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The Neurological Examination of the Horse Part II

By: Dr. Melissa Mazan

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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.

The Head

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?
Go ahead.
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.

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