COWS – my favorite mneumonic
I just finished teaching Pritzker Medical students (Class of 2015). Once again, it was fun; I learned things and had new thoughts. Teaching is indeed learning.
Today I want to share a recent teaching joy. Pritzker students gathered together on their own initiative to explore caloric testing, which I had talked about in class but never included in laboratory exercises. And I got the resulting video. Oh joy, oh happiness!
Before we get to this delightful scene, let’s start with a perennial student favorite among laboratory exercises: eliciting post-rotatatory nystagmus by twirling on a swivel chair. Here are the instructions that I give the students:
Take someone who is not a dancer or figure skater and put them in a swivel chair. Rotate them around and around (for roughly 30-60 seconds) and then stop them and look at their eyes. What do you see?
What you will see is called nystagmus. Nystagmus is an eye movement that involves a slow movement in one direction and then a fast movement in the other direction. The direction of the nystagmus is named for the direction of the fast movement because this is easier to see. Here is a video of post-rotatory nystagmus from a former Pritzker student:
You just saw a slow rightward movement and a fast leftward movement, making this a left-beating nystagmus. The nystagmus direction is named for the direction of the fast movement because the fast movement is far easier to see than the slow movement.
In this case, the nystagmus was elicited by head rotation in the yaw plane.
The three planes of the vestibular system are:
- Yaw – shaking your head no
- Roll – placing your head on your shoulder
- Pitch – nodding your head yes
The reflexive reaction to a head rotation is an eye movement in the opposite direction. This eye movement is called the vestibuloocular reflex or VOR. The VOR opposes the effect of head rotation on gaze and keeps gaze steady. Thus when the head is rotated counter clockwise (CCW, when viewed from above), the eyes rotate in the clockwise (CW) direction. Likewise, when the head is rotated CW, the eyes rotate to the left (which is CCW). The VOR is invaluable in keeping gaze steady so that we see a constant visual scene. Without a VOR, the visual scene would jump around as is the case in a video made with a hand-held camera.
Before we return to post-rotatory nystagmus, let’s take a brief look at what happens when the head is briefly rotated from facing forward to looking either right (CW rotation) or left (CCW rotation; illustrated in top row of Figure 1). The head rotates but the endolymph lags behind, setting up relative motion of the endolymph in the opposite direction. Thus a leftward head rotation (CCW) produces relative rightward (CW) movement of the endolymph.
Head rotation only affects fluid movement in semicircular canals that are contained within the plane of the rotation. For rotations in the yaw plane, the relevant canals are the horizontal semicircular canals (h-scc).
The eye movement (due to the VOR) elicited by a brief head rotation occurs in the same direction as the relative endolymph movement. Thus a CCW head rotation elicits a CW VOR and vice versa.
Now we are finally ready to understand post-rotatory nystagmus (Figure 1, bottom row). When a rotation is sustained, the head and endolymph achieve constant velocity in the direction of the rotation. Vestibular hair cells (the sensory cells) only respond when the head accelerates, not when it moves at a constant velocity. So there is no vestibular response to a steady rotation. However, when a sustained rotation is stopped , the head becomes instantaneously stationary while the fluid (endolymph) in the horizontal semicircular canals of the inner ear keeps moving in the direction of the just-ended rotation. This endolymph movement is the same as occurs during a brief head rotation in the opposite direction. And therefore a VOR in the direction opposite to the steady state head rotation occurs.
Now how do we get from a post-rotatory VOR to post-rotatory nystagmus? Well, once the VOR is accomplished, it is as though the eye has come to the edge of the orbit and a quick reset to the neutral (straight ahead) position occurs. This quick reset is a saccade and it is ultra-fast. But once back in the neutral position, the conditions that elicited the original VOR, flowing endolymph, are still present. So another VOR occurs. Then another saccade reset. Then another VOR and so on, over and over again. This repetitive eye movement pattern is nystagmus and it will continue until the endolymph stops flowing, after a minute or so.
Post-rotatory nystagmus shows that the VOR is working but it is not a test used in the clinic. Physicians and patients alike would object to the uncontrolled aspects of twirling a person on a chair… A VOR test that is used is caloric testing. In caloric testing, a person lies on their back. This posture places the horizontal semicircular canal into the vertical plane and above the ear canal (external auditory meatus in medico-speak). In the clinic, a fancy tube is placed into the ear canal and the water is irrigated in to fill the canal. Hot water circulated into the right ear canal mimics a CW head rotation. Cold water circulated in the right ear canal mimics a CCW head rotation.A nystagmus results.
Let us consider the example of irrigating the right ear with either hot or cold water:
- Hot water – VOR toward the left and then saccade back to the right
- Cold water – VOR toward the right and then saccade back to the left
More generally, hot water elicits a VOR toward the opposite side followed by a saccade back to the same side (as the ear being irrigated). Cold water elicits a VOR toward the same side followed by a saccade to the opposite side. This gives rise to my all-time favorite neurobiology-related mneumonic:
COWS – Cold-Opposite, Warm-Same.
In other words, irrigating cold water elicits a nystagmus that beats toward the opposite side (as the ear that is irrigated) whereas warm (hot is probably a bad idea…) elicits a nystagmus that beats toward the same side. [Apologies for an error that lasted for about 20 minutes until I was alerted to it by Yuri Sugano!! Thank you Yuri!]
I have been told by neurologist friends that caloric testing is used on unconscious patients. It is said that awake patients would vehemently object to caloric testing because it causes nausea as well as vertigo. In general abormal vestibular stimuli produce nausea and even vomiting. Therefore, in the interest of avoiding nausea and vomiting in the laboratory sessions, I have never tried to do caloric testing with Pritzker students.
But then a funny thing happened. Just a few days before their final exam, a group of Pritzker students got together on their own to performed caloric testing on one of their own, Philip Sossenheimer!!! They tried warm water but it “didn’t work” probably because the water used wasn’t hot enough. An understandable exercise of caution. Then they tried cold water and look at what happened:
Super cool video, don’t you agree? It was particularly interesting to me that when his classmates excitedly tell him he’s “doing it,” Philip responds, “am I really?”. I asked Philip about this, wondering whether he could tell that his gaze was unsteady and also asking whether he felt any nausea. Philip responded:
“At first I couldn’t really tell whether or not anything significant was happening. I was definitely aware that my eye was moving, but it was easy enough to correct and I thought I was just poorly fixating under pressure! After a few more seconds the nystagmus became very evident visually and I became very dizzy. It was very similar to what happens after spinning around quickly and stopping. I felt fairly dizzy, lightheaded and slightly nauseated for a few minutes after, but the effects wore off quickly and I haven’t felt anything since!”
That is the end of my story and my inevitably long post (why can’t I write short simple posts? Let me know if you know…). What moved me out of my posting silence to share this story is the marvelous image of students gathering on their own to “do neurobiology.” That brought a big smile to my face. And Philip and his colleagues were even doing this before exams. Remarkable. I am a lucky person! I revel in my students, at Pritzker and beyond.
Categories: Gaze control