Twitch Part II
by Katharine Mertens, DVM
Last month I introduced the utility of the nose twitch. I thought it would be interesting now, to illustrate another example of when this restraint tool comes in handy.
The scenario presented last month was where a non-invasive twitch just seems so much more appropriate, when only seconds of patient cooperation are needed, than subjecting the physiology of the animal to clearing a foreign chemical (i.e. a sedative medication) over several hours. Why invoke hours of chemical change for only seconds of necessity? The example last month was, I needed the horse still for a few seconds while I injected antibiotics intramuscularly.
Conversely, chemical sedation seems like a no-brainer when we need the animal safely restrained for long periods. When we are suturing a wound, for example, or performing a dental float, we’ll need the 30 minutes to an hour of chemical cooperation that injectable sedatives provide.
But there are times when we need not only mere seconds of cooperation, but also that the animal be fully alert immediately after the noxious procedure. For example, the nerve block during a lameness examination.
This scenario comes up all the time, but I remember specifically the case of “Pedro,” a four year old black Morgan gelding who was politely but adamantly opposed to my approach to his lameness exam.
Pedro’s lameness involved a foreleg, although being a horse, he could not tell me exactly where it hurt. The nerve block helps with this. Considering that the nerve track runs from the brain outwards, we block the nerve from the outermost point inwards. The “block” itself is a small amount of local anesthetic that we inject directly on top of the nerve—exactly like a dentist does to numb the mouth when necessary.
For Pedro, I placed my block just above the hoof, at the back of the pastern as close as possible to the heel bulbs. With this block, most of the hoof goes numb, as well as that nemesis of horses, the “navicular region.” (More on this another month.) Pedro stood quietly while I grasped his hoof between my knees, and one after the other, quickly inserted a needle over the nerves on either side of his pastern. At each location I injected about 3 mls of local anesthetic.
“Thanks!” I said to his owner, who held Pedro with halter and lead rope while I did my work. “Now we’ll just let that ‘cook’ a little bit,” I said, straightening up to full standing again. Pedro got to graze for a few minutes, while I extolled the benefits of yoga in keeping me flexible and strong enough to continue work with horses!
After a few minutes (I resisted the urge to bust out a downward dog right then and there) I tested my block. “Starting up the engine, are we, doc?” said Pedro’s person, watching me pull car keys from my jean’s pocket.
“Any blunt object will do in a pinch,” I said, crouching next to Pedro and using a key tip to test the feeling at his heels. He pulled away from the key I pushed against his unblocked foot; no response when I prodded at the same spot in the blocked foot.
“Okay, moment-of-truth time,” I said. “Could you please take Pedro out on the lunge line again and let’s see how he goes.”
Since Pedro’s forelimb lameness was unchanged, I concluded that the painful area was higher up the leg than the hoof and navicular regions that were now numb. (While this sounds black and white, the reality in veterinary circles is that we agonize over how much misinterpretation may come into play with blocks. What if the anesthetic travels upwards on the nerve? We may be reaching the wrong conclusions about exactly which part of the limb is numb after any one particular block.)
My next block location would be at the level of the fetlock. Placing anesthetic here would add the toe region of the hoof, as well as the pastern itself, to the areas numbed out previously.
But Pedro’s dime had run out. Each time I placed his hoof back between my knees, he’d yank it away and stomp it on the ground for good measure. He knew my game and was having none of it. Was sedation an option? No—for one thing, many of our sedatives themselves provide generalized pain relief, which could mask the very lameness I was trying to uncover. For another, I needed Pedro awake and coordinated within minutes of my block.
So the twitch was once again my answer. With the twitch in place on his upper lip, Pedro stayed calm and stood still for the seconds it took to place my block injections. The twitch was off in an instant, and Pedro got another five-minute grazing break. I tested the blocked area for numbness, and once again sent Pedro out for a trot on the lunge line.
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Katharine Mertens, DVM is the owner of Mertens Mammals, LLC, a mobile, equine veterinary practice based in Boring. You can reach the practice at 503-663-6400.