Assessing dizziness is a diagnostic challenge. There are multiple possible etiologies for dizziness—some rooted in the anatomy and physiology of the inner ear, some not—and until recently, clinician assessment of dizziness and the vestibular system was limited by a lack of diagnostic tools.
For nearly a century, the assessment of the vestibular system was been restricted to assessment of the semicircular canals. The caloric test assesses lateral semicircular canal function. Bedside head impulse testing (HIT) can assess the function of the vertical semicircular canals, but the test is not always easy to perform or interpret; bedside HIT relies on clinicians’ subjective evaluation of saccade. The otolith organs have largely been ignored because there was no way to assess them.
“We’ve been limited up until now because we’ve only been able to see a small fraction of the picture of what’s happening,” said Kristen Janky, AuD, PhD, director of the Clinical Vestibular Lab and coordinator of vestibular services at Boys Town National Research Hospital in Omaha, Neb. “We weren’t able to get a big picture of what’s going on in the balance center.”
The advent of video head impulse testing (vHIT) and vestibular evoked myogenic potential (VEMP) testing now allows researchers and clinicians to probe the function of all ten end organs, including the three semicircular canals and two otoliths on each side.
These tests open new possibilities to researchers, clinicians, and patients. They also invite debate and confusion as clinicians figure out how best to utilize these tools for the benefit of patients.
Begin with the Basics
A thorough history and physical is still the best way to begin assessing a patient’s dizziness. In fact, a thorough history is “the most crucial part in figuring out what might be causing someone’s dizziness,” said Meredith E. Adams, MD, assistant professor of otolaryngology at the University of Minnesota in Minneapolis. So take your time and ask patients specific questions to elicit details about their dizziness. Many patients are eager to discuss their current symptoms, but also ask them when, where, and how the dizziness first began.
The information you receive will help you determine what tests to order. Many patients will still require an audiogram and/or imaging study. And, despite the availability of newer tests, electronystamography (ENG) or videonystagmography (VNG) remain mainstays of vestibular assessment.
“Videonystagmography gives you ear-specific information,” Dr. Adams said. “We also have a lot of data on it in different clinical disease states, so we know the most about its results in different diseases.”
In contrast, vHIT and VEMP are both so new that researchers are still working to establish normal vs. abnormal, and to correlate vHIT and VEMP results with clinical conditions. Because the new tests assess the vestibular system in ways that were previously impossible, however, healthcare providers are beginning to incorporate them into clinical practice.
Video Head Impulse Testing (vHIT)
“In a lot of ways, video head impulse testing has revolutionized our ability to test the function of the canals in a rigorous, quantitative way,” said Yuri Agrawal, MD, assistant professor of otolaryngology-head and neck surgery at Johns Hopkins University School of Medicine in Baltimore.
The FDA approved vHIT in 2013. The test utilizes a pair of video-equipped goggles to record eye movements in response to head movement. The testing equipment records and compares both the direction and velocity of head movement and the direction and velocity of eye movement. Well tolerated by most patients, vHIT measures the function of all six semicircular canals in each ear in about 10 minutes. (Caloric testing, in contrast, takes approximately 30 minutes and cannot be tolerated by some patients.)
Clinicians can use vHIT testing to detect saccades they might otherwise miss. “The camera can detect saccade whether it’s happening overtly or covertly,” Dr. Adams said. “It also gives an objective gain measure, a number that compares the amount of eye movement to the amount of head movement.”
The vHIT equipment also provides feedback regarding head thrust technique; there’s “more real-time feedback to make sure you’re doing it correctly,” Dr. Adams said.
Because vHIT is fast, well-tolerated and objective, it has “opened our eyes to a lot of things we didn’t even know were happening,” said Devin
McCaslin, PhD, associate professor in the department of hearing and speech sciences at Vanderbilt University in Nashville. “It allows us to track impairment, to learn how the vestibular system heals itself.”
The difference between bedside head impulse testing and vHIT is so stark that Dr. McCaslin likens bedside testing to a telescope and vHIT to the Hubble Space Telescope. For the first time, researchers can get a large-scale, intricately detailed look at vestibular function. “We’ve never really had population norms,” Dr. Agrawal said. “Now we’re able to get that because we can survey a lot of people. We’re gaining a better understanding of what normal function should look like and how it should change, so when we see patients, we can interpret their findings in the context of those population norms.”
vHIT is also helping researchers and clinicians identify disease states they were previously unable to detect. In a paper recently published in the American Journal of Audiology, Dr. McCaslin reports that some patients with Meniere’s disease have an abnormal caloric test but normal vHIT; as the disease progresses, both tests may become abnormal (Am J Audiol [Published online ahead of print November 7, 2014]; doi: 10.1044/2014_AJA-14-0040). Another study of 26 patients suggests that vHIT is helpful in differentiating vestibular neuritis from stroke (Otol Neurotol [Published online ahead of print October 15, 2014]; doi: 10.1097/MAO.0000000000000638).
While it appears to be a very promising test that will likely complement current vestibular assessments, it’s important to note that uncooperative patients and patients with neck injuries may not be able to undergo vHIT.
Vestibular Evoked Myogenic Potential (VEMP)
VEMP testing allows clinicians to gather information about the function of the otolith organs, part of the vestibular system that was previously beyond clinicians’ reach. The test uses sound or vibration to stimulate the inner ear; that stimulus triggers reflexive muscle movement, which is measured via electrodes placed on the neck (in the case of cervical VEMP) or face (in the case of ocular VEMP).
Cervical VEMP primarily assesses the saccule and the inferior vestibular nerve. To assess cervical VEMP, electrodes are placed on the neck to record the muscle potentials of the sternocleidomastoid muscle. When sound is introduced to the ear, the electrodes record the degree of ipsilateral muscle relaxation.
Ocular VEMP is believed to assess the utricle and superior division of the vestibular nerve. Assessment of ocular VEMP involves a sound or vibratory stimulus and electrodes placed just below the opposite eye. When the stimulus is introduced to the ear, the vestibulo-ocular reflex excites the eye muscles on the opposite side; ocular VEMP measures the muscle potential of the inferior oblique muscle.
VEMP testing can be used to assess the presence or absence of a VEMP response, as well as to measure thresholds and amplitude. “You can compare the amplitude between the two sides to see if there’s a lower amplitude on one side versus the other, suggesting it’s abnormal. You can also look at threshold—how loud you have to make the sound for the muscle to respond. More recently, people have been looking at the tuning characteristics of VEMP, meaning what frequency elicits the best stimulus response in a patient,” Dr. Adams said.
VEMP response typically fades with age; in fact, as many as forty percent of people over the age of 60 may not have a cVEMP response, while a quarter will lack an oVEMP response; however, research suggests that adjusting the increasing the frequency may induce a better VEMP response in older adults (Ear Hear. 2013; 34:e65-e73).
At present, the primary application of VEMP is to rule out superior semicircular canal dehiscence (SSCD) syndrome. Patients with SSCD usually have a lower-than-normal threshold for the VEMP response. Recent research suggests that oVEMP amplitudes in response to sound are superior to cVEMP thresholds in the diagnosis of SSCD (Otol Neurotol. 2013;34:121-126).
“In my practice, I use this test as part of my evaluation when I suspect a third window lesion,” Dr. Adams said. “At this point, it’s very clear that VEMP testing helps with superior canal dehiscence syndrome, but we’re just now getting more information to figure out how it will be most helpful in other disorders.”
Because VEMP assesses the function of the otoliths, Dr. Janky, Dr. McCaslin, and others believe it will complement, instead of supplant, other balance tests. “We’ve started adding the VEMP test to our initial battery of testing because a VEMP and VNG look at two separate parts of the ear,” Dr. Janky said.
VEMP testing is not yet FDA approved and at least some insurance companies consider VEMP testing experimental and investigational. Some clinicians are having luck billing VEMP testing as an unspecified code.
The Future of Balance Disorder Testing
“We’re at a stage with these tests where the tests are helping us understand the normal function of the vestibular system. As the tests are developed, we learn more about the vestibular system and how it functions in disease, and then we learn more about how to use the tests,” Dr. Adams said.
More research is needed to better understand what vHIT and VEMP testing reveals; it is hoped that increased understanding of the vestibular system will also lead to more precise diagnosis and treatment.
“The clinical utility of these tests is still being worked out,” Dr. McCaslin said. Clinicians interested in using vHIT and VEMP testing in their practices are advised to stay up on the current literature. “You need to understand what the best practices are at this time, because that’s changing in real time.”
Jennifer Fink is a freelance medical writer based in Wisconsin.