Although I struggle to understand or be entertained by many of the events in the Winter Olympics, I can appreciate the dedication and athleticism required to compete in some of them. I grew up in New England where I developed a strong aversion to anything snow related, and only appreciate ice when it is floating in a glass. This post is inspired by the linked article The Brutal Neuroscience of Figure Skating: How Spinning Athletes Overcome Dizziness, which provides an excellent explanation of the physics and physiology of how Olympic skaters avoid dizziness when they do prolonged rapid spinning moves. I suspect there is a name for that spinny thing they do at the end of their routine, but like I say, ice sports do not entertain me so I don’t watch them. I think that is referred to as willful ignorance.

Here are some excerpts from the article:

In our inner ears, there are three fluid-filled tubes called “semicircular canals,” said Paul DiZio, a neuroscientist at Brandeis University who studies balance, motion and dizziness. Each one is aligned with a different axis of motion: up and down, left and right, and side to side.

“When you move your head, the fluid inside the tubes kind of flows a little bit,” DiZio told Live Science. “And then you’ve got these sensors — sensors that are like little pieces of seaweed inside the tubes — that kind of float with the fluid and sense what’s going on.”

Nod your head yes, and the sensors in one set of tubes spark to life. Shake your head no, and another set of tubes sends signals to the brain. Touch your ears to each shoulder, and the final set of sensors activates.

“Normally, the motions we make don’t last for too long,” DiZio said.

And rotational motion, in particular, tends to occur in short stretches of time — turning to look out the window, leaning your head back to crack your neck, that sort of thing. And our inner ears are built well for tracking that sort of motion.

When Mirai Nagasu hurls herself into a whirling triple axel, Nathan Chen hops balletically into the air and turns four times before landing, or Adam Rippon contorts himself through a series of fluid shapes while spinning his way on one skate through long measures of music, their wet inner ears— human beings’ motion sensors and the origins of most dizziness — slosh around just the same as mine in that rotating chair (or yours, if you gyrate fast enough).”

As a vestibular specialist, we see this phenomenon when performing rotational chair testing, which involves spinning patients at various speeds and recording the eyes responses. We do not spin them nearly as fast as a skater spins. Not even close. One of the tests we perform in the rotational chair is called Step Velocity test. In this test, we spin the patient in one direction for 45 to 60 seconds, then we stop the chair, Typically, the patient will feel the spinning sensation for about 10-15 seconds, then, even though the chair is still spinning, they have no sensation of movement until the chair stops. Once the chair stops, even though they are not moving, they feel as if they are spinning in the opposite direction for another 10-15 seconds.

This is explained by the fluid dynamics of the inner ear. As described in the article, the inner ear is very efficient for short, back and forth movements, but not prolonged rotational movements. After about 6 seconds of rotation in the same direction, the inner ear fluid has moved as much as it can, and basically “pegs” against the end of the stimulated semi-circular canal. The brain has the ability to prolong the sensation of movement for another several seconds, but eventually, the movement stops being registered. When the chair stops, the fluid that was “pegged” reverses direction and flows back into the canal, creating the sensation of movement for a short period of time.

The Step Velocity test measures not only the inner ears response, but also the efficiency of the brain. The linked article explores the techniques Olympic skaters use to train their brain to maintain orientation after a prolonged spin. Go USA!

Photo courtesy of instructables.com

Editor’s Note: Brady Workman is back this week with a follow up to a post from several months ago VEMPs: what are they good for? This week’s post will instead focus on what the scientific evidence shows VEMPs are good for.

 

 

The vestibular evoked myogenic potential (VEMP) is a relatively new means of vestibular assessment, which only received FDA approval in 2015. As such, the diagnostic utility and scientific evidence supporting the use of VEMP testing is still being investigated. The two primary VEMP responses investigated in the literature are the cervical VEMP, also known as a cVEMP, and the ocular VEMP, or oVEMP. These responses are believed to be measures of otolith function (linear sensors of the inner ear), with the cVEMP thought to assess saccular function and the inferior branch of the vestibular nerve and the oVEMP thought to assess utricular function and the superior branch of the vestibular nerve. Readers unfamiliar with these two VEMP responses are referred to one of my previous posts Vestibular Evoked Myogenic Potential (VEMP) for the basic characteristics of these measures.

Recently the American Academy of Neurology released a clinical practice guideline for the use of cVEMPS and oVEMPS with a systematic review of the available evidence. The primary objectives of this were to determine if VEMPs identify superior canal dehiscence syndrome (an abnormal thinning of bone in the superior semicircular canal), if VEMPs identify otolith dysfunction, and if VEMPs aid in diagnosis of other specific vestibular disorders.

 

Do VEMPs aid in the diagnosis of superior canal dehiscence syndrome (SCDS)?

 

The use of cVEMP threshold values was found to be 86-91% sensitive and 90-96% specific in separating individuals with SCDS confirmed by imaging from controls. The use of cVEMP amplitude measures, when appropriately corrected for electromyographic (EMG)/ muscle activity, was found to be 100% sensitive and 93% specific in separating those with SCDS confirmed by imaging from those without.

 

The use of oVEMP threshold value was found to potentially distinguish SCDS from control subjects with 77% sensitivity and 93% specificity when thresholds were found to be significantly below clinic normative data (2 standard deviations). The use of oVEMP amplitude value was found to be 77-100% sensitive and 98-100% specific at separating control subjects from those with SCDS.

 

Those with SCDS tend to have lower thresholds for VEMP measures due to the abnormal thinning of bone in the superior semicircular canal. This thinning of bone alters the fluid dynamics of the fluid filled vestibular system and allows for the measurement of VEMP responses at lower sound levels and with larger amplitudes than in patients without SCDS.

 

Do VEMPs identify otolith specific dysfunction?

 

The current evidence available did not allow the authors to determine if VEMPs specifically identified saccule and/or utricle dysfunction because no current reference standard for isolated otolith dysfunction exists. There is a general consensus, however, that the cVEMP is closely connected with saccule function and the oVEMP with utricle function.  

 

Do VEMPs aid in diagnosis of other vestibular disorders?

 

Vestibular Neuritis (inflammation of vestibular nerve)

Expert consensus supports using cVEMP and/or oVEMP to determine the extent to which vestibular neuritis has affected the vestibular system. Current evidence suggests that VEMPS cannot specifically identify affected vestibular structures alone. For example, an abnormal cVEMP on the side of caloric hypofunction would imply involvement of both the superior and inferior branches of the vestibular nerve.

 

Meniere’s Disease (fluid abnormality of the ear)

VEMPs alone are not useful in making the diagnosis of Meniere’s disease, but may be used as accessory test to quantify peripheral vestibular damage as a result of Meniere disease. 

 

Benign Paroxysmal Positional Vertigo (BPPV) (migrated ear particles)

VEMPs are not currently useful in making a diagnosis of BPPV.

 

Vestibular Migraine

Some patients with vestibular migraine have been shown to have absent or reduced amplitude VEMPS in either one or both ears; however, currently VEMPS cannot be used to aid in the diagnosis of vestibular migraine.

 

 

So, what does the evidence show?

 

Currently, VEMPs are useful in making a diagnosis of superior canal dehiscence syndrome and as an ancillary test to quantify the extent of vestibular system damage in relation to other primary vestibular disorders. More research is necessary to further determine the roles of VEMP measures in the clinical evaluation of patients with otologic and/or neurologic conditions. Standardization of test protocols is necessary to further prove VEMPs as effective clinical tools and to allow for more widespread clinical use.

 

 

*Images courtesy Canadian Audiologist, Neurology