Most lameness can be diagnosed by using our senses. If we are watching from the ground, our eyes tell us that the horse is not moving right. If we are riding, our seat bones tell us. Our ears can tell us which leg is hitting the ground the hardest. By using our hands we can make the horse flinch when we squeeze on a tender spot.
On a surprising number of horses, the exact area on the leg that hurts cannot be diagnosed this easily. We watch the horse go, identify the lame leg, and then run into a snag. No matter how much we bend, twist, push, or pull the limb, we can’t get the horse to respond. This is always a little embarrassing. “My horse hurts so bad he can’t put his heel on the ground, and my vet can’t even tell where he is sore.” It is probably no reflection on the examining veterinarian. There is an explanation.
As a horse’s foot hits the ground, the impact is sent up the leg. It travels up the boney column and when it reaches a sore spot, the horse reacts by favoring that leg. That’s simple enough. But let’s use the example of a damaged cartilage deep inside a stifle joint. As the weight of the leg passes over that spot, there is pain. We often can’t duplicate this pain by manipulating this largest of all joints. We can flex, extend, twist, and stress test the joint as much as possible, and may still not be able to get a reaction from the horse. That deep problem is not being affected by our puny efforts. There might not be sufficient damage to cause heat or swelling. This is also the reason that hoof testers sometimes aren’t any help even when we know there is a foot problem. So this leaves us with a lame horse and no idea of why.
It is vital to know exactly where the pain is coming from so that we can radiograph, ultrasound, or otherwise image to know what the source of the pain is. It is true that nuclear scans can find inflamed areas, but sometimes several areas “light up” and we are no further ahead. If our senses aren’t giving us the clues, then “blocking out” is the next logical step in lameness location. Blocking is pretty unique to horse work. It isn’t really indicated in human medicine where the patient can talk. “It hurts right here, No, a little to the left. Ouch! Yes, that’s it, right there.”
Let’s assume that we have a horse lame in the right fore but we are stumped as to where in the leg the pain is coming from. Through the process of nerve and joint blocks we can selectively numb different sections of the leg. After each injection or block we wait several minutes and then move the horse. When the correct area has been numbed the horse’s gait will improve, and the sore area is identified. This is not just a good theory. Blocking out horses is done on a daily basis in equine practice. Vets dealing with sport horses use far more numbing agents than the busiest dental practice.
Done systematically the procedure is usually begun at the level of the foot. There is a pair of nerves that run down the back of the pastern that go to the back third of the foot. When a small amount of anesthetic is injected directly over these nerves that portion of the foot loses sensation. If the horse is still lame after this block, the entire foot or the joints in that area will be next, and so on, up the leg. We start at the bottom and work up because if we started with the high (closest to the body) nerves, the whole leg would be anesthetized, and not much would have been gained. After a number of nerve blocks it might turn out that the problem is high in the leg and so a good deal of time will be invested before the lameness source is located. For this reason, the vet on your farm may only have time for a couple of blocks before recommending that you take the horse to a clinic where they can work on other horses during those waiting times for blocks to work. Ever notice how your dentist will leave for about 10 minutes after he numbs an area of your mouth? He’s waiting for that block to work, and let’s be truthful; you are in no hurry for him to get back as you want to be sure that that troublesome tooth is completely anesthetized. Your dentist does the blocks so that he can drill or extract your tooth. We do it for diagnostic purposes
When blocks do locate the area, the difference it makes is amazing. Horses that were hopping lame become sound. Of course, the relief is temporary, and the painful condition returns after the anesthetic wears off. This might take an hour or two. As soon as the area is pin pointed through the nerve blocks, X-rays, ultrasounds, or both are used to see what the problem is.
An occasional horse remains lame even after blocking up to and including the highest joint. This is frustrating and can be due to different causes. One is that some problems are mechanical in nature and don’t involve pain. An example of this is a case of navicular disease that has caused adhesions between the navicular bone of the foot and the tendon that rides over it. This causes a stumbly gait that cannot be effectively blocked out. Other cases may involve a very deep pulled muscle that may hurt, but cannot be effectively pinpointed.
The process of nerve blocking requires a very precise knowledge of anatomy. Miss a nerve by quarter of an inch and there may be little or no effect. Nerve blocks are not indicated for every lameness, but in many cases they are a valuable tool in reaching a correct diagnosis.
–David A. Jefferson, DVM