The Neurological Examination of the Horse Part II
Dr. Melissa Mazan
They brought Koenig outside, and had Suzie walk him up and down the examination area. They let him have his head as much as possible, in order to have a fair idea of how he went.
I don't see anything abnormal, said Becca after a few minutes of observation.
That's why we're going to do a few challenges, said Dr. Smith. Suzie, will you please back Koenig up for us?
Suzie backed Koenig, and they could see that he felt a little awkward. His hind feet came too close to his front feet, and he nearly tripped himself as he moved away.
Now, Suzie, continued Dr. Smith, Can you walk Koenig in large circles that slowly become smaller and smaller – but make sure to keep Koenig going forward while he circles. If he is just spinning in place, it will be hard to evaluate him.
Suzie carefully circled Koenig first to the right, and then to the left.
Now I'd like you to walk Koenig up and down while I pull steadily on his tail said Dr. Smith.
After they had performed that maneuver, they brought Koenig over to a hill and watched him walk up and down it.
I think that I see several abnormal things, said Becca to Dr. Smith.
When horses turn, they are supposed to pivot a little bit on the inside foot, but Koenig pivots almost 180 degrees on that foot. Also, his outside foot swings very wide to the outside when he circles to the left and the right. When you pulled on his tail, his hind end swayed very easily toward you. It looked as though you could have pulled him over without too much effort. Finally, when Koenig went down the hill, he knuckled over on his hind fetlocks – there were several steps where he actually walked on the front of his pastern. I didn't notice as much with the front limbs, although when he came down the hill, he did interfere several times. I think that he has proprioceptive deficits.
Can you explain to the owner what that means?
Becca thought for a moment, and then began. Conscious proprioception requires that both the signals going to the brain from the outside world – sensation – are intact, as well as the signals coming from the brain to initiate motor activity. When we see conscious proprioception, or CP deficits, such as stumbling, knuckling, standing with the legs too close together or too far apart, or swinging the leg out while circling (circumduction), or perhaps interference between the limbs, we know that some of the signals aren't getting too, or coming from the brain. A problem at the level of the spinal cord can also look like weakness – for instance, you shouldn't be able to make a big horse like Koenig stumble and almost fall over just because you pulled his tail. It looks as though the signal to brace himself against your pull just isn't reaching Koenig's muscles. Because Koenig's brain seems to be functioning well in all other aspects, we will probably assume that the problem is at the level of the spinal cord. Now we need to figure out where in the spinal cord the problem might be.
Did you think that the problem was worse in the hind limbs or the front? asked Dr. Smith.
Worse in the hind, by far, but the front was suspicious, replied Becca.
Does that give you a clue as to where it might be?
Yes, said Becca, confidently. Because there does seem to be a problem in all four limbs, I would place the lesion at the level of the cervical spinal cord. This includes the portion of the spinal cord that is enclosed by the first 7 vertebrae. Because of the way that the spinal cord itself is arranged, the part that sends signals to the hind end is affected more severely than the part that sends signals to the front end. However, if we look carefully, then we should see at least mild signs in the front end. Otherwise, I might suspect that the problem was further back, in the lumbosacral portion of the spine – the portion of the spine that is at saddle level and further back to the tail.
Very good, said Dr. Smith approvingly. Now we're going to try some tests called postural placement. These can be difficult to interpret, because horses who are well-trained, like Koenig, will sometimes allow their limbs to stay in an abnormal position simply because they want to please you. However, most normal horses will put their limbs back into a normal position within a half a minute or so. Let's see how Koenig responds.
Dr. Smith then performed a series of maneuvers. First, he took Koenig's left front leg and crossed it over the right front leg. Koenig looked somewhat bemused, but after 10 seconds, untwisted himself and put his left front leg firmly in the right place. He was slower to do the same with the right front, but even so, had his right front leg back in place in less than half a minute. Then, Dr. Smith took each of Koenig's hooves, and knuckled them over. This time, Koenig was slow to right himself. He happily left both his hind feet knuckled over, but as Dr. Smith explained, that was particularly difficult to evaluate, as horses will comfortably stand that way on their own.
Koenig has slow postural placement in both front limbs, said Dr. Smith. We interpret this as another indication of conscious proprioception deficits, and it further leads us to believe that the problem is in the cervical spinal cord. Now, let's test Koenig's sensation a little further.
Dr. Smith took out a hemostat, and gently started to pinch Koenig's skin, starting at the poll, and continuing down his body.
This is called the panniculus reflex. Be wary of where the horse's hind legs are when you do this, he instructed Becca. Some horses will think that you are a particularly annoying fly, and won't hesitate to kick you.
As Dr. Smith tested Koenig, he watched Koenig's skin twitch each time he applied the hemostat.
No abnormalities on this portion of the test, he said. However, many horses can have sensory losses that result in proprioceptive deficits without having loss of skin sensation. It just depends on how extensive the lesion is, and where it is located.
Dr. Smith turned to Becca again, and asked What else should we include in our examination?