The Neurological Examination of the Horse
Suzie's 6-year old Hanoverian gelding, Koenig, was not doing well this spring. She had great hopes for him as an Amateur-Owner jumper, and during his 5-year old training year, he had shown flashes of brilliance. He had the ability to turn on a dime and hand you back the change, and, was as quick as a cat on his feet. He seemed to look at a 4-foot oxer as below his dignity – surely he had soared at least 5 feet in the air. She gave him the winter off, as he had a series of hoof abscesses, but now that they were cleared up, she expected that Koenig would quickly return to his previous form. Suzie reflected that it had been hard to put her finger on the problem, but as time went by, Koenig's ability to clear relatively small 3-foot fences had deteriorated, and he sometimes stumbled when making a quick turn. She'd had three different opinions by three different veterinarians, and none of them seemed to be able to pinpoint the problem. One said that Koenig had a hock problem, one said that he was foot-sore because of the old abscesses, and one suspected, much to Suzie's dismay, a neurological problem.
Unfortunately, this is a relatively common scenario. Even experienced examiners may differ as to the cause of a horse's abnormal performance or gait. One reason is that many horses, which we suspect of having neurologic disease, may also be lame. The hard part is sorting out which is the most important problem. Moreover, neurologic disease often mimics the signs that we see and expect in many lamenesses. The foundation on which a diagnosis of neurologic disease rests is a thorough, methodical examination. It is important that all aspects of the horse's ability to function neurologically be examined – and understanding the rationale behind the examination will help you to understand your horse's diagnosis.
The Neurological Exam
As with any examination, the history is extremely important. Suzie brought Koenig to the nearest referral facility, a school of veterinary medicine, where they were greeted by a clinician and a senior student. They sat down at a round table, and the questions began:
Is Koenig a gelding?
Certain diseases, such as cervical vertebral instability (Cobbler's syndrome) affect males more than females.
What breed is Koenig?
Certain diseases, such as cerebellar abiotrophy, are breed-specific.
How old is Koenig?
Certain diseases, such as degenerative myeloencephalopathy, are more likely to be seen in young horses.
How long has the problem been going on?
This may help to determine if the problem is something the horse was born with, or if it was acquired over time.
Have any other horses in the barn shown similar signs?
This may help to determine if there is an infectious or toxic cause.
What do you use Koenig for?
If the horse is in work, it is likely that the problem was noticed right away. If the horse has been out at pasture, it will be harder in many cases to pinpoint when the problem started.
The goal of the examination is to determine where the neurological problem is located. During the physical examination, a series of tests are performed to evaluate methodically whether various areas of the nervous system are affected. Once we know where the defect resides, we can have a much better chance to figure out the disease process that is causing the defect.
The examination began with them watching Koenig come off the trailer. This can be a useful part of the neurological examination, as it requires a fair bit of coordination for a horse to back down a ramp.
Dr. Smith, the clinician, asked the student, Do you see any obvious abnormalities – for example, is Koenig wobbly, or is he standing with his legs in unnatural positions?
The student, Becca, watched carefully, and then answered He seems fairly normal to me at this point. I don't see any obviously abnormal behavior or gait.
Good, said Dr. Smith. I agree with you. That helps us to grade the level of neurological disease that he may have. A normal horse – grade 0 – has no abnormalities. A horse that has grade 1 disease is usually only determined to be abnormal by someone who has considerable experience in observing horses with neurological deficits. A grade 2 can be seen by most people, but only if the horse is really taxed to do something challenging – such as walking backward down a ramp, or turning in tight circles. A grade 3 has obvious abnormalities doing ordinary things, such as walking in a straight line. A grade 4 is so abnormal that you don't even have to take that horse out of the stall – you look in the stall and there he is, with his legs crossed, or pressing his head into the wall, or walking in circles – he pretty much has a sign on him that says 'I'm neurological. It's a simplistic grading system, but it works fairly well. Let's continue with our examination to determine which of these categories Koenig fits into.
I like to be methodical and always begin the next part of the examination at the head, said Dr. Smith. I ask myself, Does the horse exhibit normal demeanor? The presence of a stranger should make most horses appear bright, alert, and curious. If the horse remains dull, in the corner, paying no attention to a stranger, this may be evidence that the horse has a cortical problem – that is, with the portion of the brain that we associate with thinking and deliberate actions. We'll put Koenig in a stall, and see how he behaves.
They walked Koenig into the hospital to a stall that was waiting for him. He nervously blew air through his nostrils as he examined the shavings on the floor. He wheeled to the back of the stall and back again, and as he turned, his left hind leg swung out a little bit further than the right. Then he walked to the front of the stall and whinnied, ears forward, eyes on his owner.
