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.