where in the leg is a trickier matter. For example, many forelimb lamenesses can appear to be affecting the horse’s shoulder because they do not freely swing the leg forward from the shoulder. In fact, less than 1% of the lameness we see affects the shoulder. The same can be said of the stifle in the hindlimb, although stifle-related lamenesses are more common. The reduced range of motion that we see could be the result of pain coming from anywhere in the leg, and it is important to do good detective work to find the true problem. This begins with hoof testers, to assess hoof pain, and flexion tests. Flexion tests strain the upper and lower limb independently. A more severe response to one test may help localize the problem.
If these tests yield inconclusive results, we begin with diagnostic nerve blocks. We use carbocaine (similar to the novacaine used by dentists to numb the mouth) to decrease sensation to the hoof. The horse can still feel where his leg is, so there is no concern about stumbling; it only numbs pain. We gradually numb nerves up the leg until we reach a nerve block that alleviates the lameness. We consider 75% improvement in lameness a positive response, meaning we have isolated the problem to that area. In some cases, we will put the carbocaine into a joint such as the hock, stifle or coffin joint if we are suspicious that is the source of the pain. Once we have isolated the region of pain, we use diagnostic imaging such as digital x-ray and ultrasound to assess the problem more specifically.
Nerve blocks have other uses as well, such as helping a laminitic horse that is too painful to lift a front foot for a trim or shoe application. This allows us a brief window of time to relieve pain and accomplish any treatments.
As a side note, there are longer-acting numbing agents that can be used that last for up to 13 hours. We offer testing for these drugs at a pre-purchase exam as it is possible that a lameness has been blocked out for the exam by the seller.