Did you see that hind leg? asked Dr. Smith. That was a little suspicious. We'll have to repeat that during the rest of our examination. But what do you think of Koenig's demeanor?
Again, Becca replied that it seemed quite normal, and Dr. Smith agreed. What would you like to do next, Becca? asked Dr. Smith.
Should we test the cranial nerves?
Dr. Smith asked Becca to explain the cranial nerves to Suzie. Becca started a little hesitantly, but then gained confidence.
Well, she said, horses, like humans, have 12 cranial nerves, labeled CN 1-XII. Technically you have to start with CN I (the olfactory nerve), that governs the sense of smell – it is hard to evaluate. We usually assume that horses with a good appetite, and who visibly sniff their environments, have an intact CNI. The next is CN II (the optic nerve), that governs the ability to see. If the optic nerve or its pathway in the brain is severely affected, then signs of dysfunction may be fairly obvious – your horse may bump into objects, shy at things that have never bothered him before, or be reluctant to navigate even in areas that are familiar to him. We use the menace reflex to evaluate the optic nerve.
Becca used her hand to illustrate and test the reflex. She brought her hand toward the Koenig's eye abruptly, and he promptly blinked his eye.
That shows that his menace reflex is intact, said Becca. We know that his first and 7th cranial nerves are working fine, because he could see my hand, and he used his 7th cranial nerve – his facial nerve – to tell the muscles of his eyelid to close. CN VII governs the face's muscles of expression. Incidentally, it also gives us an idea about Koenig's cerebellar function, which we'll assess later.
Next, we're going to test CNIII, which governs the ability of the pupils to constrict. We test it with the pupillary light reflex (PLR). If the horse is in a dim environment, his pupils will dilate (as will yours). If we shine a light in Koenig's eyes, his pupils should constrict – an intact PLR tells us that CN III is functioning well – and look, it does. That's good. Next, we'll look at CNIV. We can tell that this nerve, that governs the position of the eyes, is intact in Koenig, because his eyes are normal and symmetric. Then, Becca took out a cotton swab, and gently swabbed the inside of Koenig's nostrils – he obviously didn't like it, snorted, and jerked his head away. Becca followed that by taking a blunt probe, and gently pressed various areas of Koenig's face. He responded by trying to move his head away, and twitching his skin.
Now I'm testing Koenig's 5th cranial nerve, said Becca. This is the nerve that regulates sensation in the face. Koenig shows a normal response to my probing his skin. He feels it, doesn't like it, and tries to get away.
Becca quickly assessed CN VI, which helps to govern the position of the eyes – it, too, was intact.
We already know that the branch of the facial nerve (CNVII) that goes to the eyelid is intact, she said. We can make a few other observations to help us make sure that all the branches of the nerve are fine. I'm going to test Koenig's palpebral reflex – watch, just a light touch on his eyelid makes him close his eye. This also tests the 5th cranial nerve – very handy! He uses CNV to feel the sensation, and CN VII to tell his eyelid to close. I'm also looking to see if Koenig's head is symmetrical. Both ears are up and forward when he hears a noise, and I don't see any other abnormalities such as a twisted nose or a drooping eyelid.
Becca then walked to Koenig's side and took her bundle of keys out of her pocket. She jangled them behind Koenig, and saw that he flicked his ears backward to see where the noise came from.
The 8th cranial nerve governs both hearing and balance, she explained to Suzie. The normal horse should have exquisitely acute hearing – and should react readily to any unexpected sound. The normal horse should also exhibit excellent balance (the vestibular portion of CN VIII). If the horse's head is cocked to one side, or if the eyes are moving rapidly back and forth (called nystagmus), then we may suspect vestibular disease – a problem with the horse's vestibular, or balancing system.
Suzie watched as Becca then offered Koenig a piece of carrot that she had in her pocket. Koenig eagerly chewed and swallowed the carrot, and looked for more.
That simple act of swallowing told me a lot, said Becca. The 9th cranial nerve – CN IX – helps to control the ability to swallow. The horse with a lesion in CN IX may not be able to swallow properly, and you may see food coming back out of the mouth despite the horse's ability to chew and obvious good appetite. The 10th cranial nerve, known as the vagus nerve, also helps to control swallowing, so I tested part of that nerve as well. CNX is also very important to the function of the larynx, or voicebox, so if Koenig had problems with CNX, you might notice that he had a funny-sounding whinny, or had started to be a 'roarer' – that is, had exercise intolerance and made a roaring noise when working hard because of laryngeal paralysis.
The 11th cranial nerve is harder to assess', Becca continued. He doesn't appear to have any asymmetry or atrophy of the trapezius muscle, so I'm assuming that CNXII is intact.
Becca then carefully reached inside Koenig's mouth and gently grasped his tongue. She really had to hold onto it to inspect it, because Koenig was pulling back hard.
CN XII controls the tongue, she explained. The tongue should have good musculature on both sides, and should stay inside the mouth – with the exception of a few horses who have a habit of lolling their tongues, or horses who have had an injury to the tongue. Horses have exceptionally strong tongues, and it should be difficult to grasp the tongue and gently pull it out of the mouth.
Dr. Smith was very pleased with Becca. She had done an excellent job of assessing the cranial nerves, and correctly found no abnormalities. They then decided to bring Koenig outside to finish the examination.
We've assessed a large portion of Koenig's brain function, said Dr. Smith. We still need to assess the ability of the cerebellum to control the action of Koenig's legs and indeed his entire body. The portion of the brain termed the cerebellum controls the ability of the horse to rate and control his actions. It lets the horse know how high, for instance, the foot should rise with each step, and how far forward and backward each limb should go. Horses with a lesion in the cerebellum will look a bit like a drunk three hours into happy hour – the horse will not be able to control how far each limb can go, and will stagger or over-reach dramatically. The horse will also show intention tremors – when the horse goes to perform an action intentionally, the cortical portion of the brain knows exactly what it wants to do – perhaps eat grain out of a bucket. However, the cerebellum doesn't tell the head when to limit where it is going, so it overshoots, then undershoots the location of the bucket – this action recurs very rapidly, and appears as a tremor. We can assess this best when Koenig is moving freely outside.
We also need to assess whether there is a problem at the level of Koenig's spinal cord. If we think of the central nervous system, or CNS, as a circuit, then think of the spinal cord as being downstream of the brain. The spinal cord has to transmit messages to the muscles in order for the horse to have a normal posture and gait. If there is a problem at the level of the spinal cord, then the horse may know perfectly well what he wants to do at the level of the cerebrum and cerebellum, but his muscles, and therefore his limbs, simply won't be able to carry out the brain's instructions.
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?
Should we do Koenig's other reflexes? asked Becca.
Well, it would be nice to have the information, said Dr. Smith, but in a large animal such as Koenig, it isn't practical to test what we call spinal reflexes, as this requires that the animal is lying down and relaxed – we're not about to achieve that with Koenig! If he were a dog, or even a goat, we would lay him down, and check reflexes such as his patellar reflex – your own doctor has probably tested that on you. That's the one when the examiner taps below your knee with a little hammer and your leg involuntarily strikes forward. It tells us that a certain segment of your spinal cord is intact. In horses, we have to rely on our observations during other parts of the examination.
Well, said Becca, I think that we should do a thorough physical examination. Some systemic diseases can result in neurologic disease, so we should listen to his heart and lungs carefully. We should also do a thorough ocular examination, as the optic nerve, which we can see at the back of the eye, is the only part of the central nervous system that we can view directly. Some diseases, such as equine motor neuron disease result in visible changes to the optic nerve. Finally, we should do some basic bloodwork. A complete blood count, or CBC, can tell us if there is a focus of inflammation or infection in the body.
They took blood and submitted it to the lab for testing – there were no abnormalities. Then, they dilated Koenig's eyes and looked at the back of them – everything looked normal.
Well, Suzie, said Dr. Smith. I'd like to sum up our findings for you. We've gone through a systematic, thorough neurological examination. Our goal with a neurological examination is to find out where in the central nervous system the lesion is. Then, we can have a better shot at 'who dunnit'. From our examination, we think that Koenig's neurological system is intact in all areas except the cervical spinal cord. We also find that the signs are fairly symmetric – meaning that it has affected both sides of the spinal cord evenly. We judge him to be a grade 2 out of a possible 4 – it takes challenge tests to see what is abnormal with him. We now can do several ancillary tests to try to determine the cause. Our top choices of what is wrong with Koenig include Wobbler's Syndrome, which is a type of developmental disease of the vertebral bones that sometimes doesn't show up until the horse is in hard work, and Equine Protozoal Myeloencephalopathy (EPM). In order to figure out which it is, we'll do x-rays, bloodwork, and a spinal tap. If we hadn't done our careful neurological examination, we would still have a very long list of rule-outs – and we might be quite a long way from figuring out what is wrong with Koenig. This way, we'll be pretty sure of an answer very soon